Van Maanen 2

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 2 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.

In 2016, it was discovered that a spectrographic plate of van Maanen 2 made in 1917 has evidence – the earliest known – of planetary matter outside the solar system. No actual planet has been detected, but the plate reveals the existence of a circumstellar ring of debris, and such rings in other cases have been associated with planets.

Characteristics

Van Maanen 2 is located 13.9 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 2.978″ annually along a position angle of 155.538°. 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 68% 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 giant that reached a maximum radius of 650 times the current radius of the Sun, or about 3 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 2 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.

The total mass of metals in the atmosphere of Van Maanen 2 is estimated to be around 10²¹ g—about the same mass as a large moon such as Ariel. These pollutants will sink deeper into the atmosphere on time scales of around three million years, which indicates the material is being replenished at a rate of 10⁷ g/s. These materials could have been accreted in the form of multiple planetesimals smaller than around 84 km colliding with the star.

White dwarfs with a spectrum that indicates high levels of metal contamination often possess a circumstellar disk. In the case of van Maanen 2, 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.

Based upon the space velocity of this star, it made its perihelion approach some 15,070 years ago at a separation of 3.1 ly (0.95 pc) from the Sun.

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.