Modified Urca neutrino emissivity at finite temperature

TL;DR

This paper revises calculations of modified Urca neutrino emissivity in dense, hot neutron star matter, revealing that these processes are not always suppressed relative to direct Urca processes, impacting models of neutron star thermal evolution.

Contribution

It provides new expressions and numerical evaluations of modified Urca neutrino rates considering various conditions, challenging previous assumptions about their suppression.

Findings

Modified Urca process can be comparable to direct process at certain densities and temperatures.

Numerical results confirm that modified Urca rates are not always suppressed.

Analytic approximation estimates the ratio of modified to direct Urca rates.

Abstract

Charged current neutrino-nucleon reactions, generally called Urca processes, are crucial actors of a neutron star's thermal evolution. The so-called direct processes show a pronounced threshold under which the reaction is kinematically suppressed. This suppression does not apply to "modified" Urca processes which involve interaction with an additional nucleon. Calculations of the modified Urca neutrino rates were established for cold neutron star matter and for dilute hot matter, in both cases under strong assumptions. In this paper, we revise the calculations of the modified Urca neutrino rates for dense and hot matter, and for different compositions. We study the influence of different approximations used in previous computations. We derive expressions for the rates of modified Urca neutrino emissivity within thermal field theory and perform the phase space integration numerically using mainly importance sampling Monte-Carlo techniques. The neutrino emissivity of modified and direct processes are established and compared. We find in particular that the modified Urca process is not necessarily suppressed with respect to the direct process above the threshold of the latter at moderate densities and temperatures, in contrast to what is generally assumed. Numerical results are confirmed by an estimation of the ratio of modified to direct Urca rates with a simple analytic approximation, thereby showing the regimes of suppression for the modified processes depending on temperature and density. These results show that modified Urca rates have to be considered carefully upon evaluating neutrino opacities in dense and warm matter.