Submitted to The Astrophysical Journal Further Defining Spectral Type “Y” and Exploring the Low-mass End of the Field Brown Dwarf Mass Function J. Davy Kirkpatricka, Christopher R. Gelinoa, Michael C. Cushingb, Gregory N. Macec Roger L. Griffitha, Michael F. Skrutskied, Kenneth A. Marsha, Edward L. Wrightc, Peter R. Eisenhardte, Ian S. McLeanc, Amanda K. Mainzere, Adam J. Burgasserf , C. G. Tinneyg, Stephen Parkerg, Graeme Salterg ABSTRACT We present the discovery of another seven Y dwarfs from the Wide-field In- frared Survey Explorer (WISE). Using these objects, as well as the first six WISE Y dwarf discoveries from Cushing et al., we further explore the transition between spectral types T and Y. We find that the T/Y boundary roughly coincides with the spot where the J − H colors of brown dwarfs, as predicted by models, turn back to the red. Moreover, we use preliminary trigonometric parallax measure- ments to show that the T/Y boundary may also correspond to the point at which the absolute H (1.6 µm) and W2 (4.6 µm) magnitudes plummet. We use these discoveries and their preliminary distances to place them in the larger context of the Solar Neighborhood. We present a table that updates the entire stellar and substellar constinuency within 8 parsecs of the Sun, and we show that the cur- rent census has hydrogen-burning stars outnumbering brown dwarfs by roughly a factor of six. This factor will decrease with time as more brown dwarfs are iden- tified within this volume, but unless there is a vast reservoir of cold brown dwarfs arXiv:1205.2122v1 [astro-ph.SR] 9 May 2012 aInfrared Processing and Analysis Center, MS 100-22, California Institute of Technology, Pasadena, CA 91125;
[email protected] bDepartment of Physics and Astronomy, MS 111, University of Toledo, 2801 W.