HST/STIS observations of the high-velocity interstellar cloud HVC 291.2-41.2+80: a warm, mainly ionized high-velocity cloud

Abstract

We present intermediate-resolution HST/STIS spectra of a high-velocity interstellar cloud (vLSR = +80 km s−1) towards DI 1388, a young star in the Magellanic Bridge located between the Small and Large Magellanic Clouds. The STIS data have a signal-to-noise ratio (S/N) of 20–45 and a spectral resolution of about 6.5 km s−1 (FWHM). The high-velocity cloud absorption is observed in the lines of C ii, O i, Si ii, Si iii, Si iv and S iii. Limits can be placed on the amount of S ii and Fe ii absorption that is present. An analysis of the relative abundances derived from the observed species, particularly C ii and O i, suggests that this high-velocity gas is warm (Tk∼103–104 K) and predominantly ionized. This hypothesis is supported by the presence of absorption produced by highly ionized species, such as Si iv. This sightline also intercepts two other high-velocity clouds that produce weak absorption features at vLSR = +113 and +130 km s−1 in the STIS spectra.