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

4. Interpretation

4.1. Excitation state

To explain the column densities and the relative population of the rotational states of the H₂ molecule we have to determine the mechanism which is responsible for the excitation of the molecular gas on our line of sight. Two processes dominate the population of the excited states, collisional excitation and pumping by UV photons (Spitzer & Zweibel 1974). With the column densities for the individual rotational states we were able to determine the excitation temperature for the LMC gas for . In Fig. 4 the LMC column densities divided by their statistical weight are plotted against the excitation energy . The equivalent excitation temperature of the gas can be derived by fitting theoretical population densites from a Boltzmann distribution. For we derive an equivalent excitation temperature of 470 K.

The plot of Fig. 4 shows also that the level lies above the linear relation of 470 K. Thus at least the level is populated through kinetic excitation. Fitting the Boltzmann relation to the and 1 column densities we find a kinetic temperature of 50 K. Clearly, the population of level is due to gas with a temperature K. Such very low gas temperatures have also been found by Dickey et al. (1994) and by Marx-Zimmer et al. (1998) in the LMC through analysis of H I 21 cm absorption.

Israel & Koornneef (1988) looked for emission by H₂ in the direction of N 11. They report a marginal detection of H₂ in the 1-0 line at a position very close to our line of sight. However, given the difference in direction as well as the low significance of the emission detection a connection with our absorption will be speculative. Also, the emission may be from gas more distant than the star.

4.2. Particularities of the line of sight

Our line of sight through the LMC is ending at the star LH 10:3120. The association LH 10 ionizes the H II region N 11B, the northern H II clump of the N 11 superbubble complex. The central superbubble is apparently created by the association LH 9 and is filled with hot gas. Several more patches of hot gas, partly coinciding with filamentary H shells, exist. The one X-ray patch coinciding with N 11B hints at wind driven bubbles around some massive stars inside N 11B (Mac Low et al. 1998). While N 11B indeed shows first signs of the local effects of its most massive stars (Rosado et al. 1996), it is still a relatively unevoled H II region without large-scale expansion. Our line of sight is therefore most likely illuminated by the stars of LH 10. With the relatively unevolved nature of N 11B it is tempting to relate the H₂ absorbing gas with the remainder of the cold molecular cloud which formed N 11B.

4.3. Comparison with galactic gas

The total column density found in the LMC can be related to the properties of the H₂ gas in the Milky Way. The extinction to LH 10:3120 is . Taking out the galactic forground extinction of (Parker et al. 1992), one has extinction in the LMC of . The total H₂ column density in the LMC is, compared to Milky Way gas (see Fig. 4 of Savage et al. 1977), slightly on the low side for its extinction. The smaller dust to gas content of the LMC (compared to the Milky Way; see Koornneef 1984) may have influenced the H₂ to ratio by way of a lower H₂ formation rate.

The excitation level of the higher J levels on the line of sight to LH 10:3120 is equivalent to K. Such values are also found for the higher rotational states in galactic gas (Snow 1977; Spitzer et al. 1974). The environment of N 11B has contributed to the UV pumping but not in an excessive way. The excitation of the lowest level indicates the gas is kinetically cold.

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
helpdesk.link@springer.de