Strategy for the Detection and Study of Other Planetary Systems and Extrasolar Planetary Materials 1990-2000 (1990) / Chapter Skim
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4. Observations of Extrasolar Planetary and Protoplanetary Material
4. Observations of Extrasolar Planetary and Protoplanetary Material
Pages 34-43
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From page 34... ...
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.
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From page 35... ...
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.
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From page 36... ...
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.
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From page 37... ...
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.
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From page 38... ...
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, groundbased infrared measurements had not detected dust close (<100 AU)
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From page 39... ...
· 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.
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From page 40... ...
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.
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From page 41... ...
Circumstellar dust has been observed as absorbing, reflecting, or emitting matter that surrounds cool giant and supergiant stars, planetary nebulae, hot stars, evolved stars, novae, 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.
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From page 42... ...
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.
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From page 43... ...
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.
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