Possible detection of a diffuse interstellar cloud boundary. II. HST-GHRS observations of Sirius A.

Abstract

We present HST-GHRS observations with the G160M grating in the HI Lyman α spectral range toward Sirius A and around the Civ 1550 A doublet toward three lines of sight of the local interstellar medium. The local interstellar cloud (LIC) in which the Sun is embedded and another cloud (BC) are detected toward Sirius A from high spectral resolution MgII and FeII data (Ech-B HST-GHRS, Lallement et al. 1994). Under the assumption that the Di column density is properly estimated and that the Di/Hi ratio measured in the LIC by Linsky et al. (1993) is applicable to this line of sight, the total Hi column density of these clouds (shared in a ratio LIC:BC from 1:1 to 2:1) is (3.4+/-1.0)x10^17^cm^-2^. Comparison with the Hi column density of (1.8+/-0.2)x10^18^cm^-2^ measured in the LIC toward Capella (Linsky et al. 1993), proves that the Sun is located close to an edge of the LIC. This unusually low Hi absorption, if confirmed by future observations, provides the opportunity to detect absorption excesses in the line wings. The red wing excess is interpreted as being due to a warmer (T=(1.0+/-0.1)x10^5^K) very diffuse Hi region (N_HI_=(9.5+/-0.5)x10^13^cm^-2^) moving with approximately the same radial velocity as the LIC. This hot neutral gas, intermediate between the warm (7000K), low ionized cloud envelopes and the very hot (10^6^K), fully ionized surrounding gas of the "Local Bubble", may originate from an evaporation interface around the diffuse cloud. However, upper limits have been estimated on the Civ column densities toward the three other lines of sight observed, including that of the distant star β CMa at only 7deg from Sirius. Assuming a temperature of 10^5^K, these column densities are on average smaller than 4x10^11^cm^-2^. This low limit is below what is predicted from general conductive boundary models. If it is not due to carbon depletion, it raises questions about the thermal conductivity of such interfaces and about the source of hot neutrals.