Astron. Astrophys. 359, 1111-1116 (2000)

3. Data analysis

The complex line-of-sight structure in direction of the Magellanic Clouds, with contributions from local Galactic gas (0 km s^-1), Galactic halo gas (near and km s^-1 in front of the LMC) and Magellanic Cloud gas (near km s^-1 for the LMC and km s^-1 for the SMC; see Savage & de Boer 1979, 1981; de Boer et al. 1980; Bomans et al. 1996) makes the thorough analysis of H₂ absorption lines at LMC velocities and km s^-1 resolution a difficult task. The main problem is that for the vast majority of the H₂ transitions line blends from atomic or molecular species can not be excluded, even when many of these possible blendings are not expected. As a consequence, the number of unambigously identified H₂ features at high radial velocities is strongly limited to only a few wavelength regions. Typical line strengths for low H₂ column densities (as observed in the spectra presented in the following) have values mÅ, which is (at low S/N) comparable with the strength of noise peaks. For most of these lines, only upper limits for the equivalent widths can be obtained. For diffuse H₂ clouds it is known that the process of UV pumping (Spitzer & Zweibel 1974) often leads to an excitation of the higher rotational states, particulary if the total H₂ column density is below the limit for the self-shielding effect. Thus, the rotational excitation of H₂ in the most diffuse medium often does not reflect the actual kinetic temperature of the gas. H₂ line strengths in diffuse clouds for excited rotational states might be significantly higher than for the ground states (see the Copernicus spectrum of Pup, as published by Morton & Dinerstein 1976, where the strongest H₂ lines occur for ), even if the gas is cold. In the most complex case, the equivalent widths for the ground state lines are below the detection limit, while in the same spectrum, absorption from higher rotational levels is visible.

Velocity information from metal lines as well as model spectra for the excited rotational states were used to interpret the complex H₂ absorption pattern found in the ORFEUS spectra of stars in the Magellanic Clouds.

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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