DOC

Version 1

By Marie Freeman,2014-05-19 12:36
11 views 0
Version 1

24 June 2008

    Early Science Opportunities with SOFIA

1.0 Introduction

    NASA’s new Stratospheric Observatory for Infrared (IR) Astronomy (SOFIA) is now undergoing its final phases of flight testing and will begin science operations early in 2009. A process to involve the U.S. scientific community in developing SOFIA's early science program began with a workshop at the Austin January 2008 AAS meeting. The principal objective of the workshop was to initiate a community-wide effort to identify cutting-edge scientific investigations that will require and capitalize on SOFIA’s unique capabilities. Attendees heard the latest information about the SOFIA mission and the observatory's instruments and scientific capabilities, and then divided into five Working Groups (WGs) to begin drafting a report on five main science and programmatic themes that SOFIA might fruitfully pursue during the early phases of its mission. The five themes addressed by the WGs were:

     Stars and Star Formation

     The Chemical Evolution of the Universe

     Extra-Solar and Solar System Planetary Science

     Extragalactic Science

     Community Knowledge Base (CKB) Observing Programs

    Work initiated by the WGs at the workshop was continued and expanded during the period between January 15, 2008 and April 1, 2008, when final results were presented to the workshop chairs for integration into a final report. This White Paper summarizes the deliberations of the WGs effort and represents the cumulative product of the workshop.

2.0 Stars and Star Formation (Chair: John Bally)

    The investigation of stellar and planetary birth, life, and death is a central theme in astrophysical research. Star formation determines the cosmic fate of baryonic matter.

2.1 Probing the “Cosmic Ecology” of Stars and Planetary Systems

    The star formation rate (SFR) in a galaxy determines how rapidly the interstellar medium (ISM) is converted into stars, planets, and smaller solid bodies. The initial mass function (IMF) determines how long baryons are locked-up in compact objects, precluding them from participation in the “cosmic ecology”. Massive stars convert primordial H and He into all elements of the periodic table and recycle a portion of this matter back into the ISM on a timescales of 40 megayears. Their cores collapse into neutron stars or black holes. Stars with masses ranging from 0.8 to 8 Solar masses synthesize the lighter elements, return a portion to the ISM on times-scales ranging from 40 Myr to the Hubble time, and sequester the rest in white

     1

    dwarf remnants. Stars below 0.8 Solar masses lock-up baryons for longer than the current age of the Universe; no such star has ever died of natural causes in this Universe. Since the transformation of baryons from the ISM into stars and planets and back into the ISM occurs in dust enshrouded environments at relatively low temperatures, most emitted radiation emerges at IR wavelengths.

     SOFIA provides unique access to the 1 ;m - 1 mm spectrum where most radiation

    produced by the ISM, forming and dying stars, and warm circumstellar matter emerges.

    In the two decades, SOFIA is the only facility that will provide access to the mid-IR

    between 25 and 60 ;m where warm ~100 K dust and gas radiates. SOFIA will provide

    capabilities complementary to those of other IR facilities (Gemini, Keck, VLT, LBT),

    soon-to be-built extremely large telescopes (ELTs), the new generation of synoptic-

    survey telescopes (LSST, PanSTARS), new mm/sub-mm facilities such as CARMA and

    ALMA, and space-based platforms ( JWST, Herschel). SOFIA will provide access to the

    thermal IR regime where ground-based telescopes are blind. JWST provides access only

    to wavelengths below 29 ;m and only with low spectral resolution (R < 3,000). Herschel

    provides broad-band filters down to 60 ;m and single pixel heterodyne capability at 4 and angular selected frequencies. SOFIA will provide high spectral resolution R > 10

    resolution of θ ~λ/10 in arcseconds where λ is the wavelength in ;m. min

     SOFIA will provide high spectral resolution and nearly complete access to the thermal IR

    with spectral resolution much greater than JWST, Spitzer, and Herschel (below 60 ;m).

    SOFIA will provide better angular resolution than Spitzer, IRAS, and ISO, and access to

    the bright sources where Spitzer saturates.

     SOFIA provides unique access to time-critical and synoptic observations. It can achieve

    super-resolution by using cold outer-Solar system bodies (KBOs, asteroids, etc) as

    occulters and by making multiple passes through the transit shadow.

     SOFIA will enable use of unique filters, detectors, and instruments that will be

    unavailable from space or the ground. SOFIA will serve as the first-light test-bed for

    second-generation IR instrumentation. Within the next decade, broad-band incoherent

    focal plane arrays which can simultaneously detect multiple colors (MKIDs) will become

    available. Multi-feed heterodyne cameras will enable SOFIA to take advantage of spatial

    multiplexing.

     During its first five years of operations, SOFIA will bridge the wavelength gap between

    the visual/near-IR and sub-millimeter regimes where ground-based facilities operate, and

    will provide the high spectral resolution and spatial-multiplexing not available on near-

    term space-based facilities.

    The potential of SOFIA to enable new insights into the processes of star and planet formation can be grouped into three themes: 1) How do stars and planets form?, 2) How does material

    transition from the ISM into mature planetary systems?, and 3) How is material returned to the

    ISM? Near-term SOFIA research may be driven by the results of surveys. Several mm and

     2

    submm continuum surveys of the Galactic plane and nearby star forming regions are underway or will soon start at facilities such as the CSO, JCMT, and APEX. For example, the 1.1 millimeter Bolocam Galactic Plane Survey (BGPS) has mapped 150 square degrees of sky at 30” resolution, finding about 10,000 cloud cores. By 2009, the Herschel Space Observatory and SCUBA2 surveys of the Galactic plane will be providing maps from 60 to 850 ;m.

    Complementary near-IR (< 3 ;m) and to mid-IR (3 to 24 ;m) surveys are also being obtained (e.g.

    VISTA, GLIMPSE, MIPSGAL, etc.) from the ground and with Spitzer. By 2010 to 2012, these surveys will provide new lists containing thousands of regions in our Galaxy where stars and clusters are about to, or are actively forming, or where stars and their planetary systems are dying. SOFIA will be the principle tool for investigating this “Galactic Ecology”.

    Undoubtedly, unexpected discoveries will emerge from the opening of new regions of parameter space. Emerging facilities (JWST, ELTs, LSST) and new technologies during the next decade may trigger new types of observations with SOFIA. Key areas to watch for new developments and breakthroughs include detection of dark matter particles, new constrains on the nature of dark energy, properties of ultra-high energy cosmic rays, developments in astrobiology, the identification and characterization of extra-Solar planets, the first stars and galaxies to emerge form the Universe, small-scale studies of the CMBR and its polarization, new particles detected at high energies by LHC, and cosmic-ray studies.

    These disciplines and emerging wide-field synoptic programs will drive future SOFIA research towards time-domain work. Examples include monitoring SN, time-evolution of gravitational lensing events, weather patterns on planets, monitoring of both short-timescale and long-time-scale evolution of systems such as GRBs, AGN, stellar mass-blackholes, micro-quasars, evolution of shocks in various contexts, proper motions, parallaxes, and other types of synoptic change.

Moore’s Law, developments in solid-state physics, and nano-technology are likely to lead to

    many orders-of-magnitude improvement in data processing, storage, and transmission and these technologies are likely to lead to new types of detectors, receivers, and focalplane arrays. These factors may create major upgrade opportunities to SOFIA on a 10 year horizon that will make it an increasingly important facility for astrophysical research.

    Below, we discuss likely examples of SOFIA-based studies in each of the above three theme areas as an illustration of the utility of SOFIA to enable new understanding in these areas.

2.2 The Formation of Stars and Planets

    High-R spectroscopy and spectro-imaging in the SOFIA spectral domain will illuminate many key questions in star and planet formation. Some specific observations that can be made with SOFIA include:

The structure and spectral energy distributions (SEDs) of forming stars and clusters: Key targets

    include the nearest star forming complexes and their cores. These include L1551, Taurus, Ophiucus, and Orion OMC-2, and the nearest massive star forming regions such as Orion OMC1

     3

    and Cep-A. More massive cluster forming regions such as DR21, G34.15+0.26, Sgr B2, and the Galactic Center will probe the highest mass cluster and massive star-forming regions in our Galaxy. FORCAST and HAWC will provide the highest resolution imaging and photometry capability in the 5 to 250 µm regime where most of the luminosity emerges. Key questions to answer with SOFIA mid-IR imaging are: How do clouds fragment into clusters and stars? What are the structure functions (“correlations”) of warm dust and young stars?

The physical and chemical properties of star forming regions: SOFIA will make essential mid-

    IR observations of massive star and cluster birth, including the evolution of cores into “hot-

    cores” hyper-, ultra-, and compact HII regions, into mature star complexes and associations at the peak of their emitted spectra. Continuous coverage of the mid-IR from 5 to about 100 ;m

    provides high spatial and spectral resolution where most YSO luminosity emerges. Representative targets can be observed with SOFIA spectrometers to determine their emission spectra, kinematics, and spatial structure. The spectral lines of hydrogen, nobel gases, various ions, and molecules in both gas and solid-state phases will be diagnostics of compositions, spatial and velocity structure, and the physical and chemical conditions in star forming regions.

    Energy-balance in the ISM: Owing to its broad spectral coverage, SOFIA can uniquely address the cooling rates of various phases of the ISM by means of high-spectral resolution. The broad spectral-coverage of SOFIA is especially important for the investigation interface regions and post-shock cooling layers where temperatures can range from 10s to millions of Kelvin within sub-arc-second to 10s of arc-second scales. The heating and cooling of photo dissociation regions (PDRs), Herbig-Haro objects, the diffuse ISM, molecular clouds, and hot cores is dominated by IR emission. Even the warm and hot phases of the ISM have abundant IR tracers. Key tracers include C+, OI, OIII, NII, SIII, SiII, SiIV, ions of noble gasses (Ne V, Ne III, Ne II, Ar V, Ar III, Ar II) and a variety of gas-phase and solid-state molecular transitions of common species (H2, CO, CO2, H2O, OH, etc.).

    Magnetic fields: Imaging polarimetry with SOFIA will trace grain alignment and the magnetic field geometry. Spectro-polarimetry of various spectral lines such as the 24 ;m Fe complex and

    Zeeman sensitive molecules such as OH may provide a powerful tool for the determination of strong magnetic fields expected in massive star forming regions, disks, and in ultra-dense environments such as the Galactic center.

    Massive star and cluster birth: The densest phases of massive star formation during which most of their mass is assembled requires the mid-IR observations provided by SOFIA. Do massive stars and clusters form by competitive accretion or from scale-up disk accretion. Do the most massive star clusters form in a “cooperative” where the Eddington luminosity re-directs accretion

    onto less-massive, less-luminous siblings? EXES, FIFI-LS, GREAT, CAIMIR, and SAFIRE will provide unprecedented high-R spectral coverage of the IR. Tracers such as 12 ;m [NeII] provide

    both spatial and spectral resolution. H2, CO, and fine-structure lines will probe the structure and velocity fields of jets and the primary drivers of molecular outflows beyond the Solar vicinity where extinction prevents near-IR and visual-wavelength studies. Mid-IR tracers will probe the low-shock speeds where outflows degrade into turbulent and thermal energy.

     4

The “Galactic Ecology” - Feedback and Self-Regulation of by Massive Star Formation: SOFIA

    observations are needed to probe how massive stars regulate the physical and chemical state of the ISM. Fine-structure lines, H2, other species are needed to follow the degradation energy injected in the form of photons, particle winds, and explosions into radiation and turbulent motions in the ISM. The spectro-morphology of star forming regions will be used to study how energy injection drives the transformations of the ISM phases into each other. How do OB associations, massive stars and clusters, and Galactic processes drive the ecology of the ISM?

Outflows and Feedback from Low-Mass Stars: In the absence of high-mass stars, protostellar

    winds may be the primary agent for the self-regulation of star formation. SOFIA spectroscopy of jets and molecular outflows using a variety of tracers such as fine-structure lines, molecules, H2, and recombination lines are needed to evaluate the mass, momentum, and energy injected by winds and jets into the ISM.

The Galactic Center: The center of the Milky Way provides access to the environment of the 6 M) black hole (BH). The approximately A = 30 mag nearest super-massive (M ~ 3 x 10V

    extinction precludes observations from the near-IR through soft X-rays. IR observations with SOFIA will shed unique light on the accretion processes, fueling, and growth of the BH, the formation of stars in the circum-nuclear environment where the Keplerian orbital motion about the BH dominates, and of the properties of the nuclear interstellar medium (NISM) and star systems. The Galactic NISM consists of ultra-dense molecular clouds and a variety of exotic thermal and non-thermal features such as the radio-filaments, the circum-nuclear spiral, the “Arches” filaments, energetic stellar wind bubbles from the nuclear cluster of massive stars, and ablating red-giant atmospheres. The stellar population in the inner parsec reaches its highest 7-3space density anywhere in the Galaxy (>10 stars pc), contains exotic high-velocity stars in

    orbit around the BH, and a remarkable collection of massive Wolf-Rayet and massive stars that must have formed recently in extreme conditions. Massive star birth in a circum-BH disk may be analogous to planet formation around normal stars. Many massive star remnants (neutron stars and stellar-BHs) are expected to be interacting with the NISM. For example, it has been proposed that the “Great Annihilator” – one source of 511 keV positronium line emission - may

    be a stellar mass BH accreting from the NISM. IR spectroscopy will provide unique tests of the interactions of relativistic particles with dense molecular gas. SOFIA will provide unique spectral probes of the gas and dust in the central parsecs, spectroscopy and spectral-typing of massive stars and clusters in the GC, and imaging and spectroscopy of the non-thermal filaments. Are these filaments similar to Herbig-Haro objects near the Sun, but much larger and more luminous? Do they contains strong magnetic fields? SOFIA will provide unique probes of the IR environment of stellar-mass black-holes, neutron stars, other collapsed objects. It will enable searches for gravitational lensing events of background stars by the central super-massive black hole. Finally, detailed investigations of the massive star forming regions that abound in the Central Molecular Zone will shed light on nuclear starbursts in other galaxies, and the behavior of the NISM in response to the forcing by the central stellar bar.

2.3 The Transition of Material from the ISM into Mature Planetary Systems

The closest young stars and forming planetary systems: Spitzer, HST, Chandra, and ground-

     5

    based studies have identified thousands of young stellar objects (YSOs) within 500 pc of the Sun. The formation and early evolution of planetary systems requires IR observations in the SOFIA spectral range (1 to 300 ;m) because planetary system formation occurs in disks where the

    temperature ranges from about 10 to 1000 Kelvin. High spectral resolution studies of disks around the youngest YSO will be used to determine disk sizes, structure, and composition. SOFIA spectroscopy of the closest YSO such as those in Taurus , Ophiuchus, Lupus, and Chamaeleon will provide complete spectral energy distributions (SEDs) and detailed constraints on excitation, ionization, and composition.

“Proplyds” and the first steps to planet formation: Nearby regions such as the Sco-Cen OB

    association, the Perseus Clouds, and Orion contain hundreds of low-mass stars thought to be forming planetary systems. SOFIA will be especially well suited to the study of photo-ablating proto-planetary disks (e.g. the “proplyds” such as those observed with HST in the Orion Nebula). Although these irradiated disks will be unresolved, IR spectroscopy with SOFIA will enable global properties such as dust mass (spectral distribution of dust continuum), mass-loss-rate (flux and velocity of various lines), grain size and composition (shape and strength of the 10 and 20 mm silicate feature, ices, and PAH spectra), to be determined. Emission spectroscopy will be used to infer the abundances of gas and solid state features, determine the nature of ices, and measure grain size distributions. Polarization and Zeeman studies will be used to constrain the geometry and strengths of magnetic fields.

Maturing Planetary Systems: As samples of YSOs and young stars having ages ranging from

    less than on million to nearly a billion years become available (from Spitzer, Hershel, and ground-based studies), SOFIA imaging and spectroscopy will be used to measure the lifetimes of primordial circumstellar disks as functions of stellar mass, binarity, and birth environment. SOFIA will trace the transition to debris disks, and estimate their longevity and evolutionary behavior. Do debris disks come and go as grains are lost and re-supplied by proto-planetary collisions? Are debris disks produced primarily by collisions, by the evaporation of smaller icy bodies, or the collisional erosion of planetismals? Imaging and spectro-imaging studies of more evolved YSO disks will detect the secondary dust produced by collisions of planetary embryos, and more mature protoplanets as functions of stellar age, mass, and birth environment. Spectral studies of solid state features (silicate features, ices, and various stretch and bending modes of molecules locked into grains) will enable mineralogical studies of debris disks such as AU Mic (located a mere 12 pc from the Sun), Vega, ß-Pictoris. SOFIA will search for and probe the properties of disks around multiple stars identified by other means to investigate the impact of multiplicity on disk evolution.

Balog tails: A remarkable result from Spitzer is the detection of 24 ;m (and in some cases 8 ;m,

    and in one case near the central star of Tr 14, at 3 ;m) light from 0.1 pc long dust tails driven

    away from low-mass stars that happen to be close to O stars (e.g. Balog 2006). These tails are gas-free and appear to be blown out from forming planetary systems by the intense radiation pressure of the O star. The origin of the dust is unclear. It may signal the prompt decay of a debris disk in a harsh radiation environment (“debris disk blow-out”), the rapid evaporation of

    icy parent bodies, or dust production caused by the collision of large planetary embryos. SOFIA is the only instrument capable of spectroscopy (e.g. the silicate features), to constrain models.

     6

Occultation studies (“natural coronagraphy”): SOFIA’s mobility will enable unique super-

    resolution studies of a variety of targets using asteroids and KBOs as occulters. This capability may be especially useful for the NIR investigation of debris disks systems because SOFIA mobility may enable the central star to be occulted. There is a chance that SOFIA may be able to directly image and characterize giant planets around the nearest stars by using asteroids or KBOs to suppress the light of the host star.

2.4 The Return of Material to the ISM

Winds and Outflows from Mature Stars: SOFIA spectroscopy of obscured stars and wind

    bubbles will extend our understanding of winds throughout the Galaxy, enabling the study of their dependencies on metallicity and environment. Ordinary stellar winds provide the first means by which stars recycle the products of stellar nucleosynthesis into the ISM. Massive star winds can be detected in both in the stellar spectra and by the forward shocks driven into the ISM. IR spectroscopy of emission line Be stars will probe the cool outer disk material and shed light on their unique mass loss mechanisms. Is the Be phenomenon connected to stellar magnetic activity and fast rotation? Mass-loss rates and the dredging of synthesized material increase dramatically as stars evolve off the main sequence. IR spectroscopy of K and M giants, supergiants, OH/IR stars, WR stars, LBVs, and other types of post-main sequence (PMS) objects will probe the formation and composition of grains by means of the PAH features, C-C, C-H and other stretch and bending modes, 10 and 30 ;m silicate, and other IR band bands. Gas phase

    enrichment and metallicity patterns can be probed by spectroscopy of the rich variety of noble gas and fine-structure transitions of atoms and their ions, molecular rovibrational spectra. The IR emission from objects transitioning from the main-sequence to the postmain sequence phase will provide definitive measurements of mass-loss rates as functions of stellar mass and age. Does most mass-loss occur in brief and violent eruptions during certain stages of stellar evolution? IR studies of the shells of rare luminous blue variables (LBVs), and eruptive stars such as AG-Car and P-Cyg will provide constraints.

    Binaries and mass-transfer systems: The evolution of stars in close binaries can lead to mass loss and a variety of exotic stellar behaviors such as symbiotic stars, intense variability, and the nova phenomenon. Stars in close binaries can exchange mass and alter the normal course of stellar evolution. Roche-lobe overflow can result in the formation of expanding rings and disks. Giant circumstellar rings have been discovered around a variety of stars when externally illuminated by nearby massive stars. Examples Include WeBo 1, an 80,000 AU diameter ring surrounding a barium star (Bond 2003), the eqauatorial ring around the supergiant Sher-25 in NGC 3603, and the triple ring system in SN 1987A. Multiple ring-systems have recently been discovered around several Be and LBVs (Smith et al. 2007). Since most shed circumstellar rings are cool, SOFIA IR observations are required to detect and characterize them.

    Stellar Death: Stars of all-masses enshroud themselves in dust cocoons that reprocess most of their visual and NIR light into thermal IR radiation in the SOFIA-accessible wavelength regime. High resolution spectroscopy of dusty stellar envelopes such as proto-planetary nebulae (PPN), and objects such as IRC10216, IRC10420, ?-Carina, Betelgeuse will be used to measure wind

     7

    composition, geometry, mass-loss rate and velocity history, gas-to-dust ratio, and the interaction of these flows with the surrounding ISM.

    Spectro-imaging of PPN and planetary nebulae will provide the most reliable method for determining the metallicity of recycled circumstellar matter. Noble gasses and their multiple ionization stages are accessible in the IR (e.g. Ne and Ar). Their equivalent widths in planetary nebulae are robust indicators of excitation conditions and metallicity. Solid state and ice features will probe gain formation, composition, and mineralogy.

    Stellar Remnants: The stellar graveyard consists of white dwarfs (WDs) , neutron stars (NSs), and black holes (BHs). IR emission from the circumstellar environments of remnants indicates that as the cores of dying stars collapse, not all envelope material is recycled or incorporated into the collapsed object. Some high-angular momentum core material from the dying star collapsed into a “fall-back” disk. Disks surrounding remnants can undergo processes analogous to planet

    formation, producing solid bodies observed as pulsar planets. Collisions between such bodies can produce dust and IRexcess emission. The surface pollution of WDs by the accretion of gains and gas from fall-back disks may be responsible for exotic surface compositions. SOFIA spectroscopy will probe the masses, compositions, and longevity of such fall back disks.

    Remnants might also accumulate circumstellar disks when they pass through a dense molecular cloud by Bondi-Hoyle accretion. SOFIA detections or limits on IR emission from hard X-ray sources in molecular clouds can be used to constrain the Galactic population of old stellar remnants left over fro the birth of our Galaxy. Such measurements will provide constrains on the evolution and star formation history of our Galaxy.

    Stellar remnants in close, mass-transfer systems produce exotic phenomena such as themicro-quasars (e.g. LSI +61 303, SS430), and the Great Annihilator in the Galactic center. In some cases, the winds and relativistic jets interact with the dense phases of the ISM. IR spectroscopy can reveal the unique signatures of the excitation of the dense ISM by hard X-ray, ?-rays, and relativistic particles.

Destruction of Planetary Systems: When low-mass stars swell into red giants, they will vaporize

    their planetary systems. The thousand-fold increase in the luminosity of Sun-like stars will a;so evaporate their remnant Kuiper Belt objects and Oort cloud comets, thereby injecting large amounts of water, ammonia, and dust. As the stellar envelope swells, hot Jupiters and terrestrial planets will be consumed and outer gas giants will be greatly heated. SOFIA observations of the nearest red-giants will reveal the ultimate fate of our Solar System through peculiar compositions that reflect those of the disrupted bodies. Planetary destruction may result in local chemical enhancements that might be revealed by spectro-imaging of the closest red-giants.

3.0 The Chemical Evolution of the Universe (Chair: Dan Jaffe)

    Observations with SOFIA will provide crucial information about the processes that have enriched the metal content of the universe. The current picture is that H/He rich primordial matter produced by Big Bang nucleosynthesis condensed into galaxies during the epoch of

     8

    galaxy formation. Since then, nucleosynthesis in stars and stellar explosions has created metals that are returned to the ISM by post main sequence stellar winds and the ejecta produced in the explosions of evolved stars. These materials form the building block of planetary systems, debris disks, and life itself in subsequent generations of stars.

3.1 SOFIA and its Role in Studying The Life Cycle of Dust and Gas

    The great strength of SOFIA is the enormous breadth of its capabilities and the flexibility with which those capabilities can be modified and improved. In particular SOFIA has the unique ability to carry out spectroscopy across almost all of the three decades in wavelength between 1 µm and 1 mm. This ability gives SOFIA a significant overarching mission within NASA's goal of understanding the origin of terrestrial planets, planetary systems, and their host stars. This mission is to understand the chemical evolution of solid and gaseous matter from the ejection of heavy elements from evolved stars through the formation of rocky planets and the deposition of water and organic molecules on these bodies.

3.2 From Stellar Death to the Formation of Terrestrial Planets

    We understand the basic outlines of the evolution to our present chemical environment, but many of the key mechanisms are only poorly understood. If we wish to understand some of the key steps, we need to address the many unresolved questions about the evolution of gas and dust. IR and submillimeter spectroscopy at both high and moderate resolution are the essential tools for an investigation of the remaining open questions. SOFIA has capabilities across this range that will address the unresolved questions about every stage of the evolutionary process, especially when complemented by results from other ground and space observatories with more narrowly focused strengths.

Arrival of Dust and Heavy Elements in the ISM: Dust made from heavy elements and gas-phase

    molecules other than H2 play critical roles in our origin. Without these consituents, interstellar gas would not be able to cool to form gravitationally unstable protostellar cores. Without them, the raw materials for planet formation would not be available. This latter point is made with particular force by the strong correlation between metallicity and the likelihood of extrasolar planets (Gonzalez 1997). Heavy elements are produced in stellar interiors and make their way into the interstellar medium by a variety of mechanisms during the last stages of stellar evolution. One goal of SOFIA will be to study the state this “freshly minted” solid and gaseous interstellar matter as it enters into the chain of evolution.

The Transition from Atomic to Molecular Clouds: The gas and dust initially resides in clouds

    where the dominant constituent, hydrogen, is predominantly atomic. Even when molecular material has formed, a substantial fraction of the dense ISM resides in clouds where ultraviolet photons determine the chemistry and energetics of the gas. Through its resistance to ambipolar diffusion, this material may also play a critical role in regulating star formation.

The Chemical Evolution of Molecular Clouds: In a chemical sense, molecular clouds are not

    static entities, even before protostellar collapse begins. During the molecular phase, the

     9

    abundances of the various gas-phase molecules steadily evolve. In the cold, shielded portions of the clouds, dust grains grow significant ice mantles, changing both their own characteristics and, through depletion, the chemical makeup of the gas. These changes set the stage for the formation of stars and planetary systems.

    We discuss the evolution of the solid matter and the gaseous matter across this evolutionary path separately. The interaction between the two is critical and we mention it along the way, but differences in observational techniques and physical mechanisms require a separation of the discussion at the level of specific experiments.

3.3 Evolution of Solid Matter

Evolution of Refractory Dust Grains: The cores of dust grains make a curious journey from

    crystalline to amorphous back to crystalline along there way from late-type stellar atmospheres to protoplanetary disks. The matter emerging into the ISM has a significant fractional content in crystalline material (~5%). Interstellar clouds, on the other hand, have a much lower (<1%) crystalline fraction. In protostellar disks, evidence from IR spectroscopy and from studies of comets indicates that material is somehow annealed and returned to crystalline form. This cycle involves many different chemical and physical processes which are only poorly understood. Understanding this evolutionary history requires a moderate-resolution spectroscopic capability across as broad a range as possible. Broad temperature ranges, optical depth effects, overlapping of broad features, and the presence of a range of dust types in each source, not to mention the contributions in cool sources of ice mantles, make spectroscopy across the range from 8 to 120 µm essential in studying the evolution of refractory material. With SOFIA, we have the ability to measure the shape and depth of solid-state features across this entire range in AGB star envelopes, diffuse interstellar clouds, and dense molecular clouds.

Ices as a Step Toward Protoplanets: While refractory materials dominate the story in the initial

    evolution of interstellar material, ices play a major role in the later stages. Once the density and column density of molecular clouds grow sufficiently large, many abundant molecules (water, methane, and CO, for example) rapidly form mantles on the refractory grains further depleting the gas of heavy elements and strongly altering the gas-phase chemistry. The whole subsequent chemical history of protostars unfolds with the processed and returned ices as a significant influence. The development of these ices had a strong effect on the present-day chemical state of bodies throughout the solar system. These icy mantles can be studied in transmission against background sources both in cold and moderately warm environments as star formation unfolds. To get a complete picture of the chemical and physical nature of the icy material, we need to use absorption bands throughout the near, mid, and far-IR. It is only with this broad coverage that we can determine compositions and abundances in the face of opacity and temperature differences, as well as confusion with features from refractory constituents or different ices.

PAH’s, Energetics, and Ionization Balance: Polycyclic aromatic hydrocarbons are the carriers of

    a large number of spectroscopic features in the 3-30 µm region. Difference in features show

     10

Report this document

For any questions or suggestions please email
cust-service@docsford.com