A Caltech Library Service

Numerical problems in coupling photon momentum (radiation pressure) to gas

Hopkins, Philip F. and Grudić, Michael Y. (2019) Numerical problems in coupling photon momentum (radiation pressure) to gas. Monthly Notices of the Royal Astronomical Society, 483 (3). pp. 4187-4196. ISSN 0035-8711.

[img] PDF - Published Version
See Usage Policy.

[img] PDF - Accepted Version
See Usage Policy.


Use this Persistent URL to link to this item:


Radiation pressure (RP; or photon momentum absorbed by gas) is important in a tremendous range of astrophysical systems. But we show the usual method for assigning absorbed photon momentum to gas in numerical radiation-hydrodynamics simulations (integrating over cell volumes or evaluating at cell centres) can severely underestimate the RP force in the immediate vicinity around unresolved (point/discrete) sources (and subsequently underestimate its effects on bulk gas properties), unless photon mean free paths are highly resolved in the fluid grid. The existence of this error is independent of the numerical radiation transfer (RT) method (even in exact ray-tracing/Monte Carlo methods), because it depends on how the RT solution is interpolated back onto fluid elements. Brute-force convergence (resolving mean free paths) is impossible in many cases (especially where UV/ionizing photons are involved). Instead, we show a ‘face-integrated’ method – integrating and applying the momentum fluxes at interfaces between fluid elements – better approximates the correct solution at all resolution levels. The ‘fix’ is simple and we provide example implementations for ray-tracing, Monte Carlo, and moments RT methods in both grid and mesh-free fluid schemes. We consider an example of star formation in a molecular cloud with UV/ionizing RP. At state-of-the-art resolution, cell-integrated methods underestimate the net effects of RP by an order of magnitude, leading (incorrectly) to the conclusion that RP is unimportant, while face-integrated methods predict strong self-regulation of star formation and cloud destruction via RP.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Hopkins, Philip F.0000-0003-3729-1684
Grudić, Michael Y.0000-0002-1655-5604
Additional Information:© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2018 October 24. Received 2018 October 23; in original form 2018 March 19. Published: 15 November 2018. Support for PFH and MYG was provided by an Alfred P. Sloan Research Fellowship, NSF Collaborative Research Grant #1715847 and CAREER grant #1455342. Numerical calculations were run on the Caltech compute cluster ‘Wheeler’, allocations from XSEDE TG-AST130039 and PRAC NSF.1713353 supported by the NSF and NASA HEC SMD-16-7592.
Group:TAPIR, Astronomy Department
Funding AgencyGrant Number
Alfred P. Sloan FoundationUNSPECIFIED
Subject Keywords:hydrodynamics – methods: numerical – stars: formation – galaxies: active – galaxies: formation
Issue or Number:3
Record Number:CaltechAUTHORS:20190411-130929118
Persistent URL:
Official Citation:Philip F Hopkins, Michael Y Grudić, Numerical problems in coupling photon momentum (radiation pressure) to gas, Monthly Notices of the Royal Astronomical Society, Volume 483, Issue 3, March 2019, Pages 4187–4196,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:94662
Deposited By: Tony Diaz
Deposited On:11 Apr 2019 22:16
Last Modified:03 Oct 2019 21:05

Repository Staff Only: item control page