A deep WISE search for very late type objects and the discovery of two halo/thick-disc T dwarfs: WISE 0013+0634 and WISE 0833+0052

Abstract

A method is defined for identifying late-T and Y dwarfs in WISE down to low values of signalto- noise. This requires a WISE detection only in theW2-band and uses the statistical properties of the WISE multiframe 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 Sloan Digital Sky Survey (SDSS), and identify a sample of 158 WISE W2-only candidates down to a signal-to-noise limit of eight. For signal-to-noise ranges >10 and 8–10, respectively, ∼45 and ∼90 per cent of our sample fall outside the selection criteria published by the WISE team, mainly due to the type of constraints placed on the number of individualW2 detections.We present follow-up of eight candidates and identifyWISE 0013+0634 and WISE 0833+0052, T8 and T9 dwarfs with high proper motion (∼1.3 and ∼1.8 arcsec yr−1). Both objects show a mid-infrared/near-infrared excess of ∼1–1.5mag and are K band suppressed. Distance estimates lead to space motion constraints that suggest halo (or at least thick disc) kinematics. We then assess the reduced proper motion diagram of WISE ultracool dwarfs, which suggests that late-T and Y dwarfs may have a higher thick-disc/halo population fraction than earlier objects.