Journal
ASTRONOMY & ASTROPHYSICS
Volume 646, Issue -, Pages -Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/202039952
Keywords
binaries: close; stars: winds, outflows; stars: individual: AT 2019zhd; stars: individual: M31-LRN2015; stars: individual: M31-RV; stars: individual: V838 Mon
Categories
Funding
- Royal Society - Science Foundation Ireland University Research Fellowship
- Villum FONDEN [13261, 28021]
- Independent Research Fund Denmark [8021-00170B]
- STFC [ST/P000312/1, ST/T000198/1, ST/S006109/1]
- State Research Agency (AEI) of the Spanish Ministry of Science, Innovation and Universities (MCIU)
- European Regional Development Fund (FEDER) [AYA2017-83383-P]
- Spanish Ministry of Science, Innovation and Universities [P/308614]
- General Budgets of the Autonomous Community of the Canary Islands by the Ministry of Economy, Industry, Trade and Knowledge
- UK Science and Technology Facilities Council
- NASA [NN12AR55G, 80NSSC18K0284, 80NSSC18K1575]
- National Aeronautics and Space Administration [NNX08AR22G]
- National Science Foundation [AST-1238877, AST-1440341]
- UKRI Science and Technology Facilities Council
- University of Edinburgh [ST/N002512/1]
- Queen's University Belfast within the LSST:UK Science Consortium [ST/N002520/1]
- Caltech
- IPAC
- Weizmann Institute for Science
- Oskar Klein Center at Stockholm University
- University of Maryland
- University of Washington
- Deutsches Elektronen-Synchrotron and Humboldt University
- Los Alamos National Laboratories
- TANGO Consortium of Taiwan
- University of Wisconsin at Milwaukee
- Lawrence Berkeley National Laboratories
- Science and Technology Facilities Council [ST/T000198/1, ST/S006109/1, ST/P000312/1] Funding Source: researchfish
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AT 2019zhd is the third luminous red nova event observed in M 31, showing a slow luminosity rise over about five months before a significant brightening and peak, followed by a short-duration plateau in the red bands and a rapid linear decline. The latest spectra display a red continuum resembling that of M-type stars.
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.
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