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The goal of this program is to recognize previously missed, low-luminosity (and presumably low-mass) dwarfs in the solar vicinity. Proper motion objects originally identified by Luyten --- yet still unclassified --- are being observed spectroscopically over the range 6300 to 9000 \AA. To date, several dwarfs as late as M8 have been uncovered, and spectroscopic parallaxes indicate that many are within 20 pc, making them new additions to the Catalogue of Nearby Stars (Gliese \& Jahreiss 1991).
As a result, a database is being amassed of the latest M dwarfs, where late-type targets announced by other investigators have also been observed. The list now includes over twenty objects with spectral types of M7 or later. Follow-up 1.0-2.5 \micron~spectra are also being acquired to understand better the temperature dependence of spectral features near the region of peak flux and to determine how the spectrum-to-spectrum differences seen in the red translate to the infrared.
Recent observational evidence may indicate that these latest M dwarfs are the brown dwarfs researchers have been struggling to find. The number density of the early-M dwarfs is slightly higher ($\sim$1.2$\times$) toward southern galactic latitudes than toward northern ones, demonstrating, as have a variety of other investigations, that the sun lies a few tens of parsecs north of the galactic mid-plane. The late-M dwarfs, on the other hand, show a density in the south which is many times larger than that in the north, indicating that we are looking down upon a population with a small scale height (i.e., young). It would appear either that the older analogues of this population never formed (their birth being precluded by the low metallicities present in star forming regions at earlier epochs) or that these analogues have since cooled beyond current detection limits --- which would be true if this population were substellar.
Wednesday program listing