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., 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.2022-06-022022-06-022021-02-01Astronomy and Astrophysics Volume 6461 February 2021 Article number A1190004-6361https://repositorio.unab.cl/xmlui/handle/ria/22699Indexación ScopusWe 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.enBinaries: closeOutflowsStars: individual: AT 2019zhdStars: individual: M31-LRN2015Stars: individual: M31-RVStars: individual: V838 MonStars: windsArtículo10.1051/0004-6361/202039952