A CORONAGRAPHIC SEARCH FOR BROWN DWARFS AROUND NEARBY STARS

Brown dwarf companions have been searched for around stars within 10 p c of the Sun using the Johns-Hopkins University Adaptive Optics Corona graph (AOC), a stellar coronagraph with an image stabilizer. The AOC c overs the field around the target star with a minimum search radius of 1.5'' and a field of view of 1 arcmin2. We have reached an unpreceden ted dynamic range of DELTAm = 13 in our search for faint companions at I band. Comparison of our survey with other brown dwarf searches show s that the AOC technique is unique in its dynamic range while at the s ame time just as sensitive to brown dwarfs as the recent brown dwarf s urveys. The present survey covered 24 target stars selected from the G liese catalog. A total of 94 stars were detected in 16 fields. The low -latitude fields are completely dominated by background star contamina tion. Kolmogorov-Smirnov tests were carried out for a sample restricte d to high latitudes and a sample with small angular separations. The h igh-latitude sample (b greater-than-or-equal-to 44-degrees) appears to show spatial concentration toward target stars. The small separation sample (DELTAtheta < 20'') shows weaker dependence on Galactic coordin ates than field stars. These statistical tests suggest that both the h igh-latitude sample and the small separation sample can include a subs tantial fraction of true companions. However, the nature of these puta tive companions is mysterious. They are too faint to be white dwarfs a nd too blue for brown dwarfs. Ignoring the significance of the statist ical tests, we can reconcile most of the detections with distant main- sequence stars or white dwarfs except for a candidate next to GL 475. Given the small size of our sample, we conclude that considerably more targets need to be surveyed before a firm conclusion on the possibili ty of a new class of companions can be made.