No abstract

No abstract

No abstract

No abstract

We present very high resolution (0.32 km s~1) spectra of interstellar Na I D1, D2, and Ca II K absorption toward HD 28497 obtained with the UltraÈHigh-Resolution Facility at the 3.9 m Anglo-Australian Telescope. The star is located in projection in a highly disturbed interstellar region close to a number of identiÐed features including the high galactic latitude molecular cloud MBM 20, the large Orion-Eridanus shell, seen in Ha and H I 21 cm maps, and a Ðlamentary loop structure between v LSR \ [12 and [4 km s~1 in the Berkeley H I 21 cm survey and visible on the IRAS 100 km map. Toward HD 28497 we detect 13 absorption components in the Na I spectra, to a column density limit of 2 ] 1010 cm~2, and 10 in Ca II K over a velocity range of D70 km s~1. Four absorption components in the Na I spectra show s-resolved hyperÐne structure with b-values from 0.31 to 0.40 km s~1 and column densities from 4.0 to 14 ] 1010 cm~2. If we assume the clouds represented by these components have no internal turbulent velocities, their temperatures would range between 134 and 227 K. One of these hyperÐne split (hfs) components, at km s~1, shows signiÐcant temporal variation in v LSR \ [11.1 equivalent width compared to earlier (1977) observations, making this the Ðrst interstellar sight line outside the Vela supernova remnant to show a time-varying component. The feature may be associated with the Ðlamentary loop structure seen in this region. There is poor correspondence between the Na I and Ca II proÐles : we do not detect narrow Ca II proÐles to the four hfs Na I components, and only three of the well-resolved components have the same Ca II and Na I radial velocities and consistent b-values. One of these components, at km v LSR \ [30.0 s~1, has a low Na I/Ca II ratio and arises in a region where turbulent motions dominateÈproperties consistent with the hypothesis that the cloud lies close to HD 28497. In general, however, the Na I and Ca II occupy di †erent gaseous phases in the ISM. We have compared our data with 21 cm emission proÐles obtained from the recent Leiden/Dwingeloo H I survey. Based on agreement in the velocities, the Na I/Ca II ratio, and the kinetic temperatures, we conclude that the component at km s~1 is associated with the front side of the large, v LSR \ [7.5 expanding Orion-Eridanus shell. Unexpectedly, the molecular cloud MBM 20 is not detected either in our absorption spectra or in the H I proÐles, indicating that HD 28497 lies away from the core of MBM 20. Apart from the two features at [11 and [7.5 km s~1 there is almost no agreement between the H I proÐles and the optical spectra. Although we cannot rule out the possibility that most of the H I lies behind the star, this explanation seems unlikely because many of the H I features have previously been attributed to foreground phenomena. The beam sizes of the H I and the optical studies are quite di †erent and this suggests a di †erent explanation, namely that the physical sizes of the interstellar structures we detect in Na I and Ca II are not extensive enoug...