Outbursts of luminous blue variable stars from variations in the helium opacity

Abstract

Luminous blue variables are massive, evolved stars that exhibit large variations in luminosity and size on timescales from months to years, with high associated rates of mass loss. In addition to this on-going variability, these stars exhibit outburst phases, during which their size increases and as a result their effective temperature decreases, typically to about 9,000 kelvin. Outbursts are believed to be caused by the radiation force on the cooler, more opaque, outer layers of the star balancing or even exceeding the force of gravity, although the exact mechanisms are unknown and cannot be determined using one-dimensional, spherically symmetric models of stars because such models cannot determine the physical processes that occur in this regime. Here we report three-dimensional simulations of massive, radiation-dominated stars, which show that helium opacity has an important role in triggering outbursts and setting the observed effective temperature during outbursts of about 9,000 kelvin. It probably also triggers the episodic mass loss at rates of 10 to 10 solar masses per year. The peak in helium opacity is evident in our three-dimensional simulations only because the density and temperature of the stellar envelope (the outer part of the star near the photosphere) need to be determined self-consistently with convection, which cannot be done in one-dimensional models that assume spherical symmetry. The simulations reproduce observations of long-timescale variability, and predict that convection causes irregular oscillations in the radii of the stars and variations in brightness of 10-30 per cent on a typical timescale of a few days. The amplitudes of these short-timescale variations are predicted to be even larger for cooler stars (in the outburst phase). This short-timescale variability should be observable with high-cadence observations.

Sensitivity of the lower edge of the pair-instability black hole mass gap to the treatment of time-dependent convection

“…For this grid, the convective velocity v c is obtained from MLT under the steady-state assumption, similarly to Paxton et al (2018) and Leung et al (2019), although the latter authors turn off convection during hydrodynamical phases of evolution. For this grid we employ MLT++, which is an enhancement of the convective flux in superadiabatic radiation-pressure dominated regions prone to developing density inversions (Paxton et al 2013;Jiang et al 2018) and a semiconvection efficiency of 0.01. We do not employ thermohaline mixing for numerical stability reasons.…”

Super-Eddington stellar winds driven by near-surface energy deposition

“…We acknowledge that although this is true in the models we have developed, it is possible that more realistic calculations will show that the outflow is sufficiently inhomogeneous to substantially increase the rate at which photons diffuse through the outflow (e.g., Shaviv 2001; Owocki et al 2004). Understanding this more quantitatively will ultimately require multi-dimensional radiation (magneto)-hydrodynamical simulations (e.g., Jiang et al 2015).…”

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

“…In addition to the possibility that this behaviour occurs in binary systems (certainly the case for HD 5980), two key issues stand out in the context of these unusual LBV eruptions. One is the relatively low-metallicity environ-ments, where wind mass-loss may generally be weaker, and where the potential role of the Fe opacity bump in triggering the LBV instability (Jiang et al 2018) may be diminished or may have different consequences for the amount of envelope inflation. It is quite likely that the lower metallicity may play an important role in the lack of observed temperature shifts in eruption.…”