Impact of Rotation-Driven Particle Repopulation on the Thermal Evolution of Pulsars

TL;DR

This paper investigates how the loss of rotational energy in pulsars triggers the direct Urca process, significantly affecting their thermal evolution and potentially explaining observed temperature anomalies like that of Cas A.

Contribution

It demonstrates that rotation-driven particle processes can activate the direct Urca process in neutron stars, influencing their cooling and matching observational data.

Findings

Rotation-driven direct Urca process can enhance neutron star cooling.

Conditions for the process depend on initial spin rates and mass.

Explains the temperature evolution of the neutron star in Cas A.

Abstract

Driven by the loss of energy, isolated rotating neutron stars (pulsars) are gradually slowing down to lower frequencies, which increases the tremendous compression of the matter inside of them. This increase in compression changes both the global properties of rotating neutron stars as well as their hadronic core compositions. Both effects may register themselves observationally in the thermal evolution of such stars, as demonstrated in this Letter. The rotation-driven particle process which we consider here is the direct Urca (DU) process, which is known to become operative in neutron stars if the number of protons in the stellar core exceeds a critical limit of around 11% to 15%. We find that neutron stars spinning down from moderately high rotation rates of a few hundred Hertz may be creating just the right conditions where the DU process becomes operative, leading to an observable effect (enhanced cooling) in the temperature evolution of such neutron stars. As it turns out, the rotation-driven DU process could explain the unusual temperature evolution observed for the neutron star in Cas A, provided the mass of this neutron star lies in the range of 1.5 to 1.9 \msun and its rotational frequency at birth was between 40 (400 Hz) and 70% (800 Hz) of the Kepler (mass shedding) frequency, respectively.