Mathematics – Logic
Scientific paper
Dec 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt........93k&link_type=abstract
PhD Thesis, Univ. of Hawaii (1996).
Mathematics
Logic
4
Scientific paper
We search for optical reflection nebulosity around ~100 main sequence and pre-main sequence stars to test the hypothesis that Vega-like stars possess replenished dust disks. A Lyot coronagraph is used to suppress light from the central star and to observe the circumstellar environment closer to planet-forming regions than is possible through direct imaging. A model of scattered light from axisymmetric circumstellar disks is developed to establish the sensitivity limits of our observations. Circumstellar nebulosities are detected around four main sequence stars: β Pic, BD +31o 643, HR 241, and HR 1307. No circumstellar disks are found around ~100 other main sequence stars, including Vega, Fomalhaut, HD 98800, HR 4796, and 51 Oph. Non-detections of disks in the main sequence sample, combined with the sensitivity limits, suggest that the optical scattering cross-section of dust at 102 - 103 AU radii is not strongly correlated to the thermal cross-section at 1-10 AU radii. We show that the prominence of the β Pic disk is primarily a result of its large scattering cross-section, rather than its edge-on inclination or close proximity to the Sun (Kalas & Jewitt 1996). Five types of asymmetry are identified and measured in the disk morphology (Kalas & Jewitt 1995). The observed tilt of the midplane may result from a small inclination (<= 5o) of the disk to our line of sight, combined with a non-isotropic scattering phase function. The remaining four asymmetries indicate a non-axisymmetric distribution of orbiting dust particles between 150 and 800 AU projected radius. The disk may have been gravitationally perturbed in the past 102 to 103 years, though a perturbing agent is not detected. A nebulosity imaged near the B5V double star BD +31o 643 is identified as a circumstellar disk candidate based on its morphological similarity to β Pic and our model disks (Kalas & Jewitt 1997). The disk has a position angle 131o, a projected radius of ~2000 AU, an inclination of i <= 10o, and a possible depletion of material within ~2000 AU. Radiation pressure forces expel the observed dust on timescales several orders of magnitude shorter than the lifetime of the star. We hypothesize that the dust is replenished by the erosion of larger, unseen parent bodies with the same inclination distribution as the observed dust. The reflection nebulosities imaged around HR 241 and HR 1307 resemble the Pleiades reflection nebulosities, with no disk-like morphology. Like the Pleiades, some Vega-like stars may interact transiently with the interstellar medium. We argue that far- infrared excess is not strictly correlated to replenished dust disks, and that a fraction of Vega stars are infrared cirrus hotspots. Comma-like nebulosities imaged around pre-main sequence stars have position angles parallel to the midplanes of postulated circumstellar disks. We hypothesize that some comma-like nebulosities are perturbed circumstellar disks. Gravitational perturbations on Keplerian accretion disks by stellar flybys, or by companions with eccentric orbits, may redistribute disk material to larger radii, therefore making the material accessible to detection in optical scattered light. The solid bodies around β Pic may have experienced such a perturbation, explaining the disk's optical detectability among a sample of comparably dusty main sequence stars. References: Kalas, P. & Jewitt, D. 1995, AJ, 110, 794 Kalas, P. & Jewitt, D. 1996, AJ, 111, 1347 Kalas, P. & Jewitt, D. 1997, Nature, 386, 52
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