Spectral models for binary products: Unifying subdwarfs and Wolf-Rayet stars as a sequence of stripped-envelope stars

Abstract

Stars stripped of their hydrogen-rich envelope through interaction with a binary companion are generally not considered when accounting for ionizing radiation from stellar populations, despite the expectation that stripped stars emit hard ionizing radiation, form frequently and live 10 − 100 times longer than single massive stars. We compute the first grid of evolutionary and spectral models specially made for stars stripped in binaries for a range of progenitor masses (2-20 M ) and metallicities ranging from solar to values representative for pop II stars. For stripped stars with masses in the range 0.3-7 M , we find consistently high effective temperatures (20 000-100 000 K, increasing with mass), small radii (0.2-1 R ) and high bolometric luminosities, comparable to that of their progenitor before stripping. The spectra show a continuous sequence that naturally bridge subdwarf-type stars at the low mass end and Wolf-Rayet like spectra at the high mass end. For intermediate masses we find hybrid spectral classes showing a mixture of absorption and emission lines. These appear for stars with mass loss rates of 10 −8 − 10 −6 M yr −1 , which have semi-transparent atmospheres. At low metallicity, substantial hydrogen-rich layers are left at the surface and we predict spectra that resemble O-type stars instead. We obtain spectra undistinguishable from subdwarfs for stripped stars with masses up to 1.7 M , which questions whether the widely adopted canonical value of 0.47 M is uniformly valid. Only a handful of stripped stars of intermediate mass have currently been identified observationally. Increasing this sample will provide necessary tests for the physics of interaction, internal mixing and stellar winds. We use our model spectra to investigate the feasibility to detect stripped stars next to an optically bright companion and recommend systematic searches for their UV excess and possible emission lines, most notably HeII λ4686 in the optical and HeII λ1640 in the UV. Our models are publicly available for further investigations or inclusion in spectral synthesis simulations.

Analysis of absorption lines in the high-resolution spectra of five hot post-AGB candidates

“…LSE 163 then would be the result of interaction in a binary system where it gained mass and angular momentum from a massive companion that exploded (see Renzo et al 2019, and comments in the LSE 45 paragraph), although in this scenario it is difficult to explain the high temperature at the surface. Alternatively, we may consider LSE 163 being a mildly stripped star, following the recent work by Götberg et al (2018). These authors have presented models of stripped stars produced by the interaction in a binary systems.…”

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

“…In the cases studied here, mass is primarily removed by Rochelobe overflow and not by the stellar winds, so we do not expect a large impact on our results. However, we stress that the mass-loss rates for stripped stars are uncertain (Götberg et al 2017(Götberg et al , 2018Vink 2017;Gilkis et al 2019). This should not have a large effect on our solar metallicity models, except perhaps for the highestmass models in our grid.…”

“…For our high-metallicity models we have verified that the exact choice of the initial orbital period and companion mass has very little effect on the maximum radius of stripped stars if varied within reasonable limits (cf. Götberg et al 2017Götberg et al , 2018. At low metallicity, Yoon et al (2010Yoon et al ( , 2017, Claeys et al (2011), andOuchi &Maeda (2017) show that the amount of hydrogen left at the surface of the donor star is a function of the initial orbital separation.…”

“…Not only are they compact, with typical sizes of about 1 R , but most of their radiation is emitted in the extreme ultraviolet. Their companions typically outshine them in optical wavelengths (Götberg et al 2017(Götberg et al , 2018. However, this is no longer the case for the later evolutionary phases on which this work focuses.…”

Multiplicity of the red supergiant population in the young massive cluster NGC 330

“…Massive main-sequence stars with companions of intermediate separation and periods (between around 10 up to 1500 days; Podsiadlowski et al 1992;Sana et al 2012) interact as the primary evolves off the main sequence and begins to expand dramatically. Binary stellar evolutionary models predict that interacting companions shorten the RSG lifetime by a factor of three at solar metallicities and even more at lower metallicities, resulting in hot massive stars (Eldridge et al 2008;Götberg et al 2018), that is, fewer RSGs.…”