Shedding Light on the Isolation of Luminous Blue Variables

In the traditional view of massive star evolution, luminous blue variables (LBVs) are a transitionary phase between massive O-type stars and Wolf-Rayet stars (WRs). A debate has sprouted over whether this single star evolution is flawed and perhaps a binary evolutionary track describes the LBV phase better. The root of this debate has been the question of isolation (projected angular separation) of LBVs from their nearest neighboring O-type star. LBVs, traditionally, have relatively short lifetimes, and as a result they should not disperse far from their birthplace or be isolated. A recent study found that LBVs have an isolation more similar to that of red supergiants (RSGs) than traditionally thought possible given single star evolution. A similar study, however, found the opposite result. Both of these studies used spectroscopically identified O-type stars, which for the Large Magellanic Cloud is extremely incomplete, and does not necessarily represent the high mass stars we expect to be LBV progenitors in any event. Therefore, we re-examined the question of isolation using photometric criteria to select the highest mass unevolved stars ("bright blue stars" or BBSs) to use as our comparison sample. We find that LBVs are no more isolated than BBSs or WRs, and were able to statistically rule out the possibility of LBVs coming from the same distribution as the RSGs. We also note the number of LBVs in or near OB associations is comparable to the number of BBSs or WRs, and not to that of RSGs. Therefore, we conclude that the isolation of LBVs is consistent with the traditional picture of massive single star evolution. This work was supported by the National Science Foundation through AST-1612874.