Issue |
A&A
Volume 693, January 2025
|
|
---|---|---|
Article Number | A203 | |
Number of page(s) | 13 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361/202451715 | |
Published online | 17 January 2025 |
Y Gem, a symbiotic star outshone by its asymptotic giant branch primary component
1
Instituto de Astrofísica de Andalucía, IAA-CSIC, Glorieta de la Astronomía S/N, Granada 18008, Spain
2
Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia, Michoacán, Mexico
3
Escola de Artes, Ciências e Humanidades, USP, Av. Arlindo Bettio 1000, 03828-000 São Paulo, Brazil
⋆ Corresponding author; mar@iaa.es
Received:
30
July
2024
Accepted:
21
November
2024
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.
Key words: stars: AGB and post-AGB / binaries: symbiotic / ultraviolet: stars / X-rays: binaries
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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