Lalande 21185

Lalande 21185 (also known as BD+36 2147, Gliese 411, and HD 95735) is a star in the south of Ursa Major. It is the apparent brightest red dwarf in the northern hemisphere. Despite this, and being relatively close by, it is very dim (as are all red dwarfs), being only magnitude 7.5 in visible light and thus too faint to be seen with the unaided eye. The star is visible through a small telescope or binoculars. At 8.304 light-years (2.546 parsecs) away it is one of the stars nearest to the Solar System; only the Alpha Centauri system, Barnard's Star, Wolf 359, and the brown dwarfs Luhman 16 and WISE 0855−0714 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, most commonly Gliese 411 and HD 95735. In approximately 19,900 years it will be at its closest, about 4.65 ly (1.43 pc) from the Sun, just over half its present distance. Lalande 21185 has two known exoplanets and one candidate exoplanet, making it the third closest confirmed planetary system to the Solar System.

History

French astronomer Jérôme Lalande published the celestial coordinates of Lalande 21185 in the popular star catalog Histoire céleste française during year 1801. The catalog sequence numbers for majority of the observed stars, including Lalande 21185, were introduced in its 1847 edition by Francis Baily. Today this star is one of just a few that are still commonly referred to by their Lalande catalog number. In May 1857, Friedrich Wilhelm Argelander discovered the high proper motion of the star. It was sometimes called "Argelander's second star". (The "first Argelander's star" is Groombridge 1830, whose high proper motion was discovered by Argelander earlier—in 1842). A large proper motion was recognised as an indicator for stars which lay close to the Sun, leading Argelander to request that its parallax be measured. As per this request, Friedrich August Theodor Winnecke made the first measurement of the star's parallax of 0.511 arc seconds between 1857–58 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 farther away, but it remained the second-closest-known star system until the astrophotographic discovery of two dim red dwarfs, Wolf 359 and Barnard's Star, in the early 20th century.

Properties

Lalande 21185 is a typical type-M main-sequence star (red dwarf) with about 39% of the mass and radius of the Sun. It is also much cooler than the Sun with a surface temperature of 3,550 K. With just 2.2% of the Sun's luminosity, it is intrinsically dim with an absolute magnitude of 10.48, emitting most of its energy in the infrared. 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 surface gravity of this relatively compact star is approximately 65 times greater than the gravity at Earth's surface (log g = 4.8 cgs), which is more than twice the surface gravity of the Sun. 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, and X-ray flares have been observed.

Planetary system Data published in 2017 from the HIRES system at the Keck Observatory on Mauna Kea supported the existence of a close-in planet with an orbital period of just 9.8693±0.0016 days, being at least 3.8 M🜨. Further radial velocity research with the SOPHIE échelle spectrograph and review of the original signal found that the 9.9 day period was undetectable, and instead proposed, using both datasets, an exoplanet orbiting the star with a period of either 12.95 or 1.08 days, much more likely 12.95, insofar as 1-day-period exoplanets seem to be rare in systems. This would give the planet a minimum mass of 2.99 Earth masses. It is too close to the star, and so therefore too hot, to be in the habitable zone, at all points within its eccentric orbit. The proposed planet on 12-day orbit was confirmed by CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrographs) project in 2020. A second planet with a more distant orbit was initially noticed by SOPHIE, but the baseline was not long enough to confirm the several-year-long signal. The signal was confirmed in 2021 to be a planet with mass at least 18.0+2.9−2.6 M🜨, a lower-bound estimate later revised to 14.2±1.8 M🜨. A third planet, Gliese 411 d, is suspected to orbit between Gliese 411 b and Gliese 411 c with a period of 215 days.

The habitable zone for this star, defined as the locations where liquid water could be present on an Earth-like planet, is at a radius of 0.11–0.24 AU, where 1 AU is the average distance from the Earth to the Sun. The planet b has an equilibrium temperature of 370.1+5.8−6.8 K. Other known planets are outside HZ boundaries too, but undetected low-mass ones may be orbiting in this region of this system as well.

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