Forbidden hugs in pandemic times

• Published in: Astronomy and astrophysics (Berlin) 2021-02, Vol.646
• Main Authors: Pastorello, A, Fraser, M, Valerin, G, Reguitti, A, Itagaki, K, Ochner, P, Williams, S C, Jones, D, Munday, J, Smartt, S J, Smith, K W, Srivastav, S, Elias-Rosa, N, Kankare, E, Karamehmetoglu, E, Lundqvist, P, Mazzali, P A, Munari, U, Stritzinger, M D, Tomasella, L, Anderson, J P, Chambers, K C, Rest, A
• Format: Article
• Language: English
• Subjects: Binary stars; Brightening; Emission spectra; Inspection; Light curve; Line spectra; Luminosity; M stars; Novae; Sky surveys (astronomy)
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202039952

We present the follow-up campaign of the luminous red nova (LRN) AT 2019zhd, the third event of this class observed in M 31. The object was followed by several sky surveys for about five months before the outburst, during which it showed a slow luminosity rise. In this phase, the absolute magnitude ranged from Mr = −2.8 ± 0.2 mag to Mr = −5.6 ± 0.1 mag. Then, over a four to five day period, AT 2019zhd experienced a major brightening, reaching a peak of Mr = −9.61 ± 0.08 mag and an optical luminosity of 1.4 × 1039 erg s−1. After a fast decline, the light curve settled onto a short-duration plateau in the red bands. Although less pronounced, this feature is reminiscent of the second red maximum observed in other LRNe. This phase was followed by a rapid linear decline in all bands. At maximum, the spectra show a blue continuum with prominent Balmer emission lines. The post-maximum spectra show a much redder continuum, resembling that of an intermediate-type star. In this phase, Hα becomes very weak, Hβ is no longer detectable, and a forest of narrow absorption metal lines now dominate the spectrum. The latest spectra, obtained during the post-plateau decline, show a very red continuum (Teff ≈ 3000 K) with broad molecular bands of TiO, similar to those of M-type stars. The long-lasting, slow photometric rise observed before the peak resembles that of LRN V1309 Sco, which was interpreted as the signature of the common-envelope ejection. The subsequent outburst is likely due to the gas outflow following a stellar merging event. The inspection of archival HST images taken 22 years before the LRN discovery reveals a faint red source (MF555W = 0.21 ± 0.14 mag, with F555W − F814W = 2.96 ± 0.12 mag) at the position of AT 2019zhd, which is the most likely quiescent precursor. The source is consistent with expectations for a binary system including a predominant M5-type star.