Contribution from stars stripped in binaries to cosmic reionization of hydrogen and helium

Abstract

Massive stars are often found in binary systems and it has been argued that binary products boost the ionizing radiation of stellar populations. Accurate predictions for binary products are needed to understand and quantify their contribution to Cosmic Reionization. We investigate the contribution of stars stripped in binaries since (1) they are, arguably, the best-understood products of binary evolution, (2) we recently produced the first non-LTE radiative transfer calculations for the atmospheres of these stripped stars that predict their ionizing spectra, and (3) they are very promising sources since they boost the ionizing emission of stellar populations at late times. This allows stellar feedback to clear the surroundings such that a higher fraction of their photons can escape and ionize the intergalactic medium. Combining our detailed predictions for the ionizing spectra with a simple cosmic reionization model, we estimate that stripped stars contributed tens of percent of the photons that caused cosmic reionization of hydrogen, depending on the assumed escape fractions. More importantly, stripped stars harden the ionizing emission. We estimate that the spectral index for the ionizing part of the spectrum can increase to −1 compared to −2 for single stars. At high redshift, stripped stars and massive single stars combined dominate the He ii-ionizing emission, but we expect active galactic nuclei drive cosmic helium reionization. Further observational consequences we expect are (1) high ionization states for the intergalactic gas surrounding stellar systems, such as C iv and Si iv and (2) additional heating of the intergalactic medium of up to a few thousand Kelvin. Quantifying these warrants the inclusion of accurate models for stripped stars and other binary products in full cosmological simulations.

Observed binary populations reflect the Galactic history

“…Similarly, our study implies that the sdB population has been rather different at different times in Galactic history. In the extreme example of the early universe, a formation channel similar to the one studied here would involve binaries formed from massive low-metallicity stars leading to bright, massive helium stars and possibly contributing to the reionisation of the universe (Götberg et al 2017(Götberg et al , 2020.…”

No missing photons for reionization: moderate ionizing photon escape fractions from the FIRE-2 simulations

“…These two effects suggest that binary stars may contribute a large number of ionizing photons for reionization (e.g. Ma et al 2016;Stanway et al 2016;Rosdahl et al 2018;Götberg et al 2017Götberg et al , 2019.…”

Applications of Stellar Population Synthesis in the Distant Universe

“…One focus of recent attention, particularly in the high redshift universe where stellar populations <100 million years old are more common, has been improved models for the spectra of stripped, helium-dominated star atmospheres. While either giant Wolf-Rayet stars [123][124][125][126] or naked helium dwarfs [117,127,128] may result from extreme wind-driven mass loss in single stars, they result in larger numbers from binary interactions and incorporation of these atmospheres can make significant differences to the ultraviolet spectrum of composite binary stellar populations. To a large extent, the production and survival of these extreme objects is dependent on the rate at which stars lose their mass through stellar winds [129,130].…”

“…Its focus has historically been on relatively massive stars and young starbursts, and it was the first large stellar population synthesis code to implement the rotating Geneva stellar models. It has recently incorporated a prescription for including the helium dwarf atmospheres of Götberg et al [127,128] which arise primarily from binary products.…”