# Long Gamma-Ray Burst Progenitors: Boundary Conditions and Binary Models

## TL;DR

This paper investigates the conditions under which long gamma-ray burst progenitors can form, focusing on binary star models and the effects of metallicity and stellar interactions on core angular momentum.

## Contribution

It analyzes binary evolution scenarios and identifies conditions that allow helium stars in close binaries to retain enough angular momentum to produce GRBs.

## Key findings

- Tidal synchronization occurs rapidly during core-helium burning.
- Core-envelope coupling likely prevents helium stars with main-sequence companions from producing GRBs.
- Helium stars with neutron-star or black-hole companions can retain sufficient core angular momentum.

## Abstract

The observed association of Long Gamma-Ray Bursts (LGRBs) with peculiar Type Ic supernovae gives support to Woosley`s collapsar/hypernova model, in which the GRB is produced by the collapse of the rapidly rotating core of a massive star to a black hole. The association of LGRBs with small star-forming galaxies suggests low-metallicity to be a condition for a massive star to evolve to the collapsar stage. Both completely-mixed single star models and binary star models are possible. In binary models the progenitor of the GRB is a massive helium star with a close companion. We find that tidal synchronization during core-helium burning is reached on a short timescale (less than a few millennia). However, the strong core-envelope coupling in the subsequent evolutionary stages is likely to rule out helium stars with main-sequence companions as progenitors of hypernovae/GRBs. On the other hand, helium stars in close binaries with a neutron-star or black-hole companion can, despite the strong core-envelope coupling in the post-helium burning phase, retain sufficient core angular momentum to produce a hypernova/GRB.