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

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2. Observations, data handling

The total observing time for LH 10:3120 in the ORFEUS space shuttle mission of Nov./Dec. 1996 was 6000 s in 3 pointings exploiting the integrating capabilities of the microchannel plate detector system. A detailed instrument description and information about the basic data reduction is given by Barnstedt et al. (1998). The data reduction for the 20 echelle orders has been performed by the ORFEUS team in Tübingen. The spectrum has been filtered by us with a de-noising algorithm basing on a wavelet transformation (Fligge & Solanki 1997). This leads to a slight degradation of the spectral resolution, now being equivalent to [FORMULA] km s-1. The spectra have a signal-to-noise (S/N) of [FORMULA] at the longer wavelengths of the recorded spectral range. Toward the shorter end of the spectral range, both the increased effect of the UV-extinction as well as the wavelength dependent sensitivity of the instrument leads to a reduction in S/N such, that little can be done with the spectrum at [FORMULA] Å. After the filtering absorption features in the longer part of the spectrum become clearly visible.

We first inspected spectral ranges almost devoid of atomic absorption lines and where only few H2 absorptions are expected. The reason for this is to avoid getting confused in the search for H2 by the complexity of the absorption line profiles on the line of sight to the LMC. Yet, the characteristic pattern of absorption by the Milky Way disk near 0 km s-1, by the LMC near +270 km s-1, and possibly by high-velocity clouds near +60 and +130 km s-1 all known from IUE (see Savage & de Boer 1979, 1981; de Boer et al. 1980) and HST (Bomans et al. 1995) spectra, helps to identify the absorption structures.

A section of the echelle order 51, where several H2 aborption lines have been found, is shown in Fig. 1. Many of the H2 profiles overlap in their [FORMULA] 300 km s-1 wide profile structure and decompositions are not always possible. However, in some cases the galactic absorption stayed unblended, in other cases the LMC portion was blend free. For this first analysis we took 16 H2 absorption lines from the lowest 5 rotational states for the further analysis. These lines are essentially free from any blending problems so that wrong identifications can be excluded. Absorption strengths could thus be determined for the LMC components seen in these absorption profiles. A selection of characteristic H2 absorption line profiles in velocity scale (LSR) is shown in Fig. 2.

[FIGURE] Fig. 1. A portion of the ORFEUS spectrum of LH 10:3120 near 1095 Å shows several H2 absorption lines. The strongest of these are identified by their transition below the spectrum. The markings indicate the velocity range of 0 to 270 km s-1 for each of the lines. Some weaker H2 lines as well as some atomic features are also visible but have not been marked

[FIGURE] Fig. 2. Examples of absorption profiles clearly showing the detection of H2. At the top we present a profile of the Si II 1190.416 Å line for easy reference. The H2 absorption is due to the Milky Way component near 0 km s-1 and the LMC component near +270 km s-1. The atomic line of Si II also shows the presence of the galactic halo high-velocity clouds near +60 and +130 km s-1 (LSR)

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998