A Study of Proto-Planetary System Candidates

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9520989 Cheng The origin of our own solar system is one of the most fundamental problems in astrophysics. One way to investigate this problem is to study systems that are still in the phase of planet formation. Intensive studies of Beta Pictoris (A5 IV-V) have revealed a dust disk and a dynamic gas envelope (showing both mass infall and outflow) around the star. The age of Beta Pic (200 million years) roughly corresponds to the era of planet formation and subsequent "heavy bombardment" in our solar system. Therefore, it is likely that Beta Pic's dust + gas disk is similar to the accretion disk in the primordial solar nebula. However, it is still not resolved why Beta Pic seems to have such a dynamic dust + gas disk, with about 100-200 mass infall events each year. The understanding of the accretion mechanism in Beta Pic, and of the relationship between the debris disk and its circumstellar gas, is far from complete. Is this a young planetary system still undergoing formation, or is it the remnant of a dusty proto-stellar accretion disk in its clearing phase? Other systems similar to Beta Pic with debris disks and circumstellar gas need to be identified and studied to answer the questions. The detection of circumstellar debris around nearby A type stars, such as ,Beta Pic, provides an unparalleled opportunity to explore possible proto-planetary systems that could resemble our own Solar System at an early stage of evolution. Therefore, a search for proto-planetary system candidates will be completed. The ultimate goal of this study of these systems is to ascertain if they, as a group, represent some phase of planet formation. Three major objectives will be pursued: (1) Identify nearby main-sequence A type stars that have circumstellar dust, using the Infrared Astronomical Satellite (IRAS) Faint Source Survey (FSS) data. Although it is very difficult to detect dust disks by direct imaging, one can infer the presence of circumstellar dust by the det ection of excess infrared wavelength emission that is consistent with thermal radiation from the orbiting grains. (2) Obtain high-resolution visible and ultraviolet wavelength spectra of the volume-limited A-star sample to study the presence, ionization, and kinematics of the circumstellar gas. These high-resolution spectra can be used to detect narrow circumstellar gas absorption spectra features and the amount of circumstellar gas in the line of sight will be inferred. The visible and ultraviolet wavelength circumstellar absorption lines will be compared to each other to test whether the evaporation of cometary-like bodies is responsible for the variable redshifted lines seen in spectra of Beta Pic and several other Beta Pic-like stars in the volume-limited A-star survey. (3) Test the hypothesis that gaseous accretion is occurring in the inner regions of the debris disks of these planetary system candidates. These systems might have physical properties similar to those of the primordial solar nebula. The study of the dynamic nature of these proto-planetary system's circumstellar environments will provide valuable insight into the formation of planetary systems.