A Secure W2 Detection of WD 0806-661B from CatWISE

Abstract

The WD 0806-661 system consists of a DQ white dwarf primary at 19.26 pc (Gaia Collaboration et al. 2018) and an extremely cold common proper motion companion at 130'' separation (WD 0806-661B; Luhman et al. 2011). Given its absolute magnitude at Spitzer [4.5], the cold companion is presumed to be either a Y type brown dwarf or giant planet. At a projected physical separation of ~2500 au, WD 0806-661B represents the widest known definitively planetary-mass companion, and may provide important insights into the processes of star and planet formation. Unlike most Y dwarfs, WD 0806-661B has lacked a secure detection at 4.6 μm in the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) W2 channel. WD 0806-661B was reported as a low-significance detection in the WISE All-Sky Source Catalog (Cutri et al. 2012), with W2 = 17.68 ± 0.41 (J080715.32-661851.6, w2snr = 2.5). No detection of a WD 0806-661B counterpart is reported in the AllWISE Source Catalog, although the AllWISE Reject Table contains a low-significance positional match (J080715.34-661852.1) with W2 = 17.85 ± 0.41 (Cutri et al. 2013). Both the All-Sky and AllWISE Reject best-fit W2 magnitudes, if taken at face value, would make WD 0806-661B a very blue outlier among Y dwarfs in [4.5]−W2 color, given that the W2 and [4.5] magnitudes of known Y dwarfs generally agree to within ~0.1 mag.