Fuse and Hst Stis Observations of Hot and Cold Gas in the Ab Aurigae System

We present the first observations of a Herbig Ae star with a circumstellar disk by the Far Ultraviolet Spectroscopic Explorer (FUSE ), as well as a simultaneous observation of the star obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS). The spectra of AB Aurigae show emission and absorption features arising from gasses that have a wide range in temperature, from hot O VI emission to cold H2 and CO absorption. Emissions from the highly ionized species O VI and C III present in the FUSE spectrum are redshifted, while absorption features arising from low-ionization species like O I, N I, and Si II are blueshifted and show characteristic stellar wind line-profiles. We find the total column density of H2 toward AB Aur from the FUSE spectrum, N(H2) = (6.8 ± 0.5) × 10 19 cm. The gas kinetic temperature of the H2 derived from the ratio N(J = 1)/N(J = 0) is 65 ± 4 K. The column density of the CO observed in the STIS spectrum is N(CO) = (7.1 ± 0.5) × 10 cm, giving a CO/H2 ratio of (1.04±0.11)×10 . We also use the STIS spectrum to find the column density of H I, permitting us to calculate the total column density of hydrogen atoms, the fractional abundance of H2, and the gas-to-dust ratio. Subject headings: stars: individual (AB Aurigae)—stars: atmospheres—circumstellar matter