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E.B. Jenkins, T.M. Tripp (Princeton U. Obs.), C.W. Bowers (NASA GSFC), F. Roesler, R.J. Reynolds, L.M. Haffner (U. Wisc)
We are using HST to conduct a survey of UV spectra of 21 stars over the wavelength intervals 1160 to 1361Å\ and 1626 to 1906Å, using the highest resolution echelle gratings of STIS (E140H and E230H) and its narrowest entrance aperture (0.1\times 0.03 arc sec). The target stars were selected according to the criteria (1) spectral type earlier than B2, (2) no visible companions that could complicate the acquisition, (3) large projected rotational velocity v \sin i, and (4) a UV flux that creates a count rate just below the allowed maximum. Most of the targets are within HST's continuous viewing zone, so that we can improve our efficiency by about a factor of two. As a result, most stars in the plane of the galaxy are centered about the galactic longitudes 120\circ and 300\circ. A few targets were selected at high galactic latitudes so that useful comparisons could be made with results from the Wisconsin H Alpha Mapper (WHAM). Our primary objective is to sample the thermal pressures of the interstellar medium by measuring the relative populations of neutral carbon atoms in excited fine-structure levels. Secondary objectives include the detection of H~II regions with the excited Si~II line and the measurements of various atomic abundances.
Up to now (when this abstract was written) we have observed the stars HD~109399, HD~224151 and HD~99857. The narrowest line that we have detected so far is one from C~I at -47\,{\rm km~s}-1 in the spectrum of HD~224151. This line has a width of 2\,{\rm km~s}-1 FWHM, indicating that the STIS resolution is at least 150,000. Telluric O~I* and O~I** absorption features have a width of about 4\,{\rm km~s}-1; evidently there is broadening of these lines beyond that expected for thermal broadening at 1000K. A weak velocity component at -60\,{\rm km~s}-1 in the spectrum of HD~109399 shows a pronounced deficit of N~I and O~I, relative to their expected cosmic abundances in relation to S~II. This is very similar to the effect that Jenkins, et al (1998: ApJL 492, L147) found in a STIS spectrum of shocked gas in the Vela supernova remnant in front of HD~72089.
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