A systematic search for brown dwarfs orbiting nearby stars

We have concluded a systematic search for brown dwarf and stellar companions to every known M dwarf star within 5 pc north of -30°. Using infrared speckle interferometry, we have examined a region 0″.2 to 5″ in radius around 27 stars at the infrared H (1.6 μm) and K (2.2 μm) bands, at which low-mass companions are expected to emit maximum radiation near their blackbody peaks. Two stars were found to have very low mass, previously unseen companions: G208-44, which is an astrometric binary, and GL 866. The five previously known close M dwarf binaries with well-determined masses and these two new M dwarf doubles, all with separations less than 3″, were observed at J (1.25 μm), H, and K in order to produce mass-luminosity relations at these wavelengths. All stellar and substellar objects to an M_K of 11.5 and M_H of 12.0 would have been detected around most of the target stars. For these infrared magnitudes, comparisons to the low-mass star and brown dwarf evolutionary models of D'Antona and Mazzitelli yield corresponding mass limits of 70 and 80 Jupiters at ages of 8 × 10⁸ and 8 × 10⁹ yr, which are representative ages for the sample. All astrometric companions of stars in the survey which remain undetected now have limits placed upon their infrared fluxes. We believe that all stellar companions to the survey objects have been discovered, and we find that the infrared luminosity function at the very red end of the main sequence is not falling but rising, until an abrupt cutoff corresponding to the stellar/substellar breakpoint at M_K = 10.0. The census of M dwarfs within 5 pc north of -30° is now revised to include 19 single M dwarfs, eight M dwarf binaries, one M dwarf triple system, and one M dwarf in a triple system with earlier spectral type components. We have found no definite brown dwarf companions on the scale of our planetary system (2-10 AU) around any M dwarfs within 5 pc, and we are now extending the survey to 10 pc.