Ultraviolet observations of Sirius A and Sirius B with HST-GHRS. an interstellar cloud with a possible low deuterium abundance

Abstract

We present new observations of the binary Sirius A / Sirius B performed with HST-GHRS. Two interstellar clouds are detected on this sightline, one of them being identified as the Local Interstellar Cloud (LIC), in agreement with previous HST-GHRS observations of Sirius A (Lallement et al. \cite{lalle}). The interstellar structure of this sightline, which we assume is the same toward both stars (separated by less than 4 arcsec at the time of observation), is constrained by high spectral resolution data of the species O I, N I, Si Ii, C Ii, Fe Ii and Mg Ii. Lyman alpha interstellar lines are also observed toward the two stars. But whereas the deuterium Lyman alpha line is well detected in the LIC with an abundance in agreement with that obtained by Linsky et al. (\cite{linsky93} & \cite{linsky95}), no significant D I line is detected in the other cloud. However, the Lyman alpha lines toward Sirius A and Sirius B are not trivial. An excess of absorption is seen in the blue wing of the Sirius A Lyman alpha line and interpreted as the wind from Sirius A. In its white dwarf companion, an excess in absorption is seen in the red wing and interpreted as the core of the Sirius B photospheric Lyman alpha line. A composite Lyman alpha profile can nonetheless be constructed, and allows one to measure the deuterium abundance in the second cloud 0<(D/H)_ISM<1.6*E(-5) , which is marginally in agreement with the Linsky et al. (\cite{linsky93} & \cite{linsky95}) value. This sightline appears consequently as a good candidate for a low (D/H)_ISM. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No.~NAS5-26555.