BOSS Ultracool Dwarfs I: Colors and Magnetic Activity of M and L Dwarfs

We present the colors and activity of ultracool (M7–L8) dwarfs from the Tenth Data Release of the Sloan Digital Sky Survey (SDSS). We combine previous samples of SDSS M and L dwarfs with new data obtained from the Baryon Oscillation Sky Survey (BOSS) to produce the BOSS Ultracool Dwarf (BUD) sample of 11820 M7–L8 dwarfs. By combining SDSS data with photometry from 2MASS and the Wide-field Infrared Sky Explorer (WISE) mission, we present ultracool dwarf colors from to as a function of spectral type, and extend the SDSS–2MASS–WISE color locus to include ultracool dwarfs. The , , and colors provide the best indication of spectral type for M7–L3 dwarfs. We also examine ultracool dwarf chromospheric activity through the presence and strength of Hα emission. The fraction of active dwarfs rises through the M spectral sequence until it reaches ~90% at spectral type L0. The fraction of active dwarfs then declines to 50% at spectral type L5; no Hα emission is observed in the late-L dwarfs in the BUD sample. The fraction of active L0–L5 dwarfs is much higher than previously observed. The strength of activity declines with spectral type from M7 through L3, after which the data do not show a clear trend. Using one-dimensional chromosphere models, we explore the range of filling factors and chromospheric temperature structures that are consistent with Hα observations of M0–L7 dwarfs. M dwarf chromospheres have a similar, smoothly varying range of temperature and surface coverage, while L dwarf chromospheres are cooler and have smaller filling factors