Sub-stellar objects with habitable-zone temperatures: A new search for Y dwarfs in WISE

¹ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
² Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile

^a e-mail: D.J.Pinfield@herts.ac.uk
^b e-mail: J.Gomes@herts.ac.uk
^c e-mail: adjones@das.uchile.cl

Abstract

A method is defined for using the maximum sensitivity of WISE to find late T and Y dwarfs. This requires a WISE detection only in the W2-band and uses the statistical properties of the WISE multi-frame measurements and profile fit photometry to reject contamination resulting from non-point-like objects, variables and moving sources. To trace our desired parameter space we use a control sample of isolated non-moving non-variable point sources from the SDSS, and identify a sample of 158 WISE W2-only candidates down to a signal-to-noise limit of 8. For signal-to-noise ranges >10 and 8-10 respectively, ∼45% and ∼90% of our sample fall outside the criteria published by the WISE team (Kirkpatrick et al. 2012), due mainly to the type of constraints placed on the number of individual W2 detections. We present some early follow-up of our sample and identify two new very late objects with high proper motion (∼1.3 and ∼1.8 arcsec yr⁻¹). Based on colour, brightness and proper motion, these object are either ∼T9 dwarfs with thick-disk/halo kinematics, or Y dwarfs with more disk-like kinematics. Reduced proper motion diagrams are also presented which suggest that late T and Y dwarfs may have a much higher thick-disk/halo population fraction than earlier objects.