arXiv subjects

F. Castillo

Publications and source records attributed to F. Castillo.

6 recordsLinked to original sources

arXiv2025

The advanced evolution of massive stars: I. New reaction rates for carbon and oxygen nuclear reactions

The nuclear rates for reactions involving 12C and 16O are key to compute the energy release and nucleosynthesis of massive stars during their evolution. These rates shape the stellar structure and evolution, and impact the nature of the final compact remnant. We explore the impact of new nuclear reaction rates for 12C({\alpha},{\gamma})16O, 12C+12C, 12C+16O and 16O+16O reactions for massive stars. We aim to investigate how the structure and nucleosynthesis evolve and how these processes influence the stellar fate. We computed stellar models using the GENEC code, including updated rates for 12C({\alpha},{\gamma})16O and, for the three fusion reactions, new rates following a fusion suppression scenario and new theoretical rates obtained with TDHF calculations. The updated 12C({\alpha},{\gamma})16O rates mainly impact the chemical structure evolution changing the 12C/16O ratio with little effect on the CO core mass. This variation in the 12C/16O ratio is critical for predicting the stellar fate, which is very sensitive to 12C abundance. The combined new rates for 12C+12C and 16O+16O fusion reactions according to the HIN(RES) model lead to shorter C- and O-burning lifetimes, and shift the ignition conditions to higher temperatures and densities. Theoretical TDHF rates primarily affect C-burning, increasing its duration and lowering the ignition temperature. These changes alter the core chemical structure, the carbon shell size and duration, and hence the compactness. They also affect nucleosynthesis. This work shows that accurate reaction rates for key processes in massive star evolution drive significant changes in stellar burning lifetimes, chemical evolution, and stellar fate. In addition, discrepancies between experimental and theoretical rates introduce uncertainties in model predictions, influencing both the internal structure and the supernova ejecta composition.

astro-ph.SR↗

arXiv2025

Magnetothermal evolution of neutron star cores in the `weak-coupling' regime: implications of ambipolar diffusion for the quiescent X-ray luminosity of magnetars

The high quiescent X-ray luminosity observed in some magnetars is widely attributed to the decay and evolution of their ultra-strong magnetic fields. Several dissipation mechanisms have been proposed, each operating with different efficiencies depending on the region of the star. In this context, ambipolar diffusion, i.e., the relative motion of charged particles with respect to neutrons in the neutron star core, has been proposed as a promising candidate due to its strong dependence on magnetic field strength and its capacity to convert magnetic energy into heat. We perform axisymmetric magnetohydrodynamic simulations to study the long-term magnetic evolution of a NS core composed of normal (non-Cooper paired) matter under the influence of ambipolar diffusion. The core is modeled as a two-fluid system consisting of neutrons and a charged-particle fluid (protons and electrons), coupled to the magnetic field. Simulations are performed both at constant and variable temperatures. In the latter case, a strategy that decouples the magnetic and thermal evolution is employed, enabling efficient thermal modeling across a range of initial magnetic field strengths. At constant temperature, we obtained the expected result where neutrons reach diffusive equilibrium, the Lorentz force is balanced by chemical potential gradients of charged particles, and the magnetic field satisfies a non-linear Grad-Shafranov equation. When thermal evolution is included, fields $B \gtrsim 5 \times 10^{15} \,\text{G}$ can balance ambipolar heating and neutrino cooling, delaying the evolution over $\sim 10^{3} \,[B/(5 \times 10^{15}\,\text{G})]^{-6/5}$ yr. Although the surface luminosity is enhanced compared to passive cooling, the heating from ambipolar diffusion alone is insufficient to fully explain the persistent X-ray emission observed in magnetars.

astro-ph.HE↗

arXiv2025

Validating and improving two-fluid simulations of the magnetic field evolution in neutron star cores

This paper addresses the evolution of an axially symmetric magnetic field in the core of a neutron star. The matter in the core is modeled as a system of two fluids, namely neutrons and charged particles, with slightly different velocity fields, controlled by their mutual collisional friction. This problem was addressed in our previous work through the so-called ``fictitious friction'' approach. We study the validity of our previous work and improve it by comparing the fictitious friction approach to alternatives, making approximations that allow it to be applied to arbitrary magnetic field strengths and using realistic equations of state. We assume the neutron star crust to be perfectly resistive, so its magnetic field reacts instantaneously to changes in the core, in which we neglect the effects of Cooper pairing. We explore different approaches to solve the equations to obtain the velocities and chemical potential perturbations induced by a given, fixed magnetic field configuration in the core. We also present a new version of our code to perform time-evolving simulations and discuss the results obtained with it. Our calculations without fictitious friction further confirm that bulk velocity is generally much greater than ambipolar velocity, leading to faster evolution. These findings align with those with fictitious friction, validating this approach. We also find that, in the long term, the star evolves towards a barotropic ``Grad-Shafranov equilibrium,'' where the magnetic force is fully balanced by charged particle fluid forces. Qualitatively, the evolution and the final equilibrium are independent of the magnetic field strength $B$ and the equation of state considered. The timescale to reach this equilibrium is proportional to $B^{-2}$ and becomes shorter for equations of state with a smaller gradient of the ratio between the densities of protons and neutrons.

astro-ph.HE↗

arXiv2020

Two-fluid simulations of the magnetic field evolution in neutron star cores in the weak-coupling regime

In a previous paper, we reported simulations of the evolution of the magnetic field in neutron star cores through ambipolar diffusion, taking the neutrons as a motionless uniform background. However, in real neutron stars, neutrons are free to move, and a strong composition gradient leads to stable stratification (stability against convective motions) both of which might impact on the time-scales of evolution. Here we address these issues by providing the first long-term two-fluid simulations of the evolution of an axially symmetric magnetic field in a neutron star core composed of neutrons, protons, and electrons with density and composition gradients. Again, we find that the magnetic field evolves towards barotropic "Grad-Shafranov equilibria", in which the magnetic force is balanced by the degeneracy pressure gradient and gravitational force of the charged particles. However, the evolution is found to be faster than in the case of motionless neutrons, as the movement of charged particles (which are coupled to the magnetic field, but are also limited by the collisional drag forces exerted by neutrons) is less constrained, since neutrons are now allowed to move. The possible impact of non-axisymmetric instabilities on these equilibria, as well as beta decays, proton superconductivity, and neutron superfluidity, are left for future work.

astro-ph.HE↗

arXiv2017

Magnetic field evolution and equilibrium configurations in neutron star cores: the effect of ambipolar diffusion

As another step towards understanding the long-term evolution of the magnetic field in neutron stars, we provide the first simulations of ambipolar diffusion in a spherical star. Restricting ourselves to axial symmetry, we consider a charged-particle fluid of protons and electrons carrying the magnetic flux through a motionless, uniform background of neutrons that exerts a collisional drag force on the former. We also ignore the possible impact of beta decays, proton superconductivity, and neutron superfluidity. All initial magnetic field configurations considered are found to evolve on the analytically expected time-scales towards "barotropic equilibria" satisfying the "Grad-Shafranov equation", in which the magnetic force is balanced by the degeneracy pressure gradient, so ambipolar diffusion is choked. These equilibria are so-called "twisted torus" configurations, which include poloidal and toroidal components, the latter restricted to the toroidal volumes in which the poloidal field lines close inside the star. In axial symmetry, they appear to be stable, although they are likely to undergo non-axially symmetric instabilities.

astro-ph.HE↗

arXiv2010

On the characterization of a random monolayer of particles from coherent optical reflectance

We present the viability of obtaining the particle size and surface coverage in a monolayer of polystyrene particles adsorbed on a glass surface from optical coherent reflectance data around the critical angle in an internal reflection configuration. We have found that fitting a CSM to optical reflectivity curves in an internal reflection configuration around the critical angle with a dilute random monolayer of particles adsorbed on the surface can in fact provide the particle's radius and surface coverage once the particles are sufficiently large.

physics.optics↗