Manuel Barranco

Clustering and phase transitions in hot, dense stellar matter

The nuclear Thomas-Fermi model is applied to the study of the clustering of nucleons in hot, dense matter. The several phase transitions which occur are discussed: (a) from the clustered to the homogeneous phase, (b) between the Coulomb fluid and the Coulomb crystal and, finally (c) the transition from nuclei to nucleon bubbles.

Thermal limit cycle oscillations on the surface of accreting neutron stars-X-ray bursters

With the help of a very simple two-zone model we deomonstrate the possiblity of periodic thermal relaxation (limit cycle) oscillations in the helium burning envelope of accreting neutron stars. Physically reasonable model parameters can be chosen which yield agreement with the observed features of X-ray bursts. We suggest that this limit cycle is operative in neutron stars which have an accretion rate in a specific range. For hydrogen burning a similar cycle is possible, but it operates at such high temperatures that an unrealistically large accretion rate would be required.

Multimode stellar pulsations. III. Resonances

We include the treatment of resonances in the formalism of multitime asymptotic techniques previously devised to treat stellar pulsation (monomode and multimode) with a slow evolution in the absence of resonances. The evolution can be followed toward and through a resonance, and the effects of resonances can be investigated. The technique is illustrated by a simple example, and various aspects which may be relevant to stellar pulsation are discussed.

Multimode stellar pulsations

We apply the two time method to weakly anharmonic (nonlinear) stellar pulsations. The problem contains two small parameters: ε-the ratio of the dynamical and the thermal timescale and μ-the measure of the anharmonicity in the oscillator equation. The 0(ɛ0) system is solved up to 0(μ) by the use of an asymptotic perturbation method. This solution is then used within the framework of the two time method to obtain the slow evolution of the amplitudes and entropy. Both monomode and double-mode pulsations are treated in a non-resonance case.

X-ray burst sources ? A simple two zone model

With the help of a very simple two zone model, we demonstrate the possibility of periodic thermal relaxation (limit cycle) oscillations in the helium burning envelope of accreting neutron stars. Physically reasonable model parameters can be chosen which yield agreement with the observed features of x-ray bursts and we suggest that this limit cycle is operative in neutron stars which have an accretion rate in a specific range. For hydrogen burning a similar cycle is possible, but it operates at such high temperatures that an unrealistically large accretion rate would be required.