Analysis of initial observations from sky surveys has shown that the resulting photometric catalogues , combined with far-red optical data , provide an extremely effective method of finding isolated , very low-temperature objects in the general field .
Follow-up observations have already identified more than 25 sources with temperatures cooler than the latest M dwarfs .
A comparison with detailed model predictions ( Burrows & Sharp ) indicates that these L dwarfs have effective temperatures between \approx 2000 \pm 100 K and 1500 \pm 100 K , while the available trigonometric parallax data place their luminosities at between 10 ^ { -3.5 } and 10 ^ { -4.3 } L _ { \odot } .
Those properties , together with the detection of lithium in one-third of the objects , are consistent with the majority having substellar masses .
The mass function can not be derived directly , since only near-infrared photometry and spectral types are available for most sources , but we can incorporate VLM/brown dwarf models in simulations of the Solar Neighbourhood population and constrain \Psi ( M ) by comparing the predicted L-dwarf surface densities and temperature distributions against observations from the DENIS and 2MASS surveys .
The data , although sparse , can be represented by a power-law mass function , \Psi ( M ) \propto M ^ { - \alpha } , with 1 < \alpha < 2 .
Current results favour a value nearer the lower limit .
If \alpha = 1.3 , then the local space density of 0.075 > { M \over M _ { \odot } } > 0.01 brown dwarfs is 0.10 systems pc ^ { -3 } .
In that case brown dwarfs are twice as common as main-sequence stars , but contribute no more than \sim 15 \% of the total mass of the disk .