THE CIRCUMSTELLAR NATURE OF THE METALLIC FEATURES IN A HOT DA WHITE DWARf

摘要

A new co-added IUE echelle spectrum of the bright DA white dwarf CD -38°10980, together with a newly determined radial velocity for this star, indicate that the sharp lined Si and C absorption features seen in the UV are clearly circumstellar in origin. Absorption in both excited and ground state transitions occurs at a velocity displaced by —12.1 ± 2.0 km s~(-1) with respect to the photospheric velocity. Weak features due to the Si Ⅳ doublet are seen at a velocity intermediate between that of the circumstellar features and the photosphere. First time estimates of column densities for excited and ground states of C Ⅱ, Si Ⅱ, and Si Ⅲ are derived. These quantities are used with electron density estimates derived from these species to determine the location and physical conditions of the circumstellar gas in the vicinity of CD —38°10980. If collisional excitation alone is responsible for the excited levels of Si Ⅲ observed in CD — 38°10980, then electron densities in the circumstellar gas must exceed 10~9 cm~(-3). Substantially lower electron densities are possible if the circumstellar gas is located near enough to the star so that photoexcitation is the dominant process responsible for the excited lines seen in the UV. Strong limits are placed on the photospheric abundance of Si and C in the star itself. These limits are in sharp contrast to the theoretical predictions of radiative levitation in which Si, but not C, is expected in the photosphere of a white dwarf such as CD —38°10980. The interstellar line of sight to CD -38°10980 is also investigated.