The January 2015 outburst of a red nova in M 31

Abstract

Context. M31N 2015M31N -01a (or M31LRN 2015 is a red nova that erupted in January 2015 -the first event of this kind observed in M 31 since 1988. Very few similar events have been confirmed as of 2015. Most of them are considered to be products of stellar mergers. Aims. Results of an extensive optical monitoring of the transient in the period January-March 2015 are presented. Methods. Eight optical telescopes were used for imaging. Spectra were obtained on the Large Altazimuth Telescope (BTA), the Gran Telecsopio Canarias (GTC) and the Rozhen 2 m telescope. Results. We present a highly accurate 70 d light curve and astrometry with a 0.05 uncertainty. The colour indices reached a minimum of 2−3 d before peak brightness and rapidly increased afterwards. The spectral type changed from F5I to F0I in 6 d before the maximum and then to K3I in the next 30 d. The luminosity of the transient was estimated to be 8.7 +3.3 −2.2 × 10 5 L during the optical maximum. Conclusions. Both the photometric and the spectroscopic results confirm that the object is a red nova, similar to V838 Monocerotis.

Pre-explosion Spiral Mass Loss of a Binary Star Merger

“…Depending on the shock energy set by the binary parameters, the collision might not be energetic enough to keep the hydrogen in the fast expanding shell ionized and there would not be a second peak. Metzger & Pejcha (2017) suggest that this indeed might be the case for V1309 Sco and perhaps also M31 LRN 2015 (Kurtenkov et al 2015;Williams et al 2015). If the shock cannot prevent dust formation, which should occur more readily in long-period binaries with more evolved components, the forming dust will shift the peak of the spectral energy distribution to the infrared and prolong the transient duration.…”

Common Envelope Ejection for a Luminous Red Nova in M101

“…V833 Mon, at 6.1±0.6 kpc (Sparks et al 2008) is another remarkable example of a low-mass stellar merger candidate (Soker & Tylenda 2003), including a spectacular light echo revealed by observations with the Hubble Space Telescope (HST) (Bond et al 2003). Some extragalactic examples of discoveries consistent with the merger scenario 14 are M85-OT2006OT-1, the luminous red nova in M31, reported in Kurtenkov et al (2015) and Williams et al (2015), and the massive stellar merger NGC 4490 2011OT-1 (Smith et al 2016b). Preexplosion photometry of the progenitor systems has allowed us to estimate the mass and evolutionary stage of several progenitor systems.…”

The ambiguous transient ASASSN-17hx

“…Spectroscopic confirmation came one day after the announcement (Kurtenkov et al 2017a) together with its CTIO spectral classification (Williams et al 1991(Williams et al , 1994. Reports of the 17hx light curve and spectral variations filled the ATels for about 3 months (Williams & Darnley, 2017;Saito et al 2017;Berardi et al 2017;Munari et al 2017a,b,c;Pavana et al 2017;Kuin et al 2017;Kurtenkov et al 2017b;Guarro et al 2017), while, the observations continued to build a substantial data set. In an attempt to make sense of the peculiar light curve of ASASSN-17hx, we collected spectra and in particular high resolution spectra for 2 years after discovery.…”