Viscous evolution of a massive disk surrounding stellar-mass black holes in full general relativity
Sho Fujibayashi, Masaru Shibata, Shinya Wanajo, Kenta Kiuchi, Koutarou Kyutoku, Yuichiro Sekiguchi
Abstract
Long-term viscous neutrino-radiation hydrodynamics simulations in full general relativity are performed for a massive disk surrounding spinning stellar-mass black holes with mass ${M}_{\mathrm{BH}}=4$, 6, and $10\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ and initial dimensionless spin $\ensuremath{\chi}\ensuremath{\approx}0.8$. The initial disk is chosen to have mass ${M}_{\mathrm{disk}}\ensuremath{\approx}0.1$ or $3\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ as plausible models of the remnants for the merger of black hole--neutron star binaries or the stellar core collapse from a rapidly rotating progenitor, respectively. For ${M}_{\mathrm{disk}}\ensuremath{\approx}0.1\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ with the outer disk edge initially located at ${r}_{\mathrm{out}}\ensuremath{\sim}200\text{ }\text{ }\mathrm{km}$, we find that 15%--20% of ${M}_{\mathrm{disk}}$ is ejected and the average electron fraction of the ejecta is $⟨{Y}_{e}⟩=0.30--0.35$ as found in the previous study. For ${M}_{\mathrm{disk}}\ensuremath{\approx}3\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$, we find that approximately $10%--20%$ of ${M}_{\mathrm{disk}}$ is ejected for ${r}_{\mathrm{out}}\ensuremath{\approx}200--1000\text{ }\text{ }\mathrm{km}$. In addition, the average electron fraction of the ejecta can be enhanced to $⟨{Y}_{e}⟩\ensuremath{\gtrsim}0.4$ because the electron fraction is increased significantly during the long-term viscous expansion of the disk with high neutrino luminosity until the mass ejection sets in. Our results suggest that not heavy $r$-process elements but light trans-iron elements would be synthesized in the matter ejected from a massive torus surrounding stellar-mass black holes. We also find that the outcomes of the viscous evolution for the high-mass disk case is composed of a rapidly spinning black hole surrounded by a torus with a narrow funnel, which appears to be suitable for generating gamma-ray bursts.