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ABSTRACT Supermassive stars are Population III stars with masses exceeding $$10^4\, {\rm M}_{\odot }$$ that could be the progenitors of the first supermassive black holes. Their interiors are in a regime where radiation pressure dominates the equation of state. In this work, we use the explicit gas dynamics code ppmstar to simulate the hydrogen-burning core of a $$10^4\, {\rm M}_{\odot }$$ supermassive main-sequence star. These are the first three-dimensional hydrodynamics simulations of core convection in supermassive stars. We perform a series of 10 simulations at different heating rates and on Cartesian grids with resolutions of 7683, 11523, and 17283. We examine different properties of the convective flow, including its large-scale morphology, its velocity spectrum, and its mixing properties. We conclude that the radiation pressure-dominated nature of the interior does not noticeably affect the behaviour of convection compared to the case of core convection in a massive main-sequence star where gas pressure dominates. Our simulations also offer support for the use of mixing-length theory in one-dimensional models of supermassive stars.
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Numerical Simulations of Convective Three-dimensional Red Supergiant Envelopes
Abstract We explore the three-dimensional properties of convective, luminous ( L ≈ 10 4.5 –10 5 L ⊙ ), hydrogen-rich envelopes of red supergiants (RSGs) based on radiation hydrodynamic simulations in spherical geometry using Athena ++. These computations comprise ≈30% of the stellar volume, include gas and radiation pressure, and self-consistently track the gravitational potential for the outer ≈3 M ⊙ of the simulated M ≈ 15 M ⊙ stars. This work reveals a radius, R corr , around which the nature of the convection changes. For r > R corr , though still optically thick, diffusion of photons dominates the energy transport. Such a regime is well studied in less luminous stars, but in RSGs, the near- (or above-)Eddington luminosity (due to opacity enhancements at ionization transitions) leads to the unusual outcome of denser regions moving outward rather than inward. This region of the star also has a large amount of turbulent pressure, yielding a density structure much more extended than 1D stellar evolution predicts. This “halo” of material will impact predictions for both shock breakout and early lightcurves of Type IIP supernovae. Inside of R corr , we find a nearly flat entropy profile as expected in the efficient regime of mixing-length theory (MLT). Radiation pressure provides ≈1/3 of the support against gravity in this region. Our comparisons to MLT suggest a mixing length of α = 3–4, consistent with the sizes of convective plumes seen in the simulations. The temporal variability of these 3D models is mostly on the timescale of the convective plume lifetimes (≈300 days), with amplitudes consistent with those observed photometrically.
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- Award ID(s):
- 1663688
- PAR ID:
- 10414673
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 929
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 156
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-4357/ac5ab3
