Rotation Rates of Massive Main Sequence Stars

Massive stars are generally rapid rotators. It is generally assumed that the observed projected rotation rates of main sequence stars (i.e. with stars performing their core hydrogen burning evolution) reflect their initial rotation rates, based on the short life times of these objectsreflect their initial rotation rates, based on the short life times of these objects. However, observed stars are never truly "just born", i.e., they have a non-zero age. And the rotation rate of a massive star, even on the main sequence, where its structure does not change much, can change as function of time due to various effects, such that any observed distribution of (projected) rotational velocities can deviate from the initial distribution. Such effects are in particular stellar winds, which can drag out stellar angular momentum, winds coupled to a magnetic surface field, and internal angular momentum transport. We can model all these effects using a sophisticated stellar evolution computer code, and thereby constrain the distribution function of initial rotation rates based on observed distributions of projected rotational velocities. This project has a strong relation to large recently finished and ongoing observational projects in which we are involved.

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Norbert Langer