Follow-up photometry of M101 OT2015-1

The light curve showed two distinct peaks with absolute magnitudes M_r_<=-12.4 and M_r_~-12, on 2014 November 11 and 2015 February 17, respectively. The spectral energy distributions during the second maximum show a cool outburst temperature of ~3700K and low expansion velocities (~-300km/s) for the HI, CaII, BaII, and KI lines.

From archival data spanning 15-8 years before the outburst, we find a single source consistent with the optically discovered transient, which we attribute to being the progenitor; it has properties consistent with being an F-type yellow supergiant with L~8.7x10^4^L_{sun}_, T_eff_~7000K, and an estimated mass of M1=18+/-1M_{sun}_. This star has likely just finished the H-burning phase in the core, started expanding, and is now crossing the Hertzsprung gap.