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

Abstract

Context. 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). Aims. Our goal is to uncover the true nature of Y Gem, which will help us to study the possible misidentified population of SySts. Methods. 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. Results. 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 Teff ≈ 60 000 K, L = 140 L⊙, and R = 0.11 R⊙ that very likely is an accreting WD. The late component is found to be an 1.1 M⊙ AGB star with Teff = 3350 K and R = 240 R⊙. Conclusions. 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 Ṁacc = 2.3 × 10−7 M⊙ yr−1. At this accretion rate, the accreting WD has reached the stable and steady burning phase in which no recurrent events are expected.