THA 15−31: Discovery with VLT/X-shooter and Swift /UVOT of a new symbiotic star of the accreting-only variety

• Published in: Astronomy and astrophysics (Berlin) Vol. 661; p. A124
• Main Authors: Munari, U., Alcalá, J. M., Frasca, A., Masetti, N., Traven, G., Akras, S., Zampieri, L.
• Format: Journal Article
• Language: English
• Published: 01.05.2022
• Subjects: Astronomi, astrofysik och kosmologi; Astronomy, Astrophysics and Cosmology; Fysik; Natural Sciences; Naturvetenskap; Physical Sciences
• ISSN: 0004-6361; 1432-0746; 1432-0746
• DOI: 10.1051/0004-6361/202142058

We report the discovery and characterization of a new symbiotic star of the accreting-only variety, which we observed in the optical/near-infrared (NIR) with VLT/X-shooter and in the X-rays/ultraviolet with Swift /UVOT+XRT. The new symbiotic star, THA 15−31, was previously described as a pre-main sequence star belonging to the Lupus 3 association. Our observations, ancillary data, and Gaia EDR3 parallax indicate that THA 15−31 is a symbiotic star composed of an M6III red giant and an accreting companion, is subject to E B  −  V  = 0.38 reddening, and is located at a distance of ∼12 kpc and at 1.8 kpc above the Galactic plane in the outskirts of the Bulge. The luminosity of the accreting companion is ∼100  L ⊙ , placing THA 15−31 among the symbiotic stars accreting at a high rate (2.5 × 10 −8   M ⊙ yr −1 if the accretion is occurring on a white dwarf of 1  M ⊙ ). The observed emission lines originate primarily from H I, He I, and Fe II, with no He II or other high-excitation lines observed; a sharp central absorption superimposed on the Balmer emission lines is observed, while all other lines have a simple Gaussian-like profile. The emission from the companion dominates over the M6III red giant at U and B -band wavelengths, and is consistent with an origin primarily in an optically thick accretion disk. No significant photometric variability is observed at optical or NIR wavelengths, suggesting either a face-on orbital orientation and/or that the red giant is far from Roche-lobe filling conditions. The profile of emission lines supports a low orbital inclination if they form primarily in the accretion disk. An excess emission is present in AllWISE W 3 (12 μm) and W 4 (22 μm) data, radiating a luminosity ≥35  L ⊙ , consistent with thermal emission from optically thin circumstellar dust.