Astron. Astrophys. 338, L5-L8 (1998)

3. H₂ column density in the LMC gas

The absorption profiles have been analysed and decomposed into the various velocity components. Here we limit ourselves to the absorption pertaining to the LMC. For each line the absorption equivalent width has been determined. The f-values for the further analysis have been taken from Morton & Dinerstein (1976) for the H₂ transitions and for the atomic absorption lines from the compilation of Morton (1991). Theoretical Voigt profiles were fitted to some of the identified H₂ absorption lines and we thus could derive an upper limit for the velocity dispersion of km s^-1 for the LMC gas.

Subsequently, curves of growth have been constructed for each of the absorptions by the 5 rotational states for the LMC component. The logarithmic equivalent widths for each rotational level J have been shifted horizontally to give a fit to a theoretical single cloud curve of growth as a function of log . The best fit for all 5 rotational states was obtained with km s^-1 as shown in the empirical curve of growth for all identified H₂ lines (Fig. 3). The column densities derived in this way have been collected in Table 1. The uncertainties in the column densities are based on the quality of the fits to the curve of growths. They range from 0.2 to 0.4 in the logarithm, as shown in Fig. 4. The total column density for the lowest 5 rotational states is .

Fig. 3. The H2 lines were fitted to a single cloud curve of growth, indicating km s-1. The column densities for the levels to 4 are given in Table 1

Fig. 4. The column densities for the levels to 4 of H2 are plotted against the excitation energy. The higher levels exhibit an equivalent excitation temperature of K, the excitation being mostly due to UV pumping. For the two lowest levels the kinetic excitation temperature is K; the fit drawn marks the indicated upper limit

Table 1. H₂ column densities in LMC gas toward LH 10:3120

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
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