Astrometric detection and characterization of brown dwarfs

摘要

As a result of failed star formation, brown dwarfs (BDs) do not reach the critical massto ignite the fusion of hydrogen in their cores. Different from their low-mass stellar brothers, thered dwarfs, BDs cool down with their lifetime to very faint magnitudes. Therefore, it was onlyabout 10 to 20 years ago that such ultracool objects began to be detected. Accurate astrometrycan be used to detect them indirectly as companions to stars by the signature of the so-calledastrometric wobble. Resolved faint BD companions of nearby stars can be identified by theircommon proper motion (CPM). A direct astrometric detection of the hidden isolated BDs in theSolar neighborhood is possible with deep high proper motion (HPM) surveys. This techniqueled to the discovery of the first free-floating BD, Kelu 1, and of the nearest BD, e Indi B. Bothwere meanwhile found to be binary BDs. The astrometric orbital monitoring of c Indi Ba+Bb,for which we know an accurate distance from the Hipparcos measurement of its primary, e IndiA, will allow the determination of individual masses of two low-mass BDs. Hundreds of BDshave been identified for the last decade. Deep optical sky survey (SDSS) and near-infrared skysurveys (DENIS, 2MASS), played a major role in the search mainly based on colours, since BDsemit most of their light at longer wavelengths. However, alternative deep optical HPM surveysbased on archival photographic data are not only sensitive enough to detect some of the nearestrepresentatives, they do also uncover many of the rare class of ultracool halo objects crossing theSolar neighborhood at large velocities. SSSPM 1444, with the extremely large proper motionof 3.5 arcsec/yr, is one of the nearest among these subdwarfs with masses at the substellarboundary. We present preliminary parallax results for this and two other ultracool subdwarfs(USDs) from the Calar Alto Omega 2000 parallax program.