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 et al, Anderson, JP, Chambers, KC and Rest, A (2021) Forbidden hugs in pandemic times: I. Luminous red nova AT 2019zhd, a new merger in M 31. Astronomy and Astrophysics, 646. ISSN 0004-6361

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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.

Item Type:	Article
Uncontrolled Keywords:	Science & Technology; Physical Sciences; Astronomy & Astrophysics; binaries: close; stars: winds, outflows; stars: individual: AT 2019zhd; stars: individual: M31-LRN2015; stars: individual: M31-RV; stars: individual: V838 Mon; SN HUNT 248; STELLAR MERGERS; V838 MON; OUTBURST; PHOTOMETRY; EVOLUTION; TRANSFORMATIONS; SPECTROSCOPY; Astronomy & Astrophysics; 0201 Astronomical and Space Sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	EDP Sciences
SWORD Depositor:	A Symplectic
Date Deposited:	04 Nov 2022 10:44
Last Modified:	04 Nov 2022 10:44
DOI or ID number:	10.1051/0004-6361/202039952
URI:	https://researchonline.ljmu.ac.uk/id/eprint/18014

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