The Coolest Isolated M Dwarf and Other 2MASS Discoveries

We have obtained follow-up spectroscopy of eight late dwarf candidates discovered in a 105 deg² area observed with the 2MASS Prototype Camera during test runs between 1992 and 1994. These objects were chosen because of their red infrared colors (e.g., J-K_s ≥ 1.10) and/or red OIR colors (e.g., R-K_s ≥ 6.00). All eight are late M dwarfs, six of which have spectral types later than van Biesbroeck 8 (type M7 V). Despite the fact that we have only followed up a fraction of the reddest sources discovered, the number of known M dwarfs of type M7 and cooler has been increased by 30%. Extrapolation of these results alone shows that over 2000 dwarfs of similar spectral type and with K_s ≤ 14.0 will be imaged by 2MASS over the entire sky. One of these new discoveries is astonishingly cool and has a tentative type of ≥M10 V. This dwarf, one of the least luminous objects yet discovered, could itself be a high-mass brown dwarf, thus providing another empirical data point in a regime where few such objects are now recognized. Only the substellar suspect GD 165 B and the bona fide brown dwarf GL 229 B, both discovered as companions to known stars, are cooler. Thus, this 2MASS discovery becomes the coolest isolated object so far identified.