The GALAH survey and symbiotic stars – I. Discovery and follow-up of 33 candidate accreting-only systems

Abstract

We have identified a first group of 33 new candidates for symbiotic stars (SySt) of the accreting-only variety among the 600 255 stars so far observed by the GALAH high-resolution spectroscopic survey of the Southern Hemisphere, more than doubling the number of those previously known. GALAH aims to high latitudes and this offers the possibility to sound the Galaxy for new SySt away from the usual Plane and Bulge hunting regions. In this paper we focus on SySt of the M spectral type, showing an Hα emission with a peak in excess of 0.5 above the adjacent continuum level, and not affected by coherent radial pulsations. These constraints will be relaxed in future studies. The 33 new candidate SySt were subjected to an array of follow-up confirmatory observations (X–ray/UV observations with the Swift satellite, search for optical flickering, presence of a near-UV upturn in ground-based photometric and spectroscopic data, radial velocity changes suggestive of orbital motion, variability of the emission line profiles). According to Gaia eDR3 parallaxes, the candidate new SySt are located at the tip of the Giant Branch, sharing the same distribution in M(KS) of the well established SySt. The accretion luminosities of the candidate new StSt are in the range 1−10 L⊙, corresponding to mass-accretion rates of 0.1-1 10−9 M⊙ yr−1 for WDs of 1 M⊙. The M giant of one of them presents a large Lithium over-abundance.

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

“…Assuming that the M6III giant in THA 15−31 is characterized by the same M(K S ) as the M6III giant in SU Lyn, the distance to THA 15−31 turns out to be 12±0.6 kpc, a value that we adopt in the remainder of this paper. The corresponding height above the galactic plane is 1.8 kpc, which matches the median value for known SySts believed to be old-population objects (Munari et al 2021, their Figure 7).…”

“…The orbital modulation of the radial velocity and of the line profiles would be different in the two cases, and long-term, high-resolution spectroscopic monitoring would be clearly beneficial in identifying the loca- tion of formation of the emission lines and in constraining other basic parameters of THA 15-31. As a final comment, we note that the Hα and Hβ emission lines in Figure 6 would have well satisfied the selection criteria in Munari et al (2021) if THA 15-31 were in GALAH, in addition to the spectral signatures of an M-giant and the absence of radial pulsations in the light curve. The profile of THA 15-31 Hα is a fine match to that of the 33 GALAH SySts, and the condition on equivalent widths EW(Hα)>EW(Hβ) is met within a large margin.…”

“…Supplemented by epoch photometry from NeoWISE and ASASSN, as well as other ancillary data, we now show that THA 15−31 is indeed an accretingonly symbiotic star (SySt), with an accretion luminosity (L acc ) that is much higher than that characterizing the sample of 33 A&A proofs: manuscript no. paper new, accreting-only SySts that we recently identified and studied (Munari et al 2021) among the 0.6 million stars observed by the GALAH spectroscopic survey of the southern sky (de Silva et al 2015, Buder et al 2021.…”

“…There is a clear disproportion among cataloged SySts (Allen 1984, Belczyński et al 2000, Merc et al 2019, Akras et al 2019a) in favor of the burn-SySt type which are much more easily discovered, and the known examples of the acc-SySt variety could simply be "the tip of the iceberg" (Mukai et al 2016). The acc-SySts are well distributed in accretion luminosity, from the 10−100 L ⊙ of some well-studied objects like EG And (Skopal 2005, Nuñez et al 2016) to the emerging large population of lower 1−10 L ⊙ objects exemplified by the first sample of 33 SySts discovered by Munari et al (2021) in the course of the GALAH survey. The higher L acc systems are characterized by a brighter HC (collectively including the accretion disk, its central star, and some RG wind ionized by them both) and by much stronger emission lines compared to lower L acc systems, although generally missing the higher ionization features like HeII, [FeVII], OVI, or [FeX] that adorn the spectra of the burn-SySt type.…”

Hen 3-860: New southern eclipsing symbiotic star observed in the outburst

“…Accreting-only symbiotic stars, which do not satisfy these criteria, are not discussed here, see, e.g.,Munari et al (2021) andLucy (2021) and references therein.MNRAS 000,1-9 (2021) …”