Kapteyn's Star

Kapteyn's Star is a class M1 red subdwarf about 12.76 light years from Earth in the southern constellation Pictor; it is the closest halo star to the Solar System. With a magnitude of nearly 9 it is visible through binoculars or a telescope.[2]

Kapteyn's Star
The red dot shows the approximate location of Kapteyn's Star in Pictor.
Observation data
Epoch J2000      Equinox J2000
Constellation Pictor
Right ascension  05h 11m 50s
(moves 0.43 sec/yr)
Declination −45° 02′ 30″
(moves 5.7 arcsec/yr)
Apparent magnitude (V) 8.853[1]
Characteristics
Spectral type sdM1[1]
U−B color index +1.21[2]
B−V color index +1.57[2]
Variable type BY Dra[3]
Astrometry
Radial velocity (Rv)+245.2[4] km/s
Proper motion (μ) RA: +6,505.08[5] mas/yr
Dec.: -5,730.84[5] mas/yr
Parallax (π)255.66 ± 0.91[5] mas
Distance12.76 ± 0.05 ly
(3.91 ± 0.01 pc)
Absolute magnitude (MV)10.89[1]
Details
Mass0.274[6] M☉
Radius0.291±0.025[7] R☉
Luminosity (bolometric)0.012 L☉
Luminosity (visual, LV)0.004 L☉
Surface gravity (log g)4.96[6] cgs
Temperature3550±50[8] K
Metallicity [Fe/H]−0.99±0.04[9] dex
Rotational velocity (v sin i)9.15[10] km/s
Age~11[6] Gyr
Other designations
VZ Pictoris, GJ 191, HD 33793, CD-45°1841, CP(D)-44°612, SAO 217223, LHS 29, LTT 2200, LFT 395, GCTP 1181, HIP 24186.[2]
Database references
SIMBADThe star
planet b
planet c

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 a likely 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.

History of observations
Comparison with Sun, Jupiter and Earth.
Jacobus Cornelius Kapteyn, the Dutch astronomer who discovered Kapteyn's Star.

Attention was first drawn to what is now known as Kapteyn's Star by the Dutch astronomer Jacobus Kapteyn in 1898.[11] 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.[12] 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[13], seemed to be missing. However, R.T.A. Innes found an uncatalogued star about 15 arc seconds away from the absent star's position. It became clear that the star had a very high proper motion of more than 8 arc seconds per year and had moved significantly. Later, CPD-44 612 came to be referred to as Kapteyn's Star[14] although equal credit should be accorded to Robert Innes.[15] 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.[16][6][14] In 2014, two super-Earth planet candidates in orbit around the star were announced.[8]

Characteristics

Based upon parallax measurements with the Hipparcos astrometry satellite,[5] Kapteyn's Star is 12.76 light-years (3.91 parsecs) from the Earth.[5] It came within 7.00 light-years (2.15 parsecs) of the Sun about 10,800 years ago and has been moving away since that time.[17] The star is between one quarter and one third the size and mass of the Sun and has a much cooler effective temperature at about 3500 K, with some disagreement in the exact measurements between different observers.[6] The stellar classification is sdM1,[1] 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.[9][18] 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.[3]

Kapteyn's Star is distinctive in a number of other regards: it has a high radial velocity,[14] orbits the Milky Way retrograde,[6] and is the nearest known halo star to the Sun.[19] It is a member of a moving group of stars that share a common trajectory through space, named the Kapteyn moving group.[20] 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.[6][21][22]

Visibility

The star is at an apparent magnitude of 9 and is visible through binoculars or a telescope in the constellation of Pictor, in the southern sky.[23]

Planetary system

In 2014, Kapteyn's Star was announced to host two planets, Kapteyn b and Kapteyn c. Kapteyn b is the oldest-known potentially habitable planet, estimated to be 11 billion years old.[8] However, Robertson et al. (2015) noted 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 do not rule out the existence of Kapteyn c, calling for further observation.[24] Guinan et al. (2016) (as well as earlier authors) found a lower value for the stellar rotation, which lends support to the original planetary finding. Additional data will be needed to confirm the rotation period and resolve the dispute.[25]

The purported planets are close to a 5:2 period commensurability, but resonances could not be confirmed at the time. Dynamical integration of the orbits suggests[8] that the pair of planets are in a dynamical state called apsidal co-rotation, which usually implies that the system is dynamically stable over long time scales.[26] Guinan et al. (2016) suggest that the present day star could potentially support life on Kapteyn b, but that the planet's atmosphere may have been stripped away when the star was young (~0.5 Gyr) and highly active.[25] The announcement of the planetary system was accompanied by a science-fiction short-story, "Sad Kapteyn", written by writer Alastair Reynolds.[27]

The Kapteyn's star planetary system[8]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b ≥ 4.8+0.9
−1.0 M⊕		 0.168+0.006
−0.008		 48.616+0.036
−0.032		 0.21+0.11
−0.10		 — ≥ 1.5 R⊕
c ≥ 7.0+1.2
−1.0 M⊕		 0.311+0.038
−0.014		 121.53+0.25
−0.25		 0.23+0.10
−0.12		 — —

See also

References
  1. Koen, C.; et al. (April 2010), "UBV(RI)C JHK observations of Hipparcos-selected nearby stars", Monthly Notices of the Royal Astronomical Society, 403 (4): 1949–1968, Bibcode:2010MNRAS.403.1949K, doi:10.1111/j.1365-2966.2009.16182.x
  2. "V* VZ Pic -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, archived from the original on 2016-03-03, retrieved 2009-10-14.
  3. "VZ Pic", General Catalogue of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia, archived from the original on 2011-09-27, retrieved 2009-10-14
  4. Nordström, B.; et al. (May 2004), "The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ˜14 000 F and G dwarfs", Astronomy and Astrophysics, 418 (3): 989–1019, arXiv:astro-ph/0405198, Bibcode:2004A&A...418..989N, doi:10.1051/0004-6361:20035959
  5. van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357.
  6. Kotoneva, E.; et al. (2005), "A study of Kapteyn's star", Astronomy & Astrophysics, 438 (3): 957–962, Bibcode:2005A&A...438..957K, doi:10.1051/0004-6361:20042287.
  7. Demory, B.-O.; et al. (October 2009), "Mass-radius relation of low and very low-mass stars revisited with the VLTI", Astronomy and Astrophysics, 505 (1): 205–215, arXiv:0906.0602, Bibcode:2009A&A...505..205D, doi:10.1051/0004-6361/200911976
  8. Anglada-Escudé, Guillem; et al. (2014), "Two planets around Kapteyn's star : a cold and a temperate super-Earth orbiting the nearest halo red-dwarf", Monthly Notices of the Royal Astronomical Society: Letters, 443: L89–L93, arXiv:1406.0818, Bibcode:2014MNRAS.443L..89A, doi:10.1093/mnrasl/slu076
  9. Woolf, Vincent M.; Wallerstein, George (January 2005), "Metallicity measurements using atomic lines in M and K dwarf stars", Monthly Notices of the Royal Astronomical Society, 356 (3): 963–968, arXiv:astro-ph/0410452, Bibcode:2005MNRAS.356..963W, doi:10.1111/j.1365-2966.2004.08515.x
  10. Houdebine, E. R. (September 2010), "Observation and modelling of main-sequence star chromospheres - XIV. Rotation of dM1 stars", Monthly Notices of the Royal Astronomical Society, 407 (3): 1657–1673, Bibcode:2010MNRAS.407.1657H, doi:10.1111/j.1365-2966.2010.16827.x
  11. Kapteyn, J. C. (1898), "Stern mit grösster bislang bekannter Eigenbewegung" (PDF), Astronomische Nachrichten, 145 (9–10): 159–160, Bibcode:1897AN....145..159K, doi:10.1002/asna.18981450906.
  12. Dictionary of Nomenclature of Celestial Objects. CPD entry Archived 2015-09-25 at the Wayback Machine. SIMBAD. Centre de Données astronomiques de Strasbourg.
  13. "Resultados del Observatorio Nacional Argentino, vol. 7, pg.98". Archived from the original on 2019-04-21. Retrieved 2019-04-21.
  14. Kaler, James B. (2002), "Kapteyn's Star", The Hundred Greatest Stars, Copernicus Books, pp. 108–109.
  15. Gill, D. (1899). "On the Discovery of a Certain Proper Motion". The Observatory: 99–101.
  16. Barnard, E. E. (1916), "A small star with large proper motion", Astronomical Journal, 29 (695): 181, Bibcode:1916AJ.....29..181B, doi:10.1086/104156.
  17. Bobylev, Vadim V. (March 2010), "Searching for Stars Closely Encountering with the Solar System", Astronomy Letters, 36 (3): 220–226, arXiv:1003.2160, Bibcode:2010AstL...36..220B, doi:10.1134/S1063773710030060.
  18. The abundance is given by taking the metallicity to the power of 10. From Woolf and Wallerstein (2005), [M/H] ≈ –0.86 dex. Thus:
    10−0.86 = 0.138
  19. Woolf, V. M.; Wallerstein, G. (2004), "Chemical abundance analysis of Kapteyn's Star", Monthly Notices of the Royal Astronomical Society, 350 (2): 575–579, Bibcode:2004MNRAS.350..575W, doi:10.1111/j.1365-2966.2004.07671.x.
  20. Eggen, O. J. (December 1996), "The Ross 451 Group of Halo Stars", Astronomical Journal, 112: 2661, Bibcode:1996AJ....112.2661E, doi:10.1086/118210
  21. Wylie-de Boer, Elizabeth; Freeman, Ken; Williams, Mary (February 2010), "Evidence of Tidal Debris from ω Cen in the Kapteyn Group", The Astronomical Journal, 139 (2): 636–645, arXiv:0910.3735, Bibcode:2010AJ....139..636W, doi:10.1088/0004-6256/139/2/636
  22. "Backward star ain't from round here", New Scientist, November 4, 2009, archived from the original on May 25, 2015, retrieved September 2, 2017
  23. "Kapteyn b and c: Two Exoplanets Found Orbiting Kapteyn's Star". Sci-News. Archived from the original on 3 August 2014. Retrieved 23 July 2014.
  24. Robertson, Paul; Roy, Arpita; Mahadevan, Suvrath (June 2015), "Stellar activity mimics a habitable-zone planet around Kapteyn's star", The Astrophysical Journal Letters, 805 (2): 6, arXiv:1505.02778, Bibcode:2015ApJ...805L..22R, doi:10.1088/2041-8205/805/2/L22, L22.
  25. Guinan, Edward F.; Engle, Scott G.; Durbin, Allyn (April 2016), "Living with a Red Dwarf: Rotation and X-Ray and Ultraviolet Properties of the Halo Population Kapteyn's Star", The Astrophysical Journal, 821 (2): 14, arXiv:1602.01912, Bibcode:2016ApJ...821...81G, doi:10.3847/0004-637X/821/2/81, 81.
  26. Michtchenko, Tatiana A.; et al. (August 2011), "Modeling the secular evolution of migrating planet pairs", Monthly Notices of the Royal Astronomical Society, 415 (3): 2275–2292, arXiv:1103.5485, Bibcode:2011MNRAS.415.2275M, doi:10.1111/j.1365-2966.2011.18857.x
  27. "Sad Kapteyn", Science fiction story released with the announcement of planetary system, Jun 4, 2014, archived from the original on June 6, 2014, retrieved 2014-06-04

Notes

    Further reading

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