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The spectra of brown dwarfs are key to exploring the chemistry and physicsthat take place in their atmospheres. Late-T dwarf spectra are particularlydiagnostic due to their relatively cloud-free atmospheres and deep molecularbands. With the use of powerful atmospheric retrieval tools applied to thespectra of these objects, direct constraints on molecular/atomic abundances,gravity, and vertical thermal profiles can be obtained enabling a broadexploration of the chemical/physical mechanisms operating in their atmospheres.We present a uniform retrieval analysis on low-resolution IRTF SpeX near-IRspectra of a sample of 50 T dwarfs, including new observations as part of arecent volume-limited survey. This analysis more than quadruples the sample ofT dwarfs with retrieved temperature profiles and abundances (H$_2$O, CH$_4$,NH$_3$, K and subsequent C/O and metallicities). We are generally able toconstrain effective temperatures to within 50K, volume mixing ratios for majorspecies to within 0.25dex, atmospheric metallicities [M/H] to within 0.2, andC/O ratios to within 0.2. We compare our retrieved constraints on the thermalstructure, chemistry, and gravities of these objects with predictions fromself-consistent radiative-convective equilibrium models and find, in generalthough with substantial scatter, consistency with solar composition chemistryand thermal profiles of the neighboring stellar FGK population. Objects withnotable discrepancies between the two modeling techniques and potentialmechanisms for their differences, be they related to modeling approach orphysically motivated, are discussed more thoroughly in the text.

A Uniform Retrieval Analysis of Ultra-cool Dwarfs. IV. A Statistical Census from 50 Late-T Dwarfs