The thermal index of neutron-star matter in the virial approximation

Motivated by gravitational-wave observations of binary neutron-star mergers, we study the thermal index of low-density, high-temperature dense matter. We use the virial expansion to account for nuclear interaction effects. We focus on the region of validity of the expansion, which reaches 10−3 fm−3...

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Autores: Rivieccio, Giuseppe, Nadal Matosas, Adriana, Rios Huguet, Arnau, Ruiz Revuelta, Mònica
Tipo de recurso: artículo
Estado:Versión publicada
Fecha de publicación:2025
País:España
Institución:Varias* (Consorci de Biblioteques Universitáries de Catalunya, Centre de Serveis Científics i Acadèmics de Catalunya)
Repositorio:Recercat. Dipósit de la Recerca de Catalunya
OAI Identifier:oai:recercat.cat:2445/228248
Acceso en línea:https://hdl.handle.net/2445/228248
Access Level:acceso abierto
Palabra clave:Estels de neutrons
Dispersió (Física nuclear)
Astrofísica nuclear
Neutron stars
Scattering (Nuclear physics)
Nuclear astrophysics
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spelling The thermal index of neutron-star matter in the virial approximationRivieccio, GiuseppeNadal Matosas, AdrianaRios Huguet, ArnauRuiz Revuelta, MònicaEstels de neutronsDispersió (Física nuclear)Astrofísica nuclearNeutron starsScattering (Nuclear physics)Nuclear astrophysicsMotivated by gravitational-wave observations of binary neutron-star mergers, we study the thermal index of low-density, high-temperature dense matter. We use the virial expansion to account for nuclear interaction effects. We focus on the region of validity of the expansion, which reaches 10−3 fm−3 at T = 5 MeV up to almost saturation density at T = 50 MeV. In pure neutron matter, we find an analytical expression for the thermal index, and show that it is nearly density and temperature independent, within a fraction of a percent of the noninteracting, nonrelativistic value of Γth ≈ 5/3. When we incorporate protons, electrons, and photons, we find that the density and temperature dependence of the thermal index changes significantly. We predict a smooth transition between an electron-dominated regime with Γth ≈ 4/3 at low densities to a neutron-dominated region with Γth ≈ 5/3 at high densities. This behavior is by and large independent of the proton fraction and is not affected by nuclear interactions in the region where the virial expansion converges. We model this smooth transition analytically and provide a simple but accurate parameterization of the inflection point between these regimes. When compared to tabulated realistic models of the thermal index, we find an overall agreement at high temperatures that weakens for colder matter. The discrepancies can be attributed to the missing contributions of nuclear clusters. The virial approximation provides a clear and physically intuitive framework for understanding the thermal properties of dense matter, offering a computationally efficient solution that makes it particularly well suited for the regimes relevant to neutron-star binary remnants.Institute of Physics (IOP)2026202620252026info:eu-repo/semantics/articleinfo:eu-repo/semantics/publishedVersion16 p.application/pdfhttps://hdl.handle.net/2445/228248Articles publicats en revistes (Física Quàntica i Astrofísica)reponame:Recercat. Dipósit de la Recerca de Catalunyainstname:Varias* (Consorci de Biblioteques Universitáries de Catalunya, Centre de Serveis Científics i Acadèmics de Catalunya)InglésReproducció del document publicat a: https://doi.org/10.3847/1538-4357/add3edAstrophysical Journal, 2025, vol. 987, num.67https://doi.org/10.3847/1538-4357/add3edcc-by (c) Rivieccio, Giuseppe, et al., 2025https://creativecommons.org/licenses/by/4.0/info:eu-repo/semantics/openAccessoai:recercat.cat:2445/2282482026-05-29T05:05:01Z
dc.title.none.fl_str_mv The thermal index of neutron-star matter in the virial approximation
title The thermal index of neutron-star matter in the virial approximation
spellingShingle The thermal index of neutron-star matter in the virial approximation
Rivieccio, Giuseppe
Estels de neutrons
Dispersió (Física nuclear)
Astrofísica nuclear
Neutron stars
Scattering (Nuclear physics)
Nuclear astrophysics
title_short The thermal index of neutron-star matter in the virial approximation
title_full The thermal index of neutron-star matter in the virial approximation
title_fullStr The thermal index of neutron-star matter in the virial approximation
title_full_unstemmed The thermal index of neutron-star matter in the virial approximation
title_sort The thermal index of neutron-star matter in the virial approximation
dc.creator.none.fl_str_mv Rivieccio, Giuseppe
Nadal Matosas, Adriana
Rios Huguet, Arnau
Ruiz Revuelta, Mònica
author Rivieccio, Giuseppe
author_facet Rivieccio, Giuseppe
Nadal Matosas, Adriana
Rios Huguet, Arnau
Ruiz Revuelta, Mònica
author_role author
author2 Nadal Matosas, Adriana
Rios Huguet, Arnau
Ruiz Revuelta, Mònica
author2_role author
author
author
dc.subject.none.fl_str_mv Estels de neutrons
Dispersió (Física nuclear)
Astrofísica nuclear
Neutron stars
Scattering (Nuclear physics)
Nuclear astrophysics
topic Estels de neutrons
Dispersió (Física nuclear)
Astrofísica nuclear
Neutron stars
Scattering (Nuclear physics)
Nuclear astrophysics
description Motivated by gravitational-wave observations of binary neutron-star mergers, we study the thermal index of low-density, high-temperature dense matter. We use the virial expansion to account for nuclear interaction effects. We focus on the region of validity of the expansion, which reaches 10−3 fm−3 at T = 5 MeV up to almost saturation density at T = 50 MeV. In pure neutron matter, we find an analytical expression for the thermal index, and show that it is nearly density and temperature independent, within a fraction of a percent of the noninteracting, nonrelativistic value of Γth ≈ 5/3. When we incorporate protons, electrons, and photons, we find that the density and temperature dependence of the thermal index changes significantly. We predict a smooth transition between an electron-dominated regime with Γth ≈ 4/3 at low densities to a neutron-dominated region with Γth ≈ 5/3 at high densities. This behavior is by and large independent of the proton fraction and is not affected by nuclear interactions in the region where the virial expansion converges. We model this smooth transition analytically and provide a simple but accurate parameterization of the inflection point between these regimes. When compared to tabulated realistic models of the thermal index, we find an overall agreement at high temperatures that weakens for colder matter. The discrepancies can be attributed to the missing contributions of nuclear clusters. The virial approximation provides a clear and physically intuitive framework for understanding the thermal properties of dense matter, offering a computationally efficient solution that makes it particularly well suited for the regimes relevant to neutron-star binary remnants.
publishDate 2025
dc.date.none.fl_str_mv 2025
2026
2026
2026
dc.type.none.fl_str_mv info:eu-repo/semantics/article
info:eu-repo/semantics/publishedVersion
format article
status_str publishedVersion
dc.identifier.none.fl_str_mv https://hdl.handle.net/2445/228248
url https://hdl.handle.net/2445/228248
dc.language.none.fl_str_mv Inglés
language_invalid_str_mv Inglés
dc.relation.none.fl_str_mv Reproducció del document publicat a: https://doi.org/10.3847/1538-4357/add3ed
Astrophysical Journal, 2025, vol. 987, num.67
https://doi.org/10.3847/1538-4357/add3ed
dc.rights.none.fl_str_mv cc-by (c) Rivieccio, Giuseppe, et al., 2025
https://creativecommons.org/licenses/by/4.0/
info:eu-repo/semantics/openAccess
rights_invalid_str_mv cc-by (c) Rivieccio, Giuseppe, et al., 2025
https://creativecommons.org/licenses/by/4.0/
eu_rights_str_mv openAccess
dc.format.none.fl_str_mv 16 p.
application/pdf
dc.publisher.none.fl_str_mv Institute of Physics (IOP)
publisher.none.fl_str_mv Institute of Physics (IOP)
dc.source.none.fl_str_mv Articles publicats en revistes (Física Quàntica i Astrofísica)
reponame:Recercat. Dipósit de la Recerca de Catalunya
instname:Varias* (Consorci de Biblioteques Universitáries de Catalunya, Centre de Serveis Científics i Acadèmics de Catalunya)
instname_str Varias* (Consorci de Biblioteques Universitáries de Catalunya, Centre de Serveis Científics i Acadèmics de Catalunya)
reponame_str Recercat. Dipósit de la Recerca de Catalunya
collection Recercat. Dipósit de la Recerca de Catalunya
repository.name.fl_str_mv
repository.mail.fl_str_mv
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score 15,811543

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