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Astron. Astrophys. 357, 681-685 (2000)

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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

[FIGURE] Fig. 2. The gray scale image shows the extent of the IRAS 100µm emission from MBM20. The filled squares are the stars observed by Penprase (1992) to determine the distance to the cloud and the open squares are the stars we observed to determine the distance to the cloud. The black plus symbols mark the locations of two binary T Tauri stars in MBM20. The gray scale is linear from 0 to 20 MJy sr-1. The contours are at 7 MJy sr-1 and 15 MJy sr-1. This figure is in J2000 coordinates.

We obtained the spectra with the FOCES Echelle Spectrograph at the 2.2-m telescope in Calar Alto, Spain in August 1998. The Tek[FORMULA]13c 1K[FORMULA]1K chip with the FOCES Echelle Spectrograph provided a spectral resolution, R = [FORMULA] [FORMULA] 49,000, derived from measurements of the FWHM (FWHM [FORMULA] 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 [FORMULA] 5 mÅ. This corresponds to a minimum detectable column density N(NaI ) [FORMULA] 6 [FORMULA] 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 [FORMULA] 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.

[FIGURE] Fig. 3. Spectra are displayed for the observed Hipparcos stars along the line of sight to MBM12 and MBM20. The only star which shows interstellar NaI D absorption is HIP13914. All of the other stars have broader stellar NaI D lines.


[TABLE]

Table 2. Observed Stars


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 [FORMULA] [FORMULA] 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 [FORMULA] [FORMULA] pc. Therefore the distance to MBM12 is [FORMULA] pc [FORMULA] d [FORMULA] [FORMULA] 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 [FORMULA] [FORMULA] pc. Therefore the distance to MBM20 is [FORMULA] pc [FORMULA] d [FORMULA] [FORMULA] pc.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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