ASTROPHYSICS The velocity structure of the local interstellar medium probed by ultra - high - resolution spectroscopy

We present ultra-high-resolution (0.35 km s −1 FWHM) observations of the interstellar Ca K line towards eight nearby stars (six of which are closer than 30 pc). The spectral resolution is sufficient to resolve the line profiles fully, thereby enabling us to detect hitherto unresolved velocity components, and to obtain accurate measurements of the velocity dispersions (b-values). Absorption components due to the Local Interstel-lar Cloud (LIC) and/or the closely associated 'G Cloud' are identified towards all but one star (γ Oph), but only in one case (51 Oph) are both clouds reliably detected towards the same star. Most of these nearby clouds have velocity dispersions (b ≈ 2 km s −1) which suggest physical conditions similar to those inferred for the LIC (T k ≈ 7000 K, v t ≈ 1 km s −1), although at least three lines of sight (towards γ Aqr, β Cen and ρ Cen) also sample cooler and/or less turbulent material. The spectrum of the nearby Vega-excess star 51 Oph is of particular interest, owing to evidence that several of the absorption components arise in the circumstellar environment.