Coronagraphic Survey for Companions of Stars within 8 pc

Abstract

We present the technique and results of a survey of stars within 8 pc of the Sun with declinations $\bfδ> -35\arcdeg$ (J2000.00). The survey, designed to find without color bias faint companions, consists of optical coronagraphic images of the 1\arcmin field of view centered on each star and infrared direct images with a 32\asc field of view. The images were obtained through the optical Gunn $r$ and $z$ filters and the infrared J and K filters. The survey achieves sensitivities up to four absolute magnitudes fainter than the prototype brown dwarf, Gliese 229B. However, this sensitivity varies with the seeing conditions, the intrinsic brightness of the star observed and the angular distance from the star. As a result we tabulate sensitivity limits for each star in the survey. We used the criterion of common proper motion to distinguish companions and to determine their luminosities. In addition to the brown dwarf Gliese 229B, we have identified 6 new stellar companions of the sample stars. Since the survey began, accurate trigonometric parallax measurements for most of the stars have become available. As a result some of the stars we originally included should no longer be included in the 8 pc sample. In addition, the 8 pc sample is incomplete at the faint end of the main sequence complicating our calculation of the binary fraction of brown dwarfs. We assess the sensitivity of the survey to stellar companions and to brown dwarf companions of various masses and ages.