Probing the Local ISM with Hot White Dwarfs

Far-UV echelle spectra of four hot white dwarfs, lying at distances of 60 to 125 pc, have been obtained with the Space Telescope Imaging Spectrometer (STIS). The spectra of each star show interstellar lines associated with the predicted velocity of the Local Interstellar Cloud (LIC), in which the sun is embedded, as well as velocity components corresponding to a limited number of more distant clouds. For each line of sight, we determine the total and relative column densities of H I, C II, N I, O I, Si II and S II for the component clouds. From these data it is clear that the LIC is often the dominant component of local interstellar lines of sight in many directions. The dominant role of the LIC helps to explain the high degree of homogeneity of H and He ionization noted in studies of relatively nearby white dwarfs. The LIC also differs substantially in its heavy element composition and ionization state with respect to nearby clouds. The STIS observations for each line of sight can be directly related to observations of the interstellar H and He column densities and to the ionization states of He and H determined from the EUV spectra of these stars obtained from the Extreme Ultraviolet Explorer (EUVE). This work was supported by NASA through grant GO-7296 from the Space Telescope Science Institute.