Modelling luminous-blue-variable isolation

Abstract

Observations show that luminous blue variables (LBVs) are far more dispersed than massive O-type stars, and Smith & Tombleson suggested that these large separations are inconsistent with a single-star evolution model of LBVs. Instead, they suggested that the large distances are most consistent with binary evolution scenarios. To test these suggestions, we modelled young stellar clusters and their passive dissolution, and we find that, indeed, the standard single-star evolution model is mostly inconsistent with the observed LBV environments. If LBVs are single stars, then the lifetimes inferred from their luminosity and mass are far too short to be consistent with their extreme isolation. This implies that there is either an inconsistency in the luminosityto-mass mapping or the mass-to-age mapping. In this paper, we explore binary solutions that modify the mass-to-age mapping and are consistent with the isolation of LBVs. For the binary scenarios, our crude models suggest that LBVs are rejuvenated stars. They are either the result of mergers or they are mass gainers and received a kick when the primary star exploded. In the merger scenario, if the primary is about 19 M ⊙ , then the binary has enough time to wander far afield, merge and form a rejuvenated star. In the mass-gainer and kick scenario, we find that LBV isolation is consistent with a wide range of kick velocities, anywhere from 0 to ∼ 105 km/s. In either scenario, binarity seems to play a major role in the isolation of LBVs.

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

“…same pattern of being highly concentrated in clusters. For early O-type stars outside 30 Dor, the median separation is 5-6 pc and mid and late O stars somewhat larger, still in good agreement with expectations from models of cluster dispersal with age (Aghakhanloo et al 2017), and in good agreement with general expectations for O stars residing in clusters. If spectroscopically confirmed O stars are highly clustered as expected both inside and outside 30 Dor, why, then, does the BBS sample have such a large median separation of 31 pc?…”

“…Overall, LBV environments match quite well expectations for binary evolution, where LBVs are massive blue stragglers produced either by mass accretion from companion and possible kicks, or by rejuvenation in stellar mergers (Smith & Tombleson 2015;Smith 2016;Aghakhanloo et al 2017). If one is willing to accept the notion that close binaries are so common that they may dominate the evolutionary paths of massive stars (Sana et al 2012;de Mink et al 2014;Moe & Di Stefano 2017;Eldridge et al 2017), then this result is not so surprising.…”

“…Thus, whatever the incompleteness may be, the spectroscopic O star sample behaves as expected and is not strongly affected by incompleteness in terms of its overall spatial distribition. On the other hand, Aghakhanloo et al (2017) calculate that a median separation of ∼30 pc corresponds to post-main-sequence ages of around 10 Myr and initial masses of ∼20 M⊙. According to the observed median separation of the BBS sample of 31 pc, one would conjecture that the BBS sample is dominated by evolved ∼20 M⊙ stars on average, not the most massive unevolved stars of 40-100 M⊙.…”

“…If the comparison sample is not highly clustered, then this logic dissolves. The gradual dispersal of clusters accompanied by removal of the most massive stars as they die was modeled quantitatively by Aghakhanloo et al (2017), who calculated values for the expected median separation and separation distributions of such samples. In these models, the most massive unevolved stars should have a typical separation from the nearest other O star of only a few pc, which again, is found to be in quite good agreement with the observations of spectroscopically confirmed early O-type stars.…”

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.…”