Götberg Group

Massive Binary Stars

Stars more massive than 8 times the mass of the Sun are rare, but important for the evolution of our Universe. Massive stars emit highly energetic ionizing radiation, have strong stellar winds and explode in violent supernovae, thus acting as both the mechanical engines and the chemical factories that drive galaxy evolution.

Only in the last decade, it has become clear that the standard, single stellar evolution representation for massive stars is incorrect. Instead, massive stars orbit a companion star so closely that interaction is inevitable as the stars evolve and swell. The binary interaction is typically dramatic: the stars transfer mass, engulf their companion, or even merge. As a result, many solar masses of stellar material are exchanged or ejected into the interstellar medium, completely altering the future evolution of the stars.

While the outcomes are diverse, one of the most common products of binary interaction is stars stripped of their hydrogen-rich envelopes through mass transfer or common envelope ejection. Being exposed stellar cores, these “stripped stars” are hot, small, helium-rich, and hydrogen-poor. They are thought to be the main progenitors of the hydrogen-poor core-collapse supernovae, to constitute two necessary evolutionary steps in the creation of neutron stars that merge in gravitational wave events, and to emit copious amounts of hard ionizing radiation that likely contributed to cosmic reionization.

Despite their importance, stripped stars in the intermediate-mass range (~2-8 times the mass of the Sun) remained, until recently, a theoretical prediction that lacked observational confirmation. In my research group, we pursue observational, computational and theoretical work, related to the first set of observed intermediate-mass stripped stars that we recently discovered. We use the observational measurements to better understand binary evolution and their role in stellar populations and galaxies, but also to better understand the physical processes that occur in the interiors of massive stars.