The last instrument capable of high resolution spectroscopy in the 900-1200 Å range was the Copernicus satellite (Spitzer et al. 1973), working in the 1970s. This part of the spectrum contains absorption transitions of the Lyman and Werner Bands of molecular hydrogen, H2, and of transitions of O VI and other species highly relevant for interstellar medium studies.
The ORFEUS (Krämer et al. 1990) and IMAPS (Jenkins et al. 1988) experiments on the ASTRO-SPAS space shuttle platform has provided access to the far UV spectral range in great detail. The ORFEUS telescope feeds two spectrographs. The Heidelberg-Tübingen echelle gives spectra from 912 to 1410 Å with which are recorded with a microchannel plate detector. Good S/N is achieved in exposure times of the order of 1 hr of hot objects with mag (Krämer et al. 1990). The UCB spectrograph produces spectra over the range of 390 to 1220 Å (Hurwitz et al. 1998) with a resolution too low for detailed interstellar work. IMAPS has its own telescope, works at between 970 and 1195 Å, and is in overall sensitivity limited to galactic studies (see e.g. Jenkins & Peimbert 1997).
Here we report on the detection at high spectral resolution of H2 in the spectrum of the LMC star LH 10:3120. The star, located in the association LH 10 near the western edge of the LMC, is of spectral type O5.5Vf, has mag and mag (Parker et al. 1992). The star was selected because of its very early spectral type, modest extinction (too large extinction would make the far-UV undetectable), and its proximity to an area where the molecule CO has been found in emission (Cohen et al. 1988; Israel et al. 1993).
The presence of H2 in the LMC is known since Israel & Koornneef (1988) detected the near-IR emission lines from radiatively excited H2 seen toward H II regions near hot stars. Measurements showed that H2 is abundantly available, both in the SMC (Koornneef & Israel 1985) and in the LMC (Israel & Koornneef 1991a, 1991b). Clayton et al. (1996) detected with HUT at 3 Å resolution broad depressions in the far-UV spectrum of two LMC stars, which could be fitted with H2 absorptions due to N(H2) 1-2 cm-2. Studies of H2 in the LMC and SMC are of importance because of the lower metal content of these galaxies compared to the Milky Way and the different gas to dust ratios (see Koornneef 1984).
© European Southern Observatory (ESO) 1998
Online publication: September 8, 1998