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Session 106 - Herbig Ae/Be Stars.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

[106.03] A Transient Circumstellar Accretion Event in the Herbig Ae Star HD 163296

C. A. Grady, M. R. Pérez (ARC), K. S. Bjorkman (U. Wisconsin), M. L. Sitko (U. Cincinnati)

IUE high dispersion spectra of the Herbig Ae star HD 163296 were obtained on 1995 March 9, contemporaneously with WUPPE UV spectropolarimetric observations made during the Astro-2 mission. Our recent IUE data show a comparatively low state of outflowing material together with the unexpected detection of accreting gas with velocities as high as +150 km/s. Accretion is detectable in mid- and far-UV transitions of Fe II, Cr II, Zn II, Si II, Al II, Al III, Fe III, and C IV. Mg II emission shows a systematic redshift of the emission centroid compared to observations made at epochs with outflowing material in the line of sight only. Previous studies of this star have emphasized the presence of periodically modulated outflowing gas, which was particularly striking during the 1986 and 1987 monitoring runs. Comparison of our data with the full set of archival IUE data shows that accreting gas has been previously detected. The visibility of the accreting gas is enhanced in our data due to the combined effects of weaker than average outflowing gas and the strength of the accretion. The presence of Fe III (34) and C IV absorption in the accreting gas, but not in the outflowing material, indicates that the accreting gas was collisionally ionized, with the overall degree of ionization increasing with increasing radial velocity, much as is observed in \beta Pictoris. Mg I in the 1995 data shows accreting gas visible to +100 km/s, consistent with the production of atomic gas by sublimation of circumstellar grains, which also may be responsible for the polarization position angle rotation detected by WUPPE on Astro-2. Similar features in the spectrum of the Herbig Ae star UX Orionis have been interpreted as spectroscopic signatures of the presence of a star-grazing comet or planetesimal. The predictions of this model will be compared with the available HD 163296 data.

Acknowledgement: Support for this study was provided by the NASA Long-Term Space Astrophysics Program under contract NASW-4756 to the Applied Research Corporation. KSB acknowledges support under NASA contract NAS5-26777 to the University of Wisconsin.

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