In astroparticle physics, an Urca process is a reaction which emits a

neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...

and which is assumed to take part in cooling processes in

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...

s and

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...

s. The process was first discussed by

George Gamow George Gamow (March 4, 1904 – August 19, 1968), born Georgiy Antonovich Gamov ( uk, Георгій Антонович Гамов, russian: Георгий Антонович Гамов), was a Russian-born Soviet and American polymath, theoret ...

and

Mário Schenberg Mário Schenberg (var. ''Mário Schönberg'', ''Mario Schonberg'', ''Mário Schoenberg''; July 2, 1914 – November 10, 1990) was a Brazilian electrical engineer, physicist, art critic and writer. Early life Schenberg was born in Recife, Brazil ...

while they were visiting a casino named Cassino da Urca in

Urca Urca is a traditional and wealthy residential neighborhood with nearly 7,000 inhabitants (2000 census) in Rio de Janeiro, Brazil. Although most of the neighborhood dates from the 1920s, parts of it are much older. What is now called the Forte Sã ...

Rio de Janeiro Rio de Janeiro ( , , ; literally 'River of January'), or simply Rio, is the capital of the state of the same name, Brazil's third-most populous state, and the second-most populous city in Brazil, after São Paulo. Listed by the GaWC as a b ...

. As Gamow recounts in his autobiography, the name was chosen in part to commemorate the gambling establishment where the two physicists had first met, and "partially because the Urca Process results in a rapid disappearance of thermal energy from the interior of a star, similar to the rapid disappearance of money from the pockets of the gamblers on the Casino de Urca." In Gamow's South Russian dialect, ''urca'' (russian: урка) can also mean a robber or gangster. The direct Urca processes are the simplest neutrino-emitting processes and are thought to be central in the cooling of neutron stars. They have the general form : where B and B are

baryon In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classif ...

s, is a lepton, and (and ) are (anti-)

s. The baryons can be

nucleon In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number). Until the 1960s, nucleons were ...

s (free or bound),

hyperon In particle physics, a hyperon is any baryon containing one or more strange quarks, but no charm, bottom, or top quark. This form of matter may exist in a stable form within the core of some neutron stars. Hyperons are sometimes generically re ...

s like , and , or members of the isobar. The lepton is either an

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

or a muon. The Urca process is especially important in the cooling of white dwarfs, where a lepton (usually an electron) is absorbed by the nucleus of an ion and then convectively carried away from the core of a star. Then, a beta decay occurs. Convection then carries the element back into the interior of the star, and the cycle repeats many times. Because the neutrinos emitted during this process are unlikely to be reabsorbed, this is effectively a cooling mechanism for white dwarfs. The process can also be essential in the cooling of neutron stars. If a neutron star contains a central core in which the direct Urca-process is operative, the cooling timescale shortens by many orders of magnitude.

References

{{DEFAULTSORT:Urca Process Astrophysics