# Epsilon Indi

Observation dataEpoch J2000.0      Equinox J2000.0 (ICRS) Constellation Location of ε Indi (circled) Indus 22h 03m 21.65423s[1] −56° 47′ 09.5370″[1] 4.8310±0.0005[2] K5V + T1 + T6[3] 1.00[4] 1.056±0.016[2] 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 Distance 11.87 ± 0.01 ly (3.639 ± 0.003 pc) Absolute magnitude (MV) 6.89[6] Mass 0.754±0.038[3] M☉ Radius 0.711±0.005 R☉ Luminosity 0.21±0.02 L☉ Surface gravity (log g) 4.63±0.01 cgs Temperature 4,649±84 K Metallicity [Fe/H] −0.13±0.06 dex Rotation 23 days[8] Rotational velocity (v sin i) 2.00 km/s Age 1.3[9]3.7-5.7[10] Gyr Mass Ba: ≈67.6–69.1 MJupBb: ≈50.0–54.5[11] MJup Radius Ba: ~0.080–0.081 R☉Bb: ~0.082–0.083[11] R☉ Luminosity Ba: 0.00002000 L☉Bb: 0.000005861[11] L☉ Surface gravity (log g) Ba: 5.43 - 5.45Bb: 5.27 - 5.33[11] cgs Temperature Ba: 1,352 - 1,385 KBb: 976 - 1,011[11] K 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] SIMBAD The system A Bab Bab (as X-ray source)

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.

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)
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][improper synthesis?] 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.

## Notes

1. ^ The space velocity components are: U = −77; V = −38, and W = +4. This yields a net space velocity of ${\displaystyle {\begin{smallmatrix}{\sqrt {77^{2}\ +\ 38^{2}\ +\ 4^{2}}}\ =\ 86\end{smallmatrix}}}$ 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 ${\displaystyle {\begin{smallmatrix}m\ =\ M_{v}\ +\ 5\cdot ((\log _{10}\ 3.63)\ -\ 1)\ =\ 2.6\end{smallmatrix}}}$.
3. ^ Kepler's Third Law, assuming a circular orbit gives ${\displaystyle {\begin{smallmatrix}{\frac {4\pi ^{2}}{T^{2}}}={\frac {G(M+m)}{R^{3}}}\end{smallmatrix}}}$. The mass and period are known from the paper,[14] so the semimajor axis can be calculated using ${\displaystyle {\begin{smallmatrix}R={\sqrt[{3}]{\frac {G(M+m)T^{2}}{4\pi ^{2}}}}\end{smallmatrix}}}$ .
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

1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
2. ^ a b c van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
3. ^ a b c Demory, B.-O.; et al. (October 2009). "Mass-radius relation of low and very low-mass stars revisited with the VLTI". Astronomy and Astrophysics. 505 (1): 205–215. arXiv:0906.0602. Bibcode:2009A&A...505..205D. doi:10.1051/0004-6361/200911976. S2CID 14786643.
4. ^ a b c Kollatschny, W. (1980). "A model atmosphere of the late type dwarf Epsilon INDI". Astronomy and Astrophysics. 86 (3): 308–314. Bibcode:1980A&A....86..308K.
5. ^ a b "SIMBAD Query Result: LHS 67 -- High proper-motion Star". Centre de Données astronomiques de Strasbourg. Retrieved 2007-07-11.
6. ^ Jimenez, Raul; Flynn, Chris; MacDonald, James; Gibson, Brad K. (March 2003). "The Cosmic Production of Helium". Science. 299 (5612): 1552–1555. arXiv:astro-ph/0303179. Bibcode:2003Sci...299.1552J. doi:10.1126/science.1080866. PMID 12624260. S2CID 1424666.
7. ^ a b Rains, Adam D.; et al. (April 2020). "Precision angular diameters for 16 southern stars with VLTI/PIONIER". Monthly Notices of the Royal Astronomical Society. 493 (2): 2377–2394. arXiv:2004.02343. Bibcode:2020MNRAS.493.2377R. doi:10.1093/mnras/staa282.
8. ^ Kaler, Jim. "Epsilon Indi". Stars. University of Illinois. Retrieved 2010-05-03.
9. ^ Lachaume, R.; Dominik, C.; Lanz, T.; Habing, H. J. (1999). "Age determinations of main-sequence stars: combining different methods". Astronomy and Astrophysics. 348: 897–909. Bibcode:1999A&A...348..897L. — This paper gives a median log age = 9.11, with a range of min = 8.91 and max = 9.31. This corresponds to 1.3 Gyr, with an error range of 0.8–2.0 Gyr.
10. Feng, Fabo; Anglada-Escudé, Guillem; Tuomi, Mikko; Jones, Hugh R. A.; Chanamé, Julio; Butler, Paul R.; Janson, Markus (14 October 2019), "Detection of the nearest Jupiter analog in radial velocity and astrometry data", Monthly Notices of the Royal Astronomical Society, 490 (4): 5002–5016, arXiv:1910.06804, Bibcode:2019MNRAS.490.5002F, doi:10.1093/mnras/stz2912, S2CID 204575783
11. King, R. R.; et al. (February 2010), "ɛ Indi Ba, Bb: a detailed study of the nearest known brown dwarfs" (PDF), Astronomy and Astrophysics, 510: A99, arXiv:0911.3143, Bibcode:2010A&A...510A..99K, doi:10.1051/0004-6361/200912981, S2CID 53550866, However, it seems that derivations of the mass of cool brown dwarfs are uncertain even where estimates of the effective temperature, surface gravity, and luminosity exist.
12. ^ Smith, Verne V.; Tsuji, Takashi; Hinkle, Kenneth H.; Cunha, Katia; Blum, Robert D.; Valenti, Jeff A.; Ridgway, Stephen T.; Joyce, Richard R.; Bernath, Peter (2003). "High-resolution infrared spectroscopy of the brown dwarf ε Indi Ba". The Astrophysical Journal Letters. 599 (2): L107–L110. doi:10.1086/381248. S2CID 117133193.
13. ^ a b Feng, Fabo; Tuomi, Mikko; Jones, Hugh R. A. (23 March 2018). "Detection of the closest Jovian exoplanet in the Epsilon Indi triple system". arXiv:1803.08163 [astro-ph.EP].
14. Zechmeister, M.; Kürster, M; Endl, M.; Lo Curto, G.; Hartman, H.; Nilsson, H.; Henning, T.; Hatzes, A.; Cochran, W. D. (April 2013). "The planet search programme at the ESO CES and HARPS. IV. The search for Jupiter analogues around solar-like stars". Astronomy and Astrophysics. 552: 62. arXiv:1211.7263. Bibcode:2013A&A...552A..78Z. doi:10.1051/0004-6361/201116551. S2CID 53694238.
15. ^ Scholz, Ralf-Dieter; McCaughrean, Mark (2003-01-13). "Discovery of Nearest Known Brown Dwarf". ESO. Archived from the original on September 20, 2008. Retrieved 2008-07-02.
16. ^ D'Arrest, M. (1847). "On proper motion of ε Indi". Monthly Notices of the Royal Astronomical Society. 8: 16. Bibcode:1847MNRAS...8...16D. doi:10.1093/mnras/8.1.16.
17. ^ Callandreau, O. (1886). "Revue des publications astronomiques. Heliometer determinations of Stellar parallax, in the southern hemisphere, by David Gill and W. L. Elkin". Bulletin Astronomique (in French). 2 (1): 42–44. Bibcode:1885BuAsI...2...42C.
18. ^ Shapley, Harlow (1923). "Epsilon Indi". Harvard College Observatory Bulletin. 789 (789): 2. Bibcode:1923BHarO.789Q...2S.
19. ^ Burnham, Robert; Luft, Herbert A. (1978). Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System. Courier Dover Publications. ISBN 978-0-486-23568-4.
20. ^ Lawton, A. T. (1975). "CETI from Copernicus". Spaceflight. 17: 328–330. Bibcode:1975SpFl...17..328L.
21. ^ Stahl, Jason (January 2007). "20 Things You Didn't Know About... Aliens". Discover. Archived from the original on 2007-02-21. Retrieved 2007-03-02.
22. ^ Bill Steigerwald (22 February 2019). ""Goldilocks" Stars May be "Just Right" for Finding Habitable Worlds". NASA. 'I find that certain nearby K stars like 61 Cyg A/B, Epsilon Indi, Groombridge 1618, and HD 156026 may be particularly good targets for future biosignature searches,' said Arney.
23. ^ Research Consortium On Nearby Stars, Georgia State University (January 1, 2012). "The 100 nearest star systems". RECONS. Retrieved 2012-06-11.
24. ^ Müller, Hans-Reinhard; Zank, Gary P. (2001). "Modeling the Interstellar Medium-Stellar Wind Interactions of λ Andromedae and ε Indi". The Astrophysical Journal. 551 (1): 495–506. Bibcode:2001ApJ...551..495M. doi:10.1086/320070.
25. ^ Weaver, Harold F. (1947). "The Visibility of Stars Without Optical Aid". Publications of the Astronomical Society of the Pacific. 59 (350): 232–243. Bibcode:1947PASP...59..232W. doi:10.1086/125956.
26. ^ Staff (2007-05-04). "High Proper Motion Stars: Interesting Areas to View". ESA. Retrieved 2006-08-10.
27. ^ Patrick Moore; Robin Rees (2014). Patrick Moore's Data Book of Astronomy. Cambridge: Cambridge University Press. p. 296. ISBN 978-1-139-49522-6.
28. ^ Rocha-Pinto, Helio J.; Maciel, Walter J.; Castilho, Bruno V. (2001). "Chromospherically Young, Kinematically Old Stars". Astronomy and Astrophysics. 384 (3): 912–924. arXiv:astro-ph/0112452. Bibcode:2002A&A...384..912R. doi:10.1051/0004-6361:20011815. S2CID 16982360.
29. ^ Eggen, O. J. (1971). "The zeta Herculis, sigma Puppis, ε Indi, and eta Cephei Groups of Old Disk Population Stars". Publications of the Astronomical Society of the Pacific. 83 (493): 251–270. Bibcode:1971PASP...83..251E. doi:10.1086/129119.
30. ^ Kollatschny, W. (1980). "A model atmosphere of the late type dwarf Epsilon INDI". Astronomy and Astrophysics. 86 (3): 308–314. Bibcode:1980A&A....86..308K.
31. ^ Bailer-Jones, C. A. L. (March 2015), "Close encounters of the stellar kind", Astronomy & Astrophysics, 575: 13, arXiv:1412.3648, Bibcode:2015A&A...575A..35B, doi:10.1051/0004-6361/201425221, S2CID 59039482, A35.
32. ^ Scholz, Ralf-Dieter; McCaughrean, Mark (2003-01-13). "Discovery of Nearest Known Brown Dwarf: Bright Southern Star Epsilon Indi Has Cool, Substellar Companion". European Southern Observatory. Archived from the original on October 14, 2007. Retrieved 2006-05-24.
33. ^ Scholz, R.-D.; McCaughrean, M. J.; Lodieu, N.; Kuhlbrodt, B. (February 2003). "ε Indi B: A new benchmark T dwarf". Astronomy and Astrophysics. 398 (3): L29–L33. arXiv:astro-ph/0212487. Bibcode:2003A&A...398L..29S. doi:10.1051/0004-6361:20021847. S2CID 119474823.
34. ^ Volk, K.; Blum, R.; Walker, G.; Puxley, P. (2003). "epsilon Indi B". IAU Circular. IAU. 8188 (8188): 2. Bibcode:2003IAUC.8188....2V.
35. ^ E.g., Baraffe, I.; Chabrier, G.; Barman, T.; Allard, F.; Hauschildt, P. H. (May 2003). "Evolutionary models for cool brown dwarfs and extrasolar giant planets. The case of HD 209458". Astronomy and Astrophysics. 402 (2): 701–712. arXiv:astro-ph/0302293. Bibcode:2003A&A...402..701B. doi:10.1051/0004-6361:20030252. S2CID 15838318.
36. ^ McCaughrean, M. J.; et al. (January 2004). "ε Indi Ba, Bb: The nearest binary brown dwarf". Astronomy and Astrophysics. 413 (3): 1029–1036. arXiv:astro-ph/0309256. Bibcode:2004A&A...413.1029M. doi:10.1051/0004-6361:20034292. S2CID 15407249.
37. ^ Endl, M.; Kürster, M.; Els, S.; Hatzes, A. P.; Cochran, W. D.; Dennerl, K.; Döbereiner, S. (2002). "The planet search program at the ESO Coudé Echelle spectrometer. III. The complete Long Camera survey results". Astronomy and Astrophysics. 392 (2): 671–690. arXiv:astro-ph/0207512. Bibcode:2002A&A...392..671E. doi:10.1051/0004-6361:20020937. S2CID 17393347.
38. ^ Geißler, K.; Kellner, S.; Brandner, W.; Masciadri, E.; Hartung, M.; Henning, T.; Lenzen, R.; Close, L.; Endl, M.; Kürster, M. (2007). "A direct and differential imaging search for sub-stellar companions to epsilon Indi A". Astronomy and Astrophysics. 461 (2): 665–668. arXiv:astro-ph/0611336. Bibcode:2007A&A...461..665G. doi:10.1051/0004-6361:20065843. S2CID 119442720.
39. ^ Janson, M.; et al. (August 10, 2009). "Imaging search for the unseen companion to ε Ind A – improving the detection limits with 4 μm observations". Monthly Notices of the Royal Astronomical Society. 399 (1): 377–384. arXiv:0906.4145. Bibcode:2009MNRAS.399..377J. doi:10.1111/j.1365-2966.2009.15285.x. S2CID 16314685.
40. ^ Chen, Jingjing; Kipping, David (2016). "Probabilistic Forecasting of the Masses and Radii of Other Worlds". The Astrophysical Journal. 834 (1): 17. doi:10.3847/1538-4357/834/1/17. ISSN 1538-4357. A defining feature of the Jovian worlds is that the MR power-index is close to zero (−0.04 ± 0.02), with radius being nearly degenerate with respect to mass.
41. ^ Trilling, D. E.; et al. (February 2008). "Debris Disks around Sun-like Stars". The Astrophysical Journal. 674 (2): 1086–1105. arXiv:0710.5498. Bibcode:2008ApJ...674.1086T. doi:10.1086/525514. S2CID 54940779.