On the Gaia DR2 distances for Galactic luminous blue variables

Abstract

<jats:title>ABSTRACT</jats:title> <jats:p>We examine parallaxes and distances for Galactic luminous blue variables (LBVs) in the Gaia second data release (DR2). The sample includes 11 LBVs and 14 LBV candidates. For about half of the sample, DR2 distances are either similar to commonly adopted literature values, or the DR2 values have large uncertainties. For the rest, reliable DR2 distances differ significantly from values in the literature, and in most cases the Gaia DR2 distance is smaller. Two key results are that the S Doradus instability strip may not be as clearly defined as previously thought, and that there exists a population of LBVs at relatively low luminosities. LBVs seem to occupy a wide swath from the end of the main sequence at the blue edge to ∼8000 K at the red side, with a spread in luminosity reaching as low as log(L/L⊙) ≈ 4.5. The lower-luminosity group corresponds to effective single-star initial masses of 10–25 M⊙, and includes objects that have been considered as confirmed LBVs. We discuss implications for LBVs including (1) their instability and origin in binary evolution, (2) connections to some supernova (SN) impostors such as the class of SN 2008S-like objects, and (3) LBVs that may be progenitors of SNe with dense circumstellar material across a wide initial mass range. Although some of the Gaia DR2 distances for LBVs have large uncertainty, this represents the most direct and consistent set of Galactic LBV distance estimates available in the literature.</jats:p>

Centrifugally driven mass-loss and outbursts of massive stars

“…Since our models do not rely on super-Eddington mass-loss, these low-luminosity LBVs could naturally be explained by the centrifugal mass-loss processes we have investigated. Smith et al (2019) also showed that some LBVs lie to the left of the classical S Doradus instability strip, a feature also seen in many of our models (e.g. Figs 7 and 11).…”

“…Hence, we posit that S Doradus variations and outbursts from some LBV stars are driven by centrifugal mass-loss, in agreement with the apparent rapid rotation of some LBVs (Groh et al 2006(Groh et al , 2009. Our prediction of extreme centrifugal mass-loss extending down to masses of ∼ 20 M is similar to the lower end of LBVs observed in the Milky Way (Smith et al 2019). However, the absence of such low-luminosity LBVs in the Magellanic clouds suggest metallicity may also be an important factor in driving LBV outbursts, perhaps due to a radiation-centrifugal instability (Section 4).…”

“…5. In light of the lower masses and luminosities recently measured for some LBVs (Smith & Tombleson 2015;Smith 2019;Smith et al 2019), the instability region should be revised downward. Since our models do not rely on super-Eddington mass-loss, these low-luminosity LBVs could naturally be explained by the centrifugal mass-loss processes we have investigated.…”

The changing-type SN 2014C may come from an 11-M⊙ star stripped by binary interaction and violent eruption

“…Accumulating evidence, however, shows that LBVs may have much more heterogenous origins than previously thought (e.g. Smith, et al 2019). Despite their importance, the diverse mass-loss processes are still a major uncertainty in understanding the evolution of massive stars.…”

The Blue Supergiant Progenitor of the Supernova Imposter AT 2019krl

“…Although the masses and luminosities estimated above for AT 2019krl, using the modest extinction of A V = 0.4 mag, are significantly lower than those traditionally associated with LBVs , only an additional 1-1.5 mag of extinction could easily push AT 2019krl's progenitor to higher masses, as shown in Figure 7. Moreover, recent studies with revised distances have shown that Milky Way LBVs extend to lower initial masses and luminosities than previously thought (Smith et al 2019).…”