The deuterium abundance in the Galaxy has been of interest since the first models of primordial nucleosynthesis were developed. While it is unlikely that major amounts of deuterium could be produced after the Big Bang, it is sure that deuterium is almost completely destroyed in stars. This should lead to a decrease of its abundance in the progress of galactic evolution. Every measurement of thus should give a lower limit for the primordial abundance and, according to nucleosynthesis models, an upper limit for the baryonic density in the universe. Today, these limits are set by measurements in quasar spectra. For example Levshakov et al. (1999) suggest a uniform for three lines of sight at redshifts of z between 0.7 and 3.6, but Molaro et al. (1999) find from a line of sight with . Nevertheless knowledge of abundances in the galactic ISM can be important for tracing the evolution of the Milky Way.
Since the Copernicus satellite in the 1970s no instrument was capable of high resolution spectroscopy in the 900 to 1200 Å range. There have been measurements with the IUE and the HST-GHRS , but only the Ly line was observable leading to a restriction to lines of sight with low hydrogen column densities. For an overview see e.g. Lemoine et al. (1999).
The ASTRO-SPAS space shuttle platform housed 3 spectrographs operating between 900 and 1200 Å. Of those, IMAPS has an echelle spectrograph working between 930 and 1150 Å with a resolution of . Jenkins et al. (1999) and Sonneborn et al. (1999) performed measurements with that instrument. Two spectrographs were attached to the ORFEUS telescope. The Berkeley spectrograph is designed for intermediate-resolution spectra in the range of 390 to 1220 Å. The Heidelberg-Tübingen echelle spectrograph, equipped with a microchannel plate detector, gives spectra from 912 to 1410 Å and allows investigations of the entire hydrogen and deuterium Lyman series with a resolution of (Krämer et al. 1990). We use spectra of the latter to investigate deuterium.
The large number of rotational transitions of molecular hydrogen found in the wavelength range from 1200 Å up to the Lyman edge can be used for studies on the molecular gas along the line of sight. The distribution of molecular hydrogen in the local interstellar medium especially at higher galactic latitudes is still only rudimentarily known, since the majority of the Copernicus measurements pointed towards luminous targets in the galactic plane. From the H2 transitions column densities of the different H2 rotational excitation states up to can be determined. These can be used to derive physical parameters as the excitation temperature and the ortho-to-para ratio of the molecular hydrogen.
© European Southern Observatory (ESO) 1999
Online publication: November 23, 1999