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

TL;DR

This paper develops a WISE-based method to identify very late T and Y dwarfs, successfully discovering two halo/thick-disk T dwarfs with high proper motion and infrared excess, indicating a potential higher population fraction in these galactic components.

Contribution

It introduces a new WISE detection technique for late T and Y dwarfs using only W2-band data and statistical analysis, leading to the discovery of two high-velocity halo/thick-disk T dwarfs.

Findings

Discovered two high proper motion T dwarfs with halo/thick-disk kinematics.

Developed a WISE-only detection method effective at low signal-to-noise.

Suggests a higher fraction of thick-disk/halo objects among late T and Y dwarfs.

Abstract

A method is defined for identifying late T and Y dwarfs in WISE down to low values of signal-to-noise. 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 selection 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 follow-up of eight candidates and identify WISE 0013+0634 and WISE 0833+0052, T8 and T9 dwarfs with high proper motion (~1.3 and ~1.8 arcsec/yr). Both objects show a mid-infrared/near-infrared excess of ~1-1.5 magnitudes, and are K-band suppressed. Distance estimates lead to space motion constraints that suggest halo (or at least thick disk) 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-disk/halo population fraction than earlier objects.