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Abstract
We report neutron star predictions based on our most recent equations of state. These are derived from chiral effective field theory, which allows for a systematic development of nuclear forces, order by order. We utilize high-quality two-nucleon interactions and include all three-nucleon forces up to fourth order in the chiral expansion. Our ab initio predictions are restricted to the domain of applicability of chiral effective field theory. However, stellar matter in the interior of neutron stars can be up to several times denser than normal nuclear matter at saturation, and its composition is essentially unknown. Following established practices, we extend our microscopic predictions to higher densities matching piecewise polytropes. The radius of the average-size neutron star, about 1.4 solar masses, is sensitive to the pressure at normal densities, and thus it is suitable to constrain ab initio theories of the equation of state. For this reason, we focus on the radius of medium-mass stars. We compare our results with other theoretical predictions and recent constraints.
Highlights
The equation of state (EoS) of neutron-rich matter is at the forefront of nuclear astrophysics because of its role in shaping the properties of neutron stars
For the reasons stated above regarding the limitations of chiral effective field theory (EFT), we focus on the average-mass neutron star, rather than the maximum mass of a sequence, as the latter is much more sensitive to the polytropic extensions we perform in order to complete the EoS
−1.07 −5.32 3.56 0.50 −1.25 −0.011828 −0.000010 In Figure 1, we show the EoS in neutron matter (NM) over four orders, from leading order (LO) to N3LO [6]
Summary
The equation of state (EoS) of neutron-rich matter is at the forefront of nuclear astrophysics because of its role in shaping the properties of neutron stars. Chiral EFT employs a power counting scheme in which the progression of two- and many-nucleon forces is constructed following a clear and systematic hierarchy. This allows for the inclusion of all three-nucleon forces (3NFs) which appear at a given order, eliminating the inconsistencies which are unavoidable when adopting meson-theoretic or phenomenological forces. For the reasons stated above regarding the limitations of chiral EFT, we focus on the average-mass neutron star, rather than the maximum mass of a sequence, as the latter is much more sensitive to the polytropic extensions we perform in order to complete the EoS.
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