Wolf 359 is one of the faintest and least-massive nearby stars known. At the light-emitting layer called thephotosphere, it has a temperature of ~2,800 K, low enough forchemical compounds to form and survive. Theabsorption lines of compounds such as water andtitanium(II) oxide have been observed in itsspectrum.[17] The star's surface has amagnetic field hundreds of times as strong as that of theSun, generated by its thorough internalconvection. As a result of this significant magnetic activity, Wolf 359 is aflare star that can undergo sudden and great increases in luminosity, which can persist for several minutes. These flares emit strong bursts ofX-ray andgamma ray radiation that have been observed byspace telescopes. It is a relatively young star with an estimated age of less than a billion years. No planetary companions for Wolf 359 have been confirmed so far, though there is one unverified candidate, and as yet nodebris disks have been found.[14]
Wolf 359 first came to the attention of astronomers because of its relatively high rate oftransverse motion against the background, also known as theproper motion. A high rate of proper motion can indicate that the star is located nearby, as closer stars can achieve the same rate of angular change with a lower relative speed. The proper motion of Wolf 359 was first measured in 1917 by GermanastronomerMax Wolf, aided byastrophotography. In 1919 he published a catalogue of over one thousand stars with highproper motions, including this one, that are still identified by his name.[18] He listed this star as entry number 359, and the star has since been referred to as Wolf 359, in reference to Max Wolf's work.[19]
The firstparallax measurement of Wolf 359 was reported in 1928 from theMount Wilson Observatory, yielding an annual shift in the star's position of0.407 ± 0.009arcseconds. From this position change, and the known size of the Earth's orbit, the distance to the star could be estimated. It was the faintest and least-massive star known until the discovery ofVB 10 in 1944.[20][21] Theinfrared magnitude of the star was measured in 1957.[22] In 1969, a brief flare in the luminosity of Wolf 359 was observed, linking it to a class ofvariable stars known asflare stars.[23]
The position of Wolf 359 on aradar map among all stellar objects orstellar systems within 9 light years (ly) from the map's center, the Sun (Sol). The diamond-shapes are their positions entered according toright ascension inhours angle (indicated at the edge of the map's reference disc), and according to theirdeclination. The second mark shows each's distance from Sol, with theconcentric circles indicating the distance in steps of one ly.
Wolf 359 has astellar classification of M6,[3] although various sources list a spectral class of M5.5,[24] M6.5[25] or M8.[26] MostM-type stars arered dwarfs: they are visually red because the energy emission of such stars reaches a peak in the red and infrared parts of the spectrum.[27] Wolf 359 has a very low luminosity, emitting about 0.1% of theSun's power.[28][8] If it were moved to the location of the Sun, it would appear ten times as bright as thefull Moon.[29]
The entire starundergoes convection, whereby the energy generated at the core is transported toward the surface by the convective motion of stellarplasma, rather than throughelectromagnetic radiation. This constant circulation redistributes throughout the star any excess accumulation of helium in the core generated bystellar nucleosynthesis.[33] This process allows Wolf 359 to remain on themain sequence as ahydrogen fusing star for proportionately longer than one such as the Sun, for which helium steadily accumulates in the core and is not diluted. In conjunction with a much lower rate of hydrogen consumption due to its low mass and core temperature, Wolf 359 is expected to remain a main sequence star for about eight trillion years before finally exhausting its hydrogen supply and ending up as a heliumwhite dwarf.[34]
Beyond the photosphere lies a nebulous, high temperature region known as thestellar corona. In 2001, Wolf 359 became the first star other than the Sun to have the spectrum of its corona observed by a ground-based telescope. The spectrum showedemission lines of Fe XIII, which is heavilyionized iron that has been stripped of twelve of its twenty-six electrons.[42] The strength of this line can vary over a time period of several hours, which may be evidence ofmicroflare heating.[28]
Wolf 359 is classified as aUV Ceti-typeflare star,[5] a category of stars that undergo brief, dramatic increases in luminosity due to intense magnetic field activity in their photospheres. Itsvariable star designation isCN Leonis. Wolf 359 has a relatively high flare rate. Observations with the Hubble Space Telescope detected 32 flare events within a two-hour period, with energies of 1027ergs (1020joules) and higher.[26] The mean magnetic field strength at the surface of the star is around 2.2 kG (0.22teslas), but this value varies significantly on time scales as short as six hours.[24] In comparison, the magnetic field of the Sun averages a strength of 1 gauss (100μT), although it can reach as high as 3 kG (0.3 T) in activesunspot regions.[44] During periods of flare activity, Wolf 359 has been observed to emitX-rays andgamma rays.[45][46]
Distances of thenearest stars from 20,000 years ago to 80,000 years in the future. Wolf 359 is not displayed, but it is currently at a distance of 7.9 ly and increasing, with a past minimum of 7.3 ly around 13,850 years ago.
The rotation of a star causes aDoppler shift of its spectrum, generally resulting in a broadening of theabsorption lines in its spectrum, with the lines increasing in width with higher rotational speeds. However, only the rotational velocity's component in the direction of the observer can be measured by this method, and the resulting data imposes only a lower limit on the star's rotational speed. This projectedrotational velocity of Wolf 359 at its equator is less than 3 km/s, below the threshold of detection withspectral line broadening.[6] This low rate of rotation may have been caused by the loss ofangular momentum through itsstellar wind, which increases greatly during periods of flare activity. Roughly speaking, the spin-down timescale of a star of spectral class M6 is somewhat long, at ~10 billion years, as fully convective stars lose their rotational speeds more slowly than others.[47] However, evolutionary models suggest that Wolf 359 is a relatively young star with an age of less than a billion years.[28]
Wolf 359's proper motion is 4.696 arcseconds per year, and moving away from the Sun at a velocity of ~19 km/s.[6][48] When translated into thegalactic coordinate system, the motion corresponds to aspace velocity of(U, V, W) =(−26, −44, −18) km/s.[49] This space velocity implies that Wolf 359 belongs to the population ofold-disk stars. It follows an orbit through theMilky Way that will bring it as close as 20.5 kly (6.3 kpc) and as distant as 28 kly (8.6 kpc) from theGalactic Center. The predicted galactic orbit has aneccentricity of 0.156, and the star can travel as far as 444 light-years (136 pc) away from thegalactic plane.[50] The closest stellar neighbor to Wolf 359 is the red dwarfRoss 128, at 3.79 ly (1.16 pc).[51] Approximately 13,850 years before the present day, Wolf 359 attained its minimal separation of about 7.35 ly (2.25 pc) from the Sun, and has been receding away ever since.[52]
Radial velocity measurements of the star in 2011 using the Near Infrared Spectrometer (NIRSPEC) instrument at theKeck II observatory did not reveal any variations that might otherwise indicate the presence of an orbiting companion. This instrumentation is sensitive enough to detect thegravitational perturbations of massive, short period companions with the mass ofNeptune or greater.[53]
In June 2019, an international team of astronomers led byMikko Tuomi from theUniversity of Hertfordshire, UK, submitted apreprint with the results of the first reported detection of two candidate exoplanets orbiting Wolf 359 using theradial velocity method from observations withHARPS in Chile andHIRES in Hawaii.[54] If these planets were confirmed, the setup of the system would be similar to but more extreme than that of the nearby red dwarfProxima Centauri, with both having a close-in low-mass planet and a farther out higher-mass planet. The theorized and later ruled-out inner planet, Wolf 359 c, would receive per unit area about forty times as much radiative energy as compared to Earth, making it unlikely to be a habitable planet. The as yet unconfimed Wolf 359 b, in contrast, is classified as a cool super-Neptune, receiving roughly a third to a quarter of the energy per unit area as Neptune does from the Sun.[54]
Further observations from theCARMENES survey have found that the radial velocity signal corresponding to the inner planet candidate Wolf 359 c is a false positive, resulting from the rotation of the star rather than a planetary companion.[13][55] A 2023 follow-up study using MAROON-X, CARMENES, HARPS, and HIRES radial velocity data as well as imaging data was unable to either confirm or refute the presence of Wolf 359 b. The same study ruled out the existence of anybrown dwarfs or massive gas giant companions within 10AU of the star, planets more than half the mass of Jupiter within 1 AU, and planets more massive thanUranus within 0.1 AU.[14]
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