Matteo Cantiello

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Binary Stars

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Most stars are binaries. This statement still comes as a surprise to some astronomers, but  the role of binaries in astrophysics is of primary importance. From the point of view of stellar evolution, modeling binaries poses an additional challenge, since mass transfer, angular momentum transfer and tides need to be included during the calculation.

The influence of tides

In a close binary, angular momentum and kinetic energy can be exchanged between the two  stars and their orbit through tidal torques and dynamical deformation. The system tends to  evolve towards a state of minimum mechanical energy due to dissipative processes. This is an equilibrium state, where the orbit is circular, the spins of the stars are aligned and perpendicular to the orbital plane and the stars are in synchronous rotation with the orbital motion,  such that Pspin = Porbit . Therefore in very close systems tides can spin up the stars to rapid rotation, synchronous with their orbital revolution. As a consequence rotational instabilities  are expected to rapidly mix processed material from the core to the stellar surface. Since these short-period binaries often show eclipses, their parameters can be determined with high accuracy. If their surface abundances can be measured, such systems can be extremely useful as test-cases for the concept of rotational mixing. This idea has been discussed extensively in the paper  Rotational mixing in massive binaries. Detached short-period systems (de Mink, Cantiello et al. 2009).

 

Mass accretion and spin-up

In the case of a star in a binary system, one can define a region of space within which or-biting material is gravitationally bound to the star. Such a region is called the Roche lobe. During the evolution of the star, its radius can expand past its Roche lobe, resulting in material falling toward the companion star. Part of the material is accreted from the star, resulting in mass and angular momentum transfer. The specific angular momentum transferred in such a process depends on the binary parameters (mass of the two components and orbital separation). However, the total mass and angular momentum accreted depend on how the star reacts to the in-falling material. The amount of specific angular momentum in the in-falling material is, in general, huge, andthe outer layers of the accreting star are easily spun up to critical rotation, i.e. the rotationalvelocity for which the centrifugal acceleration equalizes the gravity. When this occurs, nomore material can be added to the stellar surface until angular momentum is transferred to deeper layers of the star, for example by magnetic fields or rotational instabilities. The amount of angular momentum transferred is enough for rotational instabilities to mix the star and induce quasi-chemically homogeneous evolution. The accretor becomes a fast rotating Wolf-Rayet star and at the end of its life can form a long GRB. For more details of this scenario, take a look to the paper Binary star progenitors of long gamma-ray bursts (Cantiello et al. 2007).