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

¹ INAF-Osservatorio Astronomico di Padova, 36012 Asiago (VI), Italy
e-mail: ulisse.munari@oapd.inaf.it
² INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
³ INAF-Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁴ INAF-Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 101, 40129 Bologna, Italy
⁵ Departamento de Ciencias Físicas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile
⁶ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43 221 00 Lund, Sweden
⁷ Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, Penteli 15236, Greece

Received: 20 August 2021
Accepted: 10 February 2022

Abstract

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 W3 (12 μm) and W4 (22 μm) data, radiating a luminosity ≥35 L_⊙, consistent with thermal emission from optically thin circumstellar dust.