Van Maanen's star
Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||00h 49m 09.90175s|
|Declination||+05° 23′ 19.0117″|
|Apparent magnitude (V)||12.374|
|U−B color index||0.064|
|B−V color index||0.546|
|V−R color index||0.268|
|R−I color index||0.4|
|Radial velocity (Rv)||–38 km/s|
|Proper motion (μ)||RA: +1,236.90 mas/yr
Dec.: −2709.19 mas/yr
|Parallax (π)||231.88 ± 1.79 mas|
|Distance||14.1 ± 0.1 ly
(4.31 ± 0.03 pc)
|Absolute magnitude (MV)||14.23 ± 0.05|
|Mass||0.633 ± 0.022 M☉|
|Radius||0.013 ± 0.002 R☉|
|Surface gravity (log g)||8.19 ± 0.04|
|Temperature||6,220 ± 240 K|
Van Maanen's star (van Maanen 2) is a white dwarf. Out of the white dwarfs known, it is the third closest to the Sun, after Sirius B and Procyon B, in that order, and the closest known solitary white dwarf. Discovered in 1917 by Adriaan van Maanen, Van Maanen's star was the third white dwarf identified, after 40 Eridani B and Sirius B, and the first that was not a member of a multi-star system.
Observation history 
While searching for a companion to the large-proper-motion star Lalande 1299, in 1917 Dutch–American astronomer Adriaan van Maanen discovered a star with an even larger proper motion located a few arcminutes to the northeast. He estimated the annual proper motion of the latter as 3 arcseconds. This star had been previously recorded on a plate taken November 11, 1896 for the Carte du Ciel Catalog of Toulouse, and it showed an apparent magnitude of 12.3. The initial spectral classification was type F0.
In 1918, American astronomer Frederick Seares obtained a refined visual magnitude of 12.34, but the distance to the star remained unknown. Two years later, van Maanen published a parallax estimate of 0.246″, giving it an absolute magnitude of +14.8. This made it the faintest F-type star known at that time. In 1923, Dutch-American astronomer Willem Luyten published a study of stars with large proper motions in which he identified what he called "van Maanen's star" as one of only three known white dwarfs, a term he coined. These are stars that have an unusually low absolute magnitude for their spectral class, lying well below the main sequence on the Hertzsprung–Russell diagram of stellar temperature vs. luminosity.
The high mass density of white dwarfs was demonstrated in 1925 by American astronomer Walter Adams when he measured the gravitational redshift of Sirius B as 21 km/s. In 1926, British astrophysicist Ralph Fowler used the new theory of quantum mechanics to show that these stars are supported by electron gas in a degenerate state. British astrophysicist Leon Mestel demonstrated in 1952 that the energy emitted by a white dwarf is the surviving heat from a prior period of nuclear fusion. He showed that nuclear burning no longer occurs within a white dwarf, and calculated the internal temperature of van Maanen's star as 6 × 106 K. He gave a preliminary age estimate of 1011/A years, where A is the mean atomic weight of the nuclei in the star.
Van Maanen's star is located 14.1 light-years (4.3 parsecs) from the Sun in the constellation Pisces, about 2° to the south of the star Delta Piscium, with a relatively high proper motion of 2.98″ annually. It is too faint to be seen with the naked eye. Like other white dwarfs, it is a very dense star: its mass has been estimated to be about 63% of the Sun's, yet it has only 1% of the Sun's radius. The outer atmosphere has a temperature of approximately 6,220 K, which is relatively cool for a white dwarf. As all white dwarfs steadily radiate away their heat over time, this temperature can be used to estimate its age, thought to be around 3 billion years.
The progenitor of this white dwarf had an estimated 2.6 solar masses and remained on the main sequence for about 9 × 108 years. This gives the star a combined age of about 4.1 billion years. When this star left the main sequence, it expanded into a red supergiant that reached a maximum radius of 1,000 times the current radius of the Sun, or about 5 astronomical units. Any planets that were orbiting within this radius would have interacted directly with the star's extended envelope.
The stellar classification of Van Maanen's star is DZ8, where the DZ prefix indicates the presence of elements heavier than helium in its spectrum—what astronomers term metals. Indeed, this star is the prototype for white dwarfs of this class. Based upon physical models of white dwarfs, elements with mass greater than helium should sink below the photosphere of the star, leaving only hydrogen and helium to be visible in the spectrum. Hence, for heavier elements to appear, there must have been an external source. It is unlikely that the heavy elements were obtained from the interstellar medium. Instead, the surface of the star was likely polluted by circumstellar material, such as by the remains of a rocky, terrestrial planet.
White dwarfs with a spectrum that indicates high levels of metal contamination often possess a circumstellar disk. In the case of van Maanen's star, observations of the star at a wavelength of 24 μm do not show the infrared excess that might be generated by a dusty disk. Instead there is a noticeable deficit. The predicted flux at 24 μm is 0.23 mJy, whereas the measured value is 0.11 ± 0.03 mJy. This deficit may be explained by collision-induced absorption in the atmosphere of the star. However, this is normally only known to happen with white dwarfs that have temperatures below 4,000 K, as a result of collisions between hydrogen molecules or between hydrogen molecules and helium.
Possible companion 
The possible existence of a substellar companion remains uncertain. As recently as 2004, there was one paper confirming and one denying its detection. As of 2008, observations with the Spitzer Space Telescope appear to rule out any companions within 1,200 AU of the star that have four jupiter masses or greater.
See also 
- 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
- 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
- McCook, G. P.; Sion, E. M. (August 2006), Spectroscopically Identified White Dwarfs, Bibcode:2006yCat.3235....0M, retrieved 2010-12-04 VizieR On-line Data Catalog: III/235B
- "Van Maanen's star", SIMBAD Astronomical Object Database (Centre de Données astronomiques de Strasbourg), retrieved 2008-12-08
- Aannestad, Per A. et al. (March 1993), "Cool metallic-line white dwarfs, radial velocities, and interstellar accretion", Astronomical Journal 105 (3): 1033–1044, Bibcode:1993AJ....105.1033A, doi:10.1086/116491.
- Entry, 0049+0523, astrometry, NStars database. Accessed on line July 9, 2008.
- Burleigh, M. R. et al. (May 2008), "The 'DODO' survey - I. Limits on ultra-cool substellar and planetary-mass companions to van Maanen's star (vMa2)", Monthly Notices of the Royal Astronomical Society: Letters 386 (1): L5–L9, arXiv:0801.2917, Bibcode:2008MNRAS.386L...5B, doi:10.1111/j.1745-3933.2008.00446.x
- Gatewood, G.; Russell, J. (July 1974), "Astrometric determination of the gravitational redshift of van Maanen 2 (EG 5)", Astronomical Journal 79: 815–818, Bibcode:1974AJ.....79..815G, doi:10.1086/111613 – See §III for adopted values of mass, radius and gravity.
- Sion, Edward M. et al. (December 2009), "The White Dwarfs Within 20 Parsecs of the Sun: Kinematics and Statistics", The Astronomical Journal 138 (6): 1681–1689, arXiv:0910.1288, Bibcode:2009AJ....138.1681S, doi:10.1088/0004-6256/138/6/1681
- The One Hundred Nearest Star Systems, RECONS, 2008-01-01, retrieved 2008-12-08
- Holberg, J. B.; Oswalt, Terry D.; Sion, E. M. (May 2002), "A Determination of the Local Density of White Dwarf Stars", The Astrophysical Journal 571 (1): 512–518, arXiv:astro-ph/0102120, Bibcode:2002ApJ...571..512H, doi:10.1086/339842
- van Maanen, A. (December 1917), "Two Faint Stars with Large Proper Motion", Publications of the Astronomical Society of the Pacific 29 (172): 258–259, Bibcode:1917PASP...29..258V, doi:10.1086/122654
- Schatzman, Évry (1958), White Dwarfs, North Holland Publishing Company, p. 2
- Seares, F. H. (1918), "Magnitudes and Colors of Three Faint Stars of Large Proper Motion", Publications of the Astronomical Society of the Pacific 30 (175): 191–192, Bibcode:1918PASP...30..191S, doi:10.1086/122724
- van Maanen, Adriaan (1920), "No. 182. The photographic determination of stellar parallaxes with the 60-inch reflector. Fourth series.", Contributions from the Mount Wilson Observatory 182: 1–35, Bibcode:1920CMWCI.182....1V—van Maanen identified the star as "Anon. 1".
- Holberg, J. B. (2005), "How Degenerate Stars Came to be Known as White Dwarfs", Bulletin of the American Astronomical Society 37: 1503, Bibcode:2005AAS...20720501H
- Luyten, Willem J. (1923), "Bulletin Number 344 - A study of stars with large proper motions", Lick Observatory Bulletin 11: 1–32, Bibcode:1923LicOB..11....1L—See p. 20.
- Adams, W. S. (1925), "The relativity displacement of the spectral lines in the companion of Sirius", The Observatory 48: 337–342, Bibcode:1925Obs....48..337A
- Fowler, R. H. (1926), "On Dense Matter", Monthly Notices of the Royal Astronomical Society 87: 114–122, Bibcode:1926MNRAS..87..114F
- Harman, Peter Michael; Mitton, Simon (2002), Cambridge Scientific Minds, Cambridge University Press, pp. 230–232, ISBN 0-521-78612-6
- Mestel, L. (1952), "On the theory of white dwarf stars. I. The energy sources of white dwarfs", Monthly Notices of the Royal Astronomical Society 112: 583–597, Bibcode:1952MNRAS.112..583M
- Burnham, Robert (1978), Burnham's celestial handbook: an observer's guide to the universe beyond the solar system, Dover books explaining science 3 (2nd ed.), Courier Dover Publications, pp. 1474–1477, ISBN 0-486-23673-0
- Farihi, J. et al. (June 2010), "Rocky planetesimals as the origin of metals in DZ stars", Monthly Notices of the Royal Astronomical Society 404 (4): 2123–2135, arXiv:1001.5025, Bibcode:2010MNRAS.404.2123F, doi:10.1111/j.1365-2966.2010.16426.x
- Farihi, J.; Jura, M.; Zuckerman, B. (April 2009), "Infrared Signatures of Disrupted Minor Planets at White Dwarfs", The Astrophysical Journal 694 (2): 805–819, arXiv:0901.0973, Bibcode:2009ApJ...694..805F, doi:10.1088/0004-637X/694/2/805
- Farihi, J. (May 2005), "Cool versus Ultracool White Dwarfs", The Astronomical Journal 129 (5): 2382–2385, arXiv:astro-ph/0502134, Bibcode:2005AJ....129.2382F, doi:10.1086/429527
- Makarov, Valeri V. (2004), "A Substellar Companion to van Maanen 2", The Astrophysical Journal Letters 600 (1): L71–L73, Bibcode:2004ApJ...600L..71M, doi:10.1086/381544
- Farihi, J.; Becklin, E. E. (June 2004), "Mid-Infrared Observations of van Maanen 2: No Substellar Companion, J. Farihi and E. E. Becklin", Astrophysical Journal Letters 608 (2): L109–L112, arXiv:astro-ph/0405245, Bibcode:2004ApJ...608L.109F, doi:10.1086/422502
- Farihi, J.; Becklin, E. E.; Zuckerman, B. (July 2008), "Spitzer IRAC Observations of White Dwarfs. II. Massive Planetary and Cold Brown Dwarf Companions to Young and Old Degenerates", The Astrophysical Journal 681 (2): 1470–1483, arXiv:0804.0237, Bibcode:2008ApJ...681.1470F, doi:10.1086/588726