40 Eridani

40 Eridani is a triple star system in the constellation of Eridanus, abbreviated 40 Eri. It has the Bayer designation Omicron² Eridani, which is Latinized from ο² Eridani and abbreviated Omicron² Eri or ο² Eri. Based on parallax measurements taken by the Gaia mission, it is about 16.3 light-years from the Sun.

The primary star of the system, designated 40 Eridani A and named Keid,<ref name="IAU-LSN">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> is easily visible to the naked eye. It is orbited by a binary pair whose two components are designated 40 Eridani B and C, and which were discovered on January 31, 1783, by William Herschel.<ref name=":0">Template:Cite journal</ref>Template:Rp It was again observed by Friedrich Struve in 1825 and by Otto Struve in 1851.<ref name="heintz">Template:Cite journal</ref><ref>Template:Cite journal</ref>

In 1910, it was discovered that although component B was a faint star, it was white in color. This meant that it had to be a small star; in fact it was a white dwarf, the first discovered.<ref name="schatzman">White Dwarfs, E. Schatzman, Amsterdam: North-Holland, 1958. ^{, p. 1}</ref> Although it is neither the closest white dwarf, nor the brightest in the night sky, it is by far the easiest to observe; it is nearly three magnitudes brighter than Van Maanen's Star, the nearest solitary white dwarf, and unlike the companions of Procyon and Sirius it is not swamped in the glare of a much brighter primary.<ref name="keid">Keid Template:Webarchive, Jim Kaler, STARS web page, accessed 15/5/2007, 10/12/2011.</ref>

Nomenclature

40 Eridani is the system's Flamsteed designation and ο² Eridani (Latinised to Omicron² Eridani) its Bayer designation. The designations of the sub-components – Template:Nobr B and C – derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).<ref name="planetnaming">Template:Cite arXiv</ref> Template:Nobr also bears the variable star designation DY Eridani.

The system bore the traditional name Keid derived from the Arabic word قيض (Template:Transliteration) meaning "the eggshells", alluding to its neighbour Beid (Arabic "egg").<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In 2016, the IAU organized a Working Group on Star Names (WGSN)<ref name="WGSN"> {{#invoke:citation/CS1|citation |CitationClass=web }} </ref> to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.<ref name="TriRpt18"> Template:Cite report </ref> It approved the name Keid for the component Template:Nobr on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.<ref name="IAU-LSN"/>

Properties

File:40 Eridani.png

Amateur photo of 40 Eridani

40 Eridani A is a main-sequence dwarf of spectral type K1, 40 Eridani B is a 9th magnitude white dwarf of spectral type DA4, and 40 Eridani C is an 11th magnitude red dwarf flare star of spectral type M4.5e. When component B was a main-sequence star, it is thought to have been the most massive member of the system at Template:Val, but ejected much of its mass before it became a white dwarf.<ref name=Bond_et_al_2017/> B and C orbit each other approximately 400 AU from the primary star, A.<ref name="ss" /> Their orbit has a semimajor axis of 35 AU and is rather elliptical with an orbital eccentricity of 0.410.<ref name="heintz" />

As seen from the 40 Eridani system, the Sun is a 3.4-magnitude star in Hercules, near the border with Serpens Caput.<ref group="note">From 40 Eridani the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=Template:RA, Dec=Template:DEC, which is located near the border of Hercules (constellation) and Serpens Caput, the closest bright star being Alpha Serpentis. The absolute magnitude of the Sun is 4.85, so, at a distance of 5.04 parsecs, the Sun would have an apparent magnitude <math>\begin{smallmatrix}m\ =\ M_v\ +\ 5\cdot((\log_{10}\ 5.04)\ -\ 1)\ =\ 3.36\end{smallmatrix}</math>.</ref>

Potential for life

The habitable zone of Template:Nobr where a planet could exist with liquid water, is near 0.68 [[astronomical unit|Template:Sc]] from A. At this distance a planet would complete a revolution in 223 Earth days (according to the third of Kepler's laws) and Template:Nobr would appear nearly 20%<ref name="radiusperspective" group="note">From <math>\ h = \frac{\ a\ }{d} \,,</math> where Template:Mvar is the apparent height, Template:Mvar is the distance of the object, and Template:Mvar is the actual size of the object.</ref> wider than the Sun does on Earth. An observer on a planet in the Template:Nobr system would see the B-C pair as unusually bright white and reddish-orange stars in the night sky – magnitudes −8 and −6, slightly brighter than the appearance of Venus seen from Earth as the evening star.

It is unlikely that habitable planets exist around Template:Nobr because they would have been sterilized by its evolution into a white dwarf. As for Template:Nobr, it is prone to flares, which cause large momentary increases in the emission of X-rays as well as visible light. This would be lethal to Earth-type life on planets near the flare star.<ref name="ss">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Search for planets

40 Eridani A shows periodic radial velocity variations, which were suggested to be caused by a planetary companion. The 42-day period is close to the stellar rotation period, which made the possible planetary nature of the signal difficult to confirm.<ref name="Díaz2018"/> A 2018 study found that most evidence supports a planetary origin for the signal,<ref name="exoplanet"/> but this has remained controversial, with a 2021 study characterizing the signal as a false positive.<ref name=Rosenthal2021/> As of 2022, the cause of the radial velocity variations remained inconclusive.<ref name="Zhao2022"/>

Further studies in 2023<ref name="Laliotis2023"/>Template:Rp and 2024 concluded that the radial velocity signal very likely does originate from stellar activity, and not from a planet.<ref name="Burrows2024"/>

The candidate planet would have had a minimum mass of Template:Val, and lie considerably interior to the habitable zone, receiving nine times more stellar flux than Earth, which is an even greater amount than Mercury, the innermost planet in the Solar System, on average receives from the Sun.<ref name="exoplanet"/>

In fiction

In the Star Trek franchise, the planet Vulcan orbits 40 Eridani A.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Vulcan has been referenced in relation to the real-life search for exoplanets in this system.<ref>Template:Cite news</ref><ref name="Burrows2024"/> The hypothetical planet 40 Eridani A b is also mentioned in the book Project Hail Mary as the home of the eponymous Eridian species.<ref>Template:Cite book</ref> In the Bobiverse series by Dennis E. Taylor, a double planet is found orbiting 40 Eridani A,<ref>Template:Cite book</ref> and named Vulcan and Romulus by an uploaded intelligence in a von Neumann probe.

External links

• Keid at Jim Kaler's STARS.
• Omicron(2) Eridani entry at the Internet Stellar Database.