The luminosity function at the end of the main sequence: Results of a deep, large-area, CCD survey for cool dwarfs

J. Davy Klrkpatrick, John T. McGraw, Thomas R. Hess, James Liebert, Donald W Mccarthy

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Abstract

The luminosity function at the end of the main sequence is determined from V, R, and I data taken by the CCD/Transit Instrument, a dedicated telescope surveying an 8prime;.25 wide strip of sky centered at δ = + 28°, thus sampling Galactic latitudes of + 90° down to - 35°. A selection of 133 objects chosen via R - I and V - I colors has been observed spectroscopically at the 4.5 m Multiple Mirror Telescope to assess contributions by giants and subdwarfs and to verify that the reddest targets are objects of extremely late spectral class. Eighteen dwarfs of type M6 or later have been discovered, with the latest being of type M8.5. Data used for the determination of the luminosity function cover 27.3 deg2 down to a completeness limit of R = 19.0. This luminosity function, computed a V, I, and bolometric magnitudes, shows an increase at the lowest luminosities, corresponding to spectral types later than M6 - an effect suggested in earlier work by Reid & Gilmore and Leggett & Hawkins. When the luminosity function is segregated into north Galactic and south Galactic portions, it is found that the upturn at faint magnitudes exists only in the southern sample. In fact, no dwarfs with MI ≥ 12.0 are found within the limiting volume of the 19.4 deg2 northern sample, in stark contrast to the smaller 7.9 deg2 area at southerly latitudes where seven such dwarfs are found. This fact, combined with the fact that the Sun is located ∼ 10-40 pc north of the midplane, suggests that the latest dwarfs are part of a young population with a scale height much smaller than the 350 pc value generally adopted for other M dwarfs. These objects comprise a young population either because the lower metallicities prevalent at earlier epochs inhibited the formation of late M dwarfs or because the older counterparts of this population have cooled beyond current detection limits. The latter scenario would hold if these late-type M dwarfs are substellar. The luminosity function data together with an empirical derivation of the mass-luminosity relation (from Henry & McCarthy) are used to compute a mass function independent of theory. This mass function increases toward the end of the main sequence, but the observed density of M dwarfs is still insufficient to account for the missing mass. If the increases seen in the luminosity and mass functions are indicative of a large, unseen, substellar population, brown dwarfs may yet add significantly to the mass of the Galaxy.

Original languageEnglish (US)
Pages (from-to)749-788
Number of pages40
JournalAstrophysical Journal, Supplement Series
Volume94
Issue number2
Publication statusPublished - 1994

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Keywords

  • Stars: low-mass, brown dwarfs
  • Stars: luminosity function, mass function

ASJC Scopus subject areas

  • Space and Planetary Science

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