Local Cloud structure from HST-GHRS

Abstract

Echelle spectra at high resolution (λ/{DELTA}λ= 90,000) of the nearby stars α Aql (Altaie r), α Lyr (Vega), α Cen and δ Cas, have been obtained with the Goddard High-Resolution Spectrograph (GHRS) on board the Hubble Space Telescope (HST), during the cycles 1 and 3. In all but one spectra, interstellar Fe II and Mg II absorption lines are detected at the exact Doppler shifts expected for the Local Interstellar Cloud (LIC) velocity vector, providing new confirmations of this vector. These new results are combined with observations of α CMa (Sirius), α Aur (Capella), β Pic, G191-B2B, α PsA (Fomalhaut) and β Leo, also obtained with the GHRS. For these targets too, absorption lines are detected at the expected LIC Doppler shift. α Cen is exceptional, since its spectrum shows gas about 3km/s faster than the 26km/s velocity compatible with all other stars, confirming that towards this direction a faster second cloud, or a faster extension of the LIC, is present in the immediate vicinity of the Sun. From the measured LIC column-densities a first rough schematic view of its contours in the galactic plane can be drawn. The distance to the cloud surface is estimated depending on the actual neutral H density. At least 7 (resp. 5) cloudlets, or masses of gas with different individual motions, do exist within a sphere of 12 (resp. 7) pc radius (LIC included). We also provide upper limits on Vega and Altaie r circumstellar Fe II.