On the formation of Be stars through binary interaction

TL;DR

This study investigates the formation of Be stars through binary interactions, using binary evolution models and population synthesis to estimate their numbers and origins, highlighting mergers and mass transfer as key processes.

Contribution

It introduces detailed binary evolution calculations and population synthesis to quantify the role of binary interactions in Be star formation, including the effects of mass transfer and mergers.

Findings

Be/neutron star binaries can have Be stars with masses as low as 8 solar masses.

Mergers of main-sequence stars are a significant formation channel for isolated Be stars.

Binary interactions could account for up to 30% of all B-type stars being Be stars.

Abstract

Be stars are rapidly rotating B type stars. The origin of their rapid rotation is not certain, but binary interaction remains to be a possibility. In this work we investigate the formation of Be stars resulting from mass transfer in binaries in the Galaxy. We calculate the binary evolution with both stars evolving simultaneously and consider different possible mass accretion histories for the accretor. From the calculated results we obtain the critical mass ratios $q_{\rm cr}$ that determine the stability of mass transfer. We also numerically calculate the parameter $\lambda$ in common envelope evolution, and then incorporate both $q_{\rm cr}$ and $\lambda$ into the population synthesis calculations. We present the predicted numbers and characteristics of Be stars in binary systems with different types of companions, including helium stars, white dwarfs, neutron stars, and black holes. We find that in Be/neutron star binaries the Be stars can have a lower limit of mass $ \sim 8 M_{\odot}$ if they are formed by stable (i.e., without the occurrence of common envelope evolution) and nonconservative mass transfer. We demonstrate that isolated Be stars may originate from both mergers of two main-sequence stars and disrupted Be binaries during the supernova explosions of the primary stars, but mergers seem to play a much more important role. Finally the fraction of Be stars which have involved binary interactions in all B type stars can be as high as $ \sim 13%-30% $, implying that most of Be stars may result from binary interaction.