GHRS OBSERVATIONS OF SIRIUS-A .1. INTERSTELLAR CLOUDS TOWARD SIRIUS AND LOCAL CLOUD IONIZATION

Echelle spectra of Sirius-A have been obtained with the Goddard High-R esolution Spectrograph (GHRS) on board the Hubble Space Telescope (HST ). Interstellar absorption lines of iron and magnesium clearly show fo r the first time the presence of two distinct interstellar clouds betw een the Sun and Sirius at 2.7 pc. Their projected heliocentric velocit ies differ by 5.7 +/- 0.2 km s-1. One of these two absorbing clouds is identified as the Local Interstellar Cloud (LIC) surrounding the Sun, while the second cloud is closer to Sirius. The total amounts of Fe I I and Mg II in the LIC are about 25 % of the corresponding values towa rd Capella, showing that the edge of the Local Cloud is closer to the Sun in the direction of Sirius. The present measurements constrain the LIC temperature to be 7600 +/- 3000 K, the average value being in goo d agreement with the precise LIC temperature determination toward Cape lla and in the solar system (7000 +/- 200) K. The extremely small LIC neutral magnesium absorption is very likely detected, allowing the fir st measurement of the LIC Mg II/Mg I ratio, found to be 220 +/- 50. Ac cording to the most recent recombination rates calculations and to the local UV ionizing field measurements, this ratio implies a mean elect ron density of at least 0.03 cm-3 along this line-of-sight if the LIC were as warm as 10 600 K, and a very large density of at least 0.3 cm- 3 if the LIC temperature is as small as measured toward Capella and in the solar system. Such a large plasma density, much higher than previ ously estimated, raises a number of questions on the physical state of the local interstellar gas, but makes less unrealistic the existence of a local (extra-solar) electron density large enough to confine the heliosphere within 100-150 AU, as suggested by other recent measuremen ts.