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We present preliminary trigonometric parallaxes of 184 late-T and Y dwarfsusing observations from Spitzer (143), USNO (18), NTT (14), and UKIRT (9). Tocomplete the 20-pc census of $\ge$T6 dwarfs, we combine these measurements withpreviously published trigonometric parallaxes for an additional 44 objects andspectrophotometric distance estimates for another 7. For these 235 objects, weestimate temperatures, sift into five 150K-wide $T_{\rm eff}$ bins covering therange 300-1050K, determine the completeness limit for each, and compute spacedensities. To anchor the high-mass end of the brown dwarf mass spectrum, wecompile a list of early- to mid-L dwarfs within 20 pc. We run simulations usingvarious functional forms of the mass function passed through two different setsof evolutionary code to compute predicted distributions in $T_{\rm eff}$. Thebest fit of these predictions to our L, T, and Y observations is a simplepower-law model with $\alpha \approx 0.6$ (where $dN/dM \propto M^{-\alpha}$),meaning that the slope of the field substellar mass function is in roughagreement with that found for brown dwarfs in nearby star forming regions andyoung clusters. Furthermore, we find that published versions of the log-normalform do not predict the steady rise seen in the space densities from 1050K to350K. We also find that the low-mass cutoff to formation, if one exists, islower than $\sim$5 $M_{Jup}$, which corroborates findings in young, nearbymoving groups and implies that extremely low-mass objects have been formingover the lifetime of the Milky Way.

the astrophysical journal. supplement series/astrophysical journal. supplement series

Preliminary Trigonometric Parallaxes of 184 Late-T and Y Dwarfs and an  Analysis of the Field Substellar Mass Function into the "Planetary" Mass  Regime