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.