Astronomy:82 G. Eridani
Coordinates: 
04h 31m 11.52059s, +58° 58′ 37.4806″
  | |
| Observation data Equinox J2000.0]] (ICRS) | |
|---|---|
| Constellation | Eridanus |
| Right ascension | 03h 19m 55.651s[1] |
| Declination | −43° 04′ 11.22″[1] |
| Apparent magnitude (V) | 4.254[2] |
| Characteristics | |
| Spectral type | G6 V[3] |
| U−B color index | +0.22[4] |
| B−V color index | +0.71[4] |
| Astrometry | |
| Radial velocity (Rv) | 87.76±0.13[1] km/s |
| Proper motion (μ) | RA: 3035.017[1] mas/yr Dec.: 726.964[1] mas/yr |
| Parallax (π) | 165.5242 ± 0.0784[1] mas |
| Distance | 19.704 ± 0.009 ly (6.041 ± 0.003 pc) |
| Absolute magnitude (MV) | 5.34[2] |
| Details | |
| Mass | 0.70[5] M☉ |
| Radius | 0.92[6] R☉ |
| Luminosity | 0.74[7] L☉ |
| Surface gravity (log g) | 4.40[8] cgs |
| Temperature | 5,401[8] K |
| Metallicity [Fe/H] | −0.40[8] dex |
| Rotation | 33.19 ± 3.61 days[5] |
| Rotational velocity (v sin i) | 4.0[9] km/s |
| Age | 6.1[10]–12.7[2] Gyr |
| Other designations | |
| Database references | |
| SIMBAD | The star |
| planet b | |
| planet c | |
| planet d | |
| Exoplanet Archive | data |
| ARICNS | data |
| Extrasolar Planets Encyclopaedia | data |
82 G. Eridani (HD 20794, HR 1008, e Eridani) is a star 19.7 light-years (6.0 parsecs) away from Earth in the constellation Eridanus. It is a main-sequence star with a stellar classification of G6 V, and it hosts a system of at least three planets and a dust disk.
Observation
In the southern-sky catalog Uranometria Argentina, 82 G. Eridani (often abbreviated to 82 Eridani)[11] is the 82nd star listed in the constellation Eridanus.[12] The Argentina catalog, compiled by the 19th-century astronomer Benjamin Gould, is a southern celestial hemisphere analog of the more famous Flamsteed catalog, and uses a similar numbering scheme. 82 G. Eridani, like other stars near the Sun, has held on to its Gould designation, even while other more distant stars have not.[citation needed]
Properties
This star is slightly smaller and less massive than the Sun, making it marginally dimmer than the Sun in terms of luminosity; it is about a third more luminous than Tau Ceti or Alpha Centauri B. The projected equatorial rotation rate (v sin i) is 4.0 km/s,[9] compared to 2 km/s for the Sun. However, this value is likely overestimated and explained by the limitation of the spectrograph used. When observed by HARPS, a v sin i smaller than 2 km/s is found, compatible with a slow-rotating or inclined star. Such observation would also match the lack of a reliable rotational period detection and the absence of any magnetic cycle.[13]
82 G. Eridani is a high-velocity star—it is moving quickly compared to the average—and hence is probably a member of Population II, generally older stars whose motions take them well outside the plane of the Milky Way. Like many other Population II stars, 82 G. Eridani is somewhat metal-deficient (though much less deficient than many), and is older than the Sun. It has a relatively high orbital eccentricity of 0.40 about the galaxy, ranging between 4.6 and 10.8 kiloparsecs from the core. Estimates of the age of this star ranged from 6 to 12 billion years.[10][14]
This star is located in a region of low-density interstellar matter (ISM), so it is believed to have a large astropause that subtends an angle of 6″ across the sky. Relative to the Sun, this star is moving at a space velocity of 101 km/s, with the bow shock advancing at more than Mach 3 through the ISM.[15]
Planetary system
An infrared excess was discovered around the star by the Infrared Space Observatory at 60 μm,[16] but was not later confirmed by the Spitzer Space Telescope, in 2006. However, in 2012, a dust disk was found around the star,[17] by the Herschel Space Observatory. While not well-constrained, if assumed to have a similar composition to 61 Virginis' dust disk, it has a semi-major axis of 19 AU.[18]
On August 17, 2011, European astronomers announced the discovery of three planets orbiting 82 G. Eridani. The mass range of these planets classifies them as super-Earths; objects with only a few times the Earth's mass. These planets were discovered by precise measurements of the radial velocity of the star, with the planets revealing their presence by their gravitational displacement of the star during each orbit. None of the planets display a significant orbital eccentricity. However, their orbital periods are all 90 days or less, indicating that they are orbiting close to the host star. The equilibrium temperature for the most distant planet, based on an assumed Bond albedo of 0.3, would be about 388 K (115 °C); significantly above the boiling point of water.[5]
The number of planets in the system remains uncertain. At the time of planet c's detection, it exerted the lowest gravitational perturbation. There was also a similarity noted between its orbital period and the rotational period of the star. For these reasons the discovery team were somewhat more cautious regarding the verity of its candidate planet status than for the other two.[5] Continued observation of the star will be required to determine the exact nature of the planetary system.
Using the TERRA algorithm, developed by Guillem Anglada-Escudé and R. Paul Butler in 2012, to describe better and filter out noise interference to extract more precise radial velocity measurements, a team of scientists led by Fabo Feng, in 2017, provided evidence for up to three more planets. One such candidate, of Neptune mass, 82 G. Eridani f, may orbit in the habitable zone of the star. The team also believe that, using these noise reduction techniques, they are able to better quantify the descriptions for the earlier 3 exoplanets, but only have weak evidence of 82 G. Eridani c.[19]
A study in 2023 could only confirm planets b & d, and did not significantly detect the other planet candidates. In particular, the statistical significance of planet c would be expected to increase with additional data; the fact that this has not happened casts doubt on its existence. The 40-day radial velocity signal may instead be tied to the stellar rotation. The additional three candidates found in 2017 (e, f, g) could not be confirmed or refuted.[20]:23,44 Another 2023 study also only confirmed b & d out of the previous planet candidates (referring to them as b & c), but also detected a potential third planet farther from the star than any of the previous candidates.[13]
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (days) |
Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| Hot dust | ≲0.1 AU | — | — | |||
| g (unconfirmed) | ≥1.03+0.49 −0.30 M⊕ |
0.095 ± 0.001 | 11.86+0.01 −0.02 |
0.20+0.15 −0.19 |
— | — |
| b | ≥2.82+0.10 −0.80 M⊕ |
0.127 ± 0.001 | 18.33+0.01 −0.02 |
0.27+0.04 −0.22 |
— | — |
| c (unconfirmed) | ≥2.52+0.52 −0.83 M⊕ |
0.225+0.002 −0.003 |
43.17+0.12 −0.10 |
0.17+0.10 −0.16 |
— | — |
| d | ≥3.52+0.58 −1.01 M⊕ |
0.364 ± 0.004 | 88.90+0.37 −0.41 |
0.25+0.16 −0.21 |
— | — |
| e | ≥4.77+0.96 −0.86 M⊕ |
0.509 ± 0.006 | 147.02+1.43 −0.91 |
0.29+0.14 −0.18 |
— | — |
| f (unconfirmed) | ≥10.26+1.89 −1.47 M⊕ |
0.875 ± 0.001 | 331.41+5.08 −3.01 |
0.05+0.06 −0.05 |
— | — |
| Dust disk | ~19–~30 AU | — | — | |||
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (days) |
Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| Hot dust | ≲0.1 AU | — | — | |||
| b | ≥2.0±0.2 M⊕ | 0.13±0.01 | 18.32±0.01 | 0.09+0.08 −0.06 |
— | — |
| c | ≥4.7±0.4 M⊕ | 0.37±0.01 | 89.58+0.09 −0.10 |
0.13±0.07 | — | — |
| Dust disk | ~19–~30 AU | — | — | |||
Planned observation missions
82 G. Eridani (GJ 139) was picked as a Tier 1 target star for NASA's proposed Space Interferometry Mission (SIM) mission to search for terrestrial-sized or larger planets,[21] which was cancelled in 2010.
See also
- Map analysis of the 1961 Zeta Reticuli Incident (a purported alien abduction)
References
- ↑ 1.0 1.1 1.2 1.3 1.4 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940 Gaia DR3 record for this source at VizieR.
- ↑ 2.0 2.1 2.2 Holmberg, J.; Nordstrom, B.; Andersen, J. (July 2009), "The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics", Astronomy and Astrophysics 501 (3): 941–947, doi:10.1051/0004-6361/200811191, Bibcode: 2009A&A...501..941H
- ↑ Keenan, Philip C; McNeil, Raymond C (1989). "The Perkins Catalog of Revised MK Types for the Cooler Stars". The Astrophysical Journal Supplement Series 71: 245. doi:10.1086/191373. Bibcode: 1989ApJS...71..245K.
- ↑ 4.0 4.1 4.2 "LHS 19 -- High proper-motion Star", SIMBAD (Centre de Données astronomiques de Strasbourg), http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HD+20794&submit=SIMBAD+search, retrieved 2007-07-26
- ↑ 5.0 5.1 5.2 5.3 Pepe, F. et al. (2011), "The HARPS search for Earth-like planets in the habitable zone: I – Very low-mass planets around HD20794, HD85512 and HD192310", Astronomy & Astrophysics 534: A58, doi:10.1051/0004-6361/201117055, Bibcode: 2011A&A...534A..58P
- ↑ Johnson, H. M.; Wright, C. D. (1983), "Predicted infrared brightness of stars within 25 parsecs of the sun", Astrophysical Journal Supplement Series 53: 643–711, doi:10.1086/190905, Bibcode: 1983ApJS...53..643J — See the table on p. 653.
- ↑ Porto de Mello, Gustavo; del Peloso, Eduardo F.; Ghezzi, Luan (April 2006), "Astrobiologically Interesting Stars Within 10 Parsecs of the Sun", Astrobiology 6 (2): 308–331, doi:10.1089/ast.2006.6.308, PMID 16689649, Bibcode: 2006AsBio...6..308P
- ↑ 8.0 8.1 8.2 Sousa, S. G. et al. (August 2007), "Spectroscopic parameters for 451 stars in the HARPS GTO planet search program. Stellar [Fe/H] and the frequency of exo-Neptunes", Astronomy and Astrophysics 487 (1): 373–381, doi:10.1051/0004-6361:200809698, Bibcode: 2008A&A...487..373S
- ↑ 9.0 9.1 Schröder, C.; Reiners, Ansgar; Schmitt, Jürgen H. M. M. (January 2009), "Ca II HK emission in rapidly rotating stars. Evidence for an onset of the solar-type dynamo", Astronomy and Astrophysics 493 (3): 1099–1107, doi:10.1051/0004-6361:200810377, Bibcode: 2009A&A...493.1099S, http://goedoc.uni-goettingen.de/goescholar/bitstream/handle/1/9690/aa10377-08.pdf?sequence=2[yes|permanent dead link|dead link}}]
- ↑ 10.0 10.1 Mamajek, Eric E.; Hillenbrand, Lynne A. (November 2008), "Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics", The Astrophysical Journal 687 (2): 1264–1293, doi:10.1086/591785, Bibcode: 2008ApJ...687.1264M
- ↑ Kostjuk, N. D. (2004). "VizieR Online Data Catalog: HD-DM-GC-HR-HIP-Bayer-Flamsteed Cross Index (Kostjuk, 2002)". VizieR On-line Data Catalog: IV/27A. Originally Published in: Institute of Astronomy of Russian Academy of Sciences (2002) 4027. Bibcode: 2004yCat.4027....0K.
- ↑ Gould, Benjamin Apthorp (1879), Uranometria Argentina: brightness and position of every fixed star, down to the seventh magnitude, within one hundred degrees of the South Pole, Resultados, Universidad Nacional de Córdoba Observatorio Astronómico, 1, Observatorio Nacional Argentino, pp. 159–160, https://books.google.com/books?id=VhE1AQAAIAAJ&pg=PA159 Coordinates are for the 1875 equinox.
- ↑ 13.0 13.1 13.2 Cretignier, M. et al. (August 2023). "YARARA V2: Reaching sub-m s−1 precision over a decade using PCA on line-by-line radial velocities". Astronomy & Astrophysics 678: A2. doi:10.1051/0004-6361/202347232. Bibcode: 2023A&A...678A...2C.
- ↑ Hearnshaw, J. B. (1973), "The iron abundance of 82 Eridani", Astronomy and Astrophysics 29: 165–170, Bibcode: 1973A&A....29..165H
- ↑ Frisch, P. C. (1993), "G-star astropauses - A test for interstellar pressure", Astrophysical Journal 407 (1): 198–206, doi:10.1086/172505, Bibcode: 1993ApJ...407..198F
- ↑ Decin, G. et al. (May 2000). "The Vega phenomenon around G dwarfs". Astronomy and Astrophysics 357: 533–542. Bibcode: 2000A&A...357..533D.
- ↑ Wyatt, M. C. et al. (2012). "Herschel imaging of 61 Vir: implications for the prevalence of debris in low-mass planetary systems". Monthly Notices of the Royal Astronomical Society 424 (2): 1206. doi:10.1111/j.1365-2966.2012.21298.x. Bibcode: 2012MNRAS.424.1206W.
- ↑ 18.0 18.1 18.2 Kennedy, G. M.; Matra, L.; Marmier, M.; Greaves, J. S.; Wyatt, M. C.; Bryden, G.; Holland, W.; Lovis, C. et al. (2015). "Kuiper belt structure around nearby super-Earth host stars". Monthly Notices of the Royal Astronomical Society 449 (3): 3121. doi:10.1093/mnras/stv511. Bibcode: 2015MNRAS.449.3121K.
- ↑ 19.0 19.1 Feng, F.; Tuomi, M.; Jones, H.R.A. (September 2017). "Evidence for at least three planet candidates orbiting HD 20794". Astronomy and Astrophysics 605 (103): 11. doi:10.1051/0004-6361/201730406. Bibcode: 2017A&A...605A.103F.
- ↑ Laliotis, Katherine et al. (April 2023). "Doppler Constraints on Planetary Companions to Nearby Sun-like Stars: An Archival Radial Velocity Survey of Southern Targets for Proposed NASA Direct Imaging Missions". The Astronomical Journal 165 (4): 176. doi:10.3847/1538-3881/acc067. Bibcode: 2023AJ....165..176L.
- ↑ McCarthy, Chris (2005). "SIM Planet Search Tier 1 Target Stars". San Francisco State University. http://tauceti.sfsu.edu/~chris/SIM/.
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