| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) |
|
|---|---|
| Constellation | Pisces |
| Right ascension | 00h 49m 09.8992s[1] |
| Declination | +05° 23′ 19.007″[1] |
| Apparent magnitude (V) | 12.36[2] |
| Characteristics | |
| Spectral type | DZ8[3] |
| U−B color index | 0.02[4] |
| B−V color index | 0.56[4] |
| V−R color index | 0.5[2] |
| R−I color index | 0.4[2] |
| Variable type | None |
| Astrometry | |
| Radial velocity (Rv) | +6 ± 15[4] km/s |
| Proper motion (μ) | RA: 1231.72[1] mas/yr Dec.: -2707.67[1] mas/yr |
| Parallax (π) | 231.88 ± 1.79[5] mas |
| Distance | 14.1 ± 0.1 ly (4.31 ± 0.03 pc) |
| Absolute magnitude (MV) | 14.21[6] |
| Details | |
| Mass | 0.7 ± 0.3[4] M☉ |
| Radius | 0.013 ± 0.002[4] R☉ |
| Surface gravity (log g) | 8.19[7] |
| Temperature | 6,220[7] K |
| Age | 3.13[7] Gyr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Van Maanen's star (van Maanen 2) is a white dwarf star. 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.[6][8] Discovered in 1917 by Adriaan van Maanen,[9] 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.[10]
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While searching for a companion to the large proper motion star Lalande 1299, in 1917 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.[9]
In 1918, Frederick Seares obtained a refined visual magnitude of 12.34, but the distance to the star remained unknown.[11] 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.[12] In 1923, 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.[13] These are stars that have an unusually low absolute magnitude for their spectral class, lying well below the main sequence.[14]
The high mass density of white dwarfs was demonstrated in 1925 by Walter Adams when he measured the gravitational redshift of Sirius B as 21 km/s.[15] In 1926, Ralph Fowler used the new theory of quantum mechanics to show that these stars are supported by electron gas in a degenerate state.[16][17] 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 also gave a preliminary age estimate of 1011/A years, where A is the mean atomic weight of the nuclei in the star.[18]
Van Maanen's star is located 14.1 light years from the Sun in the constellation Pisces, about 2° to the south of the star Delta Piscium,[19] and has a relatively high proper motion of 2.98" annually.[5] It is too faint to be seen with the naked eye.[19] Like other white dwarfs, it is a very dense star: its mass has been estimated to be about 70% of the Sun's, and its radius is roughly 1% of the Sun's.[4] It has a relatively cool temperature of approximately 6,220 K. As white dwarfs cool with time, the star's temperature can be used to estimate its age, thought to be around 3 billion years.[7]
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 total age of about 4.1 billion years. When this star left the main sequence, it became a red giant that had a maximum radius of 1,000 times the current radius of the Sun, or 50 astronomical units. Any planets that were orbiting within this radius would have interacted directly with the star's extended envelope.[20]
The stellar classification of this white dwarf is DZ8, where the DZ indicates the presence of elements heavier than helium in the spectrum. 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.[21]
White dwarfs with a spectrum that indicates high levels of metal contamination often possess a circumstellar disk. However, 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.[22] However, this is normally only known to happen with white dwarfs with temperatures below 4,000 K, due to collisions between hydrogen molecules or between hydrogen molecules and helium.[23]
In the galactic coordinate system, this star is moving with space velocity components of [U, V, W] = [−2.8, −53.6, −30.3] km s−1, for a net velocity of 61.6 km s−1 relative to the Sun.[7] The star made its closest approach to the Sun about 34,300 years ago at a distance of 10.85 ± 0.44 ly (3.33 ± 0.13 pc).[24]
The possible existence of a substellar companion remains uncertain. As recently as 2004, there was one paper confirming[25] and one denying[26] 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.[27]
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