The Space Telescope Science Institute
The JWST Science Operations Design Reference Mission (SODRM)
The scientific potential of JWST is most often characterized through its four primary science themes, “Planetary Systems and the Origins of Life,” “Birth of Stars and Protoplanetary Systems,” “Assembly of Galaxies,” and “First Light.” The questions behind these themes emerged in the “Astronomy and Astrophysics in the New Millenium Decadal Survey” (2000), and set the challenges for a telescope that offered huge gains in sensitivity, resolution, and multiplexing over NASA’s Great Observatories. While JWST answers these specific challenges, its unique capabilities also offer unprecedented opportunities to advance other topics that are at the forefront of astronomical research today.
Over 50 astronomers from the Space Telescope Science Institute (STScI), Goddard Space Flight Center (GSFC), and the Science Instrument teams recently came together to develop a scientific program for JWST as a simulation of its normal science operations. This new Science Operations Design Reference Mission (SODRM) overhauls the 2005 version, which was built around simpler instruments, early operational concepts, and the four mission science themes. The new SODRM brings the observatory and instrument capabilities up to date and adds a number of science topics that have evolved rapidly in the years since the 2005 SODRM. It also provides our best estimate of the range and depth of scientific investigations that JWST will carry out (see Figure 1).1 The new program includes 112 programs, complete with Phase II files in the Astronomers Proposal Tool (APT), for a total of 849 days of observing time. These programs provide a realistic test bed for the design and implementation of the JWST ground system at STScI, and for simulating the operating schedule for the observatory and its instruments.
The new SODRM is available at http://www.stsci.edu/jwst/science/sodrm/.
The input submitted to the SODRM represents hypothetical, but realistic, scientific investigations from a broad cross-section of scientists. Not surprisingly, the results represent a balanced program in subject area (see Figure 2), including approximately equal fractions in Milky Way and stellar populations science, exoplanets, and nearby galaxies. A significant fraction of the SODRM science cases are dedicated to studies of our Solar System, while the majority of the programs focus on observations of the high redshift Universe. The distribution of SODRM targets on the sky is shown in Figure 3.
The investigations submitted to the SODRM fully exercise the diverse modes of JWST operation. For example, a rich Solar System science case emerges from combining JWST’s 20+ imaging, spectroscopic, and coronagraphic modes (see Table 1) with its vantage point at L2 and pointing control system. The latter will enable observations of objects moving at rates up to 30 milliarcseconds per second, sufficient to track planets, moons and asteroids beyond Earth’s orbit. For Mars, NIRSpec and MIRI IFUs can obtain synoptic monitoring of gases, aerosols, and dust in the atmosphere over the entire disk, over time scales from a few minutes, weeks, to any other cadence. For giant planets such as Uranus and Neptune, JWST will provide unprecedented precision in exploring the chemistry and thermal balance of their atmospheres, including an analysis of the effects of seasonally varying clouds and storms. The diversity of the satellites of giant planets can also be studied with MIRI low-resolution spectroscopy and NIRSpec fixed slit spectroscopy. The sizes and compositions of these bodies provide information on the materials that are critical for planet formation, and these observations with JWST can be synergistic with planetary missions. At the outer reaches of the Solar System, JWST can make detailed surface composition measurements for any of the icy dwarf planets. This could possibly include the first detection of the low-temperature phase of solid N2, and also will enable studies of the time-variable thermal state of the surfaces (with MIRI 25 micron photometry). Other JWST Solar System science cases include Target of Opportunity (ToO) imaging and spectroscopic observations of bright comets, imaging observations of Kuiper Belt Objects to determine their diameters and albedos, searches for organics and hydrated minerals, water ice in main belt asteroids with NIRSpec medium resolution spectroscopy, and much more. More information on the JWST Solar System science case is available at http://www.stsci.edu/jwst/science/solar-system.
JWST’s exoplanet science case has become stronger since the 2000 Decadal Survey’s prioritization. The SODRM includes details on some of the core science opportunities, such as high signal-to-noise, transient, NIRCam imaging observations of Earth analogues to determine their radii and inclination angle, NIRISS, NIRSpec and MIRI spectroscopic transit observations to determine planetary atmospheres, and continuous NIRCam imaging of a rich stellar field to determine the frequency of hot Earths. JWST’s instrument suite also includes six coronagraphs and aperture mask interferometry capabilities (e.g., see Table 1). The SODRM includes applications of these tools to find and study exoplanets around nearby stars using new techniques. For example, the NIRISS non-redundant mask can be used to find the pristine population of newborn planets in young star-forming regions such as Taurus, and to study the timescale of planet formation within the first 10 Myr. This JWST mode can also fill a gap in the study of planet demographics by finding giant planets around young low-mass stars, stars that are too faint for extreme ground-based adaptive optics investigations. The NIRISS non-redundant mask will be able to detect planets of 1-3 MJup at separations of 3-20 AU around young and nearby low mass stars.
The Milky Way and Nearby Galaxies
JWST imaging and spectroscopic observations of nearby stellar populations will offer significant advances over current understanding. The SODRM includes descriptions of over a dozen science cases that fully take advantage of JWST’s sensitivity and resolution in the infrared bandpasses to enable fundamental advances in the study of debris disks, star forming regions, molecular clouds, dust formation, and much more. The wide wavelength baseline of JWST from 0.6 – 28.3 microns enables complete spectral energy distributions to be constructed for these sources, thereby enabling improved model comparisons of the interplay of different components. The SODRM also demonstrates that JWST’s study of nearby Milky Way stellar populations will provide new insights on fundamental relations such as the initial mass function (IMF) of stars. Deep NIRCam imaging will yield the clearest measurement to date on the shape of the IMF, and map its variation with environment. These studies can easily be extended to substellar objects to test physical models of cool objects, for example, through the L/T/Y dwarf transition. The JWST NIRSpec microshutter array also offers new opportunities to study dense stellar environments with spectroscopy. The multiplexing of the array, at high spatial resolution, will yield abundances and motions for individual stars across the Milky Way.
Ultra-deep Hubble imaging of Local Group galaxies such as M31 has been used in concert with large ground-based telescopes to establish exquisite data sets of the disk, bulge, and halo of a Milky Way analogue. This can include resolved measurements of the ages, abundances, kinematics, and surface brightness profile of the galaxy’s stars, both on and off of detected substructure. These studies can provide detailed tests to N-body simulations of galaxy formation. A promising science case for JWST in the SODRM is the extension of these studies to systems outside the Local Group. JWST NIRISS and NIRCam imaging will provide the first measurements of the resolved main-sequence turnoff of galaxies in the Sculptor group. These studies will provide crucial context to the differences we see between the Milky Way and M31. Other SODRM investigations of nearby galaxies include deep NIRCam imaging and MIRI medium resolution spectroscopy of Young Stellar Objects in the Magellanic Clouds, narrow-band imaging with NIRCam of cold gas in star forming regions, NIRSpec spectroscopy of compact clusters to resolve age/metallicity degeneracies, a range of ISM studies in nearby galaxies, IFR observations of extragalactic HII and star forming regions, coronagraphic observations of AGN in more distant galaxies to establish correlations between AGN mass and host galaxy mass, and much more.
Distant Galaxies and Cosmology
“First Light” is one of the four major science themes for JWST and a key driver of its capabilities. The SODRM includes programs that push the workhorse camera, NIRCam, to discover the first observable galaxies at redshifts of 10 - 15. MIRI will observe the rest-frame optical bands of these galaxies to trace the buildup of stellar mass in the first generations of galaxies. NIRSpec and NIRISS will be able to observe star formation in these galaxies using nebular emission line spectroscopy. These ultra-deep observations have always been the driving force behind JWST’s deep infrared capabilities, and their inclusion in the SODRM ensures that the flight and ground systems can support this core JWST science. In addition, the SODRM also includes searches for the end of cosmic reionization, and the first QSOs, and unique signatures of galaxy formation in spatially resolved spectroscopy of galaxies at z = 2-6. The SODRM also includes a program for intensive followup of z > 1 SNIa for measurements of the cosmic acceleration in the rest-frame infrared where the systematic error from reddenning in the host galaxy is minimized. Finally, the SODRM will test our system for following up Targets of Opportunity (ToOs) with a program for spectroscopy of high-z gamma-ray burst afterglows that will probe the ISM of the host galaxies just after the burst and then measure the much fainter spectrum of the host galaxy after the burst fades. All this adds up to a robust program of “first light” science that elaborates this important science theme with the latest ideas in this rapidly evolving field.
A Look Forward
The SODRM is a comprehensive, science-driven simulation of the JWST mission. As our system matures and the scientific frontier advances we will improve the SODRM with new science and operational information. In later years we intend to accept programs from the larger astronomical community to ensure a broad science program and robust tests of our system. This simulated science program will have fulfilled its purpose when the actual, community-driven science program of the mission is executed and new and surprising discoveries are made with a fully optimized Observatory.
1Actual JWST observing programs in its first years will consist primarily of programs competitively selected by the Telescope Allocation Committee (TAC), plus Guaranteed Time Observations (GTO).