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

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5. H I measurements

For two of our sight lines (HD 269698 and HD 269546) we present the determination of H I column densities from the analysis of the Ly [FORMULA] absorption near 1215 Å. The values for the H I column densities along the other three lines of sight have been adopted from the literature. All H I column densities are summarized in Table 3. For HD 5980 and HD 36402, the column density of H I has been determined by Fitzpatrick & Savage (1983) and de Boer & Nash (1982), respectively, using H I emission line data in combination with the Ly [FORMULA] absorption near 1215 Å. They derive H I column densities of N(H I [FORMULA] cm-2 for the SMC gas toward HD 5980 and [FORMULA] cm-2 for the LMC gas toward HD 36402. For LH 10:3120, the LMC H I column density is [FORMULA] cm-2, obtained by a multi-component fit of the Ly [FORMULA] profile (Richter et al. 1999a).


Table 3. LMC gas properties along seven lines of sight.
1) Errors include uncertainty for the choice of the doppler parameter b
2) References: (1) de Boer et al. (1998); (2) Richter et al. (1999a); (3) Richter et al. (1998); (4) Fitzpatrick & Savage (1983);
(5) this paper; (6) de Boer & Nash (1982); (7) Gunderson et al. (1998); (8) Fitzpatrick (1985)
3) From intermediate resolution spectra with HUT

We use a similar technique for the determination of H I column densities in the LMC gas toward HD 269698 and HD 269546. For HD 269546, we fix the LMC component at a velocity of [FORMULA] km s-1, similar to the velocity for which we had determined the upper limit for the H2 column density in Sect. 4.2. The velocity structure seen in metal lines (Grewing & Schulz-Luepertz 1980), however, indicates that there are definitely additional (weaker) absorption components in front of HD 269546. We thus might slightly overestimate the H I column density in the LMC gas at [FORMULA] km s-1 by fitting one single LMC component to the Ly [FORMULA] absorption structure. The situation is even more difficult for the Ly [FORMULA] profile in the spectrum of HD 269698. IUE data of HD 269698 show the presence of three velocity components in this sight line (Domgörgen et al. 1994), near [FORMULA] and [FORMULA] km s-1. The H I emission (Rohlfs et al. 1984) shows a weak component near [FORMULA] km s-1 which could be associated with the absorption component near [FORMULA] km s-1 (see Domgörgen et al. 1994). The S II abundances found by Domgörgen et al. indicate similar total gas quantities for the two main components at [FORMULA] km s-1 and [FORMULA] km s-1. The H2 absorption was found in the latter component (see Sect. 4.1). For the Ly [FORMULA] fit, we fix the two LMC components at [FORMULA] (cloud I) and [FORMULA] (cloud II) km s-1, assuming equal H I column densities. For the fitting procedure we use a multi-component Voigt profile. Galactic foreground absorption by H I is taken into account by a fit component at 0 km s-1. The multiple velocity components are not resolved in the Ly [FORMULA] profile and we do not take into account additional absorption from Galactic intermediate and high-velocity gas. Thus, it is clear that our results derived by this method represent only rough estimates for the distribution of the H I gas in front of the stars. However, for the comparison between [FORMULA], [FORMULA] and [FORMULA], as presented in Sect. 7, the determined H I column densities are sufficiently accurate.

The Ly [FORMULA] fit for HD 269546 provides the best agreement with the data with a Galactic foreground absorption of [FORMULA]H I [FORMULA] cm-2 and an additional LMC component at [FORMULA] km s-1 of [FORMULA]H I [FORMULA] cm-2. For HD 269698, the best fit is found for [FORMULA]H I [FORMULA] cm-2 and [FORMULA]H I [FORMULA]H I [FORMULA] cm-2 for the two LMC clouds at [FORMULA] and [FORMULA] km s-1. The H I column densities in the LMC gas in these two lines of sight are significantly lower than found for HD 5980 and LH 10:3120, but comparable with the H I column density found in the LMC gas toward HD 36402.

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© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000