Catching a star before explosion: the luminous blue variable progenitor
  of SN 2015bh
  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 10000 K, luminosity in
the range ~ 1.8-4.74e6 Lsun, contained at least 25% H in mass at the surface,
and half-solar Fe abundances. The results also show that the progenitor of SN
2015bh generated an extended wind with a mass-loss rate of ~ 6e-4 to 1.5e-3
Msun/yr and a velocity of 1000 km/s. We determined that the wind extended to at
least 2.57e14 cm and lasted for at least 30 days prior to the observations,
releasing 5e-5 Msun 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 SN 2015bh was most
likely a warm luminous blue variable of at least 35 Msun before the explosion.
Considering the high wind velocity, we cannot exclude the possibility that the
progenitor was a Wolf-Rayet 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 LBV or a Wolf-Rayet star, depending on
the mass of the H envelope before the explosion. If the star exploded as a
genuine SN, 2015bh would be a remarkable case of a successful explosion after
black-hole formation in a star with a possible minimum mass 35 Msun at the
pre-SN stage.
