Epsilon Indi

Epsilon Indi, Latinized from ε Indi, is a star system located at a distance of approximately 12 light-years from Earth in the southern constellation of Indus. The star has an orange hue and is faintly visible to the naked eye with an apparent visual magnitude of 4.83.[2] It consists of a K-type main-sequence star, ε Indi A, and two brown dwarfs, ε Indi Ba and ε Indi Bb, in a wide orbit around it.[12] The brown dwarfs were discovered in 2003. ε Indi Ba is an early T dwarf (T1) and ε Indi Bb a late T dwarf (T6) separated by 0.6 arcseconds, with a projected distance of 1460 AU from their primary star.

Epsilon Indi
Location of ε Indi (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Indus
Right ascension 22h 03m 21.65423s[1]
Declination −56° 47′ 09.5370″[1]
Apparent magnitude (V) 4.8310±0.0005[2]
Characteristics
Spectral type K5V + T1 + T6[3]
U−B color index 1.00[4]
B−V color index 1.056±0.016[2]
Astrometry
Radial velocity (Rv)−40.4[5] km/s
Proper motion (μ) RA: 3967.039±0.380[1] mas/yr
Dec.: −2535.758±0.415[1] mas/yr
Parallax (π)274.8048 ± 0.2494[1] mas
Distance11.87 ± 0.01 ly
(3.639 ± 0.003 pc)
Absolute magnitude (MV)6.89[6]
Details[7]
ε Ind A
Mass0.754±0.038[3] M☉
Radius0.711±0.005 R☉
Luminosity0.21±0.02 L☉
Surface gravity (log g)4.63±0.01 cgs
Temperature4,649±84 K
Metallicity [Fe/H]−0.13±0.06 dex
Rotation23 days[8]
Rotational velocity (v sin i)2.00 km/s
Age1.3[9]
3.7-5.7[10] Gyr
ε Ind Ba/Bb
MassBa: ≈67.6–69.1 MJup
Bb: ≈50.0–54.5[11] MJup
RadiusBa: ~0.080–0.081 R☉
Bb: ~0.082–0.083[11] R☉
LuminosityBa: 0.00002000 L☉
Bb: 0.000005861[11] L☉
Surface gravity (log g)Ba: 5.43 - 5.45
Bb: 5.27 - 5.33[11] cgs
TemperatureBa: 1,352 - 1,385 K
Bb: 976 - 1,011[11] K
Other designations
UGP 544, ε Ind, CD−57°8464, CPD−57°10015, FK5 825, GJ 845, HD 209100, HIP 108870, HR 8387, SAO 247287, LHS 67[5]
Database references
SIMBADThe system
A
Bab
Bab (as X-ray source)

ε Indi A has one known planet, ε Indi Ab, with a mass of 3.3 Jupiter masses in a nearly circular orbit with a period of about 45 years. ε Indi Ab is the closest Jovian exoplanet. The ε Indi system provides a benchmark case for the study of the formation of gas giants and brown dwarfs.[10][13][14]

Observation

The constellation Indus (the Indian) first appeared in Johann Bayer's celestial atlas Uranometria in 1603. The 1801 star atlas Uranographia, by German astronomer Johann Elert Bode, places ε Indi as one of the arrows being held in the left hand of the Indian.[15]

In 1847, Heinrich Louis d'Arrest compared the position of this star in several catalogues dating back to 1750, and discovered that it possessed a measureable proper motion. That is, he found that the star had changed position across the celestial sphere over time.[16] In 1882–3, the parallax of ε Indi was measured by astronomers David Gill and William L. Elkin at the Cape of Good Hope. They derived a parallax estimate of 0.22 ± 0.03 arcseconds.[17] In 1923, Harlow Shapley of the Harvard Observatory derived a parallax of 0.45 arcseconds.[18]

During Project Ozma in 1960, this star was examined for artificial radio signals, but none were found.[19] In 1972, the Copernicus satellite was used to examine this star for the emission of ultraviolet laser signals. Again, the result was negative.[20] ε Indi leads a list, compiled by Margaret Turnbull and Jill Tarter of the Carnegie Institution in Washington, of 17,129 nearby stars most likely to have planets that could support complex life.[21]

The star is among five nearby paradigms as K-type stars of a type in a 'sweet spot' between Sun-analog stars and M stars for the likelihood of evolved life, per analysis of Giada Arney from NASA's Goddard Space Flight Center.[22]

Characteristics

ε Indi A is a main-sequence star of spectral type K5V. The star has only about three-fourths the mass of the Sun[23] and 71% of the Sun's radius.[7] Its surface gravity is slightly higher than the Sun's.[4] The metallicity of a star is the proportion of elements with higher atomic numbers than helium, being typically represented by the ratio of iron to hydrogen compared to the same ratio for the Sun; ε Indi A is found to have about 87% of the Sun's proportion of iron in its photosphere.[3]

The corona of ε Indi A is similar to the Sun, with an X-ray luminosity of 2×1027 ergs s−1 (2×1020 W) and an estimated coronal temperature of 2×106 K. The stellar wind of this star expands outward, producing a bow shock at a distance of 63 AU. Downstream of the bow, the termination shock reaches as far as 140 AU from the star.[24]

Position of Sun and α Centauri in Ursa Major as seen from ε Indi

This star has the third highest proper motion of any star visible to the unaided eye, after Groombridge 1830 and 61 Cygni,[25] and the ninth highest overall.[26] This motion will move the star into the constellation Tucana around 2640 AD.[27] ε Indi A has a space velocity relative to the Sun of 86 km/s,[4][note 1] which is unusually high for what is considered a young star.[28] It is thought to be a member of the ε Indi moving group of at least sixteen population I stars.[29] This is an association of stars that have similar space velocity vectors, and therefore most likely formed at the same time and location.[30] ε Indi will make its closest approach to the Sun in about 17,500 years when it makes perihelion passage at a distance of around 10.58 light-years (3.245 pc).[31]

As seen from ε Indi, the Sun is a 2.6-magnitude star in Ursa Major, near the bowl of the Big Dipper.[note 2]

Companions
Artist's conception of the Epsilon Indi system showing Epsilon Indi A and its brown-dwarf binary companions.

In January 2003, astronomers announced the discovery of a brown dwarf with a mass of 40 to 60 Jupiter masses in orbit around ε Indi A at a distance of at least 1,500 AU.[32][33] In August 2003, astronomers discovered that this brown dwarf was actually a binary brown dwarf, with an apparent separation of 2.1 AU and an orbital period of about 15 years.[11][34] Both brown dwarfs are of spectral class T; the more massive component, ε Indi Ba, is of spectral type T1–T1.5 and the less massive component, ε Indi Bb, of spectral type T6.[11]

Evolutionary models[35] have been used to estimate the physical properties of these brown dwarfs from spectroscopic and photometric measurements. These yield masses of 47 ± 10 and 28 ± 7 times the mass of Jupiter, and radii of 0.091 ± 0.005 and 0.096 ± 0.005 solar radii, for ε Indi Ba and ε Indi Bb, respectively.[36] The effective temperatures are 1300–1340 K and 880–940 K, while the log g (cm s−1) surface gravities are 5.50 and 5.25, and their luminosities are 1.9 × 10−5 and 4.5 × 10−6 the luminosity of the Sun. They have an estimated metallicity of [M/H] = –0.2.[11]

Measurements of the radial velocity of Epsilon Indi by Endl et al. (2002)[37] appear to show a trend that indicated a planetary companion with an orbital period of more than 20 years. A substellar object with minimum mass of 1.6 Jupiter masses and orbital separation of roughly 6.5 AU (a Jupiter-analogue) was within the parameters of the highly approximate data.

A visual search using the ESO's Very Large Telescope found one potential candidate. However, a subsequent examination by the Hubble Space Telescope NICMOS showed that this was a background object.[38] As of 2009, a search for an unseen companion at 4 μm failed to detect an orbiting object. These observations further constrained the hypothetical object to be 5–20 times the mass of Jupiter, orbiting between 10 and 20 AU and have an inclination of more than 20°. Alternatively, it may be an exotic stellar remnant.[39]

A longer study of radial (to or from Earth) velocity, using the Echelle spectrometer on the HARPS telescope, to follow up on Endl findings, was published in a paper by M. Zechmeister et al. in 2013. The findings confirm that, quoting the paper, "Epsilon Ind A has a steady long-term trend still explained by a planetary companion".[14] This refined the radial-velocity trend observed and indicates a planetary companion with an orbital period of 45 years.[10] A gas giant with a minimum mass of 0.97 Jupiter masses and a minimal orbital separation of roughly 9.0 AU could explain the observed trend.[note 3] 9.0 AU is about the same distance out as Saturn. This does not quite qualify the planet as a true Jupiter analogue because it orbits considerably further out than 5.0 AU.[14] Not only does it orbit further out, but ε Indi A is also dimmer than the Sun, so it would only receive about the same amount of energy per square meter as Uranus does from the Sun. The radial-velocity trend was observed through all the observations so far taken using the HARPS telescope but due to the long time period predicted for just one orbit of the object around ε Indi A, more than 30 years, the astrometric phase coverage is not yet complete.[14]

In March 2018, the existence of the planet was confirmed using radial velocity measurements. At a separation of 3.3 arcseconds from its host star, ε Indi Ab is a candidate for direct imaging by the James Webb Space Telescope.[13]

In October 2019, Feng et al. published an updated orbit for the planet. They show that the orbit is slightly eccentric, with an eccentricity of about 0.26. The mass of the planet is 3.25 Jupiter masses, and it orbits at a distance of 11.6 AU, with a period of 45 years.[10]

The Epsilon Indi A planetary system[10]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(years)		 Eccentricity Inclination Radius
b 3.25+0.39
−0.65 MJ		 11.55+0.98
−0.86		 45.20+5.74
−4.77		 0.26+0.07
−0.03		 64.25+13.80
−6.09°		 ~0.95 (~0.098 R☉)[note 4] RJ

No excess infrared radiation that would indicate a debris disk has been detected around ε Indi.[41] Such a debris disk could be formed from the collisions of planetesimals that survive from the early period of the star's protoplanetary disk.

See also

Notes
  1. The space velocity components are: U = −77; V = −38, and W = +4. This yields a net space velocity of  km/s.
  2. From ε Indi the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=10h 03m 21s, Dec=56° 47′ 10″, which is located near Beta Ursae Majoris. The absolute magnitude of the Sun is 4.8, so, at a distance of 3.63 parsecs, the Sun would have an apparent magnitude .
  3. Kepler's Third Law, assuming a circular orbit gives . The mass and period are known from the paper,[14] so the semimajor axis can be calculated using .
  4. From the Power-law index of Jovian worlds; R ∝ MS − where R is the radius of the planet in proportion to Jupiter, M the mass of the planet in proportion to Jupiter, and S = −0.044+0.017
    −0.019    [40]

References
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