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

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ORFEUS far UV echelle spectra have been used to investigate HI, 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 N(CO) < 3.3 10¹3 cm-2 for the CO column density is derived for the C-X band near 1088 A. The detected H₂ absorption features were used to determine a limit for the H₂/CO ratio of N(H₂)/N(CO) > 2.0 10⁵ 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.