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

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1. Introduction

There is now considerable evidence that the Sun lies within a warm ([FORMULA] K) low-density ([FORMULA] cm-3) interstellar cloud, and that this Local Interstellar Cloud (LIC) is itself located within the hot ([FORMULA] K) and empty ([FORMULA] cm-3) Local Bubble in the interstellar medium (see Cox & Reynolds 1987 and Frisch 1995 for reviews). Our knowledge of the Local Bubble has improved greatly in recent years, as a result of observations performed at optical, ultraviolet and X-ray wavelengths. Recent results from the ROSAT wide-field camera (Diamond et al. 1995), EUVE (Vennes et al. 1994), and ground-based optical spectroscopy (Welsh et al. 1994) all confirm the distinct lack of neutral gas within [FORMULA] pc of the Sun. The evidence that this volume is filled with a high-temperature plasma comes primarily from observations of the soft X-ray background (McCammon et al. 1983, McCammon & Sanders 1990), although it has been pointed out by Breitschwerdt & Schmutzler (1994) and Jelinsky, Vallerga & Edelstein (1995) that if the gas is out of ionisation equilibrium the temperature might be considerably lower than generally assumed.

Our knowledge of the low-density clouds within the Local Bubble has also improved, largely owing to the ability of modern high-resolution spectrographs to detect the weak absorption lines (equivalent widths of a few mÅ) which these clouds produce in the spectra of nearby stars. This work has revealed that, within a few tens of parsecs of the Sun, the Local Bubble contains several small clouds with characteristics apparently similar to those of the LIC (Lallement et al. 1986, 1994; Bertin et al. 1993). Using ground-based and HST-GHRS observations, Lallement et al. (1995) have deduced that the LIC is moving past the Solar System with a heliocentric velocity of [FORMULA] km s-1 towards [FORMULA], [FORMULA]. In addition, they (see also Lallement & Bertin 1992) have drawn attention to another nearby cloud, characterised by a slightly different velocity vector (29 km s-1 towards [FORMULA], [FORMULA]), which they designate as the 'G Cloud.' However, it is still unclear whether the G cloud is separate from the LIC, or is contiguous with it. As reviewed by Frisch (1995), these velocity vectors are consistent with a general outflow from the Scorpio-Centaurus OB Association ([FORMULA], [FORMULA]) which appears to dominate the large scale kinematics of the local interstellar medium (LISM).

The temperature of the LIC in the immediate vicinity of the Solar System has been determined from observations of back-scattered solar Ly- [FORMULA] ([FORMULA] K; Bertaux et al. 1985) and He I [FORMULA] 584 ([FORMULA] K; Chassefiere, Dalaudier & Bertaux 1988). An essentially identical temperature ([FORMULA] K) has been measured directly for interstellar He atoms in the outer Solar System from the Ulysses spacecraft (Witte et al. 1993). Evidence that this temperature is common to the LIC, rather than just that part of it impinging on the Solar System, is provided by Linsky et al. (1993, 1995), who obtained a value of [FORMULA] K from HST observations of interstellar D I, Fe II and Mg II towards Procyon ([FORMULA] CMi; [FORMULA] pc) and Capella ([FORMULA] Aur; [FORMULA] pc). More recently, Linsky & Wood (1995) have obtained a similar, although somewhat lower, temperature of [FORMULA] K from interstellar lines towards [FORMULA] Cen A (D=1.3 pc). These authors have also measured the rms turbulent velocities, [FORMULA], for the LIC towards these stars, and found it to be of the order of 1 km s-1 (specifically, [FORMULA] km s-1 for Capella and Procyon, and [FORMULA] km s-1 for [FORMULA] Cen; Linsky et al. 1995, Linsky & Wood 1995). [Note that in their papers, Linsky et al. characterise the turbulence by the parameter [FORMULA], where [FORMULA] as used here; cf. Equation 1 below.]

Here we report observations of the interstellar Ca K line towards eight nearby stars obtained with the Ultra-High-Resolution Facility (UHRF) at the Anglo-Australian Telescope. The UHRF is currently the world's highest resolution astronomical spectrograph, and has been described in detail by Diego et al. (1995). The maximum resolving power is [FORMULA] (0.3 km s-1 FWHM), which is more than an order of magnitude higher than most other instruments. As discussed by Crawford & Dunkin (1995), use of this very high resolving power has two main advantages for the study of the LISM: (1) it makes possible the resolution of closely-spaced velocity components, and therefore separation of the LIC from other nearby clouds; and (2) it enables us to measure reliable intrinsic line widths (b -values), thereby providing information on the temperature and turbulence within the clouds.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998