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Stellar mergers

When a binary star undergoes common-envelope (CE) evolution, its orbit can undergo a dramatic shrinkage known as a spiral-in. This can result in a very tight orbit, as is observed in double white dwarfs. If the orbit shrinks sufficiently, the companion star may fill its Roche lobe, resulting in a merger of the two binary members into a single star. Such merger products are observed in stellar clusters as Blue Stragglers, main-sequence stars that are hotter (bluer) and more luminous than they ought to be due to single-star evolution.

We wrote a simple population-synthesis code that is designed to evolve binaries through a CE stage. The code is based on a grid of single-star evolution models, computed with the STARS code by Dr. Eggleton. As its first application, we looked at the systems that merged as a consequence of that CE phase on the red-giant branch. We use a crude set of rules to determine a model for the merger remnant and followed the evolution of that remnant to the white-dwarf stage.

We assume that any mass that needs to be lost in order to spin down a super-critically spinning merger remnant is indeed blown to infinity. Hence, our merged stars usually continue their evolution (after the merger) while spinning at a critical rate, while their evolution and the corresponding expansion on the giant branch spins the star down. Interestingly, the point where the star ignites helium in the core is a natural point for most stars to contract and hence to spin up. Here, we applied the same condition that the star must keep losing mass until it is spun down below critical rotation.

We find that a fraction of the merged stars in our population lose all but their entire envelopes, resulting in fast-spinning, hot horizontal-branch stars. These stars can probably be identified with observed blue horizontal-branch stars, also known as subdwarf-B stars (sdB stars). The results of this initial study and the effect of different initial conditions, are presented in Politano et al., 2008.