Atomic and Molecular Interstellar Absorption Lines toward the High Galactic Latitude Stars HD 141569 and HD 157841 at Ultra–High Resolution

Abstract

We present ultra-high-resolution (0.32 km s-1) spectra obtained with the 3.9 m Anglo-Australian Telescope (AAT) and Ultra-High-Resolution Facility (UHRF) of interstellar Na I D1, Na I D2, Ca II K, K I, and CH absorption toward two high Galactic latitude stars HD 141569 and HD 157841. We have compared our data with 21 cm observations obtained from the Leiden/Dwingeloo H I survey. We derive the velocity structure and column densities of the clouds represented by the various components and identify the clouds with ISM structures seen in the region at other wavelengths. We further derive abundances, linear depletions, and H2 fractional abundances for these clouds wherever possible. Both stars are located in regions of IRAS 100 μm emission associated with high Galactic latitude molecular clouds (HLCs): HD 141569 lies, in projection, close to MBM 37 and the Lynds dark cloud L134N, whereas HD 157841 is in the vicinity of the MBM 151. Toward HD 141569 we detect two components in our UHRF spectra: a weak, broad b = 4.5 km s-1 component at -15 km s-1, seen only in Ca II K absorption, and another component at 0 km s-1, seen in Na I D1, Na I D2, Ca II K, K I, and CH absorption. The cloud represented by the -15 km s-1 component is warm and may be located in a region close to the star. The cloud represented by the 0 km s-1 component has a Ca linear depletion δ(Ca) = 1.4 × 10-4 and shows evidence for the presence of dust, consistent with strong 100 μm emission seen in this region. The H2 fractional abundance f(H2) derived for this cloud is 0.4, which is typically what is observed toward HLCs. We conclude that this 0 km s-1 cloud is associated with MBM 37 and L134N based on the presence of dust and molecular gas (CH) and good velocity agreement with CO emission from these two clouds. This places HD 141569 beyond MBM 37 and L134N, which are estimated to be at ≈ 110 pc. In the case of the HD 157841 sight line, a total of six components are seen on our UHRF spectra in Na I D1, Na I D2, Ca II K, K I, and CH absorption. Two of these six components are seen only in a single species. The cloud represented by the components at 1.85 km s-1 has a Ca linear depletion δ(Ca) = 2.8 × 10-4, indicating the presence of dust. The f(H2) derived for this cloud is 0.45, and there is good velocity agreement with CO emission from MBM 151. To the best of our knowledge, this 1.85 km s-1 component toward HD 157841 is the first one found to have relative line widths that are consistent with pure thermal broadening only. We associate the 1.85 km s-1 cloud seen in our UHRF spectra with MBM 151 and conclude that HD 157841 must lie beyond ~200 pc, the estimated distance to MBM 151.