Guardians of the Stars
Van Maanen's star
Van Maanen’s star (van Maanen 2) is a white dwarf. It is a dense, compact stellar remnant that is no longer generating energy, having about 68% of the Sun’s mass but only 1.1% of the Sun’s radius. 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. 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.
Van Maanen’s star 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. It is too faint to be seen with the naked eye from Earth. 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 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’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, however Van Maanen’s star does not. There is a a noticeable infrared deficit. The predicted flux at 24 μm is 0.23 mJy, whereas the measured value is 0.11 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.