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

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4. ORFEUS H2 measurements

For the lines of sight toward HD 269698, HD 269546 and HD 36402 the analysis of H2 line strengths is presented in the following. For 2 other lines of sight, ORFEUS H2 measurements of Magellanic-Cloud gas have been published recently by de Boer et al. (1998; LH 10:3120) and Richter et al. (1998, 1999a; HD 5980, LH 10:3120).

Wavelengths and oscillator strengths for the H2 lines have been taken from the list of Morton & Dinerstein (1976) 1. We measured equivalent widths ([FORMULA]) by using either trapezoidal or gaussian fits. For the error determination we used the algorithm of Jenkins et al. (1973), taking into account photon statistics and the number of pixels involved for each line. In order to estimate the uncertainty for the choice of the continuum, we fitted a maximum and a minimum continuum to the data in the vicinity of each line and derived a mean deviation. The error for [FORMULA] given in Table 2 represents the total uncertainty calculated from all contributions discussed above. Column densities were derived by using a standard curve-of-growth technique.


Table 2. H2 equivalent widths for LMC gas toward HD 269698

4.1. HD 269698

The ORFEUS spectrum of HD 269698 in the Large Magellanic Cloud shows weak H2 absoption at LMC velocities near [FORMULA] km s-1. Six lines from the two rotational ground states ([FORMULA]) with high oscillator strengths are clearly seen in the spectrum and are not blended by other transitions. For additional five lines from higher rotational states we find upper limits for the equivalent widths of [FORMULA] mÅ (Table 2). Fig. 1 shows three of the detected H2 absorption lines plotted on a velocity scale. The lack of absorption in higher rotational states indicates that the H2 gas is not strongly excited. Constructing curves of growth for each rotational state we obtain column densities of [FORMULA] cm-2 for [FORMULA] and [FORMULA] cm-2 for the [FORMULA] state, using a b value of 8 km s-1 (best fit). The total H2 column density in the LMC gas toward HD 269698, derived by summing over [FORMULA] and [FORMULA], is [FORMULA] cm-2. The error is derived from the uncertainty for the fit to the curve of growth and includes the error for the individual equivalent widths and the uncertainty for the b value. From the detection limits for the lines from [FORMULA] we can exclude the possibilty that the higher rotational states will significantly contribute to the total H2 column density.

[FIGURE] Fig. 1. Interstellar H2 line profiles for HD 269698, HD 269546 and HD 36402 are shown, plotted in counts versus LSR velocity units. For HD 269698, H2 absorption from LMC gas is clearly visible near [FORMULA] km s-1. For HD 269546, weak H2 absorption might be present near [FORMULA] km s-1, but the origin of the absorption feature in the R(3), 4-0 line remains doubtful. No H2 absorption at LMC velocities is seen in the ORFEUS spectrum of HD 36402. The LMC velocities found from the H2 line centres have been marked with dashed lines. In all three spectra, H2 absorption from Galactic gas is present near 0 km s-1. The adopted continuum is indicated for each of the profiles

HD 269698 is located in the OB association N 57 at the rim of the supergiant shell LMC 4 where the H I emission (Rohlfs et al. 1984) has a minimum. The IUE spectrum of HD 269698 (Domgörgen et al. 1994) reveals three S II components at LMC velocities in front of the star: near [FORMULA] km s-1, near [FORMULA] km s-1 and near [FORMULA] km s-1. The detected H2 lines obviously belong to the first component.

4.2. HD 269546

In the ORFEUS spectrum of the LMC star HD 269546, no clear H2 absorption is visible at LMC velocities. The presence of H2 at LMC velocities (near [FORMULA] km s-1) in ORFEUS data was indicated by Widmann et al. (1998), using a coaddition of 25 Lyman- and Werner lines. However, the Werner R(0), R(1) line-pair near 1009 Å (in Fig. 1 plotted on a velocity scale) gives no hint for the presence of H2 absorption at LMC velocities. Marginal H2 absorption might be present in the Lyman P(1), 2-0 line ([FORMULA] Å) and in the Lyman R(3), 4-0 line ([FORMULA] Å) near [FORMULA] km s-1 (Fig. 1), but these absorption features are not clearly distinguishable from noise peaks and no other H2 profiles from [FORMULA] show similar features at [FORMULA] km s-1. Metal lines in the LMC gas near [FORMULA] km s-1 have been found in the IUE spectrum of HD 269546 (Grewing & Schulz-Luepertz 1980). Moreover, the IUE data reveal absorption over the whole velocity range between 0 and [FORMULA] km s-1, most likely related to Galactic halo gas and weaker LMC components. H2 absorption at [FORMULA] km s-1 is seen in some of the stronger lines, indicating that the Galactic high-velocity gas in front of HD 269546 contains molecular gas and dust (Richter et al. 1999b). The H I emission line data from Rohlfs et al. (1984) show the LMC gas at [FORMULA] km s-1. HD 269546 is member of the OB association LH 58 in the N 144 superbubble complex northwest of 30 Doradus. The H I gas seen in 21 cm emission at [FORMULA] km s-1 is most likely in front of N 144.

Detection limits for eight H2 absorption lines at LMC velocities near [FORMULA] km s-1 are used to obtain upper limits for the column densities of [FORMULA] for [FORMULA] by fitting the values of log ([FORMULA]) to the linear part of the curve of growth. We calculate an upper limit for the total H2 column density by modeling the population of the rotational states for [FORMULA] K. From that we derive [FORMULA] cm-2 for the LMC gas toward HD 269546.

4.3. HD 36402

No H2 absorption is seen in the spectrum of HD 36402 at LMC velocities in the range [FORMULA] to [FORMULA] km s-1. In this velocity range, atomic absorption has been found by de Boer & Nash (1982). We place an upper limit on the H2 column density in the LMC gas after inspecting some of the strongest of the H2 transitions in the rotational states [FORMULA] to [FORMULA]. HD 36402 is located in the superbubble N 51D and shows hydrogen emission and metal absorption from LMC foreground gas near [FORMULA] km s-1 (de Boer & Nash 1982). Therefore we expect H2 absorption from LMC gas roughly at the same velocity. Inspecting the R(0), R(1) line-pair near 1009 Å plotted on the velocity scale (Fig. 1, lowest panel), we find weak absorption near [FORMULA] km s-1, but this feature has no significance with respect to the noise.

In the same way as for HD 269546, we determine upper limits for the individual column densities [FORMULA] for [FORMULA] from the detection limits for some of the stronger H2 transitions near [FORMULA] km s-1. We find an upper limit for the total H2 column density in the LMC gas toward HD 36402 of [FORMULA] cm-2. Again, this upper limit is calculated under the assumption that the excitation temperature of possibly existing H2 gas in the LMC would not exceed a value of 300 K.

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Online publication: July 13, 2000