The isolation of luminous blue variables resembles aging B-type supergiants, not the most massive unevolved stars

Abstract

Luminous blue variables (LBVs) are suprisingly isolated from the massive O-type stars that are their putative progenitors in single-star evolution, implicating LBVs as binary evolution products. Aadland et al. (A19) found that LBVs are, however, only marginally more dispersed than a photometrically selected sample of bright blue stars (BBS) in the Large Magellanic Cloud (LMC), leading them to suggest that LBV environments may not exclude a single-star origin. In both comparisons, LBVs have the same median separation, confirming that any incompleteness in the O-star sample does not fabricate LBV isolation. Instead, the relative difference arises because the photometric BBS sample is far more dispersed than known O-type stars. Evidence suggests that the large BBS separation arises because it traces less massive (∼20 M ⊙ ), aging blue supergiants. Although photometric criteria used by A19 aimed to select only the most massive unevolved stars, visual-wavelength color selection cannot avoid contamination because O and early B stars have almost the same intrinsic color. Spectral types confirm that the BBS sample contains many B supergiants. Moreover, the observed BBS separation distribution matches that of spectroscopically confirmed early B supergiants, not O-type stars, and matches predictions for a ∼10 Myr population, not a 3-4 Myr population. A broader implication for ages of stellar populations is that bright blue stars are not a good tracer of the youngest massive O-type stars. Bright blue stars in nearby galaxies (and unresolved blue light in distant galaxies) generally trace evolved blue supergiants akin to SN 1987A's progenitor.

A new and unusual LBV-like outburst from a Wolf–Rayet star in the outskirts of M33

“…The crowded region is also interesting from the perspective that it is actually quite unusual for LBVs; most LBVs appear to shy away from crowded regions with clusters of O-type stars (Smith & Tombleson 2015). This is thought to be the result of binary evolution, either through kicks from a companion's SN explosion or by rejuventation 5 (Aghakhanloo et al 2017;Smith & Tombleson 2015;Smith 2016Smith , 2019. Interestingly, Smith & Tombleson (2015) also found that WC stars were more isolated than expected from single-star evolution.…”

Candidate LBV stars in galaxy NGC 7793 found via HST photometry + MUSE spectroscopy

“…6.1 Isolation of cLBVs Smith (2019) argued that LBVs are isolated from locations of star formation. As explained in the introduction, this would favor a binary formation scenario for LBVs.…”

“…In this scenario, stars evolve on times scales of ≤ 3 − 4 Myr 4 and consequently do not have time to move far from their birthplaces. Thus, one would expect to find LBVs near young O-type MS stars, which observationally, are stars that appear to be clustered (Smith 2019). However, LMC observations show that LBVs are more spatially dispersed than O-type MS stars, which has led Smith & Tombleson (2015) and Smith (2019) to argue that LBVs are likely the products of close binary evolution.…”

“…Thus, one would expect to find LBVs near young O-type MS stars, which observationally, are stars that appear to be clustered (Smith 2019). However, LMC observations show that LBVs are more spatially dispersed than O-type MS stars, which has led Smith & Tombleson (2015) and Smith (2019) to argue that LBVs are likely the products of close binary evolution. This is because the larger spatial dispersion of LBVs can be explained by either SN kicks which move the LBVs far from their birthplace, or rejuvenation via mass transfer and mergers, which produce older massive stars at a time when the single O-type stars have disappeared.…”

“…We combine LEGUS and Hα LEGUS photometry with MUSE spectroscopy to search for cLBVs in NGC 7793W. Candidate LBVs are defined by Smith (2019) as stars with similar luminosities and spectra to LBVs, often with circumstellar shells that indicate a prior outburst, but which have not yet been observed photometrically to undergo LBV eruptions.…”