Circumstellar material in hot DA white dwarfs

An important result of our HST/STIS observations of hot DA white dwarfs is the identification of multiple absorption components in one or more of the strong resonance absorption lines associated with the stellar photosphere (e.g. CIV, SiIV, NV, and OV) (Bannister et al. 2001, 12 th European conference on whiter dwarfs, ASP conf. Series 226, p105). In most of the stars observed the components are blue-shifted and present in at least CIV (the figure above). While no consistent pattern has yet emerged from the small number of spectra available, the material may be from recent or ongoing mass-loss or a remnant from a now dispersed planetary nebula. In some stars, such as the well-studied prototypical hot DA G191-B2B, the blue-shifted features are barely resolved by STIS, as shown in the panel below; these features were not resolved in earlier IUE or GHRS observations. Hence, the abundance of photospheric carbon, determined from what are now known to be weaker photospheric lines, is much lower than previously supposed.

As a result, all similar pre-STIS measurements of hot DAs are now suspect. We are investigating the use of FUSE spectra to provide complementary abundance data from non-resonance transitions, but it may ultimately be necessary to obtain new STIS spectra of many stars. Circumstellar features in observed in the STIS E140M spectra of REJ 1738+665 are shown below.

Closeup of the 1550 Angstrom CIV feature in REJ 1738+665

The features in REJ 1738+665 and G191-B2B are by no means unique; several other objects including Feige 24 and REJ 0558-373 show similar phenomena. The spectra of these two stars are presented below (click for an enlarged view).