POSSIBLE DETECTION OF A DIFFUSE INTERSTELLAR CLOUD BOUNDARY .2. HST-GHRS OBSERVATIONS OF SIRIUS-A

We present HST-GHRS observations with the G160M grating in the HI Lyma n ct spectral range toward Sirius A and around the Cry 1550 Angstrom d oublet toward three lines of sight of the local interstellar medium. T he local interstellar cloud (LIC) in which the Sun is embedded and ano ther cloud (BC) are detected toward Sirius A from high spectral resolu tion MgII and FeII data (Ech-B HST-GHRS, Lallement et al. 1994). Under the assumption that the DI column density is properly estimated and t hat the DI/HI ratio measured in the LIC by Linsky et al. (1993) is app licable to this line of sight, the total HI column density of these cl ouds (shared in a ratio LIC:BC from 1:1 to 2:1) is (3.4 +/- 1.0) x 10( 17) cm(-2) Comparison with the HI column density of(1.8 +/- 0.2) x 10( 18) cm(-2) measured in the LIC toward Capella (Linsky et al. 1993), pr oves that the Sun is located close to an edge of the LIC. This unusual ly low HI absorption, if confirmed by future observations, provides th e 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) x 10(5) K) very diffuse HI region (N-HI = (9.5 +/- 0.5) x 10(13) cm(-2 )) moving with approximately the same radial velocity as the LIC. This hot neutral gas, intermediate between the warm (7000 K), low ionized cloud envelopes and the very hot (10(6) K), fully ionized surrounding gas of the ''Local Bubble'', may originate from an evaporation interfa ce around the diffuse cloud. However, upper limits have been estimated on the CIV column densities toward the three other lines of sight obs erved, including that of the distant star beta CMa at only 7 degrees f rom Sirius. Assuming a temperature of 10(5) K, these column densities are on average smaller than 4 x 10(11) cm(-2). This low limit is below what is predicted from general conductive boundary models. If it is n ot due to carbon depletion, it raises questions about the thermal cond uctivity of such interfaces and about the source of hot neutrals.