Cooling of Neutron Stars and Urca-process

Orbb

Hi,

in a lecture I was told that the cooling of neutron stars cannot happen via the 'direct' Urca process (beta decay and inverse beta decay), because the created particles cannot go into occupied states (Pauli exclusion). The 'modified' Urca process was introduced, with an additional neutron catalysing the reaction. Now what is the crucial difference here, that makes the modified Urca process an allowed one as opposed to the direct Urca?

Thank you for your answers.

Chronos

That is a hot topic [pardon the pun] in theoretical physics. The direct urca process cannot work on a neutron star for reasons as stated. But, neutron stars must eventually cool to remain in conformance with the laws of thermodynamics. The modified urca process is an attempt to satisfy this demand. I dont particularly care for the additional neutron thing as a solution. Quark soup is a messy affair.

Orbb

Okay, but why is the modified Urca allowed? Has it to do with cooper pairing leading to different statistics?

Chronos

That is one possibility. See, for example:
Neutrino Emission from Cooper Pairs and Minimal Cooling of Neutron Stars
http://arxiv.org/abs/0906.1621

cecilemangana

I think that arguments and estimates based on fundamental relations of many-body theory show that one realization of this phenomenon could produce very rapid cooling of the star via a direct nucleon Urca process displaying a T5 dependence on temperature.
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Chronos

What we see appears to suggest neutron stars cool very near the low limit required by thermodynamics. I believe that rules out the URCA process. Im guessing here, as are most theorists.