ZZ Piscis Austrinus (ZZ PsA): a bright red nova progenitor and the instability mass ratio of contact binary stars

Abstract

ZZ PsA is a neglected bright southern contact binary system with maximum V magnitude of 9.26. We present the first multi-band photometric analysis and find the system to be in deep contact (>95%) with an extremely low mass ratio of 0.078. The primary has a mass of 1.213 M⊙ in keeping with its reported spectral class of F6. In order to determine if ZZ PsA is a merger candidate we outline the current status regarding the instability mass ratio and develop new relationship linking the mass of the primary to the instability mass ratio of the system and the degree of contact. We find that ZZ PsA along with two other examples from the literature to be merger candidates while an additional three require further observations to be confirmed as potential merger candidates.

The first photometric and spectroscopic analysis of the extremely low mass ratio contact binary NSVS 5029961

“…In order to discuss the stability of our target further, we calculated the instability parameters using a series of formulas introduced by Wadhwa et al (2021). The instability mass ratio (q inst = 0.0342) was determined based on the mass of primary star and the fillout factor.…”

“…Then the radius of the primary (R 1 = 1.570 R ⊙ ) was derived with the current separation. We set the value of k 2 as Wadhwa et al (2021) did. The dimensionless gyration radius of the primary (k 1 = 0.1774) was estimated from Landin et al (2009) for a 1.80 M ⊙ star.…”

“…It is predicted that they result from cool detached binaries with short period via angular momentum loss (AML) controlled by magnetic activities (e.g., Huang 1966;van't Veer 1979;Vilhu 1982;Qian 2003;Li et al 2007;Qian et al 2017;Qian et al 2018) and evolve into extreme low mass-ratio contact configurations (e.g., Li et al 2004;Qian et al 2005;Qian et al 2006). Contact binaries are supposed to have a low mass ratio limit (e.g., Jiang et al 2010;Yang & Qian 2015;Wadhwa et al 2021). When the mass ratio is smaller than the q min , the system will merge into a fast-rotating single object (FK Com-type star or blue straggler) ultimately based on tidal instability (Webbink 1976;Hut 1980;Eggleton & Kiseleva-Eggleton 2001).…”

“…V1309 Sco is an evidence of the red nova eruption from the coalescence of contact binary (Tylenda et al 2011). Wadhwa et al (2021) gave more reliable criteria of instability mass ratio and instability separation which depend on the mass of the primary star, corresponding gyration radii, the degree of contact and other physical parameters. New merger candidates were found with the method by them.…”

Analysis of Dual Band and Survey Photometry of Two Low Mass Ratio Contact Binary Systems

“…Most of them have only been relatively recently recognised through all sky surveys such as the Automated All Sky Survey (ASAS) (Pojmanski, 2002), All Sky Automated Survey -Super Nova (ASAS-SN) (Shappee et al, 2014;Jayasinghe et al, 2020), Catalina Sky Survey (Drake et al, 2017) among many others. Since the recognition that luminous transient events dubbed red novae are the result of contact binary component mergers (Tylenda et al, 2011) there has been increasing interest in their study not only from the point of view detecting pre-merger candidates (Wadhwa et al, 2021) but also with a view to understanding evolutionary and astrophysical processes such as energy transfer and angular momentum loss (Devarapalli et al, 2020;Sriram et al, 2016;Csizmadia & Klagyivik, 2004).…”