New Brown Dwarfs and an Updated Initial Mass Function in Taurus^*

By combining infrared photometry from the Two Micron All Sky Survey with new optical imaging and spectroscopy, I have performed a search for young low-mass stars and brown dwarfs in two regions encompassing a total area of 4 deg² in the Taurus star-forming region (τ ~ 1 Myr). From this work I have discovered 15 new members of Taurus. In addition, I present seven new members outside of these areas from the initial stage of a survey of all of Taurus. These 22 objects exhibit spectral types of M4.5-M9.25 and masses of 0.3-0.015 M_☉ according to the theoretical evolutionary models of Baraffe & Chabrier, seven of which are likely to be brown dwarfs. Emission in Hα, He I, Ca II, [O I], and [S II] and excess emission in optical and near-infrared bands among some of these objects suggest the presence of accretion, outflows, and circumstellar disks. These results add to the body of work—initiated by the first detections of brown dwarf disks by Comerón and coworkers in 1998 and Luhman in 1999—indicating that disks around young brown dwarfs are relatively common. The results from the 4 deg² survey have been combined with previous studies of Taurus to arrive at an initial mass function (IMF) for a total area of 12.4 deg². As in the previous IMFs for Taurus, the updated IMF peaks at a higher mass (0.8 M_☉) than the mass functions in IC 348 and Orion (0.1-0.2 M_☉). Meanwhile, the deficit of brown dwarfs in Taurus appears to be less significant (×1.4-1.8) than that found in earlier studies (×2) because of a slightly higher brown dwarf fraction in the new IMF for Taurus and a lower brown dwarf fraction in the new spectroscopic IMF for the Trapezium from Slesnick and coworkers. The spatial distribution of the low-mass stars and brown dwarfs discovered in the two new survey areas closely matches that of the more massive members. Thus, on the degree size scales (~3 pc) probed to date, there is no indication that brown dwarfs form through ejection.