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

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 () 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 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 km s^-1. Six lines from the two rotational ground states () 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 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 cm^-2 for and cm^-2 for the 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 and , is 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 we can exclude the possibilty that the higher rotational states will significantly contribute to the total H₂ column density.

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 km s-1. For HD 269546, weak H2 absorption might be present near 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 km s^-1, near km s^-1 and near 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 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 ( Å) and in the Lyman R(3), 4-0 line ( Å) near km s^-1 (Fig. 1), but these absorption features are not clearly distinguishable from noise peaks and no other H₂ profiles from show similar features at km s^-1. Metal lines in the LMC gas near 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 km s^-1, most likely related to Galactic halo gas and weaker LMC components. H₂ absorption at 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 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 km s^-1 is most likely in front of N 144.

Detection limits for eight H₂ absorption lines at LMC velocities near km s^-1 are used to obtain upper limits for the column densities of for by fitting the values of log () 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 K. From that we derive 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 to 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 to . HD 36402 is located in the superbubble N 51D and shows hydrogen emission and metal absorption from LMC foreground gas near 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 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 for from the detection limits for some of the stronger H₂ transitions near km s^-1. We find an upper limit for the total H₂ column density in the LMC gas toward HD 36402 of 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.

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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