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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.
Thanks to the ongoing NEOWISE-Reactivation mission (Mainzer et al. 2014), the total WISE depth of exposure at W2 has increased by a factor of >5 relative to the imaging available during construction of the AllWISE catalog. We are using the first four years of WISE/NEOWISE imaging at W1 and W2 to create a full-sky WISE-selected source catalog ("CatWISE") which pushes deeper than AllWISE in these bands and features a 6.5 year time baseline, more than 10× longer than that of AllWISE. A first iteration of the CatWISE catalog has recently been completed, and is being internally vetted via quality assurance checks and initial science explorations, prior to public release in 2019.
Here we report a preliminary but secure CatWISE detection of WD 0806-661B, with w2mpro_pm = 16.82 ± 0.09 (see Figure 1). We report the best-fit W2 magnitude accounting for source proper motion (w2mpro_pm) because WD 0806-661B has a total proper motion of 0
44 yr−1, corresponding to a positional shift of 29 over our CatWISE time baseline. Adopting [4.5] = 16.84 ± 0.06 (Luhman et al. 2012), the implied [4.5]−W2 color of WD 0806-661B is consistent with the sample of known Y dwarfs. Our W2 measurement of WD 0806-661B suggests that CatWISE will be able to detect Y dwarfs at distances of up to ∼20 pc, thereby improving our census of the solar neighborhood at the latest spectral types.
Figure 1. Top left: W2 unWISE coadd used by CatWISE to enable deep source detection, at native angular resolution (Meisner et al. 2018). The red "+" denotes the CatWISE-measured centroid of WD 0806-661B. WD 0806-661B is clearly detected in this W2 coadd incorporating 4 years of WISE/NEOWISE imaging. Top right: Histogram of [4.5]−W2 colors for known Y dwarfs with these data available (black). WD 0806-661B is overplotted as a red box of unit height and width indicating the 1σ color uncertainty. WD 0806-661B is consistent with other known Y dwarfs in terms of [4.5]−W2 color. Bottom left: 2004 Spitzer [4.5] mosaic, showing that the CatWISE detection's location (red box) is not significantly contaminated by static background objects. Bottom right: Our WD 0806-661B detection extends the W2 vs. [4.5] scatterplot into a fainter regime than that inhabited by other known Y dwarfs. The dotted line follows W2 = [4.5] + 0.02; this line is only for reference, and does not represent a fit to the data.
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This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. CatWISE is funded by NASA under Proposal No. 16-ADAP16-0077 issued through the Astrophysics Data Analysis Program, and uses data from the NASA-funded WISE and NEOWISE projects. This work has also been supported by Hubble Fellowship HST-HF2-51415.001-A, NASA ADAP grant NNH17AE75I, and has benefited from the Y Dwarf Compendium maintained by Michael Cushing at https://sites.google.com/view/ydwarfcompendium.