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Astron. Astrophys. 351, 323-329 (1999)

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ORFEUS II echelle spectra: On the H2/CO ratio in LMC gas towards LH 10

P. Richter 1, K.S. de Boer 1, D.J. Bomans 2, Y.-N. Chin 3,4, A. Heithausen 4 and J. Koornneef 5

1 Sternwarte, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
2 Astronomisches Institut, Ruhr-Universität Bochum, Postfach 102148, 44780 Bochum, Germany
3 Institute of Astronomy and Astrophysics, Academia Sinica, P.O.Box 1-87, Nankang, 11529 Taipei, Taiwan
4 Radioastronomisches Institut, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
5 Kapteyn Institute, Postbus 800, 9700 AV Groningen, The Netherlands

Received 26 February 1999 / Accepted 19 August 1999

Abstract

ORFEUS far UV echelle spectra have been used to investigate H I , H2 and CO absorption lines along the line of sight towards LH 10:3120 in the LMC, extending the study presented by de Boer et al. (1998). While H2 absorption is clearly visible, no CO absorption at LMC velocities is detected, but an upper limit of [FORMULA]CO[FORMULA] cm-2 for the CO column density is derived for the C-X band near 1088 Å. The detected H2 absorption features were used to determine a limit for the H2/CO ratio of N(H2)/N(12CO) [FORMULA] for the LMC gas along this individual line of sight. Generally, the fraction of gas in molecular form in the LMC is low compared to interstellar gas in the Milky Way with the same total gas quantity. We compare the absorption spectroscopy with SEST CO emission line measurements. It is found that most of the CO emission comes from gas behind LH 10:3120. We discuss our results in view of the possible scenario, in which the low dust content limits the amount of molecular hydrogen in the diffuse LMC gas.

Key words: space vehicles – stars: individual: LH 10:3120 – ISM: molecules – galaxies: ISM – galaxies: Magellanic Clouds – ultraviolet: ISM

Send offprint requests to: prichter@astro.uni-bonn.de

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999
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