Polarimetric investigatio, on interstellar dust within 50 PC from the sun.

Abstract

We have analyzed the polarization catalogue, for 1000 stars closer than 50 Pc, which has been presented in a companion paper. Although the accuracy of the measurements is generally very good (better than 0.02% for more than 700 stars), the catalogue contains essentially unpolarized stars, which is to be related to the well known depletion of dust in the Sun's vicinity. However, setting apart some stars which are known to display intrinsic polarization (e.g. several Ap magnetic variables), we have been able to select 25 stars whose polarization is indisputable. But, looking in more detail at the (presently available) data on the distance of this sample reveals that, in almost all the cases, the distances have been under-estimated. While this result will require a confirmation with the help of the Hipparcos parallaxes, we can already say that we fully confirm the main result of the preceding investigation on this topic by Tinbergen (1982), namely the complete depletion of dust within 35 pc from the Sun. Actually, we find that the dust signature begins at 40-50 pc in a few cases, but more much frequently at 70 to 100 pc, which seems to be the boundary of the local Bubble, as far as dust is concerned. We cannot confirm the previous detection by Tinbergen of a faint, near-by dusty region around l = 0°, b = -20°. Altogether, the picture given by the polarization analysis is consistent (although more sensitive) with the data derived from reddening measurements, and, to a lesser extent, with the investigations based on the measurements of interstellar absorption lines. Extending the polarization measurements to the 50-100 pc zone would provide a very precise picture of the location of those dust clouds which appear, here and there, as one gets out from the local Bubble.