SN 2015bh: NGC 2770’s 4th supernova or a luminous blue variable on its way to a Wolf-Rayet star?

¹ Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: cthoene@iaa.es
² Dark Cosmology Centre, Niels Bohr Institute, Juliane Maries Vej 30, Copenhagen Ø, 2100, Denmark
³ Department of Particle Physics & Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
⁴ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁵ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
⁶ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, 606-8502 Kyoto, Japan
⁷ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8583 Chiba, Japan
⁸ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
⁹ Millennium Institute of Astrophysics, Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹⁰ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
¹¹ Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
¹² School of Physics, Trinity College Dublin, Dublin 2, Ireland
¹³ Space Science & Engineering Division, Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510, USA
¹⁴ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
¹⁵ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Lyngby, Denmark
¹⁶ Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK
¹⁷ Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile

Received: 27 October 2016
Accepted: 6 December 2016

Abstract

Very massive stars in the final phases of their lives often show unpredictable outbursts that can mimic supernovae, so-called, “SN impostors”, but the distinction is not always straightforward. Here we present observations of a luminous blue variable (LBV) in NGC 2770 in outburst over more than 20 yr that experienced a possible terminal explosion as type IIn SN in 2015, named SN 2015bh. This possible SN (or “main event”) had a precursor peaking ~40 days before maximum. The total energy release of the main event is ~1.8 × 10⁴⁹ erg, consistent with a <0.5 M_⊙ shell plunging into a dense CSM. The emission lines show a single narrow P Cygni profile during the LBV phase and a double P Cygni profile post maximum suggesting an association of the second component with the possible SN. Since 1994 the star has been redder than an LBV in an S-Dor-like outburst. SN 2015bh lies within a spiral arm of NGC 2770 next to several small star-forming regions with a metallicity of ~0.5 solar and a stellar population age of 7–10 Myr. SN 2015bh shares many similarities with SN 2009ip and may form a new class of objects that exhibit outbursts a few decades prior to a “hyper eruption” or final core-collapse. If the star survives this event it is undoubtedly altered, and we suggest that these “zombie stars” may evolve from an LBV to a Wolf-Rayet star over the timescale of only a few years. The final fate of these stars can only be determined with observations a decade or more after the SN-like event.