AAS 207th Meeting, 8-12 January 2006
Session 193 Recent Discoveries in the Far Ultraviolet
Special Session, Thursday, 10:00-11:30am, January 12, 2006, Delaware B

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[193.08] The Evolution of Accretion and Activity Signatures in Young A Stars

G.M. Williger (U. Louisville and JHU), C. A. Grady (Eureka Scientific and GSFC), K. Hamaguchi (USRA and GSFC), S. Hubrig (ESO/Chile), J.-C. Bouret (Laboratoire d'Astrophysique de Marseille), A. Roberge (NRC and GSFC), M. Sahu (JWST Project and GSFC), B. Woodgate, R. Kimble (NASA's GSFC)

FUV spectroscopy obtained with FUSE reveals excess continuum light in 12 lightly reddened Herbig Ae stars, as well as the routine presence of emission in a range of ionization stages sampling material from neutral atomic gas to transition region temperature plasma. The FUV excess light is correlated with the near IR colors of the stars which has previously been noted as a tracer of mass accretion rate. In several cases, sufficient data exist to demonstrate that FUV continuum variability is present and is correlated with changes in the FUV emission lines, particularly red-shifted material. Combining the FUV spectra with disk inclination data, we find that the red-shifted C III 1176 emission is seen for inclinations between 0 and 60 degrees with no dependence upon inclination in that range, as expected for funneled accretion scenarios. The FUV excess light and X-ray luminosity show the same evolutionary trend, dropping gradually over the 1st 10 Myr as long as the star is accreting material from the disk. Centrally-cleared A debris disk systems have X-ray luminosities which are at least 3 orders of magnitude fainter than the Herbig Ae stars, demonstrating that the X-ray emission is related to accretion and not to more conventional stellar activity. Plasma at transition region and chromospheric temperatures persists longer, at least in some systems. Recent magnetic field detections for 5 of the FUSE Herbig Ae stars and Beta Pictoris indicate that magnetic fields with typical field strengths of 50 to several hundred Gauss are present over the entire age range where the accretion signatures are seen.

This study is based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985. Data included in this study were obtained under FUSE GO Programs C126, D065, and the FUSE Legacy program E510. HST observations of HD 163296 and HD 104237 were obtained under HST-GO-8801, 9241, and 5495.

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