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NASA -- larger image
Kapteyn's Star is a very dim red subdwarf, somewhat bluer and
dimmer than Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.
(See a 2MASS Survey image of Kapteyn's from the NASA Star
and Exoplanet Database.)
On June 3, 2014, a team of astronomers reported the discovery of two super-Earths orbiting this ancient star. Kapteyn b might support liquid water on is surface, although it has at least 4.8 Earth-masses and completes its orbit within only 48.6 days at an average orbital distance of 0.17, with an orbital eccentricity of 0.21. Kapteyn c is even more massive at a minimum of 7.0 Earth-masses and its year lasts 121.5 days at an average orbital distance of 0.31, with an orbital eccentricity of 0.23, and should be too cold to support liquid water. (CIS news release; and Anglada-Escudé et al, 2014).
Victor Robles, James Bullock, Miguel Rocha,
Joel Primack, UC Irvine, UC Santa Cruz
Larger composite illustration.
Two super-Earths have been detected around
Kapteyn's Star (an orphan star torn from an
ancient dwarf satellite galaxy of the Milky
Way), one within its habitable zone (more).
This star is located only about 12.8 light-years (ly) from our Sun, Sol, in the part (05:11:40.58-45:01:06.27, ICRS 2000.0) of Constellation Pictor, the Painter's Easel -- northwest of Beta Pictoris. Being much smaller and cooler than Sol, however, Kapteyn's Star is not visible to the naked eye. It has the second highest known proper motion after Barnard's Star, moving to the southeast at 8.7" per year. Based on its eccentric, retrograde galactic orbital path, the star may have been born a globular cluster similar to Omega Centauri (Wylie-de Boer et al, 2009; Ken Croswell, New Scientist, November 12, 2009; Kotoneva et al, 2005; and Ken Croswell, Astronomy, 2005).
According to Ken Croswell, the extremely high proper motion of this star was discovered in 1897 by Jacobus Cornelius Kapteyn (1851-1922) of the University of Groningen and by Robert Thorburn Ayton Innes (1861-1933) of Edinburgh, Scotland. Innes was observing from the Cape of Good Hope in South Africa, probably with the 7-inch refractor at the Royal Observatory. Kapteyn, who lacked a telescope, volunteered to measure photographic plates taken by David Gill (1843-1914), also at the Cape Observatory. As a result, Kapteyn developed the Cape Photographic Durchmusterung or CP(D), a catalog of 454,875 southern stars and values for the density of stars in space as a function of distance, brightness, and spectral class. In addition, his discovery of "star streaming" led to the concept of galactic rotation. The star appears to have been renamed for Kapteyn in the 1920s, after his death, possibly because he was the first to notice that the faint star listed in an old catalogue as "Cordoba Zone 5 hours 243" and missing from one of Gill's photographic plates until Innes found a star matching its description to the east of its original position. (More on Kapteyn).
Larger illustration of NASA's
Space Interferometry Mission (SIM).
Astronomers have identified Kapteyn's Star
as a "Tier 1" target for NASA's optical
SIM mission, now indefinitely delayed.
Due to Kapteyn's proximity to Sol and its deduced ancient origin from outside the galactic disk, the system has been an object of high interest among astronomers. The star has been selected as "Tier 1" target stars for NASA's optical Space Interferometry Mission (SIM). The mission will attempt to detect planets as small as three Earth-masses within two AUs of each star. Although some summary system information and images of Kapteyn's Star may still be available from the SIM Teams, the SIM project manager announced on November 8, 2010 that the mission was indefinitely postponed due to withdrawal of NASA funding.
|Inner H.Z. Edge?||0.126||29.8||0.0||?||...||...||...||...||...|
|Outer H.Z. Edge?||0.236||76.5||0.0||?||...||...||...||...||...|
Kapteyn's Star is a dim red subdwarf or main sequence (sdM0-1.5 or V), halo star (John E. Gizis, 1997, page 809; and NASA Star and Exoplanet Database), which is thought to be originally a member of the Milky Way galaxy's luminous halo. The star has around 7.4 percent of Sol's abundance of iron and a weighted mean metallicity abundance of elements heavier than hydrogen and helium of almost 10.5 percent of Sol's (Woolf and Wallerstein, 2004). Due to the scarcity of heavier elements, the star looks a bit more bluish than a main sequence red dwarf of class M. It may have around 29 to 39 percent of Sol's mass (Ken Croswell, 2005; and RECONS), around a third (29 to 32 percent) of its diameter, and less than 4/1,000th of its luminosity. It is a variable star designated VZ Pictoris. Some useful star catalogue numbers and designations for this star are: VZ Pic, Gl 191, Hip 24186, HD 33793, CD-45 1841, CP(D)-44 612, SAO 217223, LHS 29, LTT 2200, LFT 395, GC 6369, U 628, and Cordova Zone 5 hours 243.
According to a 2014 analysis, the inner edge of Kapteyn's habitable zone is located relatively close from the star at around 0.126 AUs from the star, while the outer edge lies farther out at around 0.236 AUs (Anglada-Escudé et al, 2014, Table 2 on page 4). Accounting for the relatively greater infrared output of M-stars like Kapteyn's Star, the distance from Kapteyn's where an Earth-type planet would have liquid water on its surface is centered around only 0.158 AU -- well within the orbital distance of Mercury in the Solar System. Assuming that Kapteyn's has 39 percent of Sol's mass, such a planet would complete its orbit around the star in less than 37 days at 0.158 AU -- less than six weeks.
A super-Earth, Kapteyn b might support liquid water on is surface. It has at least 4.8 Earth-masses and completes its orbit within only 48.6 days at an average orbital distance of 0.17, with an orbital eccentricity of 0.21. (CIS news release; and Anglada-Escudé et al, 2014).
An even larger super-Earth, Kapteyn c is more massive at a minimum of 7.0 Earth-masses. Its year lasts 121.5 days at an average orbital distance of 0.31, with an orbital eccentricity of 0.23. The planet, however, is located far enough from its dim host start that it should be too cold to support liquid water. (CIS news release; and Anglada-Escudé et al, 2014).
A subdwarf and halo star? - Subdwarfs, such as nearby Kapteyn's Star, are dimmer and more bluish than younger main-sequence dwarf stars (Wing et al, 1976). They have a lower "metals" content of elements heavier than helium. This low metallicity is thought to be due to their birth in an earlier age (or region) of the galaxy when relatively few supernovae had as yet spewed their metals into surrounding dust clouds (John E. Gizis, 1997). Astronomers had found 15 subdwarfs in the Solar neighborhood by 1998 (Fuchs and Jahreiß, 1998).
Most of the stars in the central bulge and in the globular clusters of the galactic halo are old, low metals stars, and halo stars account for only 0.1 to 0.2 percent of the stars near Sol (Ken Croswell, 1995, pp. 62-63). Kapteyn's Star is the closest known halo star, a member of a local cluster of halo stars called the "Kapteyn's star group" or stellar "moving group" that may be part of a local supercluster of halo stars distributed across 2,000 to 3,000 parsecs (6,500 to 9,800 light-years) into the Milky Way's galactic halo (Olin Jeuck Eggen, 1996).
Halo stars are distributed somewhat spherically around the galactic core but most members of the halo lie far above or far below the galactic plane. With extremely elliptical galactic orbits, they can move as far away as 100,000 light-years from the galactic center and as close a few thousand ly. Including the stars of the distant globular clusters, halo stars are among the galaxy's oldest, thought to be mostly 10 billion years and older. While halo stars are only very weakly concentrated towards the galactic plane, they exhibit a strong concentration towards and including the galactic nucleus but with highly eccentric orbits. As a group, these stars as a group show little if any net rotation around the galaxy, and so a halo member has a very negative V velocity ("retrograde direction"), since the Sun's motion around the galactic center is in the positive V direction.
These stars contain a very low metals abundance of one to 10 percent of Sol's (with a mean around two percent), but Kapteyn's metals deficiency as an M-type subdwarf is surprisingly mild compared with type K and G subdwarfs (J. R. Mould, 1976). While halo stars may total as few as 0.1 percent of the stars in the solar neighborhood, they include local subdwarfs, Kapteyn's Star and Groombridge 1830 (a G8VIp with "superflares" that is now believe to be a single star -- without an M-type flare star companion). Also called Population II stars because of their later discovery, this group also includes RR Lyrae variables with periods greater than 12 hours, subdwarfs and other extremely metal-poor stars, and some red giants.
Ata Sarajedini and
Larger and jumbo
Kapteyn's Star may
have been born as
a member of the
before being perturbed
into orbit around
the Milky Way (more).
In an October 20, 2009 pre-print, a group of astronomers looking at 16 stars in the Kapetyn stellar moving group hypothesized that Kapteyn's Star is one of 14 halo stars in the group with the same elemental abundance as many members of the Omega Centauri Globular Cluster, which lies some 17,000 light-years from Sol. The most luminous globular cluster observed from Earth, 12-billion-year-old Omega Centauri has stars of different age and trace elemental abundance groups, which suggest that the cluster may be the nucleus of a dwarf galaxy that merged with the Milky Way (Bekki and Freeman, 2005; and 2003). During the ancient merger, most of the outlying stars of the dwarf galaxy were perturbed into new retrograde orbits around the Milky Way's core, which appears to include Kapteyn's Star based on kinematic and chemical analysis (Wylie-de Boer et al, 2009; and Ken Croswell, New Scientist, November 12, 2009).
The following star systems are located within 10 ly of Kapteyn's Star.
|Star System||Spectra &|
|LHS 1565||M5.5 V||3.7|
|(LP 944-20)||brown dwarf||6.1|
|Sirius 2||A0-1 Vm |
|Epsilon Eridani||K2 V||8.5|
|82 Eridani 2||G5-8 V-IV |
|Gliese 229||M2-4 V||9.3|
Up-to-date technical summaries on this star can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS, the NASA Star and Exoplanet Database, the SIMBAD Astronomical Database, and the Research Consortium on Nearby Stars (RECONS) list of the 100 Nearest Star Systems. Additional information may be available at Roger Wilcox's Internet Stellar Database.
Originally named Equuleus Pictoris, Constellation Pictor is an invention of the Abbé [Abbot] Nicholas Louis de La Caille (1713-1762), in the 1750s. Based on a crooked line, the constellation is supposed to represent a painter's easel, which some opine is a clue to Lacaille's lack of imagination in filling the southern skies with new constellations. For more information on the objects in this constellation, go to Christine Kronberg's Pictor. For an illustration, see David Haworth's Pictor.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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