- Philippe Andre
- Helen Kirk
- Rachel Friesen
- Tyler Bourke
- Eve Ostriker
- Zhi-Yun Li
- James Di Francesco
- Chang Won Lee
- Jaime Pineda
- Tom Megeath
- Rowan Smith
- Nicolas Peretto
- Qizhou Zhang
- Steve Longmore
- John Bally
Herschel Observations of Gould Belt Cores and Filaments Philippe Andre
The Herschel Space Observatory has provided us with unprecedented images of the initial and boundary conditions of the star formation process at far-infrared and submillimeter wavelengths. In particular, the Herschel data give key insight into the global properties of dense cores and the link between these properties and the structure of molecular clouds. I will give an overview of the results obtained in this area as part of the Herschel Gould Belt survey, one of the largest key projects with Herschel. The survey findings confirm the existence of a close relationship between the prestellar core mass function (CMF) and the stellar initial mass function (IMF). The Herschel images also reveal a rich network of filaments in every interstellar cloud and suggest an intimate connection between the filamentary structure of the ISM and the core formation process. Remarkably, filaments are omnipresent even in unbound, non-star-forming complexes and seem to be characterized by a narrow distribution of widths around ~ 0.1 pc, which roughly corresponds to the maximum observed size of prestellar cores. In active star-forming regions, most of the prestellar cores identified with Herschelare located within gravitationally unstable filaments above a critical threshold ~ 16 Msun/pc in mass per unit length, corresponding to Av ~ 8 in visual extinction or column density. Altogether, the Herschel results favor a scenario in which interstellar filaments and prestellar cores represent two fundamental steps in the star formation process: First, the dissipation of kinetic energy in large-scale MHD flows
(turbulent or not) generates a complex web of filaments in the cold ISM; second, the densest filaments grow and fragment into prestellar cores (and ultimately protostars) by gravitational instability.
Filamentary Flows Helen Kirk
Observations over the last few years, especially those from the Herschel Space Telescope have shown that filaments are intimately connected with dense cores. Both theory and observations are starting to suggest that filaments may provide an important reservoir of material both in the accretion of relatively isolated dense cores found along filaments, as well cluster-forming cores, which tend to be found at the junction of multiple filaments. I will review some of the recent evidence of gas flows in filaments, highlighting results from the young Serpens South cluster-forming region. I will also touch on recent analysis of numerical simulations which suggests a similar behavior.
High resolution NH3 studies of nearby star-forming regions Rachel Friesen
Stars form within dense molecular cores. These cores are often embedded within larger structures, such as clumps and filaments, particularly in clustered star-forming environments. Indeed, large-scale maps of the continuum emission from dust have revealed the ubiquity of filaments in star-forming regions. The condensation and fragmentation of cores within larger structures is therefore a critical step in the star formation process, but continuum data alone do not provide key information, such as the gas kinematics, needed to discern between evolutionary scenarios. I will present new results from large NH3 studies of nearby star-forming regions, including Taurus and Serpens South. While NH3 primarily traces high density gas, sensitive observations over Serpens South reveal extensive, low brightness emission between the prominent cores and filaments, and show directly the frequently (but not invariably) sharp transitions between turbulent and quiescent gas in the high density regions. I will discuss the hierarchical structure of the dense gas in Serpens South, with comparisons between two- and three-dimensional analysis, and analyze the importance of thermal fragmentation in the filaments and cores over a range of physical scales.
Infrared and Submilllimeter Studies of Dense Cores Tyler Bourke
Dense Cores are the birthplace of stars, and so understanding their structure and evolution is key to understanding star formation. Information on the density, temperature, and motions within cores are needed to describe these properties, and are obtained through continuum and line observations at far infrared and submm/mm wavelengths. Recent observations of dust emission with Herschel and molecular line observations with single-dish telescopes and interferometers provide the wavelength coverage and resolution to finally map core properties without appealing to spherical simplifications. Although large scale Herschel observations reveal numerous filaments in molecular clouds which are well described by cylindrical geometries, cores are still modeled as spherical entities. A few examples of other core geometries exist in the literature, and the wealth of new data on cloud filaments demand that non-spherical models receive more attention in future studies. This talk will examine the evidence for non-spherical cores and their connection to the filaments from which they form.
Theory and Numerical Simulations of Self-Gravitating Core Formation Eve Ostriker
In star-forming molecular clouds, dense cores grow and evolve due to a combination of supersonic turbulent compression and self-gravity, with the details of the dynamical processes mediated by magnetic stresses and ion-neutral drift. In classical theory, cores with sufficiently high gravitational energy compared to thermal and magnetic support undergo outside-in collapse to reach a state in which the density profile approaches a singular r^-2 power law. This collapse is evident in numerical simulations, for a wide range of initial and environmental conditions. Classical theory predicts a subsequent outside-in infall stage, which is also seen in simulations. I will discuss numerical hydrodynamic and magnetohydrodynamic simulations of core formation and evolution, concentrating on evolution up to the stage of singularity formation. Observations show that cores are found to lie within larger-scale filaments, and simulations indicate that these filaments grow at the same time as cores develop within them. Although magnetic fields are often been thought of as a significant barrier to star formation that must be surmounted via ambipolar diffusion, recent simulations show that the properties of cores formed in hydrodynamic, ideal MHD, and diffusive models are quite similar. I will discuss how this can be understood in terms of anisotropic core formation models.
The Theory of Dense Core Collapse Zhi-Yun Li
I will review the theory of dense core collapse, with an emphasis on disk formation. Disk formation, once thought to be a simple consequence of the conservation of angular momentum during hydrodynamic core collapse, is far more subtle in magnetized gas. In this case, rotation can be strongly magnetically braked. Indeed, both analytic arguments and numerical simulations have shown that disk formation is suppressed in ideal MHD at the observed level of core magnetization. I will discuss the physical reason for this so-called “magnetic braking catastrophe,” and review possible resolutions to the problem that have been proposed so far, including non-ideal MHD effects, misalignment between the magnetic field and rotation axis, and turbulence. Other aspects of core collapse, such as fragmentation and outflow generation, will also be discussed.
Identification of Super-Jeans Mass Cores James Di Francesco
Pre-stellar cores are starless cores that are gravitationally bound and hence likely to collapse into protostars. Observationally identifying examples of such cores as bound can be tricky, however, given the uncertainties involved in the opacities, dust-to-gas ratios, temperatures, and distances of cores. Nevertheless, some starless cores are seen to be "super-Jeans," i.e., more massive than its thermal Jeans mass, suggesting they may collapse or fragment into protostars. (Of course, this occurrence depends on other forms of non-thermal support.) We present evidence for several super-Jeans mass cores identified by SCUBA observations, and describe the prospects for identifying more examples from the JCMT and Herschel Gould Belt Surveys. Such cores may have interesting kinematics even prior to collapse. For example, recent JCMT observations of a selection of pre-stellar cores show either inward or outward motions. In addition, we also describe one super-Jeans starless core, L1689-SMM16, that appears to be experiencing oscillations. Indeed, oscillations may be seen in cores that are significantly super-Jeans, as compression waves pass from warm exteriors to cold interiors.
Inward Motions in Dense Molecular Cores Chang Won Lee
I will review our molecular line searches for inward motion in starless dense cores. Conducted over ten years, these studies have found such motions to be prevalent in these objects. I will discuss the statistics of this internal motion, and its relationship to core density and evolution. Blue asymmetric profiles are dominant, indicating that inward motions are prevalent. These blue profiles are more common, and their asymmetry is bluer, at core positions with higher N2H+ line emission or higher column density. Based on their molecular line maps, starless cores can be classified into four different types: contracting, oscillating, expanding, and static. Contracting cores have the highest column density, greater than 6 x 10^21 cm-2, while static cores have the lowest. Our classification may indicate an evolutionary progression: static cores in the earliest stage, expanding and/or oscillating cores in the next, and contracting cores in the final stage.
The First ALMA View of IRAS 16293-2422: Infall and Rotation Jaime Pineda
The collapse of a dense core is a crucial step in understanding the formation of a star. There have been several claims for the observation of infall onto a protostar, however, it has never been free of controversy. Here we present ALMA Science Verification observations IRAS 16293-2422 proto-binary that allow us to study the kinematic of the gas close to the protostar. Thanks to the great sensitivity and image quality obtained with ALMA, we identify an inverse P-Cygni profile towards IRAS16293 B on complex organic molecules which provide the first incontrovertible detection of infall towards a protostar. We model the profile and estimate an infall rate of 4.5 x 10^-5 Msun/year. With the same observations we study the rotation towards IRAS16293 A, which display a position-velocity diagram consistent with pure rotation around a central mass of 0.53 Msun, but no evidence for infall could be identified. Finally, I will discuss the future prospects of using ALMA to study the infall around protostars.
The Role of Environment in the Formation of Low Mass Stars: Lessons from the Orion Molecular Clouds Tom Megeath
Low mass stars form in diverse environmental conditions, and understanding how these conditions influence the fragmentation and collapse of the molecular gas into stars is of key interest. The Orion molecular clouds are a remarkable laboratory for studying low mass star formation across the full range of environments, from crowded clusters containing massive stars, to moderate sized groups ofintermediate and low mass stars, and finally to relatively isolated low mass star formation. We present results from the Herschel Orion Protostar Survey, or HOPS, a study of over 300 protostar in the Orionclouds with the Herschel, Spitzer, Hubble and APEX telescopes. These data provide the means to identify young stars and protostars, determine the properties of the protostars, and map the column density of the dense gas in their surroundings. We examine how the properties of the protostars depend on the local environment, as traced by the surface densities of YSOs and gas, and we discuss the implications for our understanding of low mass star formation.
Modeling Massive Star Formation Rowan Smith
In this contribution I will discuss how massive star forming cores might compare to their lower mass brethren using insights from theoretical models. Is there such a thing as a truly massive pre-stellar core? Do massive star forming cores grow in mass, or is the core mass fixed when a protostar is formed? What is the role of filaments in forming massive protostellar cores? After I have discussed these theoretical considerations I will then examine how such questions can be tested by observations.
Infrared Dark Clouds: Cloud Dynamics and Core Formation Nicolas Peretto
In the past ten years, Infrared Dark Clouds (IRDCs) have received considerable attention. The darkness of these clouds ensures that little protostellar feedback has managed to disturb the initial conditions for star formation. IRDCs are ideal targets to study the earliest stages of star formation. In this talk I will review recent progress which has been made on our understanding of IRDC fragmentation and core formation. I will focus my talk on IRDC core properties, and will question the role of cloud dynamics on the mass determination of star forming cores. I will further argue that large-scale cloud collapse is a key stage of massive star formation, and discuss its implications on a universal star formation process.
Fragmentation of Molecular Clumps and Formation of Massive Cores Qizhou Zhang
Massive protostars are born in parsec-scale molecular clumps that collapse and fragment, leading to the formation of a cluster of stellar objects. The interplay among gravity, turbulence and magnetic fields affects the outcome of fragmentation. The physical condition (temperature and density) in molecular clumps limits the Jeans mass to about 1 Msun. This creates a theoretical puzzle for massive star formation since dense cores much larger than 1 Msun tend to further fragment into lower mass entities. In this talk, I will present recent high resolution observations of massive molecular clumps at very early evolutionary stages. I will discuss fragmentation, physical and chemical evolution of molecular coresand the implication of these findings to current theoretical ideas of massive star and cluster formation. I will also present measurements of dust polarization of a large sample of massive molecular clumps, which suggest that magnetic fields play an important role during the collapse of molecular clumps and the formation of dense cores.
Formation of Massive Clusters in the Galactic Center: Theory and Observations Steve Longmore
The ultimate goal of star formation studies is an end-to-end understanding of stellar mass assembly as a function of environment. Existing observations have two fundamental limitations to reaching this goal: they predominantly focus on regions with similar environmental conditions, and observations of a region can only provide a single evolutionary snapshot. In this talk I will discuss a way we may be able to overcome both these problems by exploiting a causally-related system of clouds in the extreme environment of the Galactic Center. Specifically, I will present results from both observational and numerical simulation studies testing whether star formation has been triggered in these clouds by close passage to the bottom of the Galactic gravitational potential, at the location of the supermassive black hole, Sgr A*. If the scenario can be confirmed, this system of gas clouds will provide a laboratory for studying stellar mass assembly as a function of ABSOLUTE time, allowing us to: (i) directly test theoretical predictions of molecular cloud structure evolution in turbulent clouds; (ii) unambiguously determine how postulated critical density thresholds for star formation vary with environment; (iii) follow the mass assembly of gravitationally bound cores, including likely precursors to stars >100 Msun, thereby directly testing SF theories at the most extreme mass ranges.
Clumps & Cores in Massive Star-Forming Regions from Orion to the Central Molecular Zone John Bally
I will present Bolocam, Herschel Hi-GAL, and ALMA results on selected massive star and star cluster-forming clumps. While the Central Molecular Zone (CMZ) contains many dense and compact clumps sufficiently massive to form young massive clusters (YMCs) such as the Arches, it is unclear if the best studied example, G0.25+0.02 (a.k.a the "Brick") will actually do so. In the Galactic disk no "pre-stellar" clumps with sufficient mass to form a YMC have yet been found. Thus, in cluster formation, the equivalent of a "pre-stellar" core may be rare. All detected clumps sufficiently massive to form a YMC are already forming massive stars. In the Galactic disk, YMCs may be assembled by the merger of sub-clusters dragged-in by converging gas flows.