Observations of S VIII and SI VIII in the Hot Interstellar Medium

We report the observation of emission lines from highly ionized silicon and sulfur in the hot interstellar medium by the Diffuse X-ray Spectrometer (DXS). The DXS experiment flew on the STS--54 mission of the Space Shuttle Endeavour in January 1993, and obtained spectra of the soft x-ray diffuse background in the 42-84 Angstroms (0.15-0.284 eV) band. The DXS detectors have spectral resolution ~ 0.01 keV and a 15(deg) field of view that was scanned along a 160(deg) arc parallel to the galactic plane at roughly 10(deg) galactic latitude, and centered at 225(deg) galactic longitude. Two of the more prominent lines seen in the DXS data are interpreted as Si VIII (61.0 Angstroms) and S VIII (63.5 Angstroms) lines arising from a million-degree phase of the local interstellar medium. Plasma models of Raymond & Smith and Mewe et al. with solar abundances (whether or not we assume ionization equilibrium) predict a value for the Si VIII/S VIII line ratio of approximately 5, while the observed ratio is approximately 0.5. This line ratio is interpreted as evidence for dust grains which are only partly sputtered into gas in the hot emitting region. This work was supported by NASA contract 5--26078.