[Paper Review] Collapsars in Three Dimensions

Gabriel Rockefeller, Chris L. Fryer+1 more|arXiv (Cornell University)|Aug 1, 2006

Gamma-ray bursts and supernovae3 citations

TL;DR

This study presents the first 3D simulations of a collapsing 60 M⊙ zero-metallicity star, revealing that non-axisymmetric spiral wave instabilities in the accretion torus drive enhanced viscous heating and angular momentum transport, leading to a $10^{52}$ erg hypernova explosion without relativistic jets. The instabilities generate strong gravitational wave signals, with amplitudes up to 15 times stronger than in previous rotating models, though not detectable by LIGO at 1 Mpc due to low event rates.

ABSTRACT

We present the results of 3-dimensional simulations of the direct collapse to a black hole of a rotating, 60-solar-mass, zero metallicity, population III star. Because the structure of this star (angular momentum, density and temperature profiles) is similar to many collapsar gamma-ray burst progenitors, these calculations have implications beyond the fate of population III stars. These simulations provide a first 3-dimensional look at a realistic collapsar progenitor, and the results are very different from any previous 2-dimensional calculations. If the angular momentum of the progenitor is high, non-axisymmetric instabilities in the collapsing core cause spiral structures to form, and these structures shape later outflows. These outflows are driven by the imbalance between viscous heating and inefficient neutrino cooling and ultimately develop into a 1e52 erg explosion. Without magnetic fields, this collapse will not produce relativistic jets, but the explosion is indeed a hypernova. We conclude with a discussion of the implications of such calculations on the explosions, nucleosynthesis, neutrino flux and gravitational wave emission from the collapse of massive stars.

Motivation & Objective

• To investigate the 3D dynamics of core collapse in a rotating, 60 M⊙ Population III star, a plausible collapsar progenitor.
• To examine how non-axisymmetric instabilities in the accretion torus influence angular momentum transport, explosive outflows, and gravitational wave emission.
• To assess the implications for nucleosynthesis, neutrino fluxes, and gravitational wave detectability in massive star collapse.

Proposed method

• Simulations use smoothed particle hydrodynamics (SPH) to model 3D collapse of a 1D progenitor from the KEPLER stellar evolution code.
• The initial conditions are derived directly from a pre-collapse 60 M⊙ zero-metallicity star with realistic angular momentum, density, and temperature profiles.
• The simulation tracks accretion onto a central black hole down to the innermost stable circular orbit, using a simplified equation of state and neutrino cooling approximation.
• Gravitational wave signals are calculated from mass-motion and neutrino emission asymmetries using formalism from Fryer et al. (2004).
• Neutrino fluxes and electron fraction evolution are analyzed to assess nucleosynthetic yields and asymmetries.
• The results are compared to 2D axially symmetric models to isolate 3D-specific effects like spiral wave instabilities.

Experimental results

Research questions

• RQ1 How do 3D instabilities in the accretion torus affect angular momentum transport and explosive energy output in collapsar collapse?
• RQ2 What is the gravitational wave signature from 3D accretion instabilities, and how does it compare to 2D models and core-collapse supernovae?
• RQ3 To what extent do neutrino flux asymmetries and electron fraction evolution influence nucleosynthesis in the ejecta?
• RQ4 Can the collapse of a realistic, rotating Population III star produce a hypernova without relativistic jets?
• RQ5 What are the detectability prospects for gravitational waves from such collapsars with current and advanced LIGO instruments?

Key findings

• Non-axisymmetric spiral wave instabilities develop in the accretion torus due to high angular momentum, driving enhanced viscous heating and angular momentum transport.
• The explosion energy reaches approximately $10^{52}$ erg, consistent with a hypernova, driven by the imbalance between viscous heating and inefficient neutrino cooling.
• The gravitational wave amplitude from mass-motion in the rapidly-rotating model is nearly 15 times stronger than in previous rotating models, with oscillation frequencies in the 100–1000 Hz range.
• Neutrino emission asymmetries produce gravitational wave signals comparable to or up to five times stronger than in core-collapse supernovae, though not detectable by LIGO at 1 Mpc due to low event rates.
• The electron fraction in ejecta may drop below 0.5 due to preferential absorption of higher-energy anti-neutrinos, altering nucleosynthetic yields.
• The explosion is not jetted in the absence of magnetic fields, and the outflows are highly asymmetric, imprinted by spiral wave structures.