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

A. Pastorello, M. Fraser, G. Valerin, A. Reguitti, K. Itagaki, P. Ochner, S. C. Williams, D. Jones, J. Munday, S. J. Smartt, K. W. Smith, S. Srivastav, N. Elias-Rosa, E. Kankare, E. Karamehmetoglu, P. Lundqvist, P. A. Mazzali, U. Munari, M. D. Stritzinger, L. TomasellaJ. P. Anderson, K. C. Chambers, A. Rest

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1 Citation (Scopus)

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.

Original languageEnglish
Article numberA119
Number of pages12
JournalAstronomy and Astrophysics
Volume646
DOIs
Publication statusPublished - 17 Feb 2021

Bibliographical note

Funding Information:
5 iraf is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. 6 foscgui is a graphic user interface developed by E. Cappellaro, and aimed at extracting supernova spectroscopy and photometry obtained

Funding Information:
Acknowledgements. We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers world-wide and used in this research. MF gratefully acknowledges the support of a Royal Society – Science Foundation Ireland University Research Fellowship. EK and MS are supported by generous grants from Villum FONDEN (13261,28021) and by a project grant (8021-00170B) from the Independent Research Fund Denmark. SJS and KWS acknowledge funding from STFC Grants ST/P000312/1, ST/T000198/1 and ST/S006109/1. DJ acknowledges support from the State Research Agency (AEI) of the Spanish Ministry of Science, Innovation and Universities (MCIU) and the European Regional Development Fund (FEDER) under grant AYA2017-83383-P. DJ also acknowledges support under grant P/308614 financed by funds transferred from the Spanish Ministry of Science, Innovation and Universities, charged to the General State Budgets and with funds transferred from the General Budgets of the Autonomous Community of the Canary Islands by the Ministry of Economy, Industry, Trade and Knowledge. This research is based on observations made with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias; the Gran Telescopio Canarias, installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma; the Liverpool Telescope operated on the island of La Palma by Liverpool John Moores University at the Spanish Obser-vatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias with financial support from the UK Science and Technology Facilities Council; the 1.82 m Copernico Telescope and the 67/92 cm Schmidt Telescope of INAF-Osservatorio Astronomico di Padova at Mt. Ekar; and the 1.22 Galileo Galilei Telescope of the Padova University in the Asiago site. This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogues from the survey area. The ATLAS science products have been made possible through the contributions of the Uni-

Funding Information:
versity of Hawaii Institute for Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, and the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile. The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cum-bres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. Lasair is supported by the UKRI Science and Technology Facilities Council and is a collaboration between the University of Edinburgh (grant ST/N002512/1) and Queen’s University Belfast (grant ST/N002520/1) within the LSST:UK Science Consortium. This publication is partially based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. This research has made use of “Aladin sky atlas” developed at CDS, Strasbourg Observatory, France. This work made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. We also used NASA’s Astrophysics Data System.

Publisher Copyright:
© ESO 2021.

Copyright:
Copyright 2021 Elsevier B.V., All rights reserved.

Keywords

  • Binaries: close
  • Outflows
  • Stars: individual: AT 2019zhd
  • Stars: individual: M31-LRN2015
  • Stars: individual: M31-RV
  • Stars: individual: V838 Mon
  • Stars: winds

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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