Lalande 21185 is a red dwarf in the constellation of Ursa Major. Although relatively close by, it is only magnitude 7 in visible light and thus is too dim to see with the unaided eye. The star is visible through a small telescope.
At approximately 8.31 light-years (2.55 parsecs) away this star is the fifth closest stellar system to the Sun; only the Alpha Centauri system, Barnard's Star, Luhman 16 and Wolf 359 are known to be closer. Because of its proximity it is a frequent subject for astronomical surveys and other research and thus is known by numerous other designations. Research papers most commonly use the designations BD+36 2147, Gliese 411, and HD 95735 to refer to this star.
The celestial coordinates of Lalande 21185 were first published in 1801 by French astronomer Jérôme Lalande of the Paris Observatory in the star catalog, Histoire Céleste Française. The catalog sequence numbers for all of the observed stars, including this one, were added to the original catalog by a later editor in an 1847 republication. Today this star, along with a few others, is still commonly referred to by its Lalande catalog number.
Winneke is reported to have made the first measurement of the star's parallax of .511 arc seconds in 1856 and thus first identifying Lalande 21185 as the second-closest known star to the Sun, after the Alpha Centauri system. Since that time better measurements have placed the star further away but it was the still the second-closest known star system until the discovery of two dim red dwarfs, Wolf 359 and Barnard's Star, in the early twentieth century using astrophotography.
Lalande 21185 is a typical type-M red dwarf main sequence star with about 46% of the mass of the Sun and is much cooler than the Sun at 3,383 K. It is intrinsically dim with an absolute magnitude of 10.48, emitting most of its energy as infrared radiation. Lalande 21185 is a high-proper-motion star moving at about 5 arc seconds a year in an orbit perpendicular to the plane of the galaxy. The proportion of elements other than hydrogen and helium is estimated based on the ratio of iron to hydrogen in the star when compared to the Sun. The logarithm of this ratio is –0.20, indicating that the proportion of iron is about 10–0.20, or 63% of the Sun. The logarithm of the surface gravity is 4.8.
Lalande 21185 is listed as a BY Draconis type variable star in the General Catalogue of Variable Stars. It is identified by the variable star designation NSV 18593. Several star catalogs, including SIMBAD, also classify it as a flare star. This conclusion is not supported by the primary reference these catalogs all use. The observations made in this reference show that it is rather quiet in comparison to other stars of its variable type.
Lalande 21185 emits X-rays.
Claims of a planetary system
In 1951 Peter van de Kamp and his student, Sarah Lippincott, claimed the astrometric detection of a planetary system using photographic plates taken with the 24-in refractor telescope at Swarthmore College's Sproul Observatory. In 1960, Sarah Lippincott repeated the 1951 claim of a planetary system, only this time having different parameters. She used the original photographic plates and new plates taken with the same telescope. Photographic plates from this observatory, taken at the same time, were used by Van de Kamp for his erroneous claim of a planetary system for Barnard's Star. The photographic plates made with the Sproul 24-in refractor, and used for these and other studies, were later shown to be flawed. The claims of planetary companions for both stars were refuted in 1974 with astrometric measurements made by George Gatewood of the Allegheny Observatory.
In 1996 the same George Gatewood prominently announced at an AAS meeting and to the popular press the discovery of multiple planets in this system, detected by astrometry. The initial report of a planet was based on a very delicate analysis of the star's position over the years which suggested reflex orbital motion due to one or more companions. Gatewood claimed that such companions would usually appear more than 0.8 arc second from the M dwarf itself. However a paper by Gatewood published only a few years earlier and subsequent searches by others, using coronagraphs and multifilter techniques to reduce the scattered-light problems from the star, have yet to positively identify any such companions and so his claim remains unconfirmed and is now in doubt.
Refining planetary boundaries
This star's measured radial velocity is so constant that astronomer and planet hunter Geoff Marcy uses it as a perfect example of "normal" M dwarf stability levels. The negative results of this and other surveys do not preclude the presence of a planetary system entirely but they do set an upper boundary on the mass of any planets that might be present. The detection limit by current technology for this star system is a little less than the mass of the planet Jupiter. New Earth- and space-based instruments will certainly lower this limit further and possibly detect any small planets that may be present.
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