Elemental abundances associated with gamma-ray bursts: nucleosynthesis in outflows^⋆

¹ Department of Automation, Shanghai University, 200072 Shanghai, PR China
e-mail: thu@shu.edu.cn
² Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Yunnan Observatories, 650011 Kunming, PR China

Received: 16 October 2014
Accepted: 6 April 2015

Abstract

Aims. Outflows from accretion disks have been suggested to be suitable sites for producing heavy elements. The objective of this work is to investigate nucleosynthesis in the outflows launched from the inner accretion disks of collapsars that are associated with gamma-ray bursts (GRBs).

Methods. By using a collapsar model of advection-dominated accretion and an adiabatic expanding outflow model, we simulated chemical yields inside the outflows associated with GRBs. As a result of the extremely high temperature (above 10¹⁰ K), the dominated species existing in the inner region of the disk are proton, neutron, ²H, and ⁴He, which are photodisintegrated from heavy nuclei. Assuming that an explosive nucleosynthesis could operate in outflows from collapsar accretions associated with GRBs, heavier species including the iron-group will be synthesized significantly. Although various outflow models have been investigated carefully in the past, the uncertainties in predicting the outflow birthplace (ejected-radius r_j) in the accretion disk are still quite large. Unlike recent researches on this topic that located the production of heavy elements in GRBs in the outer parts of the disk, in this study we focus our attention mainly on the outflows triggered from the inner region of collapsar accretion.

Results. We find ⁵⁶Ni can be abundantly produced in all the outflow cases considered here from the inner collapsar disk. The highest ⁵⁶Ni’s mass fraction in the outflows reaches 0.463, which can imply that the ⁵⁶Ni-rich outflows will lead to an observable violent explosion of a GRB event. The mass fractions of ⁵⁷Ni, ⁵⁹Cu (decays to ⁵⁹Ni), and ⁶⁰Zn (decays to ⁶⁰Ni) could be 10^-2, and the abundance of ⁵⁸Ni reaches 10^-3. Our simulations support that the effective production of heavy elements can occur in the outflows from the inner regions of the collapsar accretion disks. Our results also show that the amount of heavy species produced in outflows depends on the eject-radius r_j of the accretion disk where the outflows take place; outflows launched at larger radii can be helpful to increase the quantities of heavy nuclei by means of the lower disk temperature, which agrees with the conclusions of recent studies. Furthermore, a recently published article reported that some nuclei, such as ²⁸Si, ³²S, ³⁶Ar, ⁴⁰Ca, ⁴⁴Ti, ⁴⁸Cr, ⁵⁴Fe, and ⁵⁵⁻⁵⁶Co, experience a dip and then rise in their mass fraction while evolving in the outflow, and this characteristic behavior has occurred in many more outflow cases, and this is reported for the first time in the literature. This characteristic phenomenon also occurs in our nucleosynthetic simulations.