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Session 47 - Circumstellar Disks & Shells.
Display session, Thursday, January 08
Exhibit Hall,
Using high S/N and high resolution spectra obtained with the GHRS aboard the Hubble Space Telescope, we have determined the temperature structure in the gaseous infall seen in the inner circumstellar disk of \beta Pic. Spectra sampling both a period of rather inactive infall and a period with pronounced redshifted circumstellar absorption are analyzed. The Fe II UV1 resonance multiplet near 2600Å\ and the Fe II UV62-63 multiplet near 2740Å\ were specifically observed. The absorption from UV62-63 originates from metastable levels approximately 1.05 eV above the ground configuration. After the intrinsic local stellar continuum was determined, the net circumstellar absorption profile and column densities versus velocity were derived for the Fe II levels sampled by this absorption. Both the ground configuration and in the excited metastable levels of Fe II are populated at or near LTE in the circumstellar gas around \beta Pic, which allows a reliable determination of temperature versus infall velocity. Results for a period of enhanced gaseous infall reveal a definite "double-peaked" temperature profile. These peaks occur near +50 km s^-1 and +120 km s^-1. The implied peak temperatures are \sim 10,000 K and 17,000 K, respectively. These temperatures are well above the implied effective temperature (T_eff = 8,500 K) of \beta Pic and imply shock heating of the infalling gas. A shock model for the inner disk of \beta Pic is presented that does not rely upon evaporating cometary bodies to produce either the deduced temperature profile or the observed gaseous infall.
