The Arduous Journey to Black Hole Formation in Potential Gamma-Ray Burst Progenitors
We present a quantitative study on the properties at death of fast-rotating massive stars evolved at low-metallicity—objects that are proposed as likely progenitors of long-duration γ-ray bursts (LGRBs). We perform one-dimensional+rotation stellar-collapse simulations on the progenitor models of Woosley and Heger, and critically assess their potential for the formation of a black hole and a Keplerian disk (namely, a collapsar) or a proto-magnetar. We note that theoretical uncertainties in the treatment of magnetic fields and the approximate handling of rotation compromise the accuracy of stellar-evolution models. We find that only the fastest rotating progenitors achieve sufficient compactness for black hole formation while the bulk of models possess a core density structure typical of garden-variety core-collapse supernova (SN) progenitors evolved without rotation and at solar metallicity. Of the models that do have sufficient compactness for black hole formation, most of them also retain a large amount of angular momentum in the core, making them prone to a magneto-rotational explosion, therefore preferentially leaving behind a proto-magnetar. A large progenitor angular-momentum budget is often the sole criterion invoked in the community today to assess the suitability for producing a collapsar. This simplification ignores equally important considerations such as the core compactness, which conditions black hole formation, the core angular momentum, which may foster a magneto-rotational explosion preventing black hole formation, or the metallicity and the residual envelope mass which must be compatible with inferences from observed LGRB/SNe. Our study suggests that black hole formation is non-trivial, that there is room for accommodating both collapsars and proto-magnetars as LGRB progenitors, although proto-magnetars seem much more easily produced by current stellar-evolutionary models.
Additional Information© 2012 The American Astronomical Society. Received 2012 March 9; accepted 2012 May 18; published 2012 July 6. We acknowledge fruitful discussions with R. Hirschi, A. Beloborodov, and T. Piro. We also thank Stan Woosley for his comments on a draft version of this paper. This research is supported in part by the National Science Foundation under grant Nos. AST-0855535 and OCI-0905046 and by the Sherman Fairchild Foundation. E.O. is supported in part by a postgraduate fellowship from the Natural Sciences and Engineering Research Council of Canada (NSERC). The computations were performed at Caltech's Center for Advanced Computing Research on the cluster "Zwicky" funded through NSF grant No. PHY-0960291 and the Sherman Fairchild Foundation.
Published - Dessart2012p19056Astrophys_J.pdf