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Session 18 - White Dwarfs.
Display session, Monday, January 15
North Banquet Hall, Convention Center
V471 Tauri is a 12.5-hr DA + K2 V post-common-envelope binary, and a probable member of the Hyades cluster. We have obtained spectroscopic observations with HST\/ and the GHRS near orbital phases 0.25 and 0.75, in a 36 Å\ region centered at Lyman-\alpha. Radial velocities of the white-dwarf component were obtained from its broad Ly-\alpha wings, and yield a velocity amplitude K_WD \simeq 160 km s^-1. Since the system is eclipsing and the amplitude of the dK component is known from ground-based observations, we obtain the masses of both components, which are both near 0.8 M_ødot. The effective temperature of the white dwarf, from Hubeny profile fits to the Ly-\alpha wings, is T_eff \simeq 37,000 K.
These results lead to an evolutionary paradox: the white dwarf is the most massive one in the Hyades cluster, and thus should be the most evolved and coolest of the WDs in the cluster. But in fact it is the hottest\/ Hyades WD. To resolve the paradox, we suggest that the white-dwarf component is still hot from a relatively recent nova outburst.
During two of the observations we witnessed transient Si III 1206 Å\ absorption, which appeared on a timescale of 2 minutes. This is probably due to wind material leaving the dK star, silhouetted in front of the white dwarf.
We have also detected absorption due to interstellar deuterium in the line of sight toward V471 Tau. Preliminary estimates of the interstellar D/H ratio in this direction will be presented.
Supported by STScI grant GO-5468.
