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

Journal
Date Issued
2021
Author(s)
•
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.
•
•
Kankare, E.
•
Karamehmetoglu, E.
•
Lundqvist, P.
•
Mazzali, P. A.
•
•
Stritzinger, M. D.
•
•
Anderson, J. P.
•
Chambers, K. C.
•
Rest, A.
DOI
Abstract
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.

Tables A.1 and A.2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/646/A119...

Volume
646
Start page
A119
Uri
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Issn Identifier
0004-6361
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open.access
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