Should One Use the Ray-by-Ray Approximation in Core-Collapse Supernova Simulations?

TL;DR

This study compares the ray-by-ray+ approximation with full multi-dimensional transport in 2D core-collapse supernova simulations, revealing that the approximation can lead to misleadingly larger shock radii and potentially incorrect explosion outcomes.

Contribution

It provides the first self-consistent 2D comparison between ray-by-ray+ and full multi-D transport methods in supernova modeling, highlighting significant discrepancies.

Findings

Ray-by-ray+ produces larger shock radii than multi-D transport.

Ray-by-ray+ can artificially favor explosion conditions.

Full multi-D transport may prevent explosions predicted by ray-by-ray+.

Abstract

We perform the first self-consistent, time-dependent, multi-group calculations in two dimensions (2D) to address the consequences of using the ray-by-ray+ transport simplification in core-collapse supernova simulations. Such a dimensional reduction is employed by many researchers to facilitate their resource-intensive calculations. Our new code (F{\sc{ornax}}) implements multi-D transport, and can, by zeroing out transverse flux terms, emulate the ray-by-ray+ scheme. Using the same microphysics, initial models, resolution, and code, we compare the results of simulating 12-, 15-, 20-, and 25-M$_{\odot}$ progenitor models using these two transport methods. Our findings call into question the wisdom of the pervasive use of the ray-by-ray+ approach. Employing it leads to maximum post-bounce/pre-explosion shock radii that are almost universally larger by tens of kilometers than those derived using the more accurate scheme, typically leaving the post-bounce matter less bound and artificially more "explodable." In fact, for our 25-M$_{\odot}$ progenitor, the ray-by-ray+ model explodes, while the corresponding multi-D transport model does not. Therefore, in two dimensions the combination of ray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2D supernova dynamics can result in quantitatively, and perhaps qualitatively, incorrect results.