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6 Future Investigations of Precursor Planetary Systems The previous chapter focused on observational initiatives whose goals are the detection and enumeration of evolved extrasolar planets. The committee turns next to the question of enhancing current knowledge of possible precursors of planetary systems and protoplanetaIy materiale.g., evolving molecular clouds, stellar nebulae, and accretion disks- through the development of advanced laboratory and observational techniques. This development may also be critical to the study of the physical environment of candidate extrasolar-system planets when, and if, they are discovered. The applicable techniques are unaging, analytic spectroscopy, and po lamely, which will be discussed here under science objectives rather than by technique, as in the foregoing chapter on planetary searches. The reason is Hat clear distinctions based on technique do not exist for this area images are taken though restrictive spectral or polarimetric filters, whereas spectra are obtained with spatial sorting or scanning. The nature of the instrumentation and subject area is such that planetary scientists will cany out those programs in association with astrophysicists. Indeed, many of the relevant projects proposed or under development spnug from the astrophysics community. In the following sections, three classes of study objects are discussed: dust, protostars, and pre-main-sequence stars. The last two were identified in Chapter 3 as the immediate precursors to evolved stellar systems, and it is in their vicinity that we can expect to observe the early stages of planet formation. The study of dust is singled out for special treatment because dust is ubiquitous and plays a unique role as a tracer of many physical 57
58 processes important in stellar evolution, as well as in the formation and evolution of planetary systems. DUST-REI^TED ASTROPHYSICAL STUDIES Recent observations, especially in the infrared, show that dust is a useful tracer of elements heavier than hydrogen and helium through the processes of star formation, evolution, and death. Dust is thought to form originally in the ejecta of evolved stars. Silicate grains have been found in the interstellar medium in regions of star formation, in pre-main-sequence stars around main-sequence stars, and in end stages of stellar evolution Characteristics of astrophysical grains were described in detail in Chap- ter 4 under Circumstellar Dust. Silicates are conspicuous to thermal in- frared spectroscopy due to prominent 10- and 20-pm emission features. Silicon carbide grains emit a characteristic foam spectral feature observed around some stars. Carbon grains, responsible for Infrared continuum em~s- sions, appear in all stages of stellar evolution as well as in the interstellar medium. Dust is an important solar system constituent. Dust grains, incor- porated into comets, asteroids, and planets during the formation of the solar system, represent material that was injected into the presolar nebula by earlier, evolved stellar systems and processed to various extents. Dust observed in the comae and tails of comets as it is expelled from the frozen nucleus is the probable source of the zodiacal cloud. Dust grains of interplanetary origin can be collected in the atmosphere or in near-Earth space and studied in the laboratory to determine com- position, mineralogy, and optical properties. COMPLEX has reviewed a catalog of infrared spectra taken of individual, microscopic dust particles that display a multiplicity of characteristic features relatable to their sep- arateh,r determined intrinsic properties and is convinced of the efficacy of techniques available for the establishment of ground truth. Dust, particu- larly through its thermal and optical properties, which can be calibrated, provides a useful diagnostic material that can probe astrophysical systems for knowledge connected to the solar system. Direct knowledge of the material present in our early solar system obtained from dust-oriented studies could be compared with observations of the dusty material thought to exist in young and evolved stellar systems. A special emphasis should be placed on observations of young stellar systems where planet formation is thought to be a likely process. The dusty constituents of these systems should be careful studied to determine how closely they resemble the material in our solar system. Resolved observations of dust disks (or shells or clouds) can address key questions regarding the connation and evolution of planetary systems.
s9 Determination of locations of dust shell edges and their sharpness (e.g., at resolutions of ~1 AU) will permit determination of how He dust is supplied, such as through the erosion of comets, and whether clearing of the inner edge by planets is a plausible model. For systems such as Vega, for example, one can envision future surveys utilizing a cryogenic infrared telescope or a large millimeter-submillimeter-wave dish in space to assess the frequency of Vega-like dust shells, determine gas-dust compositions by spectroscopy, and carry out direct and indirect searches for resonances and other phenomena related to radial structure. Such work links the observation and laboratory studies of dust with searches for planetary bodies discussed in the previous chapter. Studies of the dust ejected into the interstellar medium by evolved stellar systems such as supergiants, novae, and supernovae are required to trace the injection of heavier elements into young stellar systems and to determine the characteristics of the dust formed from the ejected material. It Is important to establish whether or not dust grains in the vicinity of the objects mentioned above are consistent with the hypothesis that such grains, formed in the ejecta of evolved stars, find their way into regions of star fonnation and young stellar systems where planets form. Obse~va- tions, particularly in the infrared region of the spectrum, are needed to provide detailed information on grain formation, composition, crystallo- graphic structure, and the relative importance of various stellar sources of interstellar dust. New observational data, combined with modeling and lab- oratory condensation experiments, should provide a better understanding of nucleation and grain growth. Currently there is considerable uncertainty as to how nucleation and grain growth proceed in circumstellar environ- ments even though these processes are actually observed in classical novae. Signatures of carbon or silicate grains appear in nova spectra ~100 days fol- lowing an outburst. It has been predicted that nucleation difficulties in the pure gaseous outflow from all dust-form~ng stars could lead to high levels of supersaturation and subsequent formation of amorphous particles, with well-mned composition. This prediction needs to be carefully evaluated in the context of some of the isotopic anomalies seers in meteontes. LABORATORY STUDY OF INTERSTELLAR MATERIALS Infrared spectra of interstellar grains are complex and difficult to in- terpret unambiguously because there may be several constituents as well as contn~utions to the shape of a given spectral feature from the lattice structure of the solid matenal. Thus it is advisable to have laboratory ground truth analysis of candidate grain samples based on the wide range of in situ measurement techniques currently available. Such analyses will provide a vital synergistic link between dust studied with purely astronom- ical techniques and material that can be studied in me laboratory using
61 low-loss EELS peak is due to plasmon effects and, in principle, is directly related to ultraviolet spectral effects. Additional work along these lines could also yield fresh insight into the 10-pm "silicate" feature, the SAC fea- ture, the 3.4-pm (organic?) feature, diffuse bands, and various unidentified infrared features. An example of the potential of this field is the recent work on a single interplanetary dust sample that, in addition to other features, showed an absorption line that correlated with the 6.8-pm interstellar feature. This interstellar feature has been postulated to be related to carbonaceous matter, but in the dust grain, the feature was definitively shown to be caused by carbonates. Carbonates were identified by electron microscopy and, after dissolution of carbonates by acid treatment, the 6.~pm feature was absent. The search for presolar grains in the laboratory materials is limited by the state of analytical technology for small samples. Grains detected by astronomical techniques are generally in the 10-~3 to 10-~6-g range, while materials analyzed in the laboratory are typically much larger. For example, a standard high-precision isotopic analysis of meteoritic oxygen requires a sample of over 10-3-g mass. Recently developed ion microprobe techniques have provided isotopic analyses of carbon and hydrogen in grains in the 10-~2-g range. In addition to the ion probe, newer developing techniques include accelerator mass spectromeny, multicollector mass spectromeDy, synchrotron x-ray microprobe analysis, ultrasensitive organic analysis, and high-resolution analytical electron microscopy. The sensitivity of techniques for analysis of extraterrestrial materials has undergone orders-of-magnitude improvement over the past mro decades, but further improvements will be ~ ~ , ~ . . required to analyze presolar grams adequately. Such advances are needed for work on current available extraterrestrial materials, and they will play a critical role when the first samples of a comet nucleus are analyzed in situ or by sample return In parallel with these present and future investigations of extraterres- trial materials themselves, it is extremely important to pursue laboratory studies of the processes that are thought to act on natural extrasolar grains during their life Cycles from condensation to destruction or accretion. Such experiments can be conducted, using a wide variety of laboratory techniques under appropriate physical conditions (for instance, under microgravity conditions for studies of grain agglomeration), on simulant materials that appear liked to represent the spectrally inferred chemical compositions, physical states, and sizes of solid particles in stellar ejecta and in inter- stellar and accretion-disk environments. One example, important for the volatile inventory of planetary systems, is investigation of the condensation, chemical-organic evolution, and erosion of icy-organic grain mantles and their constituents in the last two of these environments.
62 PROTOSTELLAR OBSERVATIONS A primary objective of observational and theoretical studies of proto- stellar and pre-main-sequence objects should be to piece together a unified picture of the temporal evolution of the physical parameters of an evolving stellar system from the onset of gravitational instability to nuclear burning on the main sequence. Such an effort is statistical in nature; many repre- sentative systems In various states of evolution (and, therefore, of different ages) must be thoroughly observed. One result of such studies would be the construction of an H-R diagram for pre-main-sequence objects. Whereas molecular clouds and pre-main-sequence stars are common astronomical objects, an example of the intermediate, protostellar phase has yet to be definitely identified. An effective search for the elusive pro- tostar is important. Candidates could be located by systematic imaging surveys at long wavelengths. Dunng the protostellar collapse, cloud tem- peratures as low as 10 to 50 K obtain, and such systems emit predominantly in the far-infrared and submillimeter spectral regions. Systems operating at these long wavelengths can and do find cold compact objects embedded within molecular cloud cores. Initial observations, particularly by IRAS, will be followed up using a variety of more advanced facilities that are now being constructed or are planned. These include ground-based sub- millimeter telescopes and interferometers, proposed airborne telescopes like the Stratospheric Observatory for Infrared Astronomy (SOFIA), and space-based facilities such as SIRTF and the large deployable reflector (LDR). Even for the nearest regions of star formation, the far-infrared and submillimeter facilities contemplated here will only resolve details as small as a few hundred to a few thousand astronomical units across. It will therefore be necessary to supplement the unaging surveys by instruments that might be expected to reveal the presence of protostellar candidates; that is, with higher-spatial-resolution imaging studies that can be conducted with very large ground-based telescopes operating in the near-infrared and thermal-infrared spectral regions. For example, the once-planned National New Technology Telescope (NNTf), with its 21-m baseline for unaging, could have resolved condensations the size of the solar system in the 10- to 20-pm spectral region for the closest regions of star formation. If large space dishes and interferometers could be optimized for this spectral region, superresolution techniques combined with interferomet~y could enable resolutions considerably better than this. Such infrared studies would enable us to examine in detail the spatial morphology of the collapsing systems. It is especially important to be able to observe evolving circumstellar disks in various stages of fragmentation
63 ~ order to identify sites where substellar- and planetaty-mass companions may form. Polarimetric imaging studies of star-forming cores should be useful for identifying the location of central heat sources and candidate protostars, and for mapping the spatial d~stn~ution of the dust ~ rejection nebulae. Spectroscopic observations of atomic and molecular lines can be used to trace the gaseous constituents and determine the spatial dependence of the physical conditions in the collapsing cloud. Dust constituents can be traced in the infrared by observing prominent solid-state emission- absorption features. It is important to recognize that imaging must be conducted at high polarimetric and spectroscopic resolutions to reveal the dynamics of and physical conditions in collapsing systems. Fundamental information on the processes of star formation is pro- vided by measurements of molecular line emission (e.g., carbon-o~ygen, oxygen-hydrogen, and hydrogen-carbon-nitrogen) at radio wavelengths. These rotational lines are collisionally excited by hydrogen molecules and therefore can be used to determine the density and pressures within molec- ular clouds and the largest-scale (and thus lowest-densi~) circumstellar disks. Doppler shift and line-width analyses provide information on ve- locities, flows, turbulence, and chemical abundances. Such studies require spectral resolution in the range A//~) = 103 to 106. At present, these tech- niques alone provide direct observational support for the rotational motion of disks. Some measurements may imply inflow within certain molecular cloud cores; hence these techniques may eventually provide unambiguous evidence for protostellar collapse and inflow. Radio measurements are currently limited by both resolution and sensitivity, especially at millimeter wavelengths where single-d~sh measurements are common OBSERVATION OF PRE-MAIN-SEQUENCE STARS In contrast to protostars, many pre-main-sequence stars have been identified. They exist in regions of dense obscuration, a factor that favors longer-wavelength studies because of lower light extinction. These objects exhibit a range of identified or inferred phenomena- mass outflows, T Mauri outbursts, accretion disks, Herbig-Haro objects, and expanding shocked shells and they are accessible to the full suite of imaging, polarimetnc, and spectroscopic studies. It is particularly important to address the key questions related to planetary evolution in studies of such objects. Studies of the immediate post-protostellar collapse phase, when the central regions of the collapsing system are heating up, can be fruitfully conducted at the highest spatial resolutions using the near-infrared and even optical techniques on the largest telescopes. Spatial scales much smaller than 0.1 arcsec, which represents a 15-AU resolution in the closest
64 regions of star formation, could be probed using interferometers in space. General imaging and photometric studies combined with polarimetry can reveal the spatial morphologies of collapsed disk systems and the outflows associated with pre-main-sequence stages of stellar evolution. Observations of the smallest structures in evolving pre-main-sequence stars will require long-baseline interferometry, perhaps from orbiting telescopes where the deleterious effects of atmospheric "seeing" can be avoided and where orbital precession can be used to provide complete spatial coverage. Using the techniques of infrared spectroscopy, imaging spectroscopy, and polarimetric studies, we can expect to obtain new structural and dy- namical information about pre-main-sequence stars. At the highest spectral resolution (~/~) = ~106t to 106), spectroscopic studies will provide infor- mation about abundances and dynamical motions. The spatial resolutions that would be available with LDR, and with ground-based facilities as competent as the once-planned NNTI, would enable studies of turbulence during earlier stages of stellar evolution. Observations of the differential rotation of nebulae can be used to address the angular momentum transfer processes. Spectroscopic imaging studies of the atomic and molecular lines and dust emission features can reveal important information about the ratio of gas to dust and the chemical abundances of these species as a function of position in a nebular system. There is ample evidence from studies of our own solar system that spatial chemical fractionation by condensation occurred during the nebular phase. An especial important line of inves- tigation will be to determine the chemical evolution of nebular systems as a function of time. Spectroscopic measurements in the near-infrared, far- infrared, and microwave spectral regions can reveal how complex organic molecules that are created in molecular cloud cores are incorporated into protostellar condensation, and how these molecules evolve as the process of stellar collapse and condensation progresses. Similar studies of the fate of the elements themselves (such as the biogenically important carbon, nitro- gen, and oxygen) can also be pursued. Spectral line studies will additionally reveal the nature of the outflows in pre-main-sequence stars and can be expected to shed light on how the outflows interact with the surrounding molecular cloud and circumstellar environment. The energetic phenomena associated with pre-main-sequence stars involve large degrees of ionization and powerful magnetic fields. Radio measurements are ideally suited to providing information on emission mechanisms, electron temperatures and populations, and magnetic field strengths and configurations. Energetic plasma phenomena vary rapidly in time, and so continuous radio monitoring can in principle follow the details of accretion~isk inflow, star spot evolution, jet acceleration, and so on.
65 At present, these measurements are camed out at instruments such as the sensitive, high-resolution (subarcsecond) Very Large Array (VLA). SPECTROSCOPIC STUDIES OF EXIRASOLAR-SYSTEM PIANElrS ANI) PREPLANETARY MATERIALS Beyond its use for detection and fundamental characterization by the techniques discussed in Chapter 5, spectroscopy is a potential tool to elucidate physical and chemical environments of planetary and precursor systems. For reasons discussed under the heading Direct Detection, this is beyond the capabilities of telescopes or other systems currently In progress or planned, although there appears to be no fundamental reason why improved systems with the needed capabilities cannot and will not be built. Application of analytic spectroscopy to physical and compositional studies will require telescopes of sufficient spatial resolution to separate emitted or reflected light from the planetary object and circumstellar material from that of the pnma~y star. If such systems were trained on extrasolar planets, spectroscopy could provide the means for ascertaining the presence of an atmosphere, deter- mining or constraining its composition, and establishing a rough tempera- ture-pressure structure. Such measurements, combined with studies of metal content of the parent star, could be diagnostic of the processes by which the secondary object formed. In addition, emission spectra from substellar objects (which should be obtainable in the infrared, for example, from SIRES:) give information on the composition, temperature, and sur- face gravity of these objects. Basic questions to be addressed might concern the frequency of substeliar objects and of planets with atmospheres and among those planets the determination of the frequency of occurrence of major atmospheric types, such as those pnmaril~r of hydrogen or carbon diomde, if the solar system is used as a guide, or perhaps even other constituents. The detection of molecular oxygen as a major atmospheric constituent may even suggest the possibility of the existence of Earth-like biological activity. Prerequisites for application of spectroscopic techniques include the existence of imaging or interferometric systems with sufficient spatial res- olution and a large enough aperture to collect the required number of photons. Thus application of spectroscopy lo physical studies will follow the development of light-collecting and sorting facilities in a natural way. The development of technology for spectroscopy of extrasolar-system plan- ets is certain to be a formidable challenge, requ~nng extensive laboratory development of narrow-band spectrometers for the visible and infrared that are analogous to the radio-frequency spectrometers used with radio tele- scope arrays. For example, development of heterodyne spectrometers for
66 the far infrared has progressed marked in recent years. Future extensions to shorter wavelengths should find wide applicability to the spectroscopy of protoplanetary nebulae and other systems whose content is primarily un molecular form. It is likely that new molecular and atomic laboratory data will be necessary to support analysis of these observations. Thus, if NASA is to be ready for further studies when other planetary systems are located, it must begin to refine spectroscopic capabilities for that purpose. SUMMARY RECOMMENDATIONS · The committee strong supports coniinua~on and expansion of ob- serving programs directed toward investigation of preplaneta~y environments and precursor materials, and for development of enhanced imaging, spectro- scopic, and polw7me~c z~st~ument capabilities relevant to such programs. In addinon, COMPLEXmakes three more specific recommendations with respect to the dust-related research discussed above, and later in Chapter 7. · Observanona] studies, collection programs and techniques, and labo- ratory u~vesagahons focused on e~raterrestriz~1 dust should be continued and refined · rhythm this general area of study, He technology of laboratory analysis of small e~raterrestnal samples should be developed to the point that submicron Gains carrying "exoac'' isotopic sig~amres suggestive of presolar origin can be individually identified arid analyze · Final, active encouragement should be given to theoreacal, observa- tional' and laboratory simulation studies of the condensing and chemical nature of dust grams us czrcumstellar environments, of grain interactions with and survival in the interstellar medium arid during infall into accretion disks, and of the chemical and physical properties of icy~rganic grain mantles