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Updated: 2 weeks 4 days ago

Proxima Centauri Sports a Fancy Dust Ring

Tue, 2017-11-28 12:00

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: ALMA Discovery of Dust Belts Around Proxima Centauri
Authors: Guillem Anglada, Pedro J. Amado, Jose L. Ortiz, and collaborators
First Author’s Institution: Institute of Astrophysics of Andalusia, Spain
Status: Accepted to ApJL, open access

Hiding behind the gleaming light of a pale red dot lies one of the world’s favorite exoplanets, Proxima b, whose orbital motion imprints a barely detectable wobble in its host star, Proxima Centauri. Following the seminal discovery of a planet in its habitable zone, this undistinguished low-mass red dwarf star became a target for tireless examination, despite the challenges involved in observing it. In today’s paper, we will see that one of these searches, however, seems to have paid off … in dust?

Figure 1. Observations of Proxima Centauri with the ALMA observatory suggest that it may not only host a rocky planet but also at least one dust ring around it. [Anglada et al. 2017]

The One Ring

Okay, I will be honest and admit that, indeed, few people in the world would be enthusiastic about dust — in space, of all things! But, let me tell you, this is exciting dust. Guillem Anglada and his collaborators (including his spacefaring self from a parallel universe, Guillem Anglada-Escudé, who is the first author of Proxima b’s discovery paper) used the Atacama Large Millimeter Array (ALMA) to observe a dust ring around Proxima Centauri. Such an object is a signpost of terrestrial planet formation and could clue us in on what its planetary system looks like and how it came to be.

You may recall that the lore of Middle Earth tells the story of The One Ring to Rule Them All™, whose (un)fortunate wearer is granted the power to become invisible. In a funny exchange of roles, the dust ring around Proxima Centauri may actually help us uncover more information about the well-hidden planet Proxima b, such as its orbital inclination and mass.

But leaving western epic novels aside, let’s talk about the instrument. The ALMA observatory has two types of antennas: the smaller 7-meter diameter detectors, and the bigger 12-meter ones. The data that comes from these antennas differ in that the smaller ones produce images with fewer details but with a wider field of view; the bigger antennas produce more detailed images but see a limited field of view.

Following analysis of the data depicted in Figure 2 below, the authors found that Proxima Centauri appears to emit more infrared light than it should, which they conclude must be produced by a “belt,” or ring, of cold (50 K) dust. The dust belt has a radius four times the distance of the Earth from the Sun (i.e. 4 AU). In reality, this dust ring seems to be an analog to the Solar System’s Kuiper Belt. They estimate that the mass contained in this belt is 1% that of Earth’s, which is similar to the mass of the Kuiper Belt around the Sun.

Figure 2. Image of Proxima Centauri (represented by the + mark) using ALMA’s 12-meter array. Although not clearly separated from the central source, the observed infrared excess strongly suggests the presence of a dust belt around the star with a radius smaller than 4 AU. Its shape also hints at the presence of a warmer belt closer to the central star, but this hypothesis needs confirmation. The identity of the detached turquoise blob to the left of the star is unknown: it could be either a real object or plain noise fluctuation (see main text). [Anglada et al. 2017]

Rings for Days

In addition to the “One Ring” described above, the elongated shape of the source in Figure 2 suggests the presence of a warmer (T ~ 90 K) dust belt with size 0.8 AU, but the authors don’t seem to be completely sure about its nature yet.

Now, if you’re asking yourself what that blob of emission to the lower left of Proxima Cen in Figure 2 could possibly be, the short answer is: we don’t know yet. The long answer is that it could either be a real source or it could just be random noise fluctuation. If it is a real source then the authors suggest many possible explanations, such as a background galaxy or a collision between large bodies. But the most intriguing explanation for this potential source is that we could be looking at the rings of a Saturn-like planet orbiting Proxima Cen. More observations will be needed to confirm this exciting possibility, though.

But wait, there’s more! Figure 3 below depicts the image as observed by the compact array configuration of ALMA, using the 7-meter antennas: what seems to be a second dust belt is seen as a series of green smudges at a distance of approximately 30 AU, represented by the white ellipse. Although the detection is very marginal, the authors propose that this could be an outer dust ring with 1/10,000 the mass of Earth.

Figure 3. Image of Proxima Centauri using ALMA’s compact array configuration (7-meter antennas). The dotted white ellipse marks the position of a dust belt of radius 30 AU. The green regions are the positions where the signal produced by the dust belt is stronger.

I know, I know, that was a lot to take in! Proxima Centauri suddenly seems a lot busier than we previously thought, likely sporting a rocky planet in its habitability zone and a Kuiper Belt-analog. The other possibilities, such as the Saturn-like ringed planet and the other dust belts are still a bit speculative, so further observations of the system are a no-brainer at this point.

About the author, Leonardo dos Santos:

Leo is an exoplanet scientist and Ph.D. candidate at the Geneva Observatory. His current research involves characterization of exoplanets, physical and chemical properties of stars similar to the Sun and developing astronomical software. Not to be confused with the constellation.

Featured Image: Connecting Theory to Observation with the FluxCompensator

Mon, 2017-11-27 12:00

One of the challenges of astronomy is connecting theoretical models of distant objects to observations. Numerical simulations can produce ideal visualizations of objects and their physical processes, but this doesn’t necessarily represent what we’ll be able to see when we look at these sources with instruments that have finite resolution and sensitivity. Two scientists with the Max Planck Institute for Astronomy in Germany, Christine Koepferl and Thomas Robitaille, have now created a tool to help us make these connections: an open-source Python package called the FluxCompensator. This software package allows scientists to post-process the output of their numerical simulations, adding observational effects to the data like telescope point-spread functions, transmission curves, finite pixel resolution, noise, and reddening. The outputs of the FluxCompensator are images more consistent with what we would expect to be able to observe with our available telescopes and instruments.

In the figure above (click for a closer look), the authors display three simulated sources: a) a young stellar object, b) a star-forming region, and c) the center of a galaxy. The top panels show synthetic single-band observations extracted directly from the models, whereas the bottom panels are synthetic three-color images produced by the FluxCompensator that mimic the observational effects expected if these sources were observed as part of the GLIMPSE survey with Spitzer (a and b) or the Hi-GAL survey with Herschel (c). To read more about FluxCompensator, check out the article below.

Citation

Christine M. Koepferl and Thomas P. Robitaille 2017 ApJ 849 3. doi:10.3847/1538-4357/aa8666

Analog of the Milky Way and the Magellanic Clouds

Wed, 2017-11-22 12:00

A hunt for merging dwarf galaxies has yielded an intriguing result: 180 million light-years away, a galaxy very similar to the Milky Way — with two dwarf-galaxy satellites just like our own Magellanic clouds.

Unusual Satellites

The Large and Small Magellanic clouds, as observed from Earth. [ESO/S. Brunier]

The Large and Small Magellanic clouds (LMC and SMC), the only bright and star-forming satellite galaxies around the Milky Way, have proven unusual in the universe: satellite pairs of LMC–SMC mass are neither common in observation nor  typically produced in numerical simulations of galaxy formation and evolution.

Since the probability of having such an interacting pair of satellites in a massive halo is so low, this raises questions about how our system came about. Did the Magellanic clouds form independently around the Milky Way and then interact? Were they more recently captured as an already-merging pair of dwarf galaxies? Or is there some other explanation?

If we could find other systems that look like the LMC–SMC–Milky-Way system, we might be able to learn more about pairs of dwarf galaxies and how they interact near the halos of large galaxies like the Milky Way. Conveniently, two researchers from Yonsei University in South Korea, Sanjaya Paudel and Chandreyee Sengupta, have now identified exactly such a system.

The UGC 4703 pair of dwarf galaxies show a stellar bridge connecting them — a sign of their past interaction, when tidal forces stripped material from them as they passed each other. [Adapted from Paudel & Sengupta 2017]

An Interacting Pair

Hunting for merging dwarf galaxies in various environments, Paudel and Sengupta found UGC 4703, an interacting pair of dwarf galaxies that are located near the isolated spiral galaxy NGC 2718. This pair of satellites around the massive spiral bear a striking resemblance to the LMC–SMC system around the Milky-Way.

The authors performed a multi-wavelength study of the system using archival images from the Sloan Digital Sky Survey, The Galaxy Evolutionary Explorer spacecraft, and the Spitzer Space Telescope. They also gather new observations of the H I gas distribution in the system using the Giant Metrewave Radio Telescope in India.

Paudel and Sengupta find that NGC 2718 and the Milky Way have similar stellar masses, and the stellar mass ratio of the UGC 4703 interacting pair is around 5:1, similar to the mass ratio of the LMC to the SMC. The separation of the UGC 4703 pair is also roughly the same as that of the LMC and SMC: ~70,000 light-years.

Similarities and Differences

The H I gas distribution in UGC 4703 reveals both similarities and differences between this system and the LMC–SMC system. [Paudel & Sengupta 2017]

The stellar bridge connecting the components of the UGC 4703 system are a sign of their past interaction, but a comparison of the optical and H I morphology between the UGC 4703 pair and the LMC–SMC pair suggests that the UGC 4703 galaxies are either interacting more slowly than the Magellanic clouds or that the interaction is at a more advanced stage than we see with the LMC–SMC.

Understanding these similarities and differences between the LMC–SMC–Milky-Way system and this analog are an important first step to studying dwarf galaxy pairs as they interact near the massive halos of their large spiral hosts. In the future, further observations of UGC 4703 and detailed modeling of the system may help continue to puzzle out how our own Magellanic clouds came about.

Citation

Sanjaya Paudel and C. Sengupta 2017 ApJL 849 L28. doi:10.3847/2041-8213/aa95bf

Protoplanetary Disks Might Be More Turbulent than Thought

Tue, 2017-11-21 12:00

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: The Effects of Protostellar Disk Turbulence on CO Emission Lines: A Comparison Study of Disks With Constant CO Abundance vs. Chemically Evolving Disks
Authors: Mo (Emma) Yu, Neal Evans, Sarah Dodson-Robinson, Karen Willacy, Neal Turner
First Author’s Institution: University of Texas at Austin
Status: Accepted to ApJ, open access

We know it happens. We see that protoplanetary disks — the birthplace of planets — spill the gaseous material at their inner edge onto the young stars around which they orbit. This process of accretion persists throughout the disk’s lifetime and within 1 to 5 million years (or 10 Myr in rare cases), a disk will feed all of its gas to its star and completely fade away — leaving behind only the planets that formed along the way and any leftover rocky material that remains.

The rate at which accretion occurs — as well as why accretion occurs — are both driving forces behind determining what types of planets can form quickly enough in a protoplanetary disk’s relatively short lifetime. While we can measure accretion rates onto stars directly, we still do not know why disks accrete! This is one of the most important unsolved problems in planet formation. Until we can solve it, our models for planet formation are incomplete.

There are two leading potential explanations for why accretion occurs: (1) turbulence, and (2) magnetic winds. In the last two years, several attempts have been made to measure turbulence in a nearby disk for the first time using CO (carbon monoxide) spectral lines. These measurements showed that the turbulence in that system is not strong enough to be responsible for accretion, winning favor for the other idea of magnetic winds.

However, today’s paper led by Emma Yu argues that those measurements were not interpreted properly since they did not take into account the relatively rapid rate at which CO depletes over time (see Figure 1). This paper asks: What can we really learn about turbulence from CO spectral lines if we include CO depletion in our model?

Figure 1: Density of CO and H2 at a range of distances from the star over time (taken from chemical models). Previous measurements of turbulence assumed the ratio of CO-to-H2 to be constant over time, but this is not true beyond 15 AU after 1 Myr. [Adapted from Yu et al. 2017]

Background: Why Do Protoplanetary Disks Accrete onto Their Stars?

It is understood that disks must accrete by transporting angular momentum outwards. Naively, we might expect this to happen through “viscous shear flow.” In this process, the gaseous disk struggles to behave like a fluid because it rotates at a slower rate further away from the star (due to Kepler’s 3rd law). As a result, material in the outer disk will try to speed up to catch up to the inner disk — thereby increasing its angular momentum. Meanwhile to compensate for that increase (and conserve angular momentum), the material in the inner disk will slow down. Since this gas is rotating too slowly to obey Kepler’s 3rd law, it will spiral inwards and eventually accrete.

There is just one problem: disks are too sparse to interact viscously like a fluid! For shear flow to occur, there must be some source of turbulence to make the disk viscous. It is widely accepted that magnetic fields can create turbulence by triggering an instability, but it is not clear if this instability actually occurs in the entire disk. If it does not occur in most of the disk, turbulence most likely would not be responsible for making the disk accrete.

Instead, magnetic winds — which transport angular momentum outward in a completely different way by flinging material out of the disk on magnetic field lines pointed away from the star while the disk rotates (see also here and here for more) — would be the favored reason for the disk accreting.

Introduction to Spectral Lines with Turbulence

We would know why protoplanetary disks accrete if we could just measure their levels of turbulence, but we did not have the telescope power to do this until the ALMA telescope array was turned on several years ago. In this astrobite, Tim discussed a new method for measuring turbulence with ALMA by looking at the shape of a specific CO spectral line.

Spectral lines are one of the most important tools in astronomy. They are used for everything from inferring compositions of atmospheres to measuring distances to distant galaxies, among many other things. Molecules (and atoms) emit spectral lines when electrons that were excited to a high energy state transition back to a lower energy state. While these lines are emitted at the precise wavelength corresponding to a transition, they never appear perfectly thin. They always exhibit some level of “broadening” due to the gas molecules moving towards us or away from us as they emit.

The velocities of individual molecules are mostly random. Thermal effects create most of the random motion for gas molecules. Besides that, turbulence also creates a little bit of random motion, causing spectral lines to broaden more than normal. As a result, we can use the broadened shapes of spectral lines (that are very well-resolved) to measure turbulence!

Spectral Lines with CO Depletion

The paper covered in Tim’s astrobite found that turbulence can be probed by measuring the CO spectral line’s peak-to-trough ratio (see Figure 2). Specifically, disks that are less turbulent have higher peak-to-trough ratios. However, this analysis assumed that the amount of CO in the disk relative to hydrogen is fixed over time, whereas chemical models predict it should actually drop significantly in older disks (see Figure 1).

Figure 2: CO spectral line profiles with different levels of turbulence, with lower levels producing a higher peak-to-trough ratio. (Note: The double-peak structure arises from part of the disk rotating towards us and part of the disk rotating away from us.) [Adapted from Yu et al. 2017]

Coincidentally, Emma Yu et al. find that the depletion of CO also creates spectral lines with higher peak-to-trough ratios (see Figure 3). As a result, the authors of the other study may have thought that they detected low levels of turbulence by measuring a high peak-to-trough ratio when they may really only be seeing evidence that CO has been depleted!

Figure 3: CO spectral line profiles at different stages in the disk lifetime. Left: Includes CO depletion. Right: Assumes constant CO-to-H2 ratio. With a constant CO ratio, the peak-to-trough ratio does not change after 1 Myr. With CO depletion, the peak-to-trough ratio increases over time just like with decreasing levels of turbulence. [Adapted from Yu et al. 2017]

Figures 2 and 3 (and also Figure 7 of the paper) show how easy it is to confuse a disk with no turbulence at all with a disk that has a moderate level of turbulence. If that extra turbulence is present, it might be strong enough to explain the observed levels of accretion in the previous study, giving favor to the long-assumed idea that turbulence drives accretion — not magnetic winds. However, the paper shows that our current measurements may not be able to distinguish these levels of turbulence, leaving the question of why disks accrete still unsolved.

Future Work

All hope is not lost! The authors point out that a different isotopologue of carbon monoxide (C18O) may be more useful for measuring turbulence. However, its spectral lines are weaker and thus, more difficult to resolve. They also point out that CO would be more helpful if we could also measure the precise level of CO depletion in a disk (rather than infer it from chemical models).

This is an exciting time for studying protoplanetary disks because we are finally beginning to scratch the surface of measuring turbulence. Getting proper measurements though, will require more digging.

About the author, Michael Hammer:

I am a 3rd-year graduate student at the University of Arizona, where I am working with Kaitlin Kratter on simulating planets, vortices, and other phenomena in protoplanetary disks. I am from Queens, NYC; but I’m not Spider-Man…

Kepler Planets Tend to Have Siblings of the Same Size

Mon, 2017-11-20 12:00

After 8.5 years of observations with the Kepler space observatory, we’ve discovered a large number of close-in, tightly-spaced, multiple-planet systems orbiting distant stars. In the process, we’ve learned a lot about the properties about these systems — and discovered some unexpected behavior. A new study explores one of the properties that has surprised us: planets of the same size tend to live together.

Orbital architectures for 25 of the authors’ multiplanet systems. The dots are sized according to the planets’ relative radii and colored according to mass. Planets of similar sizes and masses tend to live together in the same system. [Millholland et al. 2017]

Ordering of Systems

From Kepler’s observations of extrasolar multiplanet systems, we have seen that the sizes of planets in a given system aren’t completely random. Systems that contain a large planet, for example, are more likely to contain additional large planets rather than additional planets of random size. So though there is a large spread in the radii we’ve observed for transiting exoplanets, the spread within any given multiplanet system tends to be much smaller.

This odd behavior has led us to ask whether this clustering occurs not just for radius, but also for mass. Since the multiplanet systems discovered by Kepler most often contain super-Earths and mini-Neptunes, which have an extremely large spread in densities, the fact that two such planets have similar radii does not guarantee that they have similar masses.

If planets don’t cluster in mass within a system, this would raise the question of why planets coordinate only their radii within a given system. If they do cluster in mass, it implies that planets within the same system tend to have similar densities, potentially allowing us to predict the sizes and masses of planets we might find in a given system.

Insight into Masses

Led by NSF graduate research fellow Sarah Millholland, a team of scientists at Yale University used recently determined masses for planets in 37 Kepler multiplanet systems to explore this question of whether exoplanets in a multiplanet system are more likely to have similar masses rather than random ones.

Millholland and collaborators find that the masses do show the same clustering trend as radii in multiplanet systems — i.e., sibling planets in the same system tend to have both masses and radii that are more similar than if the system were randomly assembled from the total population of planets we’ve observed. Furthermore, the masses and radii tend to be ordered within a system when the planets are ranked by their periods.

The host star’s metallicity is correlated with the median planetary radius for a system. [Adapted from Millholland et al. 2017]

The authors note two important implications of these results:

  1. The scatter in the relation between mass and radius of observed exoplanets is primarily due to system-to-system variability, rather than the variability within each system.
  2. Knowing the properties of a star and its primordial protoplanetary disk might allow us to predict the outcome of the planet formation process for the system.

Following up on the second point, the authors test whether certain properties of the host star correlate with properties of the planets. They find that the stellar mass and metallicity have a significant effect on the planet properties and the structure of the system.

Continuing to explore multiplanet systems like these appears to be an excellent path forward for understanding the hidden order in the broad variety of exoplanets we’ve observed.

Citation

Sarah Millholland et al 2017 ApJL 849 L33. doi:10.3847/2041-8213/aa9714

LIGO Finds Lightest Black-Hole Binary

Fri, 2017-11-17 12:00

Wednesday evening the Laser Interferometer Gravitational-wave Observatory (LIGO) collaboration quietly mentioned that they’d found gravitational waves from yet another black-hole binary back in June. This casual announcement reveals what is so far the lightest pair of black holes we’ve watched merge — opening the door for comparisons to the black holes we’ve detected by electromagnetic means.

A Routine Detection

The chirp signal of GW170608 detected by LIGO Hanford and LIGO Livingston. [LIGO collaboration 2017]

After the fanfare of the previous four black-hole-binary merger announcements over the past year and a half — as well as the announcement of the one neutron-star binary merger in August — GW170608 marks our entry into the era in which gravitational-wave detections are officially “routine”.

GW170608, a gravitational-wave signal from the merger of two black holes roughly a billion light-years away, was detected in June of this year. This detection occurred after we’d already found gravitational waves from several black-hole binaries with the two LIGO detectors in the U.S., but before the Virgo interferometer came online in Europe and increased the joint ability of the detectors to localize sources.

Mass estimates for the two components of GW170608 using different models. [LIGO collaboration 2017]

Overall, GW170608 is fairly unremarkable: it was detected by both LIGO Hanford and LIGO Livingston some 7 ms apart, and the signal looks not unlike those of the previous LIGO detections. But because we’re still in the early days of gravitational-wave astronomy, every discovery is still remarkable in some way! GW170608 stands out as being the lightest pair of black holes we’ve yet to see merge, with component masses before the merger estimated at ~12 and ~7 times the mass of the Sun.

Why Size Matters

With the exception of GW151226, the gravitational-wave signal discovered on Boxing Day last year, all of the black holes that have been discovered by LIGO/Virgo have been quite large: the masses of the components have all been estimated at 20 solar masses or more. This has made it difficult to compare these black holes to those detected by electromagnetic means — which are mostly under 10 solar masses in size.

GW170608 is the lowest-mass of the LIGO/Virgo black-hole mergers shown in blue. The primary mass is comparable to the masses of black holes we have measured by electromagnetic means (purple detections). [LIGO-Virgo/Frank Elavsky/Northwestern]

One type of electromagnetically detected black hole are those in low-mass X-ray binaries (LMXBs). LMXBs consist of a black hole and a non-compact companion: a low-mass donor star that overflows its Roche lobe, feeding material onto the black hole. It is thought that these black holes form without significant spin, and are later spun up as a result of the mass accretion. Before LIGO, however, we didn’t have any non-accreting black holes of this size to observe for comparison.

Now, detections like GW170608 and the Boxing Day event (which was also on the low end of the mass scale) are allowing us to start exploring spin distributions of non-accreting black holes to determine if we’re right in our understanding of black-hole spins. We don’t yet have a large enough comparison sample to make a definitive statement, but GW170608 is indicative of a wealth of more discoveries we can hope to find in LIGO’s next observing run, after a series of further design upgrades scheduled to conclude in 2018. The future of gravitational wave astronomy continues to look promising!

Citation

LIGO collaboration, submitted to ApJL. https://arxiv.org/abs/1711.05578

A Look at the Milky Way’s Outskirts

Wed, 2017-11-15 12:00

Studying the large-scale structure of the Milky Way is difficult given that we’re stuck in its interior — which means we can’t step back for a broad overview of our home. Instead, a recent study uses distant variable stars to map out a picture of what’s happening in the outskirts of our galaxy.

Mapping with Tracers

Phase-folded light curve for two of the RR Lyrae stars in the authors’ sample, each with hundreds of observations over 7 years. [Cohen et al. 2017]

Since observing the Milky Way from the outside isn’t an option, we have to take creative approaches to mapping its outer regions and measuring its total mass and dark matter content. One tool used by astronomers is tracers: easily identifiable stars that can be treated as massless markers moving only as a result of the galactic potential. Mapping the locations and motions of tracers allows us to measure the larger properties of the galaxy.

RR Lyrae stars are low-mass, variable stars that make especially good tracers. They pulsate predictably on timescales of less than a day, creating distinctive light curves that can easily be distinguished and tracked in wide-field optical imaging surveys over long periods of time. Their brightness makes them detectable out to large distances, and their blue color helps to separate them from contaminating stars in the foreground.

Best of all, RR Lyrae stars are very nearly standard candles: their distances can be determined precisely with only knowledge of their measured light curves.

Locations on the sky of the several hundred outer-halo RR Lyrae stars in the authors’ original sample. The red curve shows the location of the Sagittarius stream, an ordered structure the authors avoided so as to only have unassociated stars in their sample. [Cohen et al. 2017]

Distant Variables

In a new study led by Judith Cohen (California Institute of Technology), the signals of hundreds of distant RR Lyrae stars were identified in observations of transient objects made with the Palomar Transient Factory (PTF) survey. Cohen and collaborators then followed up with the Keck II telescope in Hawaii to obtain spectra for a narrower sample of 122 RR Lyrae stars.

The stars in the sample lie at whopping distances of ~150,000–350,000 light-years from us. For comparison, we’re about 25,000 light-years from the center of the galaxy, and the stellar disk of the galaxy is only thought to be perhaps 100,000 light-years across — so these variable stars lie firmly in the Milky Way’s outer halo. The spectra of the stars reveal their radial velocity, providing us with precise measurements of how objects in the outer halo move.

More Space in the Suburbs?

Histogram with distance for the ~450 RR Lyrae stars in the authors’ broader sample. When the authors include their estimates for the completeness of their sample, the best fit scales with distance as r-4, shown by the red line. [Cohen et al. 2017]

After reporting the velocity dispersions that they measure — which can be used to make more precise estimates of the Milky Way’s total mass — Cohen and collaborators discuss the stellar density implied by their sample. They find that the density of stars in the outer halo of the Milky Way scales with their distance as r-4. This is similar to the drop-off in density we’ve measured in the inner halo, and it contradicts some studies that have predicted a much sharper drop in stellar density in the Milky Way’s outermost regions.

The work presented in this study goes a long way toward building our view of the galaxy’s outer halo. Future catalogs like the Pan-STARRS RR Lyrae catalog and upcoming surveys like LSST should also significantly increase the tracer sample size and measurement accuracy, further allowing us to map out the outskirts of the Milky Way.

Citation

Judith G. Cohen et al 2017 ApJ 849 150. doi:10.3847/1538-4357/aa9120

Can We Detect Auroral Emission from Proxima b?

Tue, 2017-11-14 12:00

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: The Detectability of Radio Auroral Emission from Proxima b
Authors: Blakesley Burkhart & Abraham Loeb
First Author’s Institution: Harvard-Smithsonian Center for Astrophysics
Status: Published in ApJL, open access

Dazzling auroral displays are not uncommon in our solar system. In fact, when the Sun sends highly energetic particles out in to the solar system, any planets with a substantial magnetic field will interact with the particles, resulting in the emission of radio waves. Detecting this emission allows us to determine many interesting planetary properties, such as orbital parameters, habitability, plate tectonics and atmospheric compositions. Yet we have not observed any auroral activity from planets that lie outside our solar system (we have, however, detected radio auroral emission on a brown dwarf star!).

Figure 1: Artist’s impression showing Proxima b orbiting its red dwarf host Proxima Centauri. [ESO/M.Kornmesser]

Today’s bite investigates our nearest stellar neighbour, Proxima Centauri, to answer one very important question: can we detect auroral emissions from its exoplanet, Proxima b?

What Are We Looking For?

To answer this question, the authors first employ radiometric Bode’s law to estimate the magnitude of radio waves released during stellar wind and magnetosphere interactions. Radiometric Bode’s law, derived from observations of magnetic planets within our own solar system, indicates that the brightness of radio waves increases with size of the planetary magnetosphere. Blackett’s law then suggests the size of the magnetosphere scales with mass and rotation speed, which translates to a predicted radio power of 1013 for Proxima b. When combined with Proxima b’s estimated magnetic field strengths of 0.007 – 1 G, comparable to the 0.5 G observed at the Earth’s equator, scientists expect the frequency of these radio waves to be between 0.02 – 2.8 MHz.

Figure 2: Predicted brightness of radio waves versus emission frequency, in accordance with Bode’s law, for 106 exoplanets. The expected values for Proxima b are highlighted in the blue square, with the yellow star representing a magnetic field value of 0.3 G. The black dashed line indicates the cut-off frequency for Jupiter. [Burkhart & Loeb 2017]

Figure 2 highlights the first hurdle for observations of radio auroral emissions from Proxima b. All radiometric modelling points to radio waves being emitted at frequencies between 0.02 – 2.8 MHz for this exoplanet. However observations below 10 MHz are not possible from Earth, as these wavelengths are blocked by our atmosphere — so even at the extremes of likely magnetic field strengths, we cannot use ground-based instrumentation. A secondary issue with observing aurorae on Proxima b is that we’re now observing in a low-frequency regime that tends to be absorbed by the interstellar medium (ISM).

Modelling a Magnetosphere

So far, the authors have only estimated the brightness of auroral activity on Proxima b. With its close-in orbit creating a highly variable magnetosphere radius, we expect the variation in the observed radio waves to be rather large. To characterise this variability, authors implemented models of the wind and magnetic field around Proxima b as a function of various orbital parameters. An example of the results obtained from the simulations is shown below in Figure 3.

Figure 3: Variation in the observed radio brightness as the radius of the magnetosphere caries over Proxima b’s short 11.2 day orbit. Eccentricity = 0 and inclination = 10 degrees. [Burkhart & Loeb 2017]

Here we see how the simulated brightness of radio waves changes over one orbit. The two dips correspond to periods where the magnetospheric radius suddenly changes as the planet passes through streamer regions, located near Proxima Centauri’s equator, where stellar winds become denser. The authors also considered four different magnetic field strengths and found that a weaker magnetic field, and therefore lower emission frequency, results in brighter radio emission.

From modelling the variability in radio flux for Proxima b, the authors concluded two things:

  1. Proxima b’s radio auroral emissions vary by almost an order of magnitude over one full orbit.
  2. The amplitude of the variation depends on orbital parameters — the eccentricity and inclination of the orbit — as well as on parameters of the stellar wind and planetary magnetic field.
Can We Detect Aurorae Around Proxima b?

There are a number of issues associated with undertaking observations of Proxima b’s radio emission. As previously mentioned, the ISM is highly problematic, as electrons within the ISM are more likely to absorb photons at low frequencies due to free-free absorption. Thankfully, this shouldn’t be an issue above 0.3 MHz for nearby planetary systems, like Proxima b, as there is less ISM to contend with. We also have to overcome the 10 MHz atmospheric cut-off introduced by absorption of photons in the ionosphere. Clearly the only solution here is to make observations from space. The authors mention several interesting proposals including a radio observatory on the Moon (one example given is ROLSS) and clusters of low-cost CubeSats to form a very large telescope (think of experiments like ALMA which combined lots of smaller telescopes to form one big one, but in space).

Figure 4: Example CubeSat with hands for scale. [NASA]

The take-home message of this paper is that the brightness of radio emissions around Proxima b are substantial enough to be detected here on Earth. This is fantastic because if aurorae, caused by interactions between stellar winds and the planet’s magnetosphere, are detected on Proxima b, we will be able to further constrain the planet’s orbital inclination, eccentricity and generally gain insight into its magnetosphere. Now we just have to wait for the right instrumentation to put into space, allowing scientists to overcome the pesky 10 MHz limit imposed by our own atmosphere.

About the author, Amber Hornsby:

First year postgraduate researcher based in the Astronomy Instrumentation Group at Cardiff University. Currently I am working on detectors for future observations of the Cosmic Microwave Background. Other interests include coffee, Star Trek and pizza.

Featured Image: Nuclear Star Clusters in Virgo

Mon, 2017-11-13 12:00

This collection of images (click for the full view) from the Hubble Space Telescope reveals the nuclear star clusters of early-type galaxies located in the Virgo cluster. These dense clusters of stars are only ~10 light-years in size, and they have been found to lie at the core of galaxies throughout the universe. A recent study led by Chelsea Spengler (University of Victoria, Canada) presents an analysis of 39 of these nuclei and their hosts in the Virgo cluster, exploring the masses, metallicities and ages of the nuclei. The authors used their observations to better understand how nuclei form: are they the result of smaller star clusters falling to the center of their host galaxies and merging? Or were they formed in situ from gas funneled into the galactic centers? To learn more about what the authors discovered, check out the paper below.

Citation

Chelsea Spengler et al 2017 ApJ 849 55. doi:10.3847/1538-4357/aa8a78

Use Authorea or Overleaf to Submit to AAS Journals

Fri, 2017-11-10 12:00

Do you use collaborative document preparation software like Authorea or Overleaf? If so, submitting to AAS journals just got easier. Both systems are now partnered with the American Astronomical Society so that you can directly submit to AAS journals from within Authorea or Overleaf.

What Are Authorea and Overleaf?

Authoring tools like Authorea and Overleaf make jointly preparing a scientific document online easier. [Overleaf]

Authorea and Overleaf are both collaborative, online word processors for technical documents — like scientific articles. For scientists used to writing up their research articles in LaTeX, Authorea and Overleaf provide a way of working in that same, familiar environment while building the paper online simultaneously with multiple collaborators, much like in a Google Doc. This software allows users to write, track changes via version control, host data, provide commentary, and ultimately publish their research.

For more information on how Authorea or Overleaf can be used for scientific manuscript preparation, you can check out this Authorea demo of “The ‘Paper’ of the Future” led by Alyssa Goodman (Harvard University), or read up on Overleaf’s benefits for writing and collaborating.

What Does This Partnership Mean?

The partnership between the AAS and Authorea and Overleaf is intended to make life easier for AAS authors who work with these collaborative authoring tools and wish to submit their article to AAS journals when the manuscript is complete.

For initial submissions to all AAS Journals — The Astronomical Journal, The Astrophysical Journal, The Astrophysical Journal Supplements, The Astrophysical Journal Letters, and Resarch Notes of the AAS — authors can now submit their LaTeX manuscript and all their figures directly from Authorea or Overleaf to the AAS journal peer-review system when the files are complete. The direct submission from Authorea or Overleaf will also pre-populate the submission forms with some of the metadata, reducing the information that the authors will need to enter by hand when they go to the peer-review system to complete the submission process.

What Else Should You Know?

Authorea and Overleaf now allow you to directly submit your manuscript to an AAS journal when it is ready. [Authorea]

The AAS provides LaTeX templates for author use in preparing manuscripts for submission to AAS journals (for more on this, see previous posts about AASTex 6.0 and AASTex 6.1), and these templates are now integrated into Authorea and Overleaf for use preparing manuscripts for AAS journal submission with these tools. You can find the templates for the two services here:
Authorea templates for submission to AAS journals
Overleaf templates for submission to AAS journals

Instructions on how to submit to AAS journals via Authorea can be found here. Any questions about the Overleaf submission process can be directed to the Overleaf contact page.

If you would like more information about the details of submitting to AAS journals from within Authorea or Overleaf, you can visit the AAS journals FAQ on collaborative authoring services here.

Carrying Energy to the Corona with Waves

Wed, 2017-11-08 12:00

The solar corona has a problem: it’s weirdly hot! A new study explores how magnetic waves might solve the mystery of the unusually hot corona by transporting energy to the outer atmosphere of the Sun.

The Problem with the Corona

The temperatures of different layers of the Sun. Click for a closer look. [ISAS/JAXA]

The corona, the outer layer of the Sun’s atmosphere, has typical temperatures of 1–3 million K — significantly hotter than the cool 5,800 K of the photosphere, the surface of the Sun far below it. Since temperatures ordinarily drop the further you get from the heat source (in this case, the Sun’s atom-fusing center), this so-called “coronal heating problem” poses a definite puzzle.

As is the case for many astronomical mysteries, the answer may have something to do with magnetic fields. Alvén waves, magnetohydrodynamic waves that travel through magnetized plasma, could potentially carry energy from the convective zone beneath the Sun’s photosphere up into the solar atmosphere. There, the Alfvén waves could turn into shock waves that dissipate their energy as heat, causing the increased temperature of the corona.

The Daniel K. Inouye Solar Telescope, located on the summit of Haleakala in Hawaii, is scheduled to be completed in 2018. [Ekrem Canli]

Predicting Observations

Alfvén waves as a means of delivering heat to the corona makes for a nice picture, but there’s a lot of work to be done before we can be certain that this is the correct model. Observational evidence of Alfvén waves has thus far been limited to specific conditions — and the observations have not yet been enough to convince us that Alfvén waves can deliver enough energy to explain the corona’s temperature.

Lucas Tarr, a scientist at the Naval Research Laboratory, argues that upcoming solar telescopes may make it easier to detect these waves — but first we need to know what to look for! In a recent study, Tarr uses a simplified analytic model to show which frequencies of waves are likely to carry power when magnetic field lines in the corona are pertubed.

A Promising Future

The power carried by Alfvén waves as a function of frequency, as a result of an initial perturbation, plotted for several different initial conditions (such as the size of the perturbation or the length of the loop on which it is introduced). [Tarr 2017]

Tarr modeled the effects of a minor perturbation — like a local magnetic reconnection event in the corona — on a coronal arcade, a common structure of magnetic field loops found in the corona. Tarr determined that such a disturbance would peak in power at a low frequency (maybe tens of millihertz, or oscillations on scales of minutes), but a substantial portion of the power is carried by waves of higher frequencies (0.5–4 Hz, or oscillations on scales of seconds).

Tarr’s findings confirm that with the cadence and sensitivity of current instrumentation, we would not expect to be able to detect these Alfvén waves. The results do indicate, however, that high-cadence observations with future telescope technology — like the instrumentation at the upcoming Daniel K. Inouye Solar Telescope, which should be completed in 2018 — may have the ability to reveal the presence of these waves and confirm the model of Alfvén waves as the means by which the Sun achieves its mysteriously hot corona.

Citation

Lucas A. Tarr 2017 ApJ 847 1. doi:10.3847/1538-4357/aa880a

Feeding Black Holes Through Galactic Bars

Tue, 2017-11-07 12:00

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Galaxy-Scale Bars in Late-Type Sloan Digital Sky Survey Galaxies Do Not Influence the Average Accretion Rates of Supermassive Black Holes
Authors: A.D. Goulding, E. Matthaey, J.E. Greene, et al.
First Author’s Institution: Princeton University
Status: Accepted to ApJ, open access

When it comes to picking their host galaxies, active galactic nuclei (or AGN) are rather promiscuous. They reside in all types of galaxies: ellipticals, irregulars, and spirals. AGN of the same feather tend to flock together — the more luminous and radio-loud ones are found in elliptical galaxies while the lower luminosity ones are more often found in spiral galaxies. This is a manifestation of the black hole mass-host galaxy luminosity correlation, where spiral galaxies like our Milky Way tend to have less massive black holes than elliptical galaxies. Besides spiral arms, spiral galaxies sometimes also boast of having bars, if the right mood strikes. How are bars related to their AGN? Could they trigger the central black holes to light up as AGN?

Galactic bars are thought to contribute to the dynamical evolution of their host galaxies. Numerical studies show that they can funnel in gas from the outskirts to the central regions of the galaxies, triggering star formation and possibly AGN activity. It is still unclear whether bars actually help trigger AGN, as previous studies have produced conflicting results and tend to suffer from small number statistics and biased AGN diagnostics. In today’s paper, the authors bring better tools to bear on the problem, by utilizing the large wealth of information from the SDSS Galaxy Zoo citizen science project and X-ray stacking analyses.

The authors first selected a sample of ~100,000 local spiral galaxies from SDSS that have been visually classified as such in Galaxy Zoo by at least 20 people. This sample is later divided into three redshift bins that are each complete in stellar mass, i.e. there are galaxies that span the whole range of stellar masses in each redshift bin. This is to ensure the final results are representative and unbiased. They distinguished sources with and without bars in these three redshift bins using information from Galaxy Zoo, based on the fraction of votes to the question “Is there a sign of a bar feature through the center of the galaxy?” If the fraction of votes is equal or greater than 0.25, the galaxy is defined to have a bar; if the fraction of votes is less than 0.1, the galaxy is defined to be bar-less. Galaxies with fraction of votes between these two numbers are defined to be ambiguous. Figure 1 shows some examples of spiral galaxies with bars, no bars, and ambiguous bars in their sample.

Fig. 1: Sample unbarred (blue borders), ambiguously barred (yellow borders), and barred (red borders) spiral galaxies from the Galaxy Zoo project, as determined by fbar, which is the fraction of votes by citizen scientists for the presence of bars. [Goulding et al. 2017]

In contrast to optical light which is absorbed by dust, X-rays from an AGN can more easily pierce through dust obscuration. Stacking lots of X-ray observations help to reveal heavily-obscured or low-luminosity AGN. Using data from the Chandra X-ray observatory, the authors performed X-ray stacking analyses to investigate the presence of AGN in their barred, unbarred, and ambiguously barred samples. Figure 2 shows the X-ray luminosity of their samples after subtracting the contributions from star formation processes. The three types of spiral galaxies do not show obvious differences in their X-ray luminosities, suggesting that AGN are no more common in one type of galaxy than the others. For galaxies with X-ray detections (i.e. hosting AGN), the authors further investigated the distributions of their specific black hole accretion rates, which are the X-ray luminosities divided by the host galaxy stellar mass shown in Figure 3. This ratio removes the dependence on stellar mass and instead probes the dependence on the host galaxy properties. There is again no difference in the accretion rates between the barred and unbarred samples.

Fig. 2: Star-formation subtracted X-ray luminosities vs. redshift in three X-ray energy bands. As with Figure 1, blue markers refer to unbarred galaxies, yellow to ambiguously barred, and red to barred galaxies. Open markers are sources with X-ray detections while filled markers are the luminosities produced by stacking sources without X-ray detections. As a comparison, the predicted mean X-ray luminosities due to stellar processes are shown by the dotted lines. [Adapted from Goulding et al. 2017]

Fig. 3: Distributions of specific black hole accretion rates for galaxies with AGN. The different line colors again refer to the presence or absence of bars. Dashed, solid, and dotted lines refer to different cuts in the X-ray luminosity. [Goulding et al. 2017]

Well, all that is a bummer — the presence of AGN in spirals seems to be independent of the presence of bars. For those with AGN, there is also no difference in the specific accretion rates of their host galaxies on the basis of a bar existence. As stacking analyses tend to wash away short timescale events, any bar contributions to AGN activities would need to be very short-lived. This study shows that over the lifetime of the galactic bars, they do not play significant role in triggering AGN — astronomers need to turn their eyes to other means of growing black holes in spiral galaxies.

About the author, Suk Sien Tie:

I am a third year PhD student at the Department of Astronomy at The Ohio State University. I am currently working on quantitative analyses of various quasar selection methods using the Dark Energy Survey (DES) and quasar variability via microlensing.

Did Triton Destroy Neptune’s First Moons?

Mon, 2017-11-06 12:00

Neptune’s moon system is not what we would expect for a gas giant in our solar system. Scientists have now explored the possibility that Neptune started its life with an ordinary system of moons that was later destroyed by the capture of its current giant moon, Triton.

An Odd System

Our current understanding of giant-planet formation predicts a period of gas accretion to build up the large size of these planets. According to models, the circumplanetary gas disks that surround the planets during this time then become the birthplaces of the giant planets’ satellite systems, producing systems of co-planar and prograde (i.e., orbiting in the same direction as the planet’s rotation) satellites similar to the many-moon systems of Jupiter or Saturn.

Triton’s orbit is tilted relative to the inner Neptunian satellite orbits. [NASA, ESA, and A. Feild (STScI)]

Neptune, however, is quirky. This gas giant has surprisingly few satellites — only 14 compared to, say, the nearly 70 moons of Jupiter — and most of them are extremely small. One of Neptune’s moons is an exception to this, however: Triton, which contains 99.7% of the mass of Neptune’s entire satellite system!

Triton’s orbit has a number of unusual properties. The orbit is retrograde — Triton orbits in the opposite direction as Neptune’s rotation — which is unique behavior among large moons in our solar system. Triton’s orbit is also highly inclined, and yet the moon’s path is nearly circular and lies very close to Neptune.

The distribution of impact velocities in the authors’ simulations for primordial satellite interactions with Triton, in three cases of different satellite mass ratios. In the low-mass case — a third of the mass ratio of the Uranian satellite system — 88% of simulations ended with Triton surviving on its high-inclination orbit. The survival rate was only 12% in the high-mass case. [Adapted from Rufu et al. 2017]

How did this monster of a satellite get its strange properties, and why is Neptune’s system so odd compared to what we would expect for a gas giant’s satellites? Two scientists, Raluca Rufu (Weizmann Institute of Science, Israel) and Robin Canup (Southwest Research Institute), propose an explanation in which Triton long ago wreaked havoc on a former system of satellites around Neptune.

Destruction After Capture

Rufu and Canup explore the scenario in which Neptune once had an ordinary, prograde system of moons around it that resembled those of the other gas giants. Triton, the authors suggest, may have been a former Kuiper belt object that was then captured by Neptune. The ensuing interactions between retrograde Triton and Neptune’s original, prograde satellite system may have then resulted in the destruction of this original system, leaving behind only Triton and Neptune’s other current satellites.

Nereid, a small irregular moon of Neptune, orbits at an average distance of more than 15 times that of Triton. Models of Triton’s orbital evolution must also account for the preservation of satellites like this one. [NASA]

Using N-body simulations that model a newly captured Triton and a likely primordial prograde system of moons, Rufu and Canup show that if the moons have a mass ratio similar to that of Uranus’s system or smaller, Triton’s interactions with it have a substantial likelihood of reproducing the current Neptunian satellite system. They even demonstrate that the interactions decrease Triton’s initial semimajor axis quickly enough to prevent smaller, outer satellites like Nereid from being kicked out of the system.

If the authors’ picture is correct, then it neatly explains why Neptune’s satellite system looks so unusual compared to Jupiter’s or Saturn’s — which means that our models of how primordial systems of moons form around gas giants still hold strong.

Citation

Raluca Rufu and Robin M. Canup 2017 AJ 154 208. doi:10.3847/1538-3881/aa9184

Tracing the Fuel for Forming Stars

Fri, 2017-11-03 12:00

Huge reservoirs of cold hydrogen gas — the raw fuel for star formation — lurk in galaxies throughout the universe. A new study examines whether these reservoirs have always been similar, or whether those in distant galaxies are very different from those in local galaxies today.

Left: Optical SLOAN images of the five HIGHz galaxies in this study. Right: ALMA images of the molecular gas in these galaxies. Both images are 30” wide. [Adapted from Cortese et al. 2017]

Molecular or Atomic?

The formation of stars is a crucial process that determines how galaxies are built and evolve over time. We’ve observed that star formation takes place in cold clouds of molecular gas, and that star-formation rates increase in galaxies with a larger surface density of molecular hydrogen — so we know that molecular hydrogen feeds the star-forming process.

But not all cold gas in the interstellar medium of galaxies exists in molecular form. In the local universe, only around 30% of cold gas is found in molecular form (H2) and able to directly feed star formation; the rest is atomic hydrogen (H I). But is this true of galaxies earlier in the universe as well?

Studying Distant Galaxies

Cosmological simulations have predicted that earlier in our universe’s history, the ratio of molecular to atomic hydrogen could be larger — i.e., more cold hydrogen may be in a form ready to fuel star formation — but this prediction is difficult to test observationally. Currently, radio telescopes are not able to measure the atomic hydrogen in very distant galaxies, such as those at the peak of star formation in the universe, 10 billion years ago.

Recently, however, we have measured atomic hydrogen in closer galaxies: those at a redshift of about z ~ 0.2–0.4, a few billion years ago. One recent study of seven galaxies at this distance, using a sample from a survey known as COOL BUDHIES, showed that the hydrogen reservoirs of these galaxies are dominated by molecular hydrogen, unlike in the local universe. If this is true of most galaxies at this distance, it would suggest that gas reservoirs have drastically changed in the short time between then and now.

But a team of scientists from the International Centre for Radio Astronomy Research in Australia, led by Luca Cortese, has now challenged this conclusion.

Top: molecular vs. atomic hydrogen gas in galaxies between z = 0 and z = 1.5. Bottom: the evolution of the molecular-to-atomic mass ratio with redshift. [Adapted from Cortese et al. 2017]

Adding to the Sample

Cortese and collaborators combined observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and Arecibo to estimate the ratio of molecular to atomic hydrogen in five HIGHz-survey massive star-forming galaxies at a redshift of z ~ 0.2. They then combine these results with those of the COOL BUDHIES survey; they argue that, since the two surveys use different selection criteria, the combination of the two samples provides a fairer view of the overall population of star-forming galaxies at z ~ 0.2.

Intriguingly, the HIGHz galaxies do not show the molecular-gas dominance that the COOL BUDHIES galaxies do. Cortese and collaborators demonstrate that the addition of the HIGHz galaxies to the sample reveals that the gas reservoirs of star-forming disks 3 billion years ago are, in fact, still the same as what we see today, suggesting that star formation in galaxies at z ~ 0.2 is likely fueled in much the same way as it is today.

As telescope capabilities increase, we may be able to explore whether this continues to hold true for more distant galaxies. In the meantime, increasing our sample size within the range that we can observe will help us to further explore how galaxies have formed stars over time.

Citation

Luca Cortese et al 2017 ApJL 848 L7. doi:10.3847/2041-8213/aa8cc3

RNAAS: A Unique Journal Joins the Family

Wed, 2017-11-01 12:00

Null results — research outcomes that show what doesn’t work, rather than what does — are a crucial part of science. It’s imperative that these results are shared widely, so that researchers can learn from each others’ experiences instead of unnecessarily repeating work.

RNAAS joins the suite of AAS journals. [AAS Publishing]

Unfortunately, null results are often difficult to publish in traditional venues, as they represent the steady march of science in the background rather than exciting new discoveries. And null results aren’t alone — there are a number of other types of scientific research that are of interest to the astronomical community, and yet they cannot easily be shared, archived, or cited.

A Home for Non-Traditional Communications

Enter Research Notes of the American Astronomical Society (RNAAS) — a new and unique journal that just joined the AAS journal family this week. RNAAS provides a means of sharing with the astronomical community work that may not fit into traditional publication outlets. There are many types of submissions that could be appropriate for RNAAS, such as:

  • Null results
  • Timely reports of observations (like the spectrum of a supernova)
  • Brief observations (like the discovery of a single exoplanet or contributions to the monitoring of a variable source)
  • Work in progress or projects of limited scope (like the results of a summer undergraduate research project)
Why Publish a Research Note?

RNAAS is a non-peer-reviewed, non-edited journal that is moderated by one of the AAS journals’ lead editors, Dr. Chris Lintott (University of Oxford). Communications published in RNAAS are brief — they are limited to <1000 words, with space for one table or figure. Research Notes have the benefit of being:

  • An example of a recently submitted Research Note. [AAS Publishing]

    Searchable and citable
    Since Research Notes are indexed by ADS, this ensures that researchers can easily find work that might otherwise have gone unshared. And since Research Notes are assigned a DOI, this means that information from Research Notes can be referenced in future publications.
  • Archived for perpetuity
    Publishing data and results in RNAAS — part of the AAS suite of journals and hosted alongside them by Institute of Physics Publishing — prevents the risk that this less formal information is unintentionally lost to the community as a result of institution changes, outdated websites, etc. (a common problem in academia!).
  • Quick to publish
    Need to notify the community of something in a hurry, and don’t have time to wait for a traditional journal’s publication process? Research Notes are typically available online within 72 hours of when they are received.
  • Free to access (and to publish!)
    RNAAS is not behind a paywall, so Research Notes can be read by anyone and do not require an institutional or personal subscription to AAS journals to access. What’s more, the AAS is currently suspending charges for all submissions to RNAAS, so publishing a Research Note costs nothing at this time.
Find Out More and Submit

Intrigued? You can go see for yourself what people are submitting to RNAAS.
Convinced? We look forward to receiving your submission to RNAAS!

Citation

Ethan T. Vishniac and Chris Lintott 2017 Res. Notes AAS 1 1. doi:10.3847/2515-5172/aa93da

Modeling Limitless Skies

Tue, 2017-10-31 12:00

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Redox Evolution via Gravitational Differentiation on Low Mass Planets: Implications for Biosignatures, Water Loss and Habitability
Authors: R. Wordsworth, L. Schaefer, R. Fischer
First Author’s Institution: Harvard University
Status: Submitted to ApJ, open access

Looking for Life

If you’ve been tuning into astronomy news lately, you’ve probably heard about a number of cool new exoplanet discoveries, like those in the TRAPPIST-1 system, continuously rolling in from our telescopes hard at work. But no matter how, when, and where a new exoplanet is discovered, there’s always that question burning at the back of our minds: could this exoplanet have Earth-like life?

Figure 1: A screenshot from the open-universe space exploration video game called No Man’s Sky. Even with Earth-like constraints, the sky’s the limit in terms of the many living worlds we can imagine are out there. [<a href=”http://www.hipsthetic.com/9-jaw-dropping-no-mans-sky-planets/”>No Man’s Sky</a>].

This question is certainly not an easy one to answer. For an exoplanet to house life (as we understand life so far), there’s a long checklist of requirements (like those discussed here and also here) that we need the exoplanet to fulfill. For example, life as we know it survives and thrives on liquid water, so we require that the exoplanet has the ability to hold liquid water.

Today’s astrobite focuses on another important requirement: the exoplanet’s atmospheric composition. Here on Earth, for instance, we have a lot of wonderful plant-based and plant-like creatures (like trees) that produce oxygen through photosynthesis. Then other creatures here (like humans) use that oxygen to survive and thrive. So for an exoplanet to have Earth-like life, we expect it to have a buildup of oxygen.

You might imagine, then, that we should be looking for oxygen on these exoplanets, as a sign of Earth-like life — and you’d be right, mostly! But unfortunately it’s not clear that a buildup of oxygen will “always” be a sign of Earth-like life. Instead, an exoplanet could possibly accumulate oxygen from purely chemical, completely not-organic-or-life-related (aka, abiotic) processes. This means that if we find an exoplanet that has a buildup of oxygen, we need to be cautious and somehow make sure that we haven’t just discovered a false positive (i.e., a case where what we think to be true is actually very false).

Today’s authors present a nifty theoretical framework for thinking about and modeling the atmospheres of exoplanets, which can help both characterize and predict the atmospheres of the many new exoplanets we’ve observed and continue to observe. The authors did many, many other cool things in their paper, but here we will focus on understanding the backbone of their theoretical framework.

A Little Chemistry

The authors use the concepts of ‘redox’ as the main variable of their model. ‘Redox’ is an abbreviation for ‘oxidation-reduction reaction‘, which is a fancy term for a reaction involving the exchange of electrons between two chemical species. The atom, molecule, or ion gaining an electron(s) is ‘reduced’, while the atom, molecule, or ion giving up an electron(s) is ‘oxidized’. Combustion is a common example of a redox. When you burn firewood at, say, a campfire, the carbon (C) from the wood reacts with the oxygen (O2) in the air, producing carbon dioxide (CO2). In this case, the carbon gives up four electrons and is oxidized, while the oxygen collectively gains four electrons and is reduced.

If a planet has an atmosphere that is very oxidizing, that means there is, in a sense, a net demand for electrons. That means we would expect O2, which wants to take electrons, to abiotically build up in an oxidizing atmosphere, because there would be no net supply of chemical species floating around for the O2 to react with and take electrons from. So to evaluate if an exoplanet would likely abiotically build up O2, we want to check if and when its atmosphere is likely to be oxidizing — and that depends on the planet’s chemical composition.

Figure 2 shows different elements from the periodic table as a function of electronegativity (which is basically how oxidizing versus reducing the elements are) plotted against atomic mass and abundance. With Earth as our example, we expect that the less massive elements like hydrogen (H), carbon (C), nitrogen (N), and oxygen (O) largely hang out in the volatile layer (which includes the atmosphere) and upwards towards space; magnesium (Mg), silicon (Si), and sulfur (S) largely get caught up in the planetary crust and mantle; and the heavy element iron (Fe) is largely trapped down in the core.

Figure 2: Electronegativity (x-axis) versus atomic mass (y-axis) for the most abundant elements in our solar system. Each circle corresponds to an element, which are labeled according to their periodic table abbreviations (H is hydrogen, for example). The sizes of the circles reflect the relative logarithmic abundances of these elements in our solar system. The faint gray asterisks in the background label elements less than 10% of the abundance of silicon (Si). Reducing elements are towards the left of the plot, while oxidizing elements are towards the right. And overall, the less massive elements rise towards space, while the more massive elements fall towards the planet’s core. [Wordsworth et al. 2017]

Studies have shown that, just like Earth, planets of around 1 to 10 Earth masses tend to split into these three major layers: volatile, mantle, and core. So to understand how oxidizing the atmosphere of a planet in this mass range is, we need to understand how these three planetary layers interact and exchange these different elements.

Putting It All Together

The authors quantify how oxidizing a planetary layer is in terms of the layer’s total oxidizing power using the following equation (Equation 1 in the paper):

Where N is the total oxidizing power, Ni is the number of atoms of some element ‘i’ (like carbon or oxygen), and pi is the oxidizing potential of element ‘i’. An atom of oxygen, for example, takes two electrons, so it has an oxidizing potential of +2. An atom of hydrogen, on the other hand, gives away one electron, so it has a negative oxidizing potential of -1. Finally, the Σ in the equation says that we want to sum over all elements ‘i’ in the layer. The authors drew out a model of the three-layer planetary system, as shown in Figure 3, and assigned a total oxidizing power N for each layer.

Figure 3: A three-layer model for a planet of about 1 to 10 Earth masses after formation. The N at the bottom-left of each layer represents that layer’s total oxidizing power. The k terms represent the exchange between these layers over time. And the E at the top represents the escape of the less-heavy elements, especially the reducing element hydrogen, out into space. Figure 2 in the paper.

To calculate how oxidizing the atmosphere is with this model, we must track the flow of elements between these three layers over time. The authors discuss a lot of the cool processes that can allow transport of material between these layers at different times in the planet’s evolution. For example, when the planet is young, its mantle layer can be partially or fully ultra-hot and molten, like an ocean of magma. During this phase, the liquidy mantle layer and the atmosphere-containing volatile layer can interact and exchange materials much more easily. But once the planet cools down and the mantle layer hardens, the interactions get more complicated. Depending on the planet’s geodynamic structure (such as any churning plate tectonics and gas-spewing volcanoes), the mantle layer can shift around and still allow material to flow between these planetary layers. And over time the much-less massive elements, especially the reducing element of hydrogen, can leak out of a planet’s atmosphere and escape to space (that’s the ‘E’ term in Figure 2), also changing how oxidizing the atmosphere is.

Once we have a good handle on how these conditions and processes play out on other planets — such as answers to the tough question of how geodynamics evolve on planets other than Earth — we can apply this framework to dig out any planetary scenarios where compounds like O2 could build up abiotically over time. And for those seemingly habitable exoplanets that appear very unlikely to abiotically build up O2, we can be cautiously optimistic that any oxygen buildup we do see might come from a biotic source — exactly what we’ve been searching for all along.

About the author, Jamila Pegues:

Hi there! I’m a 2nd-year grad student at Harvard. I focus on the evolution of protoplanetary disks and extra-solar systems. I like using chemical/structural modeling and theory to explain what we see in observations. I’m also interested in artificial intelligence; I like trying to model processes of decision-making and utility with equations and algorithms. Outside of research, I enjoy running, cooking, reading stuff, and playing board/video games with friends. Fun Fact: I write trashy sci-fi novels! Stay tuned — maybe I’ll actually publish one someday!

Featured Image: Extinction in Our Inner Galaxy

Mon, 2017-10-30 12:00

In this map of the innermost galaxy, which spans only a few square degrees at the Milky Way’s center, we can see the locations of more than 31 million objects obtained from the VISTA Variables in the Vía Láctea (VVV) survey. This near-infrared atlas traces stellar populations in the inner Milky Way that are dimmed and reddened by interstellar dust and gas — a process known as extinction — in a predictable way. Led by Javier Alonso-García (University of Antofagasta and the Millennium Institute of Astrophysics in Chile), a team of scientists has now used the VVV measurements of these stars to better understand the distribution of gas and dust that causes extinction in our inner galaxy — particularly in the most central, highly reddened, and crowded areas of the Milky Way. For more information, check out the paper below.

Citation

Javier Alonso-García et al 2017 ApJL 849 L13. doi:10.3847/2041-8213/aa92c3

Observing a Strange Pulsar in X-ray and Radio

Fri, 2017-10-27 12:00

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Simultaneous Chandra and VLA Observations of the Transitional Millisecond Pulsar PSR J1023+0038: Anti-correlated X-ray and Radio Variability
Authors: Slavko Bogdanov, Adam T. Deller, James C. A. Miller-Jones, et al.
First Author’s Institution: Columbia University
Status: Submitted to ApJ, open access

What’s more interesting than a rapidly spinning neutron star that emits electromagnetic radiation parallel to its magnetic poles? One that doesn’t exactly behave as expected, of course. One such weirdly acting pulsar, PSR J1023+0038, is a transitional millisecond pulsar (tMSP) — which is fancy speak for a pulsar with a millisecond rotational period that switches between radio and X-ray emission on a several-year timescale. The fact that this pulsar emits in both X-ray and radio on these longer timescales isn’t what piques the interest of astronomers, however, in the case of the study in this astrobite.

Weird Pulsar Behavior

Figure 1: Radio emissions (black) and x-ray emissions (blue) recorded by the VLA and Chandra respectively over time. This shows that when radio emissions drop off, X-ray emissions pick up.

Pulsars can typically fall into one of the following categories: radio pulsars are powered by exchanging rotational energy from the spinning neutron star into emitting radiation. This means that their rotation slows and their pulse length increases. Meanwhile, X-ray pulsars are accretion powered, meaning they turn heated infalling matter into X-ray emission. What distinguishes PSR J1023+0038 from the background of pulsars that switch between accretion-powered X-ray and rotation-powered radio pulsars is that it has a simultaneous anti-correlated X-ray and radio emission. The authors looked at about 5 hours of overlapping and concurrent observations from the Chandra X-ray Observatory and the Very Large Array (VLA) to try and understand this weird relationship between the X-ray and radio emissions. This is very clearly shown in Fig. 1 where we can see a tiny sample of time of overlapping X-ray and radio flux measurements. The anti-correlation is quite strong, meaning that when the X-ray emissions are weakest, the radio emission is strongest.

Figure 2: (Top) Chandra x-ray observations over a period of 5 hours.The 3 x-ray modes can be seen as the one large peak (~12.7 hrs), the low (minima), and high (steady maxima).    (Middle) The simultaneous observation as seen from the VLA. (Bottom) The overlapped top and middle observations show the anti-correlation between x-ray and radio emissions.

But wait…there’s more! When we zoom out on the flux/time series observations (Fig. 2) we can not only see that the anti-correlation is persistent, but we can also see that the X-ray emission has at least 3 unique modes of operation. The authors classify these X-ray emission modes as (1) sporadic flaring (~12.7 hrs in Fig. 2), (2) high, and (3) low modes. The difference between high and low in this context is the magnitude of the luminosity.

Trying to Explain Away the Strangeness

This complex and weird behavior unfortunately does not come with an easy or readily available answer. What we know about pulsars and how we can model pulsar accretion doesn’t shed any new light on the situation. The authors do suggest that the switching between high and low modes might occur due to a changing unstable magnetosphere. They also propose that the increase in radio emission can be explained by an outflow of plasma that emits synchrotron radiation as it travels. Additionally, when comparing PSR J1023+0038 to a low-mass X-ray-emitting binary black hole (BH LMXB) (Fig. 3), we can see that the low mode of this tMSP falls into the binary BH region. This is unusual because there is a pretty clear separation between the X-ray/radio luminosity relationship of neutron stars and BHs. Knowing this now, it may call into question whether some BH LMXBs could have been misidentified.

Figure 3: PSR J1023+0038 low (red diamond) and high (red pentagon) X-ray modes closely follow the accreting low-mass X-ray binary black hole systems, making it indistinguishable in the low mode.

Now you may be asking, “so what did we actually discover?”, which is a completely valid question. Well for one, we learned that there in fact do exist strange and unique pulsars that exhibit odd behavior. But the more exciting result is that we may not have a great understanding of pulsars in general. This is exciting because it can spur new astrophysical theories and models; ones that can more generally explain even the weirdest behaviors. Like most of astronomy (and science in general) however, before we can fully claim any specific mechanism for causing the anti-correlated X-ray and radio emissions and the switching between emission modes, we’ll probably need more observations.

About the author, Joshua Kerrigan:

I’m a 3rd Year PhD student at Brown University studying the early universe through the 21cm neutral hydrogen emission. I do this by using radio interferometer arrays such as the Precision Array for Probing the Epoch of Reionization (PAPER) and the Hydrogen Epoch of Reionization Array (HERA).

Little Eyes on Large Solar Motions

Wed, 2017-10-25 12:00

Images taken during the solar eclipse in 2012. The central color composite of the eclipsed solar surface was captured by SDO, the white-light view of the solar corona around it was taken by the authors, and the background, wide-field black-and-white view is from LASCO. The white arrows mark the “atypical” structure. [Alzate et al. 2017]

It seems like science is increasingly being done with advanced detectors on enormous ground- and space-based telescopes. One might wonder: is there anything left to learn from observations made with digital cameras mounted on ~10-cm telescopes?

The answer is yes — plenty! Illustrating this point, a new study using such equipment recently reports on the structure and dynamics of the Sun’s corona during two solar eclipses.

A Full View of the Corona

The solar corona is the upper part of the Sun’s atmosphere, extending millions of kilometers into space. This plasma is dynamic, with changing structures that arise in response to activity on the Sun’s surface — such as enormous ejections of energy known as coronal mass ejections (CMEs). Studying the corona is therefore important for understanding what drives its structure and how energy is released from the Sun.

Though there exist a number of space-based telescopes that observe the Sun’s corona, they often have limited fields of view. The Solar Dynamics Observatory AIA, for instance, has spectacular resolution but only images out to 1/3 of a solar radius above the Sun’s limb. The space-based coronagraph LASCO C2, on the other hand, provides a broad view of the outer regions of the corona, but it only images down to 2.2 solar radii above the Sun’s limb. Piecing together observations from these telescopes therefore leaves a gap that prevents a full picture of the large-scale corona and how it connects to activity at the solar surface.

Same as the previous figure, but for the eclipse in 2013. [Alzate et al. 2017]

To provide this broad, continuous picture, a team of scientists used digital cameras mounted on ~10-cm telescopes to capture white-light images from the solar surface out to several solar radii using a natural coronagraph: a solar eclipse. The team made two sets of observations: one during an eclipse in 2012 in Australia, and one during an eclipse in 2013 in Gabon, Africa. In a recent publication led by Nathalia Alzate (Honolulu Community College), the team now reports what they learned from these observations.

Building Atypical Structures

The authors’ image processing revealed two “atypical” large-scale structures with sharp edges, somewhat similar in appearance to what is seen near the boundaries of rapidly expanding polar coronal holes. But these structures, visible in the southeast quadrant of the images taken during both eclipses, were not located near the poles.

By analyzing their images along with space-based images taken at the same time, Alzate and collaborators were able to determine that the shape the structures took was instead a direct consequence of a series of sudden brightenings due to low-level flaring events on the solar surface. These events were followed by small jets, and then very faint, puff-like CMEs that might otherwise have gone unnoticed.

Impact of the passage of a series of puff-like CMEs (shown in the LASCO time sequence in the bottom panels) on coronal structures. [Alzate et al. 2017]

The fact that such innocuous transient events in the Sun’s lower atmosphere can be enough to influence the corona’s large-scale structure for timescales of 12–48 hours is a significant discovery. There are roughly 3 CMEs per day during solar maximum, suggesting that atypical structures like the ones discovered in these images are likely very common. These results therefore have a significant impact on our understanding of the solar corona — which goes to show that there’s still a lot we can learn with small telescopes!

Citation

Nathalia Alzate et al 2017 ApJ 848 84. doi:10.3847/1538-4357/aa8cd2

A Neptune in the Nearby Hyades

Tue, 2017-10-24 12:00

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: K2-NnnA b: A Binary System In The Hyades Cluster Hosting A Neptune-Sized Planet
Authors: David R. Ciardi, Ian J. M. Crossfield, Adina D. Feinstein et al.
First Author’s Institution: Caltech/IPAC-NASA Exoplanet Science Institute
Status: Submitted to AJ, open access

The authors of today’s paper report the discovery of a Neptune-sized planet in the nearby Hyades Cluster orbiting within a binary star system. A binary consists of two stars mutually orbiting their center of gravity; the larger star is called the primary and the smaller is the secondary. In a close binary where the stars are separated only by a short distance, each of the two stars affects the evolution and size of the other.

The binary system in today’s paper is referred to as EPIC 247589423 (also called LP 358-348), but the primary and secondary stars within this system are called K2-NnnA and K2-NnnB, respectively. Since the planet discovered in this system orbits just the primary star, the authors of this paper refer to it as K2-NnnA b: exoplanets are generally named after their host star and then have another letter added to the end. However, this planet does not yet officially have a name, so there is hope that it will be called something that rolls off the tongue a bit more easily.

Significance to Planetary Formation and Evolution

Planets in binaries form under the influence of two stars — an extreme environment for planet formation, particularly if the two stars are close. The system EPIC 247589423 has a very close separation of 40 AU, which is roughly the distance from Pluto to the Sun. Using our solar system as an example, if we replaced Pluto with a star, it is easy to see that the rotation and generally everything about the planets would be drastically affected.

Studying binary-orbiting planets like K2-NnnA b allows us to test the robustness of planet formation, since the planets have to survive in an extreme environment for a long time. These systems also let us test how often planets are retained by their host star rather than being destroyed by the changing gravitational field. What’s more, by finding and studying planets in star clusters, we may begin to understand how planetary systems form and evolve and find out the timescale for such events.

K2 and Follow-up Observations

K2, or as I like to call it, Zombie Kepler, is the current mission for the Kepler Space Telescope. The Kepler Space Telescope was launched in 2009 with the mission of finding exoplanets through transit photometry. The method of transit photometry means recording the brightness of the star for an extended period of time, then looking for any small periodic ‘dips’ in the brightness that occur when an exoplanet eclipses a small part of the star. This data is generally referred to as a light curve, since there is a rounded dip where the exoplanet eclipses the star. After a mechanical mishap in 2013 with the Kepler Space Telescope, the scientists working on this mission still found a way to use the telescope to look for transiting exoplanets. Instead of observing one single part of the sky for an extended period of time as before, K2 now observes smaller patches of the sky for shorter amounts of time. Figure 1 shows several stages of the light curve analysis from K2.

Figure 1: This shows the light curve of EPIC 247589423 in various stages of analysis from K2. The topmost panel shows the light curve with the telescope rotation removed. The second panel shows the binned version of the top panel. The third shows the data with the stellar variability removed, and the lowest panel shows the folded and binned result for the planet transit. [Ciardi et al. 2017]

Scientists discovered the short-period exoplanet in the EPIC 247589423 system by examining the star’s K2-obtained light curve. Since K2-NnnA b orbits a binary system, the researchers had to first remove the stellar variability inherent in the data. The two stars within the binary already eclipse each other and cause dips in the light curve, which can make it difficult to find a much smaller dip from the planet. After removing the eclipse variability, they found the planet and solved for its period and radius.

After the initial discovery with K2, the researchers followed up their detection with new observations and archival data to confirm the planet’s presence. They used the archival data from 1950 Palomar Observatory Sky Survey to rule out any other object that could be causing the dip in the light curve — for example, another eclipsing binary behind the system, which could create variation in the light curve that could be mistaken for a planet — and then made additional observations of the system using the Keck I telescope.

K2-nnnA b: Planet Properties

The EPIC 247589423 binary system is located in the nearby Hyades cluster — a cluster roughly 750 Myr old and the nearest star cluster to the Sun. The two stars are separated by about 40 AU. The planet orbits the star K2-NnnA with a period of 17.3 days, and its transit lasts roughly 3 hours.

K2-NnnA b is one of the first Neptune-sized planets that has been observed orbiting in a binary system within an open cluster. The discovery of this planet can provide us with a better understanding of the planet population in stellar clusters and allow us to place more limits on planetary formation and evolution.

The authors of this paper say planets discovered in nearby star clusters ‘provide snapshots in time and represent the first steps in mapping out [planetary]evolution,’ and I wholeheartedly agree. The discovery of K2-NnnA b brings with it new understanding of planet formation in star clusters and in binary systems. The possibilities of planet formation and evolution are certainly not limitless, and with more and more discoveries like K2-NnnA b, we can hopefully find the extremes of planetary systems.

About the author, Mara Zimmerman:

Mara is working on her PhD in astronomy at the University of Wyoming. She has done research with Heartbeat binary stars and currently works on modeling debris disks.

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