
Modeling Differentially Rotating Stars
John Momberg
Be stars are a type of star that rotate so fast, they throw material into space and form an orbiting disk. In order to throw material into space, we would expect that these stars need to rotate fast enough for their equators to reach the orbital velocity. However, observations show that many Be stars rotate at only 70% of the required velocity, which shouldn’t be fast enough to eject material and form the disks we see around these stars. Is there a way for a star to appear to be rotating slowly, yet its equator is actually traveling fast enough to enter orbit? One possibility is that the equator could be rotating much faster than the rest of the star. This would allow the equator to throw material into space, while the lower velocity across the rest of the surface could cause the velocity we observe to appear lower. This situation is called differential rotation, which is when the angular velocity can vary across the surface of the star (as opposed to solid body rotation, where the angular velocity is constant everywhere). In order to test this hypothesis, we calculated the shape of a differentially rotating star and modeled the spectra it would produce. We tried various rotation profiles to see which ones could be used to explain the Be star observations.
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