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

Abstract

We present the follow-up campaign of the luminous red nova (LRN) <ASTROBJ>AT 2019zhd</ASTROBJ>, the third event of this class observed in <ASTROBJ>M 31</ASTROBJ>. 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<SUB>r</SUB> = -2.8 ± 0.2 mag to M<SUB>r</SUB> = -5.6 ± 0.1 mag. Then, over a four to five day period, <ASTROBJ>AT 2019zhd</ASTROBJ> experienced a major brightening, reaching a peak of M<SUB>r</SUB> = -9.61 ± 0.08 mag and an optical luminosity of 1.4 × 10<SUP>39</SUP> erg s<SUP>-1</SUP>. 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<SUB>eff</SUB> ≈ 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 <ASTROBJ>V1309 Sco</ASTROBJ>, 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<SUB>F555W</SUB> = 0.21 ± 0.14 mag, with F555W - F814W = 2.96 ± 0.12 mag) at the position of <ASTROBJ>AT 2019zhd</ASTROBJ>, which is the most likely quiescent precursor. The source is consistent with expectations for a binary system including a predominant M5-type star. <P />Tables A.1 and A.2 are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/646/A119">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/646/A119</A>...