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{{short description|Phenomenon in astroparticle physics}}
In [[astroparticle physics]], an '''Urca process''' is a reaction which emits a [[neutrino]] and which is assumed to take part in cooling processes in [[neutron star]]s and [[white dwarf]]s.  The process was first discussed by [[George Gamow]] and [[Mário Schenberg]] while they were visiting a casino named [[Cassino da Urca]] in [[Urca]], [[Rio de Janeiro]].  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."<ref>{{cite book|last=Gamow|first=George|title=My World Line: An Informal Autobiography|publisher=The Viking Press|year=1970|location=New York|page=137}}</ref> In Gamow's South Russian dialect, ''urca'' ({{langx|ru|урка}}) can also mean a robber or gangster.<ref>
{{Cite encyclopedia
 |author=D. Darling
 |title=Urca process
 |url=http://www.daviddarling.info/encyclopedia/U/Urca_process.html
 |encyclopedia=The Internet Encyclopedia of Science
 |access-date=2010-03-02
}}</ref><ref>
{{Cite journal
 |author=D. K. Nadyozhin
 |date=1995
 |title=Gamow and the physics and evolution of stars
 |journal=[[Space Science Reviews]]
 |volume=74 |issue=3–4 |pages=455–461
 |doi=10.1007/BF00751432
|bibcode = 1995SSRv...74..455N |s2cid=122987210
 |bibcode-access=free}}</ref>

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
:{|
| [[Baryon|B{{su|b=1}}]] ||   ||                                       || → || B{{su|b=2}} || + || {{SubatomicParticle|Lepton|link=yes}} || + || {{SubatomicParticle|Lepton antineutrino|link=yes}},
|------------------------------------------
| B{{su|b=2}}            || + || {{SubatomicParticle|Lepton|link=yes}} || → || B{{su|b=1}} || + ||{{SubatomicParticle|Lepton neutrino|link=yes}},
|}

where B{{su|b=1}} and B{{su|b=2}} are [[baryon]]s, {{SubatomicParticle|Lepton}} is a [[lepton]], and {{SubatomicParticle|Lepton neutrino}} (and {{SubatomicParticle|Lepton antineutrino}}) are (anti-)[[neutrino]]s. The baryons can be [[nucleon]]s (free or bound), [[hyperon]]s like {{SubatomicParticle|Lambda}}, {{SubatomicParticle|Sigma}} and {{SubatomicParticle|Xi}}, or members of the {{SubatomicParticle|link=yes|Delta}} [[Isobar (nuclide)|isobar]]. The [[lepton]] is either an [[electron]] 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.<ref>
{{Cite web
 |author      = M. Brueggen
 |author2     = A. Kercek
 |date        = 10 January 2000
 |title       = The convective URCA process in white dwarfs
 |url         = http://www.mpa-garching.mpg.de/HIGHLIGHT/2000/highlight0001_e.html
 |publisher   = [[Max Planck Institute]]
 |access-date  = 2010-02-05
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20100112000659/http://www.mpa-garching.mpg.de/HIGHLIGHT/2000/highlight0001_e.html
 |archive-date = 12 January 2010
 |df          = dmy-all
}}</ref>

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.<ref>
{{Cite journal
 |author=P. Haensel
 |date=1995
 |title=Urca processes in dense matter and neutron star cooling
 |journal=[[Space Science Reviews]]
 |volume=74 |issue=3–4 |pages=427–436
 |doi=10.1007/BF00751429
|bibcode = 1995SSRv...74..427H |s2cid=115313950
 |bibcode-access=free}}</ref>

==References==
{{Reflist}}

{{neutron star}}
{{White dwarf}}
{{Use dmy dates|date=April 2020}}

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[[Category:Concepts in astrophysics]]