The expansion of stripped-envelope stars: Consequences for supernovae and gravitational-wave progenitors

Abstract

Massive binaries that merge as compact objects are the progenitors of gravitational-wave sources. Most of these binaries experience one or more phases of mass transfer, during which one of the stars loses part or all of its outer envelope and becomes a strippedenvelope star. The evolution of the size of these stripped stars is crucial in determining whether they experience further interactions and their final fate. We present new calculations of stripped-envelope stars based on binary evolution models computed with MESA. We use these to investigate their radius evolution as a function of mass and metallicity. We further discuss their pre-supernova observable characteristics and potential consequences of their evolution on the properties of supernovae from stripped stars. At high metallicity we find that practically all of the hydrogen-rich envelope is removed, in agreement with earlier findings. Only progenitors with initial masses below 10 M expand to large radii (up to 100 R ), while more massive progenitors stay compact. At low metallicity, a substantial amount of hydrogen remains and the progenitors can, in principle, expand to giant sizes (> 400 R ), for all masses we consider. This implies that they can fill their Roche lobe anew. We show that the prescriptions commonly used in population synthesis models underestimate the stellar radii by up to two orders of magnitude. We expect that this has consequences for the predictions for gravitational-wave sources from double neutron star mergers, in particular for their metallicity dependence.

SN2019dge: A Helium-rich Ultra-stripped Envelope Supernova

“…However, the large inferred radius of the progenitor requires an orbital separation a  200 R e , implying an orbital period  P 100 d orb at the time of explosion, depending on the companion mass. We note that the large inferred radius of the progenitor is near the maximum photospheric radius reached by low-mass M  3 M e helium stars (Kleiser et al 2018a;Woosley 2019;Laplace et al 2020), potentially challenging the assertion that case BB mass transfer has occurred. However, exploding stars without case BB mass loss retain significantly larger ejecta masses than our constraints forSN2019dge (Tauris et al 2015;Woosley 2019), producing light curves that rise and fade too slowly (Kleiser et al 2018a).…”

“…So, the nature of the companion star remains unclear, and both main-sequence stars and compact objects are possible. What is clear is that case BB mass transfer was stable in this system, because the post-common envelope orbital separation of unstable case BB mass transfer would be much smaller than the inferred radius of the progenitor ofSN2019dge (Laplace et al 2020).…”

“…The kinetic energy is then calculated to be (see Equations (B2) and (B3)), and hence we have an exponential heating function We derive t d ≈15.4 days, which implies (  R 170 ) corroborates that the progenitor of SN2019dge is a star with an extended envelope. Indeed, stellar evolution models predict envelope radii of 10-200 R e for helium stars with zero-age helium core masses within 2.5-3.2 M e that have stripped all of the hydrogen-rich envelope (Woosley 2019;Laplace et al 2020). Therefore, the early-time shock cooling light curve serves as strong evidence that SN2019dge is the explosion of a star with inflated radius (not a compact object).…”

Supernovae Ib and Ic from the explosion of helium stars

“…not all of the hydrogen envelope (Gilkis et al 2019). However, for the metallicity and mass range considered here, the assumption that the ensuing strong stellar wind blows off any remaining hydrogen within a short time remains a good approximation (Laplace et al 2020). Notably, we did not consider the lowest mass helium stars that could still produce SNe despite their low mass, and that may (Yoon et al 2010) or may not (Antoniadis et al 2020) retain their hydrogen.…”

“…Our models were evolved at solar metallicity. Metallicity might play a role, though probably secondary, in the evolution of binaries, and thus in the formation of stripped-envelope stars (Gilkis et al 2019;Laplace et al 2020;Shenar et al 2020). However, metallicity will probably influence primarily the WR mass loss rate so that higher metallicities will favor the production of SNe Ic relative to Ib (Georgy et al 2009).…”