The isolation of luminous blue variables: on subdividing the sample

Abstract

A debate has arisen concerning the fundamental nature of luminous blue variables (LBVs) and their role in stellar evolution. While Smith & Tombleson proposed that their isolated environments indicate that LBVs must be largely the product of binary evolution, Humphreys et al. have recently expressed the view that the traditional single-star view still holds if one appropriately selects a subsample of LBVs. This paper finds the claim of Humphreys et al. to be quantitatively unjustified. A statistical test of 'candidate' as opposed to 'confirmed' LBVs shows no significant difference (<1 sigma) between their environments. Even if the sample is further subdivided as proposed, the three most luminous LBVs are spatially dispersed similar to late O-type dwarfs, which have much longer median lifetimes than expected for classical LBVs. The lower luminosity LBVs have a distribution associated with red supergiants (RSGs), but these RSGs are dominated by stars of 10-15 M-circle dot initial mass, with much longer lifetimes than expected for those lower luminosity LBVs. If one's view is restricted to the highest luminosity LBVs, then the appropriate comparison is with early O-type stars that are their presumed progenitors; when this is done, it is clear that even the high-luminosity LBVs are more dispersed than expected. Humphreys et al. also suggest that velocities of LBVs support the single-star view, being inconsistent with runaways. A quantitative analysis of the radial velocity distribution of LBVS in M31 and M33 contradicts this; modest runway speeds expected from mass gainers in binary evolution are consistent with the observed velocities, although the data lack the precision to discriminate.