3. Spectroscopic observations
Since there are additional Hipparcos stars at intermediate distances to the previous estimates in the direction of both MBM12 and MBM20, we obtained high-resolution spectroscopic observations near the NaI D lines of some of these stars to refine the distances to both clouds (cf. Hobbs et al. 1986). The stars used by Hobbs et al. (1986), Hobbs et al. (1988), and by us to determine the distance to MBM12 are displayed on an IRAS 100µm image of the region in Fig. 1 and the stars used by Penprase (1993) and by us to determine the distance to MBM20 are displayed on an IRAS 100µm image of the region around MBM20 in Fig. 2
We obtained the spectra with the FOCES Echelle Spectrograph at the 2.2-m telescope in Calar Alto, Spain in August 1998. The Tek13c 1K1K chip with the FOCES Echelle Spectrograph provided a spectral resolution, R = 49,000, derived from measurements of the FWHM (FWHM 0.12 Å) of several well isolated emission lines of the comparison spectra near the NaI D lines. As a rule, the smallest measurable equivalent width in units of mÅ is approximately the reciprocal dispersion in units Å mm-1 (Jaschek & Jaschek 1987). Therefore, given the 24µm pixels of the CCD we used for the observations, the smallest measurable equivalent width for these spectra is 5 mÅ. This corresponds to a minimum detectable column density N(NaI ) 6 1010 cm-2 in the optically thin case. All spectra were given an initial inspection at the telescope. If a particular integration produced fewer than 1000 cts pixel-1, at least one additional integration was performed.
The observed stars are listed in Table 2 with their coordinates, apparent magnitude, spectral type, Hipparcos distance, and whether the observed spectrum contains interstellar NaI features. We observed HIP13914, although it was already known to show interstellar NaI absorption, to ensure that we could detect the lines, if present. Although we made no attempt to correct for telluric features, our measured equivalent widths of the interstellar NaI D1 and D2 lines 129 mÅ and 167 mÅ, respectively, in HIP13914 are in good agreement with the measurements of Hobbs et al. (1986), 127 mÅ and 149 mÅ which were performed at the same spectral resolution. All of the observed spectra are displayed in Fig. 3.
None of the observed stars along the line of sight to MBM12 except HIP13914 showed interstellar NaI absorption, therefore the upper limit on the distance to the cloud remains pc. Since HIP13631 is the only star we observed that is superimposed directly on the cloud and it does not show interstellar NaI absorption, we can place a lower limit on the distance to the cloud of pc. Therefore the distance to MBM12 is pc d pc.
The observed stars along the line of sight to MBM20 are all projected on the central region of that cloud. Since we did not detect interstellar NaI D lines in any of the observed stars, we can place a lower limit on the distance to MBM20 of pc. Therefore the distance to MBM20 is pc d pc.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000