Forbidden hugs in pandemic times

I. Luminous red nova AT 2019zhd, a new merger in M 31^⋆

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: andrea.pastorello@inaf.it
² School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
³ Universitá degli Studi di Padova, Dipartimento di Fisica e Astronomia, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
⁴ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
⁵ Millennium Institute of Astrophysics (MAS), Nuncio Monsenor Sótero Sanz 100, Providencia, Santiago, Chile
⁶ Itagaki Astronomical Observatory, Yamagata 990-2492, Japan
⁷ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Quantum, Vesilinnantie 5, 20014 Turku, Finland
⁸ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
⁹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹⁰ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
¹¹ Astrophysics Research Group, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, UK
¹² Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
¹³ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
¹⁴ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
¹⁵ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
¹⁶ Astrophysics Research Institute, Liverpool John Moores University, IC2, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁷ Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
¹⁸ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
¹⁹ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
²⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²¹ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA

Received: 20 November 2020
Accepted: 10 December 2020

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 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 × 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 (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_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.