A1 Journal article – refereed

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

List of Authors: Pastorello A, Fraser M, Valerin G, Reguitti A, Itagaki K, Ochner P, Williams SC, Jones D, Munday J, Smartt SJ, Smith KW, Srivastav S, Elias-Rosa N, Kankare E, Karamehmetoglu E, Lundqvist P, Mazzali PA, Munari U, Stritzinger MD, Tomasella L, Anderson JP, Chambers KC, Rest A, Rest A


Publication year: 2021

Journal: Astronomy and Astrophysics

Journal name in source: ASTRONOMY & ASTROPHYSICS

Journal acronym: ASTRON ASTROPHYS

Volume number: 646

Number of pages: 12

ISSN: 0004-6361

eISSN: 1432-0746

DOI: http://dx.doi.org/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 1.4x10(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 alpha becomes very weak, H beta 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 approximate to 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-F555W=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.

Downloadable publication

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail. Please cite the original version.

Last updated on 2021-24-06 at 10:43