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Session 47 - Circumstellar Disks & Shells.
Display session, Thursday, January 08
The bright Herbig Ae star AB Aur has long served as the prototypical intermediate-mass pre-Main Sequence star, and has been extensively observed at all wavelengths. Recent mm- interferometry (Mannings amp; Sargent 1997) of the disk, coupled with detection of a polarization position angle rotation in UV-optical spectropolarimetry (Miroshnichenko amp; Bjorkman 1997, this AAS meeting) have indicated that this object is viewed closer to the disk mid-plane than had previously thought. With this geometry, AB Aur is an ideal system to test the hypothesis that all young pre-Main Sequence stars with this geometry exhibit infalling circumstellar gas indicative of the presence of star-grazing exo-comets.
As befits the prototype of its class, AB Aur is the most extensively observed Herbig Ae star in the UV, with UV spectra obtained by IUE (309 observations), HST GHRS (2 spectra), HUT, and WUPPE. Previous UV data studies focussed on the mid-UV signatures of a strong stellar and/or disk wind, particularly at Mg II (Praderie et al. 1986) and have reported 32-hour periodicities in the outflow. A dynamic spectrum, prepared from synoptic observations made in 1990 reveals, instead of the expected wind modulation, a transient infall event lasting at least 35 hours. Similar variability is seen in contemporary far UV data in transitions of both super-refractory, refractory, semi-refractory, and volatile elements. Closer inspection of the FUV data reveals the presence of infalling, circumstellar gas in all IUE spectra spanning 1978-1995 and the GHRS C IV data from 1996. Covering factors for the more typical infalling gas profiles are similar to the high infall state in the older HD 100546 system (Grady et al. 1997). The infall detections are discussed in the context of the falling evaporating body model (see Beust et al. 1996) and magnetospherically- channelled accretion models (Edwards et al. 1994).
This study is supported by the NASA LTSA program via NASA Contract NASW 4756 to Eureka Scientific.
Program listing for Thursday