Probing the Nature of LBVs in M31 and M33: Blasts from the Past

Luminous Blue Variables {LBVs} are a short-lived, but critical stage in the evolution of the most massive stars. Episodic outbursts during the LBV phase may provide the dominant mass-loss mechanism for evolution to the Wolf-Rayet stage. However, these large mass-loss outbursts {accompanied by large changes in the visual magnitude} take place on timescales of order 1000 years or so: the archetypical LBVs P Cyg and Eta Car had their last major outbursts in the 17th and 19th centuries, respectively. Were these stars located in nearby galaxies would we know of them today? Only six LBVs have been confirmed in M31 and M33 through detection of outbursts, although 175 stars have now been identified as LBV "candidates" in these galaxies. These LBV candidates are spectroscopically indistinguishable from the known LBVs, but no large-scale {>2 mag} photometric outbursts have been found, although many of the candidates do show smaller photometric variability and/or spectroscopic variability. Rather than wait 1000 years for an outburst, we instead here propose to look for signatures of past outbursts in the form of ejecta nebulae close to the stars. The high spatial resolution of STIS will be sensitive to nebulae with radii larger than 0.3-0.4 pc, which corresponds to ages of >500-1000 yrs. These data will allow us to determine the frequency of ejecta among our LBV candidates compared to those of the known LBVs, and determine the physical characteristics of these past mass-loss events. Ultimately, this will help us constrain the lifetime and total mass lost in the LBV phase.