The L to T Dwarf Transition

At least three explanations have been proposed to explain the rapidity of the L to T dwarf transition. These ideas have included a continuously sinking, thin cloud layer, horizontally patchy cloudiness, and a rapid increase in the cloud sedimentation efficiency at a given effective temperature. With the advent of measured parallaxes for a large number of late L through early T dwarfs we now know that the transition takes place at roughly constant effective temperature, which places severe constraints on all of the proposed mechanisms. In addition high quality spectral datasets from Spitzer Space Telescope/IRS and IRTF/SpeX, now provide a wealth of new information about objects at the L to T transition and substantially increase the wavelength range over which the effects of clouds can be modeled. After comparing our model spectra with these datasets I will discuss how well the various mechanisms can account for the observed spectra of L and T dwarfs and other observables. In addition I will consider how gravity signatures among the L/T transition objects can further constrain the transition mechanism. I will argue that the preponderance of evidence favors models in which the cloud behavior undergoes fundamental changes at the transition. Mass or gravity selection effects do not adequately account for all of the available data.