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 straigthforward. Here we present observations of a luminous blue variable (LBV) in NGC 2770 in outburst over more than 20 years that experienced a possible terminal explosion as type IIn SN in 2015, named SN 2015bh. This possible SN or "main event" was preceded by a precursor peaking $\sim$ 40 days before maximum. The total energy release of the main event is $\sim$1.8$\times$10$^{49}$ erg, which can be modeled by a $<$ 0.5 M$_\odot$ shell plunging into a dense CSM. All 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 this second component with the possible SN. Since 1994 the star has been redder than during a typical S-Dor like outburst. SN 2015bh lies within a spiral arm of NGC 2770 next to a number of small star-forming regions with a metallicity of $\sim$ 0.5 solar and a stellar population age of 7-10 Myr. SN 2015bh shares many similarities with SN 2009ip, which, together with other examples may form a new class of objects that exhibit outbursts a few decades prior to "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 a very short timescale of only a few years. The final fate of these types of massive stars can only be determined with observations years after the possible SN.
Highlights
Massive stars towards the end of their short life-span are unstable and can lose large amounts of mass in the form of eruptions
Luminous blue variables (LBVs) are a type of very massive star showing different types of variabilities: During S-Dor outbursts, which occur over time scales from months to decades, the star drops in temperature while staying constant or decreasing in luminosity (Groh et al 2009) and goes back to its original state
We monitored the progenitor of SN 2015bh/iPTF13evf/SNhunt 275 over more than 20 yr from a phase of LBV-like eruptions to a possible terminal explosion or hyper eruption in May 2015
Summary
Massive stars towards the end of their short life-span are unstable and can lose large amounts of mass in the form of eruptions. Luminous blue variables (LBVs) are a type of very massive star showing different types of variabilities: During S-Dor outbursts, which occur over time scales from months to decades, the star drops in temperature while staying constant or decreasing in luminosity (Groh et al 2009) and goes back to its original state. We present an extensive dataset of an LBV star in the galaxy NGC 2770 that has possibly exploded after experiencing multiple outbursts over the last 21 yr. For NGC 2770 we apply a luminosity distance of 27 Mpc corresponding to a distance modulus of 32.4 mag
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