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4 Observations of Extrasolar Planetary and Protoplanetary Material INTRODUCTION No widely accepted, confirmed discoveries of extrasolar planets (M < 2 M~upl~er) have been made as of the writing of this report, but searches are continuing with techniques that currently are capable of detecting high- mass planets in favorable orbits. There is, however, abundant evidence for a varieW of planetary and preplaneta~y materials associated with stars, pre- main-sequence stars, and what may well be protostars. Present observing programs are likely to augment this evidence rapidly, as it is generally true in astronomy that the discovery of a new type of object or phenomenon presages the identification of a new class. This chapter gives the status of current research on the observable properties of extrasolar planetary materials. These include possible planets and substellar objects, of course, but also gas and dust around stars, both in their formative phases and in their main-sequence lifetimes. Dust created and ejected into interstellar space by stars during their later stellar evolution is of interest as well, since it constitutes one type of raw material for subsequent generations of star formation. The committee examines each of these topics in the following sections. 34
35 MATERIAL ASSOCIATED VVITH PROTOSTARS AND PRE-MAIN-SEQUENCE STARS The observable properties of protesters and pre-main-sequence stars, collectively known as young stellar objects, are very different from those that characterize stars in their later life. Disklike structures and collimated bipolar outflows of material are seen in the pre-main-sequence phase, evolving on rapid time scales to the much more tenuous concentrations of c~rcumstellar matter observed in somewhat older pre-main-sequence systems. The distinctive appearance of young stellar objects Is associated with their evolutionary properties, which involve the contraction of a star toward a stable main-sequence configuration (see The Evolution of the Central Star in Chapter 3~. The order of events and their manifestations are dependent on mass. Pre-main-sequence stars of low to moderate mass, defined here as less than 3 M<~>, go through the T Tauri phase. T Run stars are very young; the optically thick circumstellar structures that characterize this evolutionary stage are observed at inferred stellar ages ranging from <3 million yr up to roughly 10 million yr. Since the range of survival times for these structures is only a small fraction of their central star lifetimes, such objects are relatively rare. T Run stars are characterized by a variety of phenomena, many of which distinguish them from ordinary main-sequence stars: (1) emission from hot (1000 to 1500 K) dust (termed "an infrared excess" because the strength of the emission exceeds that of the underlying stellar con- tinuum); (2) large optical, infrared, and x-ray variabilities on a variebr of time scales; (3) 10-pm silicate emission features, occasionally observed in absorption; and (4) occasional strong linear polarization in the optical and near-infrared continuum. The variable infrared excesses do not correlate with the amount of observed visible reddening (due presumably to cir- cumstellar dust absorption), implying that the variation is intrinsic to the star. The variability is probably ultimately due to active regions (starspots) on the young stellar objects. These active regions are presumably regions of intense magnetic field acting generated by vigorous convection and, possibly, by differential rotation induced by magnetic braking. Hot dust is sufficient to explain the infrared radiation from T lbun stars; whatever hot gas is present is not dominant. According to current thin~g, veIy young pre-main-sequence stars with a wide range of stellar masses are thought to be associated with a class of objects charactenzed by strong collimated outflows of gas and energetic interactions with the molecular clouds in which they are embedded. These regions of interactions, known as Herbig-Haro objects, are seen both optically and in carbon monoxide line emission. The presence of disks has been inferred for stars covering the mass range from 0.2 to ~5 Mo.
36 Evidence of molecular outflows are found for both optically obscured sources, still located within their protostellar cores, and visible sources throughout this mass range. Stars more massive than ~5 Me have not been observed optically in the pre-main-sequence phase, presumably because of their short contraction times. Instead, at this stage, they are seen only as luminous infrared sources, embedded near the centers of molecular clouds. The characteristics and properties of disks associated with young stellar objects are studied by a variety of observational techniques, each sensitive to particular aspects of disk structure. In general, disks actually appear as elongated or barlike structures that are interpreted to be disks in projection. In only a few cases do molecular line measurements indicate rotation appropriate to a disk or torus. In other cases, the presence of a disk is largely based on models of bipolar outflow, with disks and bipolar outflow formation being causally linked, or on observations of one outflow direction being blocked by a disk or a disklike object. At the largest scales, disks are apparently manifested as elongated molecular clouds ~103 to 104 AU across. Molecular hydrogen number densities (nip) are estimated to be ~103 to 104 cm~3 at a typical radius of ~104 AU. The degree of inferred flattening of these disklike structures is not large; the ratio of the hydrogen column density along a radius parallel to the elongation to that perpendicular to it (the axial density ratio) is typically only a few. The inferred flattening is considered reasonably good evidence for rotational or magnetic support of a disk, although flattening by itself is not robust evidence for either. While disks of this scale are clearly larger than those usually thought of as circumstellar disks (radu <1000 AU) or inner accretion disks from which planets may be born (radu <100 AU), it is reasonable to assume that these smaller-scale structures are embedded in the larger-scale ones, especially as the initial collapse phase may occur in a larger-scale cloud. In fact, microwave continuum observations show that within the larger disk structures, smaller (radii ~2,000 AU) elongated structures are embedded with the same orientation. Molecular hydrogen abundances are increased, with net densities of at least 106 cm~3 In the plane. These structures are presumably more flattened, with axial ratios of kilo to 100. Inner, circumstellar "disks" or elongations can be indirectly detected by observing infrared radiation scattered from young stellar objects. Speckle interferometry and maximum entropy techniques have been used to infer the existence of disks in the approximately 100 to 1000-AU radius range. We anticipate that use of infrared array cameras will be of value in detecting scattering disks toward young stellar objects. Disk'; can be directly detected at radio wavelengths. Radio continuum and millimeter-wave observations are sensitive to thermal emission by dust in disks, and molecular line observations are sensitive to emission and
37 absorption by gas. HE Lou is a particularly intriguing object for study by these methods. Here, the total amount of gas and dust lying in a rotating disk is inferred to be as high as Gel to 1.0 Me. Thus it is possible that a small disk with dimensions comparable to those of our planetary system may contain sufficient matter to build a system similar to our own. Molecular outflows associated with young stellar objects, detected by means of radio emission lines, characteristically extend for ~104 to 105 AU. These aligned, cortical structures often have collimated infrared and optical manifestations as well. Carbon monoxide flow velocities are typically 10 km s~i ranging up to 100 km sol, but velocities in highly collimated optical jets (often linear chains of Herb~g-Haro objects) range up to several hundred kilometers per second. Estimated lifetimes of the molecular outflows are between 104 and 105 years, a small fraction of the duration of the T Tauri phase. The mechanical luminosity of these flows is significant compared to the bolometric luminosity of the young stellar object, implying a driving mechanism far more powerful than radiation pressure or ordinary stellar coronal winds. Some of the most significant observations are geometric: collimated outflows of neighboring stars are often aligned with each other, aligned with cloud or disk magnetic fields, or oriented perpendicularly to the long axis of circumstellar disks or molecular clouds. The opening angles of the outflows are generally less than 45° and become smaller with increasing resolution of the source area. Also, blue-shifted, collisionally excited emission lines of the optical outflows are seen in preference to red-shifted ones; the inferred red-shifted companion flow is believed to be obscured by dust. The central pre-main-sequence star can be obscured as well, possibly implying the presence of a disk or torus. The observations above fit within the theoretical framework of a young stellar object surrounded by a circumstellar dish The existence of a central accretion disk of ~100 AU scale is inferred, but no certain example has been directly resolved and studied in detail Cloud magnetic fields may determine the orientation of the rotation axes of stars and disks. All current theoretical models of the mechanism of bipolar outflows invoke an accretion disk, either in an active role of providing kinetic energy for the flow or in a passive role as a barrier to a more isotropic flow whose energy is provided by the young stellar object itself. When these disks are seen edge on, the radiation Is linearly polarized; some researchers have inferred a magnetic field oriented perpendicularly to the disk HL lbu and DO Thu are edgers examples. In general, newly forming substellar objects or planets within these disks cannot be seen due to inadequate observational sensitivity. One exception is T Tori itself. It has an infrared companion, ~80 AU distant. Based on very large array (VLA) radio measurements, both objects have powerful ionized stellar winds, but the companion dominates the T Mauri
38 outflow. One estimated mass of the T Run companion is substellar, and its luminosity can be interpreted to result from capture of matter from an accretion disk Although further infrared and radio searches for substellar or protosubstellar companions to T Tauri stars have not been definitive, such searches are actively under way and have led to some tantalizing results. The existence of substellar or even planetary companions in the process of formation does not appear to be an unreasonable supposition. Observations of disks and jets around young stars stand as interesting and largely unexplained phenomena. And as detailed in Chapter 7, they may have some connection to the physics involved In collimated bipolar outflows of drastically different scales and energies, associated with such objects as SS433, Seyfert galaxies, active galactic nuclei and quasars. GAS AND DUST DISKS AROUND MAIN-SEQUENCE STARS As young stellar objects evolve onto the main sequence and begin to age, it is anticipated that they shed their associated disks of matter and become essentially unobscured. The naked T Tauri stars observed in association with the generally younger classical T Tauri stars seem very likely to represent just such an evolutionary stage of disk dissipation in these stars. It is therefore of considerable interest that dust and gas have been detected around at least some moderately evolved main-sequence stars. A central question that these observations raise is whether we are observing the remnants of a primordial circllmstellar dust and gas disk and thus the remnants of an accretion disk. One plausible class of remnants is planets. Many of these circumstellar dust observations were made by the IRAS, which detected large infrared excesses associated with what had appeared to be ordinary main-sequence stars. In particular, higher-resolution, ground- based infrared measurements had not detected dust close (<100 AU) to these stab. First reported was an extended region around Vega. Since then, dozens of stars have been classified as Vega-like on the basis of their 60-pm and 100-pm flux excesses, although spatial information is not generally obtainable. About half of these stars are spectral type A (1.5 to 2.0 May, but excesses are also seen in more solarlike F. G. and K stars (0.5 to 1.5 Mop. Given the likely selection effects caused by the volume and flux limitations of the survey, it is entirely possible that most main-sequence stars have such dust clouds. We may have only detected the thicker, more massive ones. Further discoveries will better define how cloud properties depend on stellar properties such as spectral type, duplicity, age, rotation, metal content, and so on. Three stars for which spatial information is currently available are Vega (c'Lyrae), Beta Pictons, and Fomalhaut (copse).
39 1. Vega: the dust source extends to ~ radius of ~80 AU, perhaps as far as 250 AU. The size of the dust particles is ~80 Em based on infrared emission characteristics, and the dust has a color temperature of SS K Thus it Is possible that these dust grains may be icy. The ~80- ,um particle diameter Is some 2 orders of magnitude larger than typical interstellar grain sizes of 0.1 to 1 ~m, indicating that the dust is not primary (i.e., interstellar) and that local coagulation-condensation processes have occurred. The minimum mass of the dust region is estimated to be ~1025 g (~10-3 Me), but much more could exist in larger bodies. No gas adsorptions are seen in the visual or ultraviolet 2. Beta Pictons: optical charge-coupled device (CCD) images show an elongated source extending to beyond a SOD-AU radius in two opposing directions. Modeling based on visual chronographic and infrared studies suggests a disk inner edge at ~17 AU. The minimum dust mass to account for the observations is ~4 x 1025 g (~10-2 Meg; again, much more mass could exist in larger bodies. Remarkably, narrow calcium and sodium lines are seen in visual and ultraviolet absorption; there is apparently a gas disk that accompanies the infrared and visible dust dish The mass of this gas is estimated at <2 Me, with a characteristic na2~105 cm~3. 3. Fomalha;ut: material Is detected out to a radius of ~140 AU and possibly beyond. Certain interstellar gas adsorptions may actually be circumstellar. It is plausible that all three of these systems have gas disks, but Vega and Fomalhaut lack adsorptions because they are not seen close to edge on. Details of other detections continue to be reported in the literature. A number of questions need to be addressed before astrophysicists can conclude that any of these stars has planets: · Do we observe gas and dust because these systems are young (the lifetimes of A-type stars are <109 yr) and are thus direct descendants of a preplanetary (protosolar) nebula? Recent data suggest that a number of old (>109 yr) main-sequence stars may have disks. · Does the absence of obscuration and emission from hot dust within the central regions of some of the systems imply that these regions have been cleared of dust? Relatively sharp edges on spatially resolved disks imply some clearing process. · Could this clearing process be due to planetesimal accretion or to a more prosaic phenomenon such as gas drag? · Do the observed dust disks require replenishment from an invisible swarm of larger bodies? · And could type A stars go through an evolutionary phase, unrelated to the standard protostellar and pre-main-sequence phases, that results in the formation of equatorial shells of gas and dust? Rapid removal rates
40 for dust strongly suggest that renewal is required. A recent interpretation that such disks are resupplied by relatively large, but subplanetary, bodies (perhaps analogous to asteroids or comets) and truncated at the inner edge by planets, has been advanced based on analyses of IRAS data. SUBSTELLAR OBJECIS The preceding section emphasized the detection of dust and gas around early-type (and thus luminous) man-sequence stars. Such observations are currently much more difficult around the more common, faint late-type dwarf stars unless they are close, and will probably remain so until the Space Infrared Telescope Facility (SIRU;) deployment brings objects of this kind, even at distances of hundreds of parsecs, into effective observational range. Nevertheless the possibilities for detection of massive planetary and substellar companions around faint low-mass stars are enhanced, both because of more favorable secondary to primary luminosity ratios, and because astrometric wobbles and radial velocity Doppler shifts due to stellar reflex motions are greater for a given companion mass and separation. These topics are treated in greater detail in Chapter 5. Ongoing astrometric and radial velocity work points to the existence of companions ranging in mass from small stellar to substellar. Near-infrared speckle interferometry has generally confirmed the presence of low-mass stellar companions with separations of >0.06 arcsec. Of the substellar companions reported, one (HD114762b, M ~ 10 MJ,,~er) appears at this writing to be relatively solid. Claims of planetary companions are con- siderably more uncertain at the present time. Several of the unconfirmed companions have predicted separations of <0.06 arcsec, and some would not necessarily be luminous enough in the infrared to be directly detected in any event. It is also quite possible that many of the weaker astrometric perturbations are not real. The first claimed direct detection of a substellar mass object, a compan- ion (VB 8B) to Van B~esbroeck 8 (VB 8), was made in 1985 by two~olor infrared speckle interferometry on three occasions by one observational team. Theoretical bounds on its mass ranged between ~40 and 80 Mixups More recent observations have, however, failed to confirm the existence of VB 8B. Near-inlrared speckle interferometIy has also been applied to nearby stars not suspected of having astrometnc companions. None were found, and the limits on companion luminosity generally preclude objects resembling the putative Van Biesbroeck substellar body. The controversy surrounding VB 8B illustrates the great difficulty of directly observing such faint objects with current techniques. Because substelIar objects more luminous than VB 8B occupy a rather narrow regime in age-mass space, definitive identification of such objects, either
41 as binary companions, components of multiple-star systems, or perhaps as isolated objects, will continue to be a tremendous challenge over the next decade. With the exception of the very recent report of substellar bodies in [bums, at the time of this wnting, only three other candidates Gliese 569B, GD 165B, and LIES 2924have emerged from imaging or spectroscopic observations. The first two are cool companions to white or red dwarf stars, all have possible masses as low as ~50 Mope or so, and none are considered proven as yet. LHS 2924 is cooler and less luminous than any known star, but it has a peculiar spectrum, which hampers its positive identification. Deep CCD integrations in the red and near-infrared in limited areas have turned up no examples. Data from continuing searches for indirect stellar reflex effects (as- trometric wobble and Doppler shifts In the velocity of the orbital reflex, as opposed to direct imaging or spectroscopy) are encouraging in a few cases, and in general leave open the possibility that substellar objects (and perhaps planets) may exist around other nearby stars. More measurements over a longer time period are needed to clears confirmor refuteclaims of detection. CIRCUMSTEI'AR DUST It is appropriate at this point to consider the properties of c~rcum- stellar and interstellar dust in greater detail. Circumstellar dust has been observed as absorbing, reflecting, or emitting matter that surrounds cool giant and supergiant stars, planetary nebulae, hot stars, evolved stars, no- vae, supernovae, and pre-main-sequence stars. Most of the dust around pre-main-sequence stars is probably preexisting interstellar material, but for the majority of other stars mentioned above, where they are observed, circumstellar dust shells appear to be condensates formed in situ from gas derived from the central star. Circumstellar dust is important to studies of planetary systems because it is the original source of interstellar grains, and because close examination of dust shells may give detailed information on condensation processes and products that occur in gases of various temperature, density, and composition. At the present time, however, it is not clear what direct links there might be between grains observed to form in circumstellar envelopes and particles that eventually accrete to form planetary materials. Evolutionary processes that occur in the interstellar medium may drastically alter interstellar grains between the time they leave, for example, the atmosphere of an M giant and the time they are incor- porated into a solar nebulalike environment where planetary bodies could form. Current models of grain sputtering by shock fronts indicate that grain destruction may occur on a time scale that is a factor of 10 shorter than the billion-year period required to replenish the interstellar medium
42 with dust observed to escape circumstellar envelopes. If this is actually the case, then dust must reform in the interstellar medium. The existence of circumstellar dust is most often revealed in the infrared. The dust is heated by radiation from the central star and, in cases where active disk structures are present, possibly by accretion of material through the disk. The observational evidence consists of excess infrared radiation from the heated dust above the underlying stellar continuum, or of spectral features due to silicates, ices, or carbon-rich materials. In the visual range, dust leads to reddening, extinction, and polarization or can be directly imaged in reflection nebulae. The infrared excesses range from a small change of continuum slope, usually beyond 5 ~m, to cases where the infrared flux far exceeds the observed visual luminosity. The magnitude and spectral energy distribution of the excess can be used to estimate the dust mass surrounding the central star, and to diagnose dust temperature distributions and optical depth structures. For late-type giants the total mass of the dust envelope correlates with the gas-loss rate determined by radio carbon oxygen measurements. The ratio of gas to dust implied from these measurements is ~200 to 300, indicating that virtually all condensable refractory elements have formed grains. At least for late-type giants, condensation from outflowing gas seems to be a very efficient process. This is confirmed by the remarkable efficiency of condensation actually observed to occur during nova outbursts. Information on the composition of grains can in principle be deter- mined from the temperatures of their formation and stability. In practice this is complicated by kinetic effects, nonlocal thermodynamic equilibrium environments, and the complexity of gas outflow. Infrared spectral fea- tures provide rather direct information on grain composition. The 9.7-pm and loom features seen in emission and absorption around o~ygen-rich stars have been attributed to the stretching and bending vibrational modes of silicon-o~ygen bonds in silicates. The shape of the "loom feature" is not consistent with that of single-phase crystalline silicates that have been studied in the laboratory, and the suggestion has been made that the circumstellar silicates are amorphous or poorly ordered. In rare systems, a 3.1-pm feature is seen that is consistent with water or ammonia ice at temperatures of less than 150 K Around carbon-rich stars where the ratio of carbon to oxygen is greater than 1, the grain chemistry is quite different. In these environments, nearb', all of the gas-phase oxygen is bound up in carbon oxygen, and the excess of carbon, coupled with the lack of water vapor, favors the formation of carbon-rich and reduced grains while depressing the formation of silicates. In the carbon-rich stars a broad feature at 10.5 to 12 I'm has been identified as stretching-mode vibrational emission by silicon carbon and another feature near 25 I'm has been associated with magnesium
43 sulfur. Because the silicon abundance is small compared to that of carbon, silicon carbon cannot be the dominant grain in these systems. The major part of the infrared excess in carbon stars is probably due to fairly pure carbon in either amorphous or crystalline form. The apparent absence In the observational spectra of a predicted resonance feature at 11.5 Am indicates that the carbon Is not graphitic. The previously unidentified infrared emission features at 3.3, 6.2, 7.7, 8.7, and 11.3 Am in carbon-nch outflows have recently been attributed to emission by partially hydrogenated polycyclic aromatic hydrocarbons (PAHs). The PAHs may in addition be responsible for other infrared features and may be the source of the diffuse interstellar bands seen In the visible. Future efforts from a variety of disciplines are needed to determine possible links between circumstellar dust grains and materials found in meteorites and interplanetary dust. Although no individual grains that have been studied in the laboratory have been shown to be interstellar or circumstellar in origin, there Is fairly strong isotopic evidence that materials from circumstellar envelopes did survive transport to the solar nebula and were incorporated into some meteorite parent bodies. The most direct evidence for this is the survival of nearly pure 22Ne (neon-E) in carbon-rich separations from carbonaceous chondrites. If this exotic neon component is the product of the decay of 22Na (half-life = 2.6 yr), then the sodium must have been incorporated into solid grains within a few years of its synthesis. New infrared observations of novae suggest that both 22Na production and grain formation may be observed in binary novae invoking matter accreting on an o~ygen-magnesium-neon white dwarf. The neon-E component is the most obvious example of probable sunival of circumstellar dust and incorporation into the solar system, but many of the other isotopic effects in meteorites may have had a similar origin. Heavy carbon, light nitrogen, and s-process krypton and xenon are also found in carbon-rich separates. These isotopic effects are consistent with fo~ation from a material derived from a red giant. The apparently correlated effects in the neutron-rich equilibrium isotopes 48Ca, 50Ti and 54Cr may also be tracers for circumstellar material ejecta from supernovae. Large deuterium/hydrogen enrichments, a factor of 3 or larger, have been seen in carbonaceous chondntes, in the organic fraction of unequill~rated ordirlaIy chondntes, and in interplanetary dust No known solar system process can produce such large isotopic fractionation in hydrogen, but similar and even larger effects are seen in molecular clouds, where they are believed to be produced by ion and molecule reactions. The heavy hydrogen in meteoritic materials is believed to be a direct link to the chemical evolutional process that occurred in molecular clouds. This type of information has important implications for the evolution of biogenic elements and then incorporation into planetary materials.