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Title: Further Defining Spectral Type "Y" and Exploring the Low-mass End of the Field Brown Dwarf Mass Function
Authors: J. Davy Kirkpatrick, Christopher R. Gelino, Michael C. Cushing, Gregory N. Mace, Roger L. Griffith, Michael F. Skrutskie, Kenneth A. Marsh, Edward L. Wright, Peter R. Eisenhardt, Ian S. McLean, Amanda K. Mainzer, Adam J. Burgasser, C. G.Tinney, Stephen Parker, Graeme Salter

We present the discovery of another seven Y dwarfs from the Wide-field Infrared 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 colours of brown dwarfs, as predicted by models, turn back to the red. Moreover, we use preliminary trigonometric parallax measurements to show that the T/Y boundary may also correspond to the point at which the absolute H (1.6 m) and W2 (4.6 m) magnitudes plummet. We use these discoveries and their preliminary distances to place them in the larger context of the Solar Neighbourhood. We present a table that updates the entire stellar and substellar constituency within 8 parsecs of the Sun, and we show that the current 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 identified within this volume, but unless there is a vast reservoir of cold brown dwarfs invisible to WISE, the final space density of brown dwarfs is still expected to fall well below that of stars. We also use these new Y dwarf discoveries, along with newly discovered T dwarfs from WISE, to investigate the field substellar mass function. We find that the overall space density of late-T and early-Y dwarfs matches that from simulations describing the mass function as a power law with slope -0.5 < alpha < 0.0; however, a power-law may provide a poor fit to the observed object counts as a function of spectral type because there are tantalising hints that the number of brown dwarfs continues to rise from late-T to early-Y. More detailed monitoring and characterisation of these Y dwarfs, along with dedicated searches aimed at identifying more examples, are certainly required.

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Scientists using data from NASA's Wide-field Infrared Survey Explorer (WISE) have discovered six "Y dwarfs"-- star-like bodies with temperatures as cool as the human body.
Astronomers hunted these dark orbs for more than a decade without success. When viewed with a visible-light telescope, they are nearly impossible to see. WISE's infrared vision allowed the telescope to finally spot the faint glow of a half dozen Y dwarfs relatively close to our sun, within a distance of about 40 light-years.

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Title: The Discovery of Y Dwarfs Using Data from the Wide-field Infrared Survey Explorer (WISE)
Authors: Michael C. Cushing, J. Davy Kirkpatrick, Christopher R. Gelino, Roger L. Griffith, Michael F. Skrutskie, Amanda K. Mainzer, Kenneth A. Marsh, Charles A. Beichman, Adam J. Burgasser, Lisa A. Prato, Robert A. Simcoe, Mark S. Marley, D. Saumon, Richard S. Freedman, Peter R. Eisenhardt, Edward L. Wright

We present the discovery of seven ultracool brown dwarfs identified with the Wide-field Infrared Survey Explorer (WISE). Near-infrared spectroscopy reveals deep absorption bands of H_2O and CH_4 that indicate all seven of the brown dwarfs have spectral types later than UGPS J072227.51-054031.2, the latest type T dwarf currently known. The spectrum of WISEP J182831.08+265037.8 is distinct in that the heights of the J- and H-band peaks are approximately equal in units of f_lambda, so we identify it as the archetypal member of the Y spectral class. The spectra of at least two of the other brown dwarfs exhibit absorption on the blue wing of the H-band peak that we tentatively ascribe to NH_3. These spectral morphological changes provide a clear transition between the T dwarfs and the Y dwarfs. In order to produce a smooth near-infrared spectral sequence across the T/Y dwarf transition, we have reclassified UGPS J0722-0540 as the T9 spectral standard and tentatively assign WISEP J173835.52+273258.9 as the Y0 spectral standard. In total, six of the seven new brown dwarfs are classified as Y dwarfs: four are classified as Y0, one is classified as Y0 (pec?), and WISEP J1828+2650 is classified as >Y0. We have also compared the spectra to the model atmospheres of Marley and Saumon and infer that the brown dwarfs have effective temperatures ranging from 300 K to 500 K, making them the coldest spectroscopically confirmed brown dwarfs known to date.

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UGPS J072227.51-054031.2, WISEPC J014807.25-720258.8, WISEP J041022.71+150248.5, WISEPC J140518.40+553421.5, WISEP J154151.65-225025.2, WISEP J173835.52+273258.9, WISEP J1828+2650, WISEPC J205628.90+145953.3



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NASA's Wise Mission Discovers Coolest Class of Stars

Scientists using data from NASA's Wide-field Infrared Survey Explorer (WISE) have discovered the coldest class of star-like bodies, with temperatures as cool as the human body.
Astronomers hunted these dark orbs, termed Y dwarfs, for more than a decade without success. When viewed with a visible-light telescope, they are nearly impossible to see. WISE's infrared vision allowed the telescope to finally spot the faint glow of six Y dwarfs relatively close to our sun, within a distance of about 40 light-years.

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