Wind anisotropies and GRB progenitors

We study the effect of wind anisotropies on the stellar evolution leading to collapsars. Rotating models of a 60 M$_\odot$ star with $Ω/Ω_{\rm crit}=0.75$ on the ZAMS, accounting for shellular rotation and a magnetic field, with and without wind anisotropies, are computed at $Z$=0.002 until the end of the core He-burning phase. Only the models accounting for the effects of the wind anisotropies retain enough angular momentum in their core to produce a Gamma Ray Burst (GRB). The chemical composition is such that a type Ic supernova event occurs. Wind anisotropies appear to be a key physical ingredient in the scenario leading to long GRBs.

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