Theoretical nuclear astrophysics at the University of Tennessee, Knoxville and Oak Ridge National Laboratory

Supported by: |

- PI: Joint Fac. Asst. Prof. Andrew W. Steiner
- Postdoc: Dr. Sophia Han
- Graduate students: Spencer Beloin, Xingfu Du, and Will Moffitt

My research demonstrates that neutron star observations can be used to understand how neutrons and protons interact. Also, I have shown that nuclear physics is critical for describing neutron star observations and many astrophysical processes including core-collapse supernovae, X-ray bursts and giant flares in magnetars. Recently, I determined how neutron star radius observations improve our knowledge of nuclear three-body forces, the neutron skin thickness of lead, and the nuclear symmetry energy.

In a new paper posted to the arxiv, Matt Mumpower, Gail McLaughlin, and Rebecca Surman, and I show that the abundances in the rare earth peak require a specific pattern of nuclear masses in order to reproduce observed abundances depending on the astrophysical site that one assumes for the r-process. Many of the relevant nuclear masses are too far from stability to measure, but a few of them are within reach of FRIB. A combination of nuclear mass measurements, and models like ours which directly connect these measurements to r-process abundances, may determine the astrophysical site of the rare earth peak in the near future.

In a collaboration with Or Hen, Eli Piasetzky, and Larry Weinstein, we show that using a correlated Fermi gas (CFG) model for the symmetry energy results is as compatible with neutron star mass and radius observations as the naive Fermi gas (FFG) model. The demarcation of kinetic and potential energy parts of the symmetry energy is drastically different the CFG and FFG models, but when compared at the same values of S and L, the density dependence of the sum is very similar. Our paper is posted on the arXiv.

The rare earth peak is a peak in the observed abundances of r-process nuclei in the A=165 mass region. In a collaboration with Matt Mumpower, Gail McLaughlin, and Rebecca Surman, we determine the nuclear masses of neutron-rich nuclei which are required to produce the rare earth peak. For example, for a very neutron-rich cold r-process, a strong kink in the nuclear masses near Z=60 and N=100 is required to reproduce observations. Matt's method is particularly important because it treats the beta-decay and neutron-capture rates self-consistently, instead of treating the reaction rates as completely separate from the nuclear structure input. Our paper is on the arXiv.

In work led by Joonas Nättilä at the University of Turku in Finland, we determine the radii of neutron stars using PRE X-ray bursts in the hard state. The cooling tail of the burst is matched to a quantitative model of the neutron star atmosphere to obtain a faithful description of the neutron star structure. We find that the radius of a 1.4 \( \mathrm{M}_{\odot} \) neutron star is, to within 95% confidence, between 10.5 and 12.8 km. Our paper is published in Astron. & Astrophys. and posted at the arXiv.

Sophia Han and her colleagues recently discovered (article in Phys. Rev. C) why Nambu-Jona-Lasinio models tend to disfavor hybrid neutron stars where deconfined quarks appear in the core. Assuming that the surface tension at the interface between hadronic matter and quark matter is large, they find that quark matter either destabilizes the star (thus leading to a black hole) or results in a very short hybrid star branch in the mass-radius relation. The constant-sound-speed parameterization (originally created for Han's work in Phys. Rev. D), shows that the microphysical origin of this behavior is a strong first-order phase transition with a large transition pressure. This article is also available on the arXiv.

In a Rev. Mod. Phys. colloquium paper written by a team led by Anna Watts, we review the potential of hard X-ray timing instruments to determine neutron star masses and radii. In particular, instruments which have an effective area of 10 \( \mathrm{m}^2 \) in the 2-30 keV band (as would be provided by the Large Observatory for X-ray Timing) can potentially lead to neutron star mass and radius constraints on the few percent level. Our article is also available on the arXiv.

Jim Lattimer, Ed Brown, and I show how prior distributions affect our the relationship between, for example, the radius of a 1.4 \( \mathrm{M}_{\odot} \) neutron star, the maximum mass, and the pressure of the EOS at various densities. We find that neutron star radii are unlikely to be smaller than 10 km, independent of the choice of prior distribution. We also examine new universal relations between moment of inertia, binding energy, compactness, and tidal deformability. Our paper is published in EPJA.

In a collaboration led by Chris Fryer at LANL, we determine how the fate of a neutron star merger is connected with the maximum mass of the cold non-rotating neutron star. In our article in ApJ, we show that if the non-rotating maximum mass is smaller than 2.3-2.4 \( \mathrm{M}_{\odot} \), then the remnant will likely be a black hole. Our paper was highlighted in the AAS publication Nova. The arXiv version is here.

In an article published in Phys. Rev. C, Paulo Bedaque and I use a phenomenological model of hyperons in nuclear matter and quantify the location at which the interaction between Lambda hyperons and dense matter must become repulsive (e.g. through \( n n \Lambda \) or \( n \Lambda \Lambda \) interactions) in order to ensure the creation of a two solar mass neutron star. This paper is also available on the arXiv.

In a new article, at arXiv:1407.0100, I describe how Bayesian analysis has been used to analyze neutron star observations and why the best fit and a covariance matrix is not enough to describe the likelihood. This leads to a proposal on improving fits to low-energy nuclear data and a method for modern constraints on the nuclear symmetry energy. This article is now published in J. Phys. G.

Paulo
Bedaque and I show, in an article in
Physical
Review Letters,
that the measurement of a two solar mass neutron star
implies that the speed of sound must be larger than \(
c/\sqrt{3} \) somewhere inside neutron star interiors.
Neutron stars appear to be the *only place in the
universe* where the speed of sound is this large.
This page has a more detailed
description of our result for non-scientists.