On the nature of candidate luminous blue variables in M 33

Luminous blue variables (LBVs) are a class of highly unstable stars that have been proposed to play a critical role in massive stellar evolution as well as being the progenitors of some of the most luminous supernovae known. However the physical processes underlying their characteristic instabilities are curren tly unknown. In order to provide observational constraints on this behaviour we have initiated a pilot study of the population of (candidate) LBVs in the Local Group galaxy M33. To accomplish this we have obtained new spectra of 18 examples within M33. These provide a baseline of≥4 yr with respect to previous observations, which is well suited to identifying LBV outbursts. We also employed existing multi-epoch optical and mid-IR surveys of M33 to further constrain the variability of the sample and search for the pr esence of dusty ejecta. Combining the datasets reveals that spectroscopic and photometric variability appears common, although in the majority of cases further observations will be needed to distinguish between an origin for this behavour in short lived stochasti c wind structure and low level photospheric pulsations or coherent long term LBV excursions. Of the known LBVs we report a hitherto unidentified excursion of M33 Var C between 2001-5, while the transi tion of the WNLh star B517 to a cooler B supergiant phase between 1993-2010 implies an LBV classification. Proof-of-concept quan titative model atmosphere analysis is provided for Romano’s star; th e resultant stellar parameters being consistent with the fin ding that the LBV excursions of this star are accompanied by changes in bolometric luminosity. The combination of temperature and luminosity of two stars, the BHG [HS80] 110A and the cool hypergiant B324, appear to be in violation of the empirical Humphreys-Davidson limit. Mid-IR observations demonstrate that a number of candidates appear associated with hot circumstellar dust, alt hough no objects as extreme asη Car are identified. The combined dataset suggests that the cr iteria employed to identify candidate LBVs results in a heterogeneous sample, also containing stars demonstrating the B[e] phenomenon. Of these, a subset of optically faint, low luminosity stars associated with hot dust are of particular interest si nce they appear similar to the likely progenitor of SN 2008S and the 2008 NGC300 transient (albeit suffering less intrinsic extinction). The results of such a mult iwavelength observational approach, employing multiplexing spectrographs and supplemented with quantitative model atmosphere analysis, appears to show considerable promise in both identifying and characterising the physical properti es of LBVs as well as other short lived phases of massive stellar evolution.