Backyard Worlds: Planet 9 Discovery of an Unusual Low-mass Companion to an M Dwarf at 80 pc

Abstract

We present the discovery of CWISE J203546.35-493611.0, a peculiar M8 companion to the M4.5 star APMPM J2036-4936 discovered through the citizen science project Backyard Worlds: Planet 9. Given CWISE J203546.35-493611.0's proper motion ($\mu_{\alpha}$, $\mu_{\delta}$) = ($-$126$\pm$22, $-$478$\pm$23) and angular separation of 34.2$''$ from APMPM 2036-4936, we calculate a chance alignment probability of $1.15 \times 10^{-6}$. Both stars in this system appear to be underluminous, and the spectrum obtained for CWISE J203546.35-493611.0 shows a triangular H band. Further study of this system is warranted to understand these peculiarities.