Kapteyn's Star

Kapteyn's Star

Location of Kapteyn's Star in the constellation Pictor
Observation data Epoch J2000      Equinox J2000
Constellation	Pictor
Right ascension	Template:RA
Declination	Template:DEC
Apparent magnitude (V)	Template:Val
Characteristics
Spectral type	sdM1 or M1.5V
U−B Template:Engvar	+1.21
B−V Template:Engvar	Template:Val
Variable type	BY Dra
Astrometry
Radial velocity (Rv)	Template:Val km/s
Proper motion (μ)	RA: Template:Val mas/yr Dec.: Template:Val mas/yr
Parallax (π)	254.1986±0.0168 mas
Distance	Template:Rnd ± Template:Rnd ly (Template:Rnd ± Template:Rnd pc)
Absolute magnitude (MV)	10.89
Details
Mass	Template:Val Template:Solar mass
Radius	Template:Val Template:Solar radius
Luminosity	0.012 Template:Solar luminosity
Surface gravity (log g)	Template:Val cgs
Temperature	Template:Val K
Metallicity [Fe/H]	Template:Val dex
Rotation	Template:Val
Rotational velocity (v sin i)	~0.2 km/s
Age	Template:Val Gyr
Metallicity [Fe/H]	{{{metal_fe2}}} dex
Other designations
Template:Odlist
Database references
SIMBAD	The star
	planet b
	planet c
Exoplanet Archive	data

Kapteyn's Star is a class M1 red subdwarf about 12.83 light-years from Earth in the southern constellation Pictor; it is the closest halo star to the Solar System. With an apparent magnitude of nearly 9, it is visible through binoculars or a telescope.

Its diameter is 30% of the Sun's, but its luminosity just 1.2% that of the Sun. It may have once been part of the globular cluster Omega Centauri, itself the likely core of a dwarf galaxy swallowed up by the Milky Way in the distant past. The discovery of two planets—Kapteyn b and Kapteyn c—was announced in 2014, but had a mixed history of rejections and confirmations, until a 2021 study refuted both planets. The "planets" are in fact artifacts of the star's rotation and activity.

History of observations

Attention was first drawn to what is now known as Kapteyn's Star by the Dutch astronomer Jacobus Kapteyn in 1898. Under the name CPD-44 612 it was included in the Cape photographic Durchmusterung for the equinox 1875 (−38 to −52) by David Gill and Jacobus Cornelius Kapteyn in 1897.Template:R This catalogue was based on Gill's observations from the Cape Observatory in 1885–1889 and was created in collaboration with Kapteyn. While he was reviewing star charts and photographic plates, Kapteyn noted that a star, previously catalogued in 1873 by B. A. Gould as C.Z. V 243, seemed to be missing. However, Robert T. A. Innes found an uncatalogued star about 15 arcseconds away from the absent star's position. It became clear that the star had a very high proper motion of more than 8 arcseconds per year and had moved significantly.

Later, CPD-44 612 came to be referred to as Kapteyn's Star although equal credit should be accorded to Robert Innes. At the time of its discovery it had the highest proper motion of any star known, dethroning Groombridge 1830. In 1916, Barnard's Star was found to have an even larger proper motion. In 2014, two super-Earth planet candidates in orbit around the star were announced, but later refuted.

Characteristics

Based upon parallax measurements, Kapteyn's Star is Template:Convert from the Earth. It came within Template:Cvt of the Sun about 10,900 years ago and has been moving away since that time. Kapteyn's Star is distinctive in a number of regards: it has a high radial velocity, orbits the Milky Way retrograde, and is the nearest-known halo star to the Sun. It is a member of a moving group of stars that share a common trajectory through space, named the Kapteyn moving group. Based upon their element abundances, these stars may once have been members of Omega Centauri, a globular cluster that is thought to be the remnant of a dwarf galaxy that merged with the Milky Way. During this process, the stars in the group, including Kapteyn's Star, may have been stripped away as tidal debris.

Kapteyn's Star is between one quarter and one third the size and mass of the Sun and has a much cooler effective temperature at about Template:Val, with some disagreement in the exact measurements between different observers. The stellar classification is sdM1, which indicates that it is a subdwarf with a luminosity lower than that of a main-sequence star at the same spectral type of M1. The abundance of elements other than hydrogen and helium, what astronomers term the metallicity, is about 14% of the abundance in the Sun. It is a variable star of the BY Draconis type with the identifier VZ Pictoris. This means that the luminosity of the star changes because of magnetic activity in the chromosphere coupled with rotation moving the resulting star spots into and out of the line of sight with respect to the Earth.

The star has a mass of Template:Solar mass, a radius of Template:Solar radius and has about 1.2% of the Sun's luminosity. It has a surface temperature of Template:Val and is roughly 11 billion years old. In comparison, the Sun is about 4.6 billion years old and has a surface temperature of Template:Val. Stars like Kapteyn's Star have the ability to live up to 100–200 billion years, ten to twenty times longer than the Sun will live.

Search for planets

In 2014, Kapteyn's Star was announced to host two planets, Kapteyn b and Kapteyn c, based on Doppler spectroscopy observations by the HARPS spectrometer which is housed at the European Southern Observatory's La Silla Observatory in Chile, at the Keck Observatory in Hawaii, and at the PFS Observatory, also in Chile. Kapteyn b was described as the oldest-known potentially habitable planet, estimated to be 11 billion years old, while Kapteyn c was described as beyond the host star's habitable zone. The announcement of the planetary system was accompanied by a science-fiction short-story, "Sad Kapteyn", written by writer Alastair Reynolds.

However, subsequent research by Robertson et al. (2015) found that the orbital period of Kapteyn b is an integer fraction (1/3) of their estimated stellar rotation period, and thus the planetary signal is most likely an artifact of stellar activity. The authors did not rule out the existence of Kapteyn c, calling for further observation. This refutation was questioned by the team that published the exoplanet discovery paper. Guinan et al. (2016) (as well as earlier authors) found a lower value for the stellar rotation, which lent support to the original planetary finding.

In 2021, a new analysis found no evidence for either planet, and found that the observed radial velocity signals are in fact artifacts of the star's rotation and activity, after the rotational period of the star was refined, with a rotational period very similar to that of candidate c. There is currently no evidence for planets orbiting Kapteyn's Star.

See also

• List of nearest stars and brown dwarfs
• Stars named after people

Further reading

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External links

• SolStation.com: Kapteyn's Star
• Press release on planetary system