FURTHER DEFINING SPECTRAL TYPE “Y” AND EXPLORING THE LOW-MASS END OF THE FIELD BROWN DWARF MASS FUNCTION

Abstract

We present the discovery of another seven Y dwarfs from the Wide-field\nInfrared Survey Explorer (WISE). Using these objects, as well as the first six\nWISE Y dwarf discoveries from Cushing et al., we further explore the transition\nbetween spectral types T and Y. We find that the T/Y boundary roughly coincides\nwith the spot where the J-H colors of brown dwarfs, as predicted by models,\nturn back to the red. Moreover, we use preliminary trigonometric parallax\nmeasurements to show that the T/Y boundary may also correspond to the point at\nwhich the absolute H (1.6 um) and W2 (4.6 um) magnitudes plummet. We use these\ndiscoveries and their preliminary distances to place them in the larger context\nof the Solar Neighborhood. We present a table that updates the entire stellar\nand substellar constituency within 8 parsecs of the Sun, and we show that the\ncurrent census has hydrogen-burning stars outnumbering brown dwarfs by roughly\na factor of six. This factor will decrease with time as more brown dwarfs are\nidentified within this volume, but unless there is a vast reservoir of cold\nbrown dwarfs invisible to WISE, the final space density of brown dwarfs is\nstill expected to fall well below that of stars. We also use these new Y dwarf\ndiscoveries, along with newly discovered T dwarfs from WISE, to investigate the\nfield substellar mass function. We find that the overall space density of\nlate-T and early-Y dwarfs matches that from simulations describing the mass\nfunction as a power law with slope -0.5 < alpha < 0.0; however, a power-law may\nprovide a poor fit to the observed object counts as a function of spectral type\nbecause there are tantalizing hints that the number of brown dwarfs continues\nto rise from late-T to early-Y. More detailed monitoring and characterization\nof these Y dwarfs, along with dedicated searches aimed at identifying more\nexamples, are certainly required.\n