Astronomy:Kapteyn's Star

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Short description: Subdwarf star in the constellation Pictor
Kapteyn's Star
Kapteyn's Star is located in 100x100
Kapteyn's Star
Kapteyn's Star

Location of Kapteyn's Star in the constellation Pictor
Observation data
Equinox J2000.0]] (ICRS)
Constellation Pictor
Right ascension  05h 11m 40.58984s[1]
Declination −45° 01′ 06.3617″[1]
Apparent magnitude (V) 8.853±0.008[2]
Characteristics
Spectral type sdM1[3] or M1.5V[2][4]
U−B color index +1.21[5]
B−V color index 1.57±0.012[2]
Variable type BY Dra[6]
Astrometry
Radial velocity (Rv)245.05±0.13[1] km/s
Proper motion (μ) RA: +6,491.223[1] mas/yr
Dec.: −5,708.614[1] mas/yr
Parallax (π)254.1986 ± 0.0168[1] mas
Distance12.8308 ± 0.0008 ly
(3.9339 ± 0.0003 pc)
Absolute magnitude (MV)10.89[3]
Details[2]
Mass0.281±0.014 M☉
Radius0.291±0.025 R☉
Luminosity0.012 L☉
Surface gravity (log g)4.96±0.13 cgs
Temperature3,570±80 K
Metallicity [Fe/H]−0.86±0.05 dex
Rotation124.71±0.19 d[7]
Rotational velocity (v sin i)9.15[8] km/s
Age11.5+0.5
−1.5 Gyr
Other designations
VZ Pictoris, CD−45°1841, CP(D)-44°612, GJ 191, HD 33793, HIP 24186, SAO 217223, LFT 395, LHS 29, LTT 2200[5]
Database references
SIMBADThe star
planet b
planet c

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.[9]

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,[10] 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.[7]

History of observations

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, 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[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.[14][16][17] In 2014, two super-Earth planet candidates in orbit around the star were announced,[10] but later refuted.[7]

Characteristics

A visual band light curve for VZ Pictoris, adapted from Guinan et al. (2016).[2] The red curve shows the sine function that best fits the data.

Based upon parallax measurements, Kapteyn's Star is 12.83 light-years (3.93 parsecs) from the Earth.[1] It came within 7.0 ly (2.1 pc) of the Sun about 10,900 years ago and has been moving away since that time.[18] Kapteyn's Star is distinctive in a number of regards: it has a high radial velocity,[14] orbits the Milky Way retrograde,[17] 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.[17][21][22]

Comparison with Sun, Jupiter and Earth

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 3500 K, with some disagreement in the exact measurements between different observers.[17] The stellar classification is sdM1,[3] 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.[23][24] 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.[6]

The star has a mass of 0.27 M☉, a radius of 0.29 R☉ and has about 1.2% of the Sun's luminosity. It has a surface temperature of 3,550 K and is roughly 11 billion years old.[10] In comparison, the Sun is about 4.6 billion years old[25] and has a surface temperature of 5,778 K.[26] 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.[27]

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.[28] Kapteyn b was described as the oldest-known potentially habitable planet, estimated to be 11 billion years old,[10] while Kapteyn c was described as beyond the host star's habitable zone.[29][30][10]

Kapteyn b was thought to make a complete orbit around its parent star about every 48.62 days at a distance of 0.17 AU, with an eccentricity of 0.21, meaning its orbit is mildly elliptical. Kapteyn c was thought to orbit with a period of 121.5 days at a distance of 0.31 AU, with an eccentricity of 0.23. Both planets were thought to be super-Earths, with minimum masses of 4.8 and 7.0 M⊕, respectively.[10]

The purported planets were thought to be close to a 5:2 period commensurability, but resonances could not be confirmed. Dynamical integration of the orbits suggested[10] 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.[31] Guinan et al. (2016) suggested 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.[2] The announcement of the planetary system was accompanied by a science-fiction short-story, "Sad Kapteyn", written by writer Alastair Reynolds.[32]

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.[33] This refutation was questioned by the team that published the exoplanet discovery paper.[34] Guinan et al. (2016) (as well as earlier authors) found a lower value for the stellar rotation, which lended support to the original planetary finding.[2]

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.[7] There is currently no evidence for planets orbiting Kapteyn's Star.

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940  Gaia DR3 record for this source at VizieR.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 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, doi:10.3847/0004-637X/821/2/81, 81, Bibcode: 2016ApJ...821...81G. 
  3. ↑ 3.0 3.1 3.2 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, doi:10.1111/j.1365-2966.2009.16182.x, Bibcode: 2010MNRAS.403.1949K 
  4. ↑ Demory, Brice-Olivier; Ségransan, Damien; Forveille, Thierry; Queloz, Didier; Beuzit, Jean-Luc; Delfosse, Xavier; Di Folco, Emmanuel; Kervella, Pierre 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. doi:10.1051/0004-6361/200911976. Bibcode: 2009A&A...505..205D. 
  5. ↑ 5.0 5.1 "V* VZ Pic -- Variable Star", SIMBAD (Centre de Données astronomiques de Strasbourg), http://simbad.u-strasbg.fr/simbad/sim-id?Ident=NAME+Kapteyn%27s+star, retrieved 2009-10-14 .
  6. ↑ 6.0 6.1 "VZ Pic", General Catalogue of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia, http://www.sai.msu.su/groups/cluster/gcvs/cgi-bin/search.cgi?search=VZ+pic, retrieved 2009-10-14 
  7. ↑ 7.0 7.1 7.2 7.3 Bortle, Anna et al. (2021). "A Gaussian Process Regression Reveals No Evidence for Planets Orbiting Kapteyn's Star". The Astronomical Journal 161 (5): 230. doi:10.3847/1538-3881/abec89. Bibcode: 2021AJ....161..230B. 
  8. ↑ 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, doi:10.1111/j.1365-2966.2010.16827.x, Bibcode: 2010MNRAS.407.1657H 
  9. ↑ "Kapteyn b and c: Two Exoplanets Found Orbiting Kapteyn's Star". Sci-News. http://www.sci-news.com/astronomy/science-kapteyn-b-c-two-exoplanets-kapteyns-star-01965.html. 
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 10.6 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, doi:10.1093/mnrasl/slu076, Bibcode: 2014MNRAS.443L..89A 
  11. ↑ Kapteyn, J. C. (1898), "Stern mit grösster bislang bekannter Eigenbewegung", Astronomische Nachrichten 145 (9–10): 159–160, doi:10.1002/asna.18981450906, Bibcode: 1897AN....145..159K, https://zenodo.org/record/1424761 .
  12. ↑ Dictionary of Nomenclature of Celestial Objects. CPD entry . SIMBAD. Centre de Données astronomiques de Strasbourg.
  13. ↑ "Resultados del Observatorio Nacional Argentino, vol. 7, pg.98". http://adsbit.harvard.edu//full/1884RNAO....7...96./0000124.000.html. 
  14. ↑ 14.0 14.1 14.2 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, doi:10.1086/104156, Bibcode: 1916AJ.....29..181B .
  17. ↑ 17.0 17.1 17.2 17.3 Kotoneva, E. et al. (2005), "A study of Kapteyn's star", Astronomy & Astrophysics 438 (3): 957–962, doi:10.1051/0004-6361:20042287, Bibcode: 2005A&A...438..957K .
  18. ↑ Bailer-Jones, C. A. L. (March 2015), "Close encounters of the stellar kind", Astronomy & Astrophysics 575: 13, doi:10.1051/0004-6361/201425221, A35, Bibcode: 2015A&A...575A..35B. 
  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, doi:10.1111/j.1365-2966.2004.07671.x, Bibcode: 2004MNRAS.350..575W .
  20. ↑ Eggen, O. J. (December 1996), "The Ross 451 Group of Halo Stars", Astronomical Journal 112: 2661, doi:10.1086/118210, Bibcode: 1996AJ....112.2661E 
  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, doi:10.1088/0004-6256/139/2/636, Bibcode: 2010AJ....139..636W 
  22. ↑ "Backward star ain't from round here", New Scientist, November 4, 2009, https://www.newscientist.com/article/mg20427334.100-backward-star-aint-from-round-here.html, retrieved September 2, 2017 
  23. ↑ 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, doi:10.1111/j.1365-2966.2004.08515.x, Bibcode: 2005MNRAS.356..963W 
  24. ↑ 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
  25. ↑ Cain, Fraser (16 September 2008). "How Old is the Sun?". Universe Today. http://www.universetoday.com/18237/how-old-is-the-sun/. 
  26. ↑ Cain, Fraser (September 15, 2008). "Temperature of the Sun". Universe Today. http://www.universetoday.com/18092/temperature-of-the-sun/. 
  27. ↑ Adams, Fred C.; Laughlin, Gregory; Graves, Genevieve J. M.. "Red Dwarfs and the End of the Main Sequence". Revista Mexicana de Astronomía y Astrofísica. pp. 46–49. Bibcode: 2004RMxAC..22...46A. 
  28. ↑ Wall, Mike (3 June 2014). "Found! Oldest Known Alien Planet That Might Support Life". Space.com. http://www.space.com/26115-oldest-habitable-alien-planet-kapteyn-b.html. 
  29. ↑ David Dickinson, Discovered: Two New Planets for Kapteyn’s Star (June 4, 2014).
  30. ↑ Kapteyn's Star at SolStations.com.
  31. ↑ 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, doi:10.1111/j.1365-2966.2011.18857.x, Bibcode: 2011MNRAS.415.2275M 
  32. ↑ "Sad Kapteyn", Science fiction story released with the announcement of planetary system, Jun 4, 2014, http://ph.qmul.ac.uk/sad-kapteyn, retrieved 2014-06-04 
  33. ↑ 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, doi:10.1088/2041-8205/805/2/L22, L22, Bibcode: 2015ApJ...805L..22R. 
  34. ↑ Anglada-Escudé, G.; Tuomi, M.; Arriagada, P.; Zechmeister, M.; Jenkins, J. S.; Ofir, A.; Dreizler, S.; E. Gerlach et al. (2016). "No Evidence for Activity Correlations in the Radial Velocities of Kapteyn's Star" (in en). The Astrophysical Journal 830 (2): 74. doi:10.3847/0004-637X/830/2/74. ISSN 0004-637X. Bibcode: 2016ApJ...830...74A. http://stacks.iop.org/0004-637X/830/i=2/a=74. 

Further reading

External links


Coordinates: Sky map 05h 11m 41s, −45° 01′ 06″



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