L Dwarfs and the Substellar Mass Function

Analysis of initial observations from near-infrared sky surveys has shownthat the resulting photometric catalogues, combined with far-red optical data,provide an extremely effective method of finding isolated, very low-temperatureobjects in the general field. Follow-up observations have already identifiedmore than 25 sources with temperatures cooler than the latest M dwarfs. Acomparison with detailed model predictions (Burrows & Sharp) indicates thatthese L dwarfs have effective temperatures between ~2000\pm100 K and 1500\pm100K, while the available trigonometric parallax data place their luminosities atbetween 10^{-3.5} and 10^{-4.3} L_solar. Those properties, together with thedetection of lithium in one-third of the objects, are consistent with themajority having substellar masses. The mass function cannot be deriveddirectly, since only near-infrared photometry and spectral types are availablefor most sources, but we can incorporate VLM/brown dwarf models in simulationsof the Solar Neighbourhood population and constrain Psi(M) by comparing thepredicted L-dwarf surface densities and temperature distributions againstobservations from the DENIS and 2MASS surveys. The data, although sparse, canbe represented by a power-law mass function, Psi(M) ~ M^{-alpha}, with 1 M/M_solar > 0.01 brown dwarfs is0.10 systems pc^{-3}. In that case brown dwarfs are twice as common asmain-sequence stars, but contribute no more than ~15% of the total mass of thedisk.