consequence, their surface gravities ($g=GM/R^2$) can be substantially lower than those of older, more massive dwarfs of the same spectral type.

Theoretical models show that the luminosities of brown dwarfs decrease rapidly over time as they radiate away the internal energy supplied by gravity during the formation process (e.g. @burrows01, hereafter B01). Therefore, many analyses have focussed on young clusters, where the higher luminosities of brown dwarfs at young ages allows the mass function to be derived for masses as low as 5-15M$_{\rm{Jup}}$.

Among the uncertainties in these models are those dealing with the ages of the objects and the accuracy of the models themselves. These uncertainties are most significant in the cases of very young brown dwarfs in pre-main sequence clusters.

One uncertainty concerns a brown dwarf’s initial energy. A ‘hot start’ brown dwarf forms from the collapse of a molecular cloud. A brown dwarf formed in a ‘cold start’ via core accretion has less gravitational potential energy stored as internal energy so that, mass for mass, the brown dwarf is cooler and less luminous than a ‘hot start’ brown dwarf. After $\sim$2 Myr, runaway gas accretion produces a peak in the luminosity of ‘cold start’ brown dwarfs [@marley07]. 