Catching a star before explosion: the luminous blue variable progenitor of SN 2015bh

Abstract

In this paper we analyse the pre-explosion spectrum of SN2015bh by performing radiative transfer simulations using the CMFGEN code. This object has attracted significant attention due to its remarkable similarity to SN2009ip in both its pre- and post-explosion behaviour. They seem to belong to a class of events for which the fate as a genuine core-collapse supernova or a non-terminal explosion is still under debate. Our CMFGEN models suggest that the progenitor of SN2015bh had an effective temperature between 8700 and 10 000 K, had a luminosity in the range ≃1.8−4.74 × 106 L⊙, contained at least 25% H in mass at the surface, and had half-solar Fe abundances. The results also show that the progenitor of SN2015bh generated an extended wind with a mass-loss rate of ≃6 × 10−4 to 1.5 × 10−3 M⊙ yr−1 and a velocity of 1000km s−1. We determined that the wind extended to at least 2.57 × 1014 cm and lasted for at least 30 days prior to the observations, releasing 5 × 10−5 M⊙ into the circumstellar medium. In analogy to 2009ip, we propose that this is the material that the explosive ejecta could interact at late epochs, perhaps producing observable signatures that can be probed with future observations. We conclude that the progenitor of SN2015bh was most likely a warm luminous blue variable of at least 35 M⊙ before the explosion. Considering the high wind velocity, we cannot exclude the possibility that the progenitor was a Wolf–Rayet (WR) star that inflated just before the 2013 eruption, similar to HD5980 during its 1994 episode. If the star survived, late-time spectroscopy may reveal either a similar luminous blue variable (LBV) or a WR star, depending on the mass of the H envelope before the explosion. If the star exploded as a genuine supernova (SN), 2015bh would be a remarkable case of a successful explosion after black hole formation in a star with a possible minimum mass 35 M⊙ at the pre-SN stage.