ORFEUS II Echelle spectra: Molecular hydrogen at high velocities toward HD 93521

Abstract

Absorption lines of interstellar molecular hydrogen in the far ultraviolet (FUV) have been observed in the spectrum of the O9.5 halo star HD 93521, located some 1500 pc from the Galactic plane. During the second flight of the ORFEUS-SPAS mission a spectrum with the Echelle spectrometer has been recorded with a total integration time of 1740 s. The resolution achieved was about 10.000 with a signal-to-noise ratio of up to 25. For the first time two components of molecular hydrogen have been observed in absorption at velocities of -12 km/s in the Galactic disk and at -62 km/s located presumably in the Galactic halo. The column densities derived from a standard curve of growth analysis were found to be N(H_2) = 10^17.0 cm^-2 for the disk component and N(H_2) = 10^14.6 cm^-2 respectively for the component located in the Galactic halo.