Forbidden hugs in pandemic times: I. Luminous red nova AT 2019zhd, a new merger in M 31

• Published in: Astronomy and astrophysics (Berlin) 2021-02, Vol.646, p.A119
• 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: binaries: close; outflows; stars: individual: AT 2019zhd; stars: individual: M31-LRN2015; stars: individual: M31-RV; stars: individual: V838 Mon; stars: winds
• ISSN: 0004-6361; 1432-0746; 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 M r  = −2.8 ± 0.2 mag to M r  = −5.6 ± 0.1 mag. Then, over a four to five day period, AT 2019zhd experienced a major brightening, reaching a peak of M r  = −9.61 ± 0.08 mag and an optical luminosity of 1.4 × 10 39 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, H α becomes very weak, H β 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 ( T eff ≈ 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 ( M F 555 W  = 0.21 ± 0.14 mag, with F 555 W  −  F 814 W  = 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.