The Luminosity Function of Field M Dwarfs: Evidence for a Population of Substellar Objects?
Session 51 -- Brown Dwarfs and Very Low Mass Stars
Oral presentation, Wednesday, 1, 1994, 8:30-12:30

## [51.05] The Luminosity Function of Field M Dwarfs: Evidence for a Population of Substellar Objects?

J. D. Kirkpatrick (U. Texas)

The luminosity function at the lower end of the main sequence is determined from V, R, and I data taken by the CCD/Transit Instrument on Kitt Peak, a dedicated telescope surveying an 8.25-arcminute wide strip of sky centered at $\delta = +28{\deg}$. The data cover 27.3 square degrees down to a completeness limit of R = 19.0. The derived luminosity function, computed at V, I, and bolometric magnitudes, shows an upturn at the lowest luminosities, corresponding to spectral types later than M6 --- an effect suggested by several earlier surveys of the field. However, when this luminosity function is segregated into north and south galactic portions, it is found that the upturn at faint magnitudes exists only in the southern sample. No dwarfs with $M_I \ge 12.0$ are found within the limiting volume of the 19.4-square-degree northern sample, in stark contrast to the smaller 7.9-square-degree area at southerly latitudes where seven such dwarfs are found. This fact, combined with the fact that the sun is located $\sim$10 to 40 parsecs north of the galactic 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 earlier M dwarfs. It may be that this population appears young because its older members have cooled beyond current detection limits --- a scenario which would hold true if these late-type M dwarfs are substellar .

Other observational evidence in support of this hypothesis is also presented.