Y Gem, a symbiotic star outshone by its asymptotic giant branch primary component

TL;DR

This study uncovers Y Gem as a symbiotic star with a white dwarf accreting matter from an AGB companion, using multiwavelength data to analyze its stellar components and accretion processes.

Contribution

First optical spectrum of Y Gem and detailed analysis confirming it as an S-type symbiotic star with a white dwarf accretor.

Findings

Y Gem hosts a hot accreting white dwarf with T_eff≈60,000 K.

The AGB star in Y Gem has a mass of 1.1 M_sun and T_eff=3350 K.

Y Gem's white dwarf is in a stable burning phase, with an accretion rate of 2.3×10^{-7} M_sun/yr.

Abstract

A considerable number of asymptotic giant branch (AGB) stars exhibit UV excess and/or X-ray emission that indicates a binary companion. AGB stars are so bright that they easily outshine their companions. This almost prevents their identification. Y Gem has been known for some decades to be an AGB star that is bright in the far-UV and X-rays, but it is unclear whether its companion is a main-sequence star or a white dwarf (WD) in a symbiotic system (SySt). Our goal is to uncover the true nature of Y Gem, which will help us to study the possible misidentified population of SySts. Multiwavelength IR, optical, UV, and X-ray observations were analyzed to investigate the properties of the stellar components and the accretion process in Y Gem. In particular, an optical spectrum of Y Gem is presented here for the first time, while X-ray data are interpreted by means of reflection models produced by an accretion disk and material in its vicinity. The optical spectrum exhibits the typical sawtooth-shaped features of molecular absorptions in addition to narrow recombination and forbidden emission lines. The emission lines and the analysis of the extinction-corrected UV spectrum suggest a hot component with $T_\mathrm{eff}\approx$60,000 K, $L$=140 L$_{\odot}$, and $R$=0.11 R$_{\odot}$ that very likely is an accreting WD. The late component is found to be an 1.1 M$_\odot$ AGB star with $T_\mathrm{eff}$=3350 K and $R$=240 R$_\odot$. Using IR, optical, UV, and X-ray data, we found that Y Gem is an S-type SySt whose compact component is accreting at an estimated mass-accretion rate of $\dot{M}_\mathrm{acc}=2.3\times10^{-7}$ M$_\odot$ yr$^{-1}$. At this accretion rate, the accreting WD has reached the stable and steady burning phase in which no recurrent events are expected.