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

7. Discussion

Values of H I , H and for all five lines of sight measured with ORFEUS (summarized in Table 3) are used to investigate the diffuse molecular ISM of the Magellanic Clouds. In order to extend our data, we include recent results from Gunderson et al. (1998) for two lines of sight toward Sk -66 19 and Sk -69 270 in the LMC. They estimate, on the basis of low dispersion spectra with the Hopkins Ultraviolet Telescope (HUT ), column densities of molecular hydrogen in the LMC gas by fitting H₂ line profiles (see Table 3).

Fig. 2 presents correlations between , , /, and = . The left panel shows log H plotted versus . In principle, we find the typical relation known from the Copernicus H₂ survey from S77 for Galactic gas. In both Milky Way and the Magellanic Clouds the logarithmic H₂ column density (log H) undergoes a transition from low to to high values at (dashed line) due to the self-shielding effect of H₂ (Federman et al. 1979).

Fig. 2. Correlations between atomic hydrogen, molecular hydrogen and colour excess for Magellanic Cloud gas along seven lines of sight. LMC stars measured with ORFEUS are labeled by filled dots, the one SMC target is given by a filled square. Data from Gunderson et al. (1998) for two additional lines of sight to the LMC have been included, here given as open circles. The plots are discussed in Sect. 7

It is known that the Magellanic Clouds have a significantly lower dust content than the Milky Way. Typical gas-to-dust ratios / in the Magellanic Clouds are 4 times (LMC) and 8 times (SMC) higher than in Milky Way gas (Koornneef 1982; Bouchet et al. 1985, respectively). In our sample, we find gas-to-dust ratios as high as cm^-2 for the gas in the LMC and SMC (see Table 3), consistent with these results. For HD 36402, HD 269698 and HD 269546, the ratios are significantly lower, but note that these values most likely represent lower limits due to the large uncertainty for the gas-to-dust ratio near the zero point of the scale. With respect to the generally lower dust content and the relation between H₂ column density and (Fig. 2, left panel) one should expect that the fraction of gas in molecular form is significantly lower in the Magellanic Cloud than in the Milky Way. From more theoretical considerations, Elmegreen (1989) concluded that interstellar clouds in Magellanic type irregular galaxies should be mostly atomic, since their lower metallicity directly influences the shielding function S for the cloud layers. This author also showed that the H to H₂ conversion also depends sensitively on the pressure and radiation field in the ISM (Elmegreen 1993). Accordingly, typical sight lines through the Magellanic Clouds might not contain any measureable column density of H₂, except for those, whose column density in H I is high enough to allow a significant fraction of the hydrogen to convert into molecular form. As the right panel of Fig. 2 shows, the discussed effects are slightly visible in the FUV absorption line data. The figure shows the molecular fraction f plotted against the total hydrogen column density H I +H. The Copernicus sample (S77) shows that the transition from low () to high () molecular fractions in the local Galactic gas is found at a total hydrogen column density (`transition column density' (H I )) near cm^-2 (right panel, dashed line). We find high total hydrogen column densities ( cm^-2) but low molecular hydrogen fractions () for the Magellanic Clouds gas along two of seven lines of sight. The data points of these two lines of sight toward LH 10:3120 and HD 5980 indicate that the transition column density from low to high molecular fractions could be indeed higher in the Magellanic Clouds than in the Milky Way. Only for sight lines with a very high total hydrogen column density (Sk -66 19 and Sk -69 270), the molecular fractions exceeds values above 1 percent. For sight lines with cm^-2 the molecular fractions in the Magellanic Cloud gas seem to be negligible.

Additional sight line measurements toward the Magellanic Clouds, however, are required to investigate these relations on a statistically more significant level. With a larger data set it might be possible to determine the transition column density from low to high molecular fractions as a function of the overall metallicity. Since it is known that the SMC is even more metal-poor than the LMC, it is of special interest to also investigate differences in the molecular gas fractions between LMC and SMC. For that, the FUSE satellite, launched in June 1999, holds the prospect for fresh H₂ absorption line data in the near future.

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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