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Title: Constraining the Speed of Sound inside Neutron Stars with Chiral Effective Field Theory Interactions and Observations

Abstract

Here, the dense matter equation of state (EOS) determines neutron star (NS) structure but can be calculated reliably only up to one to two times the nuclear saturation density, using accurate many-body methods that employ nuclear interactions from chiral effective field theory constrained by scattering data. In this work, we use physically motivated ansatzes for the speed of sound c S at high density to extend microscopic calculations of neutron-rich matter to the highest densities encountered in stable NS cores. We show how existing and expected astrophysical constraints on NS masses and radii from X-ray observations can constrain the speed of sound in the NS core. We confirm earlier expectations that c S is likely to violate the conformal limit of $${c}_{S}^{2}\leqslant {c}^{2}/3$$, possibly reaching values closer to the speed of light c at a few times the nuclear saturation density, independent of the nuclear Hamiltonian. If QCD obeys the conformal limit, we conclude that the rapid increase of c S required to accommodate a 2 M NS suggests a form of strongly interacting matter where a description in terms of nucleons will be unwieldy, even between one and two times the nuclear saturation density. For typical NSs with masses in the range of 1.2–1.4 M , we find radii between 10 and 14 km, and the smallest possible radius of a 1.4 M NS consistent with constraints from nuclear physics and observations is 8.4 km. We also discuss how future observations could constrain the EOS and guide theoretical developments in nuclear physics.

Authors:
ORCiD logo [1]; ORCiD logo [2]; ORCiD logo [2]; ORCiD logo [1]
  1. Univ. of Washington, Seattle, WA (United States); Michigan State Univ., East Lansing, MI (United States)
  2. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Publication Date:
Research Org.:
Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE Laboratory Directed Research and Development (LDRD) Program
OSTI Identifier:
1481144
Report Number(s):
LA-UR-17-31455
Journal ID: ISSN 1538-4357
Grant/Contract Number:  
AC52-06NA25396
Resource Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 860; Journal Issue: 2; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; Astronomy and Astrophysics; dense matter; equation of state; stars: neutron

Citation Formats

Tews, I., Carlson, Joseph Allen, Gandolfi, Stefano, and Reddy, S. Constraining the Speed of Sound inside Neutron Stars with Chiral Effective Field Theory Interactions and Observations. United States: N. p., 2018. Web. doi:10.3847/1538-4357/aac267.
Tews, I., Carlson, Joseph Allen, Gandolfi, Stefano, & Reddy, S. Constraining the Speed of Sound inside Neutron Stars with Chiral Effective Field Theory Interactions and Observations. United States. https://doi.org/10.3847/1538-4357/aac267
Tews, I., Carlson, Joseph Allen, Gandolfi, Stefano, and Reddy, S. Wed . "Constraining the Speed of Sound inside Neutron Stars with Chiral Effective Field Theory Interactions and Observations". United States. https://doi.org/10.3847/1538-4357/aac267. https://www.osti.gov/servlets/purl/1481144.
@article{osti_1481144,
title = {Constraining the Speed of Sound inside Neutron Stars with Chiral Effective Field Theory Interactions and Observations},
author = {Tews, I. and Carlson, Joseph Allen and Gandolfi, Stefano and Reddy, S.},
abstractNote = {Here, the dense matter equation of state (EOS) determines neutron star (NS) structure but can be calculated reliably only up to one to two times the nuclear saturation density, using accurate many-body methods that employ nuclear interactions from chiral effective field theory constrained by scattering data. In this work, we use physically motivated ansatzes for the speed of sound c S at high density to extend microscopic calculations of neutron-rich matter to the highest densities encountered in stable NS cores. We show how existing and expected astrophysical constraints on NS masses and radii from X-ray observations can constrain the speed of sound in the NS core. We confirm earlier expectations that c S is likely to violate the conformal limit of ${c}_{S}^{2}\leqslant {c}^{2}/3$, possibly reaching values closer to the speed of light c at a few times the nuclear saturation density, independent of the nuclear Hamiltonian. If QCD obeys the conformal limit, we conclude that the rapid increase of c S required to accommodate a 2 M ⊙ NS suggests a form of strongly interacting matter where a description in terms of nucleons will be unwieldy, even between one and two times the nuclear saturation density. For typical NSs with masses in the range of 1.2–1.4 M ⊙, we find radii between 10 and 14 km, and the smallest possible radius of a 1.4 M ⊙ NS consistent with constraints from nuclear physics and observations is 8.4 km. We also discuss how future observations could constrain the EOS and guide theoretical developments in nuclear physics.},
doi = {10.3847/1538-4357/aac267},
journal = {The Astrophysical Journal (Online)},
number = 2,
volume = 860,
place = {United States},
year = {Wed Jun 20 00:00:00 EDT 2018},
month = {Wed Jun 20 00:00:00 EDT 2018}
}

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