L dwarfs and the substellar mass function

Analysis of initial observations sky surveys has shown that the resulting p hotometric catalogs, combined with far-red optical data, provide an extreme ly effective method of finding isolated, very low-temperature objects in th e general held. Follow-up observations have already identified more than 25 sources with temperatures cooler than the latest M dwarfs. A comparison wi th detailed model predictions (Burrows & Sharp 1999) indicates that these L dwarfs have effective temperatures between approximate to 2000 +/- 100 K a nd 1500 +/- 100 K, while the available trigonometric parallax data place th eir luminosities at between 10(-3.5) and 10. Those properties, together wit h the detection of lithium in one-third of the objects, are consistent with the majority having substellar masses. The mass function cannot be derived directly, since only near-infrared photometry and spectral types are avail able for most sources, but we can incorporate VLM/brown dwarf models in sim ulations of the solar neighborhood population and constrain Psi(M) by compa ring the predicted L dwarf surface densities and temperature distributions against observations from the Deep Near-Infrared Survey (DENIS) and 2 Micro n All-Sky Survey (2MASS) surveys. The data, although sparse, can be represe nted by a power-law mass function, Psi(M) proportional to M-alpha, with 1 < alpha < 2. Current results favor a value nearer the lower limit. If alpha = 1.3, then the local space density of 0.075 > M/M-. > 0.01 brown dwarfs is 0.10 systems pc(-3). In that case, brown dwarfs are twice as common as mai n-sequence stars but contribute no more than similar to 15% of the total ma ss of the disk.