IRAS 18357-0604 - an analogue of the galactic yellow hypergiant IRC +10420?
Yellow hypergiants represent a short-lived evolutionary episode experienced by massive stars as they transit to and from a red supergiant phase. As such, their properties provide a critical test of stellar evolutionary theory. The galactic yellow hypergiant IRC +10420 is a cornerstone system for understanding this phase, since it is the strongest post-RSG candidate known, has demonstrated real-time evolution across the Hertzsprung-Russell diagram and been subject to extensive mass loss. Here we report on the discovery of a twin of IRC +10420 - IRAS 18357-0604. Optical and near-IR spectroscopy are used to investigate the physical properties of IRAS 18357-0604, while IR photometry probes the nature of its circumstellar environment. Pronounced spectral similarities between IRAS 18357-0604 and IRC +10420 suggest comparable temperatures and wind geometries, while photometric data reveals a similarly dusty circumstellar environment, although historical mass loss appears to have been heavier in IRC +10420. The systemic velocity implies a distance compatible with the red supergiant dominated complex at the base of the Scutum Crux arm; the resultant luminosity determination is consistent with a physical association but suggests a lower initial mass than inferred for IRC +10420 (<20Msun versus ~40Msun). Evolutionary predictions for the physical properties of supernova progenitors derived from ~18-20Msun stars - or ~12-15Msun stars that have experienced enhanced mass loss as red supergiants - compare favourably with those of IRAS 18357-0604, which in turn appears to be similar to the the progenitor of SN2011dh; it may therefore provide an important insight into the nature of the apparently H-depleted yellow hypergiant progenitors of some type IIb SNe.