The January 2015 outburst of a red nova in M 31^⋆

¹ Department of Astronomy, University of Sofia, 5 James Bourchier Blvd., 1164 Sofia, Bulgaria
e-mail: al.kurtenkov@gmail.com
² Instituto de Astrofísica de Canarias, 38200, La Laguna, Tenerife, Spain
e-mail: peter.pessev@gtc.iac.es
³ Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
e-mail: tomov@umk.pl
⁴ Special Astrophysical Observatory, Nizhnij Arkhyz, Russia
⁵ Konkoly Observatory, Hungarian Academy of Sciences, 1121 Budapest, Konkoly Thege Miklós út 15-17, Hungary
⁶ Astronomical Institute, Academy of Sciences, 251 65 Ondřejov, Czech Republic
⁷ Departamento de Astrofísica, Universidad de La Laguna, 38206, La Laguna, Tenerife, Spain
⁸ Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
⁹ Sternberg Astronomical Institute, M.V. Lomonosov Moscow State University, Universitetskii pr. 13, 119992 Moscow, Russia
¹⁰ Las Cumbres Observatory Global Telescope Network, 6740 Cortona Dr., Suite 102, Goleta, CA 93117, USA
¹¹ Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK
¹² European Space Astronomy Centre, PO Box 78, 28692 Villanueva de la Cañada, Madrid, Spain
¹³ Department of Astronomy, San Diego State University, San Diego, CA 92182, USA
¹⁴ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
¹⁵ Kazan Federal University, 420008 Kazan, Russia
¹⁶ Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, C/Can Magrans s/n, 08193 Cerdanyola del Valles, Spain

Received: 20 May 2015
Accepted: 28 May 2015

Abstract

Context. M31N 2015-01a (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 × 10⁵ 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.