Session DE: Nuclear Astrophysics I

Constraining Nuclear Symmetry Energy from Converting Neutron Star EoS into HI Collisions

The symmetry energy expansion is important for connecting nuclear experiments to neutron stars.  However, the expansion coefficients have significant error bars.  In our study, we take the symmetry energy expansion with 4 coefficients and focus specifically on the possibility that neutron stars may have a large bump in the speed of sound, motivated by the observation of heavy neutron stars. 

A natural question is whether this behavior is compatible with heavy-ion experimental data. We explore the entire range allowed by the experimentally obtained coefficients to produce a prior distribution for the speed of sound functional using charge constraints compatible with heavy-ion experiments. Only a fraction of the resulting equations of state are causal and stable. By imposing stability and causality, we find in the posterior distribution that a large bump in the speed of sound can further constrain the symmetry energy expansion coefficients.    

Measuring the 15O(α, γ)19Ne reaction rate in Type I X-ray bursts using the GADGET II TPC

A neutron star can accrete H/He-rich material from a low-mass stellar companion (e.g., a main sequence star or a red giant). This can lead to thermonuclear runaway, which manifests as a Type I X-ray burst in a space-based telescope. Sensitivity studies have shown that the ¹⁵O(α, γ)¹⁹Ne reaction is one of the most important reaction rate uncertainties affecting the modeling of the resulting light curve. This reaction is expected to be dominated by the 4.03 MeV resonance in ¹⁹Ne. This state has a well known lifetime, so only a finite value for the alpha-particle branching ratio is needed to determine the reaction rate. Previous measurements have shown that this state is populated in the decay sequence of ²⁰Mg. ²⁰Mg(βpα)¹⁵O events through the key ¹⁵O(α, γ)¹⁹Ne resonance yield a characteristic signature: the emission of a proton and alpha particle. To identify these coincidence events we have upgraded the GADGET Proton Detector into a time projection chamber to form the GADGET II detection system. GADGET II has been constructed, and is in the final testing phase before FRIB PAC approved experiment 21072. Additionally, machine learning will be leveraged in analysis by fine-tuning a pre-trained convolutional neural network to identify the proton-alpha events of interest.   

Neutron star mergers as a probe of the quark matter equation of state

We perform the first simulations of binary neutron-star mergers which adopt various parameterizations of the quark-hadron crossover (QHC) equation of state (EOS). These are constructed from combinations of a hardronic EOS (up tp 2 times the nuclear satuation density) and a quark-matter EOS (2-5 times the saturation density). At the crossover densities the QHC EOSs have a gradually increasing stiffness approaching the stiffness of the strongly correlated quark matter. This enhanced stiffness leads to significantly longer lifetimes of the hypermassive neutron star than that for a pure hadronic EOS. We find a dual nature of these EOSs such that their maximum chirp gravitational wave frequencies fall into the category of a soft EOS while the dominant peak frequencies of the postmerger stage falls in between that of a soft and stiff hadronic EOS. An observation of this kind of dual nature in the characteristic gravitational wave frequencies will provide crucial evidence for the existence of strongly interacting quark matter at the crossover densities.   

Evolution of Urca Pairs in the Crusts of Highly Magnetized Neutron Stars

The effects of strong magnetic fields on neutrino emission in the modified Urca process are studied. Landau levels on the various Urca pairs affects the neutrino emission spectrum and leads to an angular asymmetry in the neutrino emission on neutron star surfaces. For low magnetic fields the Landau levels have almost no effect on the cooling. However, as the field strength increases, the electron chemical potential increases resulting in a lower density at which Urca pairs can exist. For intermediate field strength there is an interesting interference between the Landau level distribution and the Fermi distribution. For high enough field strength, the entire electron energy spectrum is eventually confined to single Landau level producing dramatic spikes in the emission spectrum. As a result, neutrino emissivity depends on polar angle on the stellar surface with respect to the magnetic poles. The overall luminosity is affected. In addition, because the electron phase space is altered in the presence of a magnetic field Urca pairs may be prohibited in a magnetic field, new Urca pairs may emerge, and Urca pairs involving highly-excited states may evolve.   

C++ zero temperature Chiral Mean Field implementation with spinodal solutions

The CMF model, which is used to describe neutron stars, neutron star mergers, and heavy-ion collisions, is a very successful model for the equation of state at large baryon densities. This proprietary was initially written in Fortran77 and relies on legacy routines. In this work, we report the updates of a C++ implementation of the CMF model for zero temperature within the MUSES collaboration. GNU GSL has been adopted for integration and multidimensional root-finding algorithms. In our code, metastable solutions are reachable; therefore, spinodal solutions of the field can be studied. Finally, this new code allows the computation of higher-order derivatives required for the nuclear matter hydrodynamical evolution codes.

    

Binary Love Relation for the Inspiral of Heavy Neutron Stars

Because of recent measurements of heavy neutron stars of at least 2 solar masses, there appears to be a strong preference for equations of state of neutron stars that have a sharp rise in the speed of sound as a function of baryon number density above the conformal limit.  It was shown in [1] that the Binary Love relation depends on a specific feature of the EoS, which is the baryon number density at which the rise in the speed of sound occurs.  Specifically, it was found that one could measure a change in the slope of the Binary Love relation. Here we use a modified Gaussian process to study ~100K equations of state with sharp rises in the speed of sound to study the connection between the rise in the speed of sound and the slope change in the binary love relation.

[1] H. Tan, et al, Phys.Rev.Lett. 128 (2022) 16, 161101   

r-Process Radioisotopes from Near-Earth Supernovae and Kilonovae

The astrophysical sites where ??-process elements are synthesized remain mysterious: it is clear that neutron-star-mergers (kilonovae, KNe) contribute, and some classes of core-collapse supernovae (SNe) are also possible sources of at least the lighter ??-process species. The discovery of 60Fe on the Earth and Moon implies that one or more astrophysical explosions have occurred near the Earth within the last few Million years (Myr), probably SNe. Intriguingly, 244Pu has now been detected, mostly overlapping with 60Fe pulses. However, the 244Pu flux may extend to before 12Myr ago, pointing to a different origin. Motivated by these observations and difficulties for ??-process nucleosynthesis in SN models, we propose that ejecta from a KN enriched the giant molecular cloud that gave rise to the Local Bubble where the Sun resides. Accelerator Mass Spectrometry (AMS) measurements of 244Pu and searches for other live isotopes could probe the origins of the ??-process and the history of the solar neighborhood, including triggers for mass extinctions, e.g., at the end of the Devonian epoch, motivating the calculations of the abundances of live ??-process radioisotopes produced in SNe and KNe that we present here. Given the presence of 244Pu, other ??-process species such as 93Zr, 107Pd, 129I, 135Cs, 182Hf, 236U, 237Np and 247Cm should be present. Their abundances and well-resolved time histories could distinguish between SN and KN scenarios, and we discuss prospects for their detection in deep-ocean deposits and lunar regolith. We show that AMS 129I measurements in Fe-Mn crusts already constrain a possible nearby KN scenario. Thus, we urge searches for ??-process radioisotopes in deep-ocean Fe-Mn crusts, and in the lunar regolith samples brought to Earth recently by the Chang'e-5 lunar mission and upcoming missions including Artemis.   

Structure and Stability of Charm Quark Stars

In this work, we investigate the properties of quark matter, which is electrically neutral and in chemical equilibrium. It has been hypothesized that such matter could exist in the ultradense cores of neutron stars and could also lead to the existence of a new class of compact stars known as (strange) quark stars. The focus of our investigation is on the possible existence of such objects which, in addition to up, down, and strange quarks, also contain charm quarks. Such objects could possibly represent a new family of stable compact stars if the phase transition for charm quark production is discontinuous (Maxwell-like). The theoretical conditions under which stable charm stars could form are investigated and the implications of the existence of such objects for astrophysics is pointed out.   

Model constraints for degenerate neutron capture rates in neutron star crusts

Heavy element synthesis within stellar bodies typically manifests in explosive environments such as neutron star mergers. However, at the low temperature and high density conditions of a neutron star crust, degenerate neutrons provide alternate synthesis pathways compared to conventional systems. In this work, we study the effect of this degeneracy on neutron capture rates by several rp-process ashes and neutron-rich nuclei within accreting neutron stars. We consider strongly interacting asymmetric nuclear matter and its effect on the neutron chemical potential and therefore on the capture rates. We then investigate variations in the nuclear physics input which constructs the absorption cross section, and their effects on the reaction rate in the context of degenerate neutron capture. Finally, we propose an analytic approximation for highly degenerate neutron capture rates. Our results may help interpret the abundance evolution of rp-process ashes.