Wind-envelope interaction as the origin of the slow cyclic brightness variations of luminous blue variables

Abstract

Luminous blue variables (LBVs) are hot, very luminous massive stars displaying large quasi-periodic variations in brightness, radius, and photospheric temperature on timescales of years to decades. The physical origin of this variability, called S Doradus cycle after its prototype, has remained elusive. We study the feedback of stellar wind mass-loss on the envelope structure in stars near the Eddington limit. We calculated a time-dependent hydrodynamic stellar evolution, applying a stellar wind mass-loss prescription with a temperature dependence inspired by the predicted systematic increase in mass-loss rates below 25 kK. We find that when the wind mass-loss rate crosses a well-defined threshold, a discontinuous change in the wind base conditions leads to a restructuring of the stellar envelope. The induced drastic radius and temperature changes, which occur on the thermal timescale of the inflated envelope, in turn impose mass-loss variations that reverse the initial changes, leading to a cycle that lacks a stationary equilibrium configuration. Our proof-of-concept model broadly reproduces the typical observational phenomenology of the S Doradus variability. We identify three key physical ingredients that are required to trigger the instability: inflated envelopes in close proximity to the Eddington limit, a temperature range where decreasing opacities do not lead to an accelerating outflow, and a mass-loss rate that increases with decreasing temperature, crossing a critical threshold value within this temperature range. Our scenario and model provide testable predictions, and open the door for a consistent theoretical treatment of the LBV phase in stellar evolution, with consequences for their further evolution as single stars or in binary systems.

Stellar winds and metal enrichment from fast-rotating Population III stars

“…Regardless of rotation, some other effects may be responsible for mass loss episodes during the evolution of massive stars, rather independent of metallicity, and thus occuring even in Pop III or very metal poor stars. An example may be the mechanism responsible for the strong mass loss experienced by Luminous Blue Variables (Smith & Owocki 2006;Zhao & Fuller 2020;Grassitelli et al 2021). Pulsation, either induced when the star is in a red supergiant stage (Yoon & Cantiello 2010), or in a blue supergiant phase (Saio et al 2013), may be linked to violent mass loss episodes.…”

“…To evaluate whether Pop III winds can make a difference in primordial metal enrichment, we further con- sider still more extreme cases where winds go deeper into the He core. Such strong mass loss may occur from pre-SN pulsations and instabilities (Smith & Owocki 2006;Woosley et al 2007;Yoon & Cantiello 2010;Fuller 2017;Fuller & Ro 2018;Leung & Fuller 2020;Zhao & Fuller 2020;Wu & Fuller 2021;Grassitelli et al 2021). We adopt as the maximum amount that can be lost by winds the mass above the incipient stellar remnant.…”

The contribution by luminous blue variable stars to the dust content of the Magellanic Clouds

“…Although the physical mechanisms underlying these instabilities are possibly different (e.g. the vicinity to the Eddington limit, sub-photospheric instabilities, bistability jump, envelope inflation, wind-envelope interaction, fast rotation, binarity, stellar merger; Humphreys & Davidson 1994;Gallagher 1989; Vink et al 1999;Groh et al 2009;Gräfener et al 2012;Portegies Zwart & van den Heuvel 2016;Owocki et al 2017;Grassitelli et al 2021), observations suggest increases in the mass-loss rates by about a factor of three in S Doradus-type variability (e.g. Vink & de Koter 2002) or by several orders of magnitude in giant eruptions, like that of η Carinae (e.g.…”

Modeling the Remnants of Core-collapse Supernovae from Luminous Blue Variable stars

“…Although we still do not fully understand the physical mechanism that drives LBV variability, some progress has been made in the last years. Recently Grassitelli et al (2021) have developed a model that reproduces the typical observational phenomenology of the S Doradus variability. According to their model, the instability responsible for the observed variability can be triggered when some physical conditions are met, involving inflated envelopes in proximity to the Eddington limit, a temperature range that does not lead to accelerating outflows, and a mass-loss rate that increases with decreasing temperature (see Grassitelli et al 2021, for more details).…”

“…Recently Grassitelli et al (2021) have developed a model that reproduces the typical observational phenomenology of the S Doradus variability. According to their model, the instability responsible for the observed variability can be triggered when some physical conditions are met, involving inflated envelopes in proximity to the Eddington limit, a temperature range that does not lead to accelerating outflows, and a mass-loss rate that increases with decreasing temperature (see Grassitelli et al 2021, for more details). The causes of the sporadic and violent massloss events, however, are still poorly understood and a physical driving mechanism has not been clearly identified yet (Smith et al 2018).…”