From astrobites: “Did stripped stars re-ionize the Universe?”

Astrobites bloc


Apr 25, 2018
Mathieu Renzo, University of Amsterdam

Title: Ionizing spectra of stars that lose their envelope through interaction with a binary companion: role of metallicity
Authors: Y. Götberg, S. E. de Mink, J. H. Groh
First Author’s Institution: Institute for Astronomy, University of Amsterday, The Netherlands
Status: Published by A&A, open access

One long standing puzzle about the evolution of the Universe is the epoch of reionization. After the recombination of electrons and protons that created the cosmic microwave background [CMB], the universe was opaque and light could not get through. But roughly ~500 million years after the Big Bang, something turned on and started producing ionizing radiation, causing protons and electrons to re-separate. This ended the “cosmic dark ages” and made the universe transparent again.

Reionization era and first stars, Caltech

Cosmic Background Radiation per ESA/Planck

ESA/Planck 2009 to 2013

It is generally believed that this “something” was the population of massive stars (although AGN are also considered). Most stars have a thick hydrogen-rich envelope, which absorbs the ionizing photons they produce, but stars more massive than ~30 solar masses can lose to stellar winds their entire hydrogen-rich envelope. By doing so, these stars reveal their hot helium-rich core and become Wolf-Rayet stars, which can release ionizing radiation. However, current estimates of the massive star populations in the early Universe can hardly produce enough ionizing photons from Wolf-Rayet stars to explain the epoch of re-ionization.

In today’s paper, Götberg et al. propose another way to remove the ionizing-radiation-blocking envelope from stars: mass transfer in binaries. In the local universe, the vast majority of massive stars are in binaries. Interactions with a companion can change the properties of stars. In particular, it is expected that ~30% of massive stars will lose their hydrogen-rich envelope to their companion after the end of their main sequence phase, when they expand to become giants. By losing their envelope, these stars also expose their helium core, and become “stripped stars”. Stripped stars might have contributed or even dominated the epoch of re-ionization, if massive stars preferred to be in binaries also in the early universe.

Figure 1: Evolution on the HR diagram of a single 12 solar mass star (A-B-C-gray track), and the same star in a binary (colored line, from A to H). The single star never enters the shaded region, where a significant amount of ionizing radiation is emitted. Instead, the binary donor spends all its core helium burning duration there, roughly 10% of its total lifetime. The binary mass transfer phase is marked by the black contour, and the color of the binary track indicates the core helium abundance: long live phases correspond to where the colors change faster. Source: Figure 2 from today’s paper.

See the full article here .

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