Redder than Red: Discovery of an Exceptionally Red L/T Transition Dwarf

Abstract

We present the discovery of CWISE J050626.96$+$073842.4 (CWISE J0506$+$0738), an L/T transition dwarf with extremely red near-infrared colors discovered through the Backyard Worlds: Planet 9 citizen science project. Photometry from UKIRT and CatWISE give a $(J-K{)}_{\mathrm{MKO}}$ color of 2.97$\pm$0.03 mag and a $J_{\mathrm{MKO}}-$W2 color of 4.93$\pm$0.02 mag, making CWISE J0506$+$0738 the reddest known free-floating L/T dwarf in both colors. We confirm the extremely red nature of CWISE J0506$+$0738 using Keck/NIRES near-infrared spectroscopy and establish that it is a low-gravity late-type L/T transition dwarf. The spectrum of CWISE J0506$+$0738 shows possible signatures of CH${}_4$ absorption in its atmosphere, suggesting a colder effective temperature than other known, young, red L dwarfs. We assign a preliminary spectral type for this source of L8${\hat{I}}^3$-T0${\hat{I}}^3$. We tentatively find that CWISE J0506$+$0738 is variable at 3-5 $\hat{I}¼$m based on multi-epoch WISE photometry. Proper motions derived from follow-up UKIRT observations combined with a radial velocity from our Keck/NIRES spectrum and a photometric distance estimate indicate a strong membership probability in the ${\hat{I}}^2$ Pic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object.

Accepted to ApJ Letters