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Beyond Earthly Skies

Planet Candidate Circling a Rapidly Rotating Star

7 May 2014, 22:00 UTC
Planet Candidate Circling a Rapidly Rotating Star
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Main sequence stars of spectral types earlier than ~ F6 (i.e. stars > 1.5 times the Sun’s mass) are expected to be rapid rotators as their outermost layers are radiative instead of convective. Stars like the Sun have convective outermost layers that drive surface magnetic activity, generating strong stellar winds that with time carry away the star’s angular momentum and spin down the star. Stars of spectral types earlier than ~ F6 retain their high angular momentum (i.e. rapid rotation rates) since their radiative outermost layers do not produce strong stellar winds that sap the star’s angular momentum with time. Some of these stars are known to spin at near their break-up speeds.The rapid rotation of ~ F6 type and earlier stars can make then significantly oblate - flattened at the poles and bulging around the equator. This causes the star’s surface gravity to exhibit a pole-to-equator gradient, with the poles having a higher surface gravity, and thus higher temperature and brightness. The star’s photosphere can be several thousand Kevin hotter at the poles than at the equator. As a result, the poles are “gravity-brightened” and the equator “gravity-darkened”. This effect is called gravity darkening and it was first predicted ...

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