The "Giraffe": Discovery of a stripped red giant in an interacting binary with a $\sim 2M_{\odot }$ lower giant

Abstract

We report the discovery of a stripped giant + lower giant binary, 2M04123153+6738486 (2M0412), identified during a search for non-interacting compact object-star binaries. 2M0412 is an evolved ($T_{\mathrm{eff},\mathrm{giant}}\simeq 4000$ K), luminous ($L_{\mathrm{giant}}\simeq 150L_{\odot }$) red giant in a circular $P=81.2$ day binary. 2M0412 is a known variable star previously classified as a semi-regular variable. The cross-correlation functions of follow-up Keck/HIRES and LBT/PEPSI spectra show an RV-variable second component with implied mass ratio $q=M_{\mathrm{giant}}/M_{\mathrm{comp}}\simeq 0.20\pm 0.01$. The ASAS-SN, ATLAS, TESS and ZTF light curves show that the giant is a Roche lobe filling ellipsoidal variable with an inclination of $49.4^{\circ }\pm 0.3^{\circ }$, and a giant mass of $M_{\mathrm{giant}}=0.38\pm 0.01M_{\odot }$ for a distance of $\simeq 3.7$ kpc. The mass of the giant indicates that its envelope has been stripped. The giant companion on the lower red giant branch has a mass of $M_{\mathrm{comp}}=1.91\pm 0.03M_{\odot }$ with $T_{\mathrm{eff},\mathrm{comp}}\simeq 5000$ K, $L_{\mathrm{comp}}\simeq 60L_{\odot }$ and $R_{\mathrm{comp}}\simeq 11R_{\odot }$. We also identify an orbital phase dependent, broad $\mathrm{H}\hat{\mathrm{I}}\pm$ emission line which could indicate ongoing accretion from the stripped red giant onto the companion.

Revised version of the paper following comments from the referee, submitted to MNRAS. arXiv admin note: text overlap with arXiv:2101.02212