The Castor Moving Group, or Castor stream, is a

moving group In astronomy, stellar kinematics is the Observational astronomy, observational study or measurement of the kinematics or motions of stars through space. Stellar kinematics encompasses the measurement of stellar Velocity, velocities in the Milky W ...

of stars sharing similar velocities and directions. The stars that have been identified as part of the group include Castor, Fomalhaut,

Vega Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only from the Sun, an ...

α Cephei Alpha Cephei (α Cephei, abbreviated Alpha Cep, α Cep), officially named Alderamin , is a second magnitude star in the constellation of Cepheus near the northern pole. The star is relatively close to Earth at 49 light years (''ly''). ...

and α Librae. While the reported group members are young (typically with ages of hundreds of millions of years), recent results suggest that the velocity differences amongst the moving group stars are too large for them to have possibly shared a common origin.

Discovery and constituents

The moving group was first proposed by J. P. Anosova, and V. V. Orlov in 1990. Anosova and Orlov originally proposed 15 members. In 1999, Barrado y Navascues presented a membership list of 16 stars, and estimated a group age of 200 million years. The membership of the group has not been well-established and varies quite a bit amongst studies.

Physicality of the Group

There is controversy over whether the Castor Moving Group constitutes a physical group of stars of shared origin (e.g. an "association") or a group of stars of heterogeneous age and chemical composition that happen to have somewhat similar velocities (e.g. a "dynamical stream"). In their 2013 paper describing the discovery of Fomalhaut C, Mamajek et al. point out that the 3D velocities of the famous Castor Moving Group members Fomalhaut,

, Castor, and

LP 944-20 LP 944-20 is a dim brown dwarf of spectral class M9 located about 21 light-years from the Solar System in the constellation of Fornax. With a visual apparent magnitude of 18.69, it has one of the dimmest visual magnitudes listed on the ...

are now very well-determined. They find that the star-to-star velocity differences are so large (typically ~5–10 km/s) that the stars were not within 100 pc of each other as recently as 10 Myr ago, let alone at the purported group age of ~200 million years. The local density of purported Castor Moving Group members is very low (roughly 0.4% of the mass density of the local Galactic disk), hence the systems are not bound to one another, and the high velocity dispersion can not be attributed to the dynamical interactions amongst the moving group members themselves. They conclude that at least those famous "members" of the Castor Moving Group ( Fomalhaut,

, Castor,

) are extremely unlikely to have shared a common origin.

Proposed member stars

* ζ Leporis *

* α¹ Librae * α² Librae * Castor *

DX Cancri DX Cancri is a variable star in the northern zodiac constellation of Cancer (constellation), Cancer. With an apparent visual magnitude of 14.81, it is much too faint to be seen with the naked eye. Visually viewing this star requires a t ...

* Fomalhaut * HD 51825 * ψ Velorum * Gliese 351A * Gliese 426 AB * GJ 408 * GJ 4247 * HD 117934 *

HD 119124 HD 119124 is a wide binary star system in the circumpolar constellation of Ursa Major. With an apparent visual magnitude of 6.3, it lies below the normal brightness limit of stars that are visible with the naked eye under most viewing con ...

* HD 162283 * HD 181321 * Gliese 842.2 *

Gliese 896 AB Gliese may refer to: * Rochus Gliese (1891—1978), a German actor, director, production designer, and art director * Wilhelm Gliese (1915–1993), a German astronomer, best known for the Gliese Catalogue of Nearby Stars * Gliese Catalogue of Nea ...

TW Piscis Austrini TW Piscis Austrini (also Fomalhaut B) is a dwarf star in the constellation Piscis Austrinus. It lies relatively close to the Sun, at an estimated distance of 24.8 light-years. To an observer on Earth the star is visually separated from ...

* κ Phoenicis *

References

{{Reflist, colwidth=30em, refs= {{cite journal , display-authors=1 , last1=Lestrade , first1=J.-F. , last2=Wyatt , first2=M. C. , last3=Bertoldi , first3=F. , last4=Dent , first4=W. R. F. , last5=Menten , first5=K. M. , title=Search for cold debris disks around M-dwarfs , journal=Astronomy and Astrophysics , volume=460 , issue=3 , pages=733–741 , date=December 2006 , doi=10.1051/0004-6361:20065873 , bibcode=2006A&A...460..733L , arxiv = astro-ph/0609574 {{cite journal , author = J. P. Anosova , author2=V. V. Orlov , date=December 1991 , title = The dynamical evolution of the nearby multiple stellar systems ADS 48, ADS 6175 (Alpha Geminorum = Castor), Alpha Centauri, and ADS 9909 (Xi Scorpii) , journal = Astronomy & Astrophysics , volume = 252 , pages = 123–126 , bibcode = 1991A&A...252..123A {{cite journal , author = D. Barrado y Navascues , year = 1998 , title = The Castor Moving Group: The age of Fomalhaut and Vega , journal = Astronomy & Astrophysics , volume = 339 , pages = 831–839 , arxiv = astro-ph/9905243 , bibcode = 1998A&A...339..831B {{cite journal , last1=Mamajek , first1=Eric E. , last2=Bartlett , first2=Jennifer L., author2-link=Jennifer Bartlett (astronomer) , last3=Seifahrt , first3=Andreas , title=The Solar Neighborhood XXX: Fomalhaut C , journal=

The Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...

, volume= 146, pages= 154, year=2013 , doi= 10.1088/0004-6256/146/6/154 , arxiv=1310.0764 , bibcode= 2013AJ....146..154M {{cite journal , title=Details of the spatial structure and kinematics of the Castor and Ursa Major streams , last1=Vereshchagin , first1=S. V. , last2=Chupina , first2=N. V. , journal=Baltic Astronomy , volume=24 , pages=421–425 , date=2015 , arxiv=1603.03159 , bibcode=2015BaltA..24..421V , doi=10.1515/astro-2017-0244

Discovery and constituents

Physicality of the Group

Proposed member stars

See also

References