Nuclear Burning in Collapsar Accretion Disks

TYPEAstrophysics Seminar
Speaker:Yossef Zenati
Time:14:30 - 15:30
LocationZoom LINK
The core collapse of massive, rapidly-rotating stars are thought to be the progenitors of long-duration gamma-ray bursts (GRB) and their associated hyper-energeticsupernovae (SNe). At early times after the collapse, relatively low angular momentummaterial from the infalling stellar envelope will circularize into an accretion disk lo-cated just outside the  black hole horizon, resulting in high accretion rates necessaryto power a GRB jet. 
Temperatures in the disk midplane at these small radii are sufficiently high to dissociate nuclei, while outflows from the disk can be neutron-rich andmay synthesizer-process nuclei.However, at later times, and for high progenitor an-gular momentum, the outer layers of the stellar envelope can circularize at larger radii >1e7cm, where nuclear reactions can take place in the disk midplane (e.g.4He +16O20Ne +γ).
Here we explore the effects of nuclear burning on collapsar accretiondisks and their outflows by means of hydrodynamicalα-viscosity torus simulationscoupled to a 19-isotope nuclear reaction network, which are  designed to mimic thelate infall epochs in collapsar evolution when the viscous time of the torus has becomecomparable to the envelope fall-back time. 
Our results address several key questions,such as the conditions for quiescent burning and accretion versus detonation and the generation of 56Ni in disk outflows, which may contribute to powering GRB supernovae. Being located in the slowest, innermost layers of  the ejecta, the latter could provide the radioactive heating source necessary to make the spectral signatures ofr-process elements visible in late-time GRB-SNe spectra.