1. M dwarf/L dwarf Binaries Resolved with SpeX Adam J. Burgasser (MIT) Michael W. McElwain (UCLA) “Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. I. DENIS J220002.05-303832.9AB” Burgasser & McElwain 2006, AJ, 131, 1007 “Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. II. 2MASS J17072434-0558249AB” McElwain & Burgasser 2006, AJ, submitted

2. M dwarfs and L dwarfs are stellar spectroscopic classes encompassing the lowest temperature, lowest mass Hydrogen burning stars. At the transition between these classes (T eff ~ 2400 K), gases such as TiO, VO and Fe condense into solid and liquid “grains” (e.g., perovskite, enstatite; e.g., Lodders 1999), forming “floating clouds of dirt”. The process of cloud formation (e.g., Ackerman & Marley 2001) and the properties of these clouds remains controversial, but they do have a profound impact on the spectral properties and atmospheric chemistry of L dwarfs.

3. As part of a program to obtain low resolution, near infrared spectra of late-type stars and brown dwarfs with SpeX, we uncovered two M dwarf/L dwarf binaries with separations of roughly 1 arcsecond: 2MASS 1707-0558AB and DENIS 2200-3038AB 2MASS 1707-0558AB DENIS 2200-3038AB J JJ H H K K

4. The pairs were both sufficiently resolved to obtain spectra of their individual components. These are among the few low mass binaries with resolved spectroscopy of any kind.

5. Spectra were classified by comparison to NIR spectral standards: 2MASS 1707-0558AB is an M9+L3 binary, DENIS 2200-3038AB is an M9+L0 binary

6. Optical spectroscopy of 2MASS 1707-0558AB shows no signature of the 6708 Å Li I line, implying that the primary component must have a mass > 0.065 M sun. This provides a constraint on the age of the system and mass of the secondary.

7. With projected separations of 17 and 38 AU, these systems are relatively wide compared to most very low mass binary systems, which are generally separated by 20 AU or less. However, if the maximum separation of binaries is mass- dependent, these systems match a trend a max = 1400xM tot 2 AU previously identified amongst brown dwarf binaries (Burgasser et al. 2003). As presumably coeval pairs, 2MASS 1707- 0558AB and DENIS 2200-3038AB are valuable new probes of atmospheric processes across the M dwarf/L dwarf transition.

8. Breaking News! We have uncovered a new L dwarf binary selected from a near infrared proper motion survey using 2MASS data. 2MASS 1520-4422 is composed of an L1.5+L4.5 with a 1.17” separation. Like 2MASS 1707-0558 and DENIS 2200-3038, it is a prime source for resolved spectroscopic studies tracing trends across the M and L dwarf regime. Burgasser, Looper, Kirkpatrick & Liu 2006, AJ, submitted