The evolution of luminous red nova AT 2017jfs in NGC 4470

Abstract

We present the results of our photometric and spectroscopic follow-up of the intermediate-luminosity optical transient <ASTROBJ>AT 2017jfs</ASTROBJ>. At peak, the object reaches an absolute magnitude of M<SUB>g</SUB> = -15.46 ± 0.15 mag and a bolometric luminosity of 5.5 × 10<SUP>41</SUP> erg s<SUP>-1</SUP>. Its light curve has the double-peak shape typical of luminous red novae (LRNe), with a narrow first peak bright in the blue bands, while the second peak is longer-lasting and more luminous in the red and near-infrared (NIR) bands. During the first peak, the spectrum shows a blue continuum with narrow emission lines of H and Fe II. During the second peak, the spectrum becomes cooler, resembling that of a K-type star, and the emission lines are replaced by a forest of narrow lines in absorption. About 5 months later, while the optical light curves are characterized by a fast linear decline, the NIR ones show a moderate rebrightening, observed until the transient disappears in solar conjunction. At these late epochs, the spectrum becomes reminiscent of that of M-type stars, with prominent molecular absorption bands. The late-time properties suggest the formation of some dust in the expanding common envelope or an IR echo from foreground pre-existing dust. We propose that the object is a common-envelope transient, possibly the outcome of a merging event in a massive binary, similar to <ASTROBJ>NGC 4490-2011OT1</ASTROBJ>. <P />Table A.1 is 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/qcat?J/A+A/625/L8">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/625/L8</A>