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Astron. Astrophys. 317, 889-897 (1997)

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5. Conclusions

We have used the Ultra-High-Resolution Facility (UHRF) at the Anglo-Australian Telescope to study interstellar Ca K lines towards eight nearby stars. The spectral resolution (0.35 km s-1, FWHM) was sufficient for us to fully resolve the intrinsic line profiles. This has enabled us to detect hitherto unresolved velocity structure in the LISM, and to obtain accurate measurements of the velocity dispersions (b -values) of the local clouds. Our principal conclusions are as follows: (1) Absorption components at the velocities expected for the LIC and/or the G cloud (Lallement et al. 1995) have been detected towards all but one star ([FORMULA] Oph). The column density upper limits deduced for [FORMULA] Oph ([FORMULA] cm -2) are consistent with the LIC and/or G cloud being present towards this star, but only by assuming order-of-magnitude spatial and/or density inhomogeneities on sub-parsec scales. Only in the case of one star (51 Oph) is there a fairly secure detection of both the LIC and G clouds. (2) The velocity dispersions deduced for the LIC/G cloud components are generally consistent with other determinations of the physical conditions prevailing in the LIC ([FORMULA] K, [FORMULA] km s-1). However, the weak LIC component towards [FORMULA] Aqr is narrower than expected ([FORMULA] +-00:6:4), suggesting that the LIC in this direction is cooler and/or less turbulent than elsewhere. Observations of other nearby stars, close to [FORMULA] Aqr on the sky, are needed to determine the extent of this apparently anomalous region. (3) The velocity dispersions of the other (i.e. non-LIC/G cloud) velocity components suggest temperature/turbulence regimes similar to that of the LIC. However, the two most distant stars in the present sample ([FORMULA] Cen and [FORMULA] Cen) also exhibit narrower components which must arise in a somewhat cooler and/or less turbulent environment. (4) Five absorption components were identified towards the nearby Vega-excess star 51 Oph. Two were identified with the LIC and G clouds (Table 2). Of the others, that at -15.8 km s-1 is almost certainly circumstellar, owing to its association with an excited metastable state of Fe II (Fig. 2b; Dunkin et al. 1996). The two remaining components (at -21.2 and -20.3 km s-1, which are here resolved for the first time; cf. Fig. 2a) are very narrow by LISM standards, and may also arise in the circumstellar environment (Sect. 4.3). However, allowing for the differences in spectral resolution, we found no evidence for any change in the Ca K line profile between the observations reported here and those obtained by Lagrange-Henri et al. (1990).

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998