82 G. Eridani

82 G. Eridani
Observation data
Epoch J2000      Equinox J2000
Constellation Eridanus
Right ascension 03h 19m 55.65093s[1]
Declination −43° 04′ 11.2175″[1]
Apparent magnitude (V) 4.254[2]
Characteristics
Spectral type G8 V[3]
U−B color index +0.21[4]
B−V color index +0.71[4]
Astrometry
Radial velocity (Rv) +87.2[2] km/s
Proper motion (μ) RA: 3038.34[1] mas/yr
Dec.: 726.58[1] mas/yr
Parallax (π) 165.47 ± 0.19[1] mas
Distance 19.71 ± 0.02 ly
(6.043 ± 0.007 pc)
Absolute magnitude (MV) 5.34[2]
Details
Mass 0.97[5] M☉
Radius 0.92[6] R☉
Surface gravity (log g) 4.40[7]
Luminosity 0.62[8][9] L☉
Temperature 5,401[7] K
Metallicity [Fe/H] –0.40[7] dex
Rotational velocity (v sin i) 4.0[10] km/s
Age 6.1[11]–12.7[2] Gyr
Other designations
Database references
SIMBAD data
NStED data
ARICNS data
Extrasolar Planets
Encyclopaedia		 data

82 G. Eridani is a star about 20 light years away from Earth in the constellation Eridanus. It is a main sequence star with a stellar classification of G8.[3]

Contents

Observation

In the southern-sky catalog Uranometria Argentina, 82 G. Eridani (sometimes abbreviated to "82 Eridani") 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.

Properties

This star is slightly smaller and less massive than the Sun, making it marginally dimmer than the Sun in terms of luminosity; about 20% more luminous than Tau Ceti or Alpha Centauri B. The projected equatorial rotation rate (v \sin i) is 4.0 km/s,[10] compared to 2 km/s for the Sun.

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.[13][11]

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.[14]

Planetary system

On August 17, 2011, European astronomers announced the discovery of three planets orbiting 82 G. Eridani (HD 20794). 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 measurement 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 displays 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), which is above the boiling point of water.[15]

The 82 G. Eridani system[16]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity
b 2.7 ± 0.3 M⊕ 0.1207 ± 0.002 18.315 ± 0.008 0
c 2.4 ± 0.4 M⊕ 0.2036 ± 0.0034 40.114 ± 0.053 0
d 4.8 ± 0.6 M⊕ 0.3499 ± 0.0059 90.309 0

Habitability

In his book Habitable Planets for Man,[17] Stephen Dole gave 82 G. Eridani his highest estimate for habitability: 5.7%. Four other stars had this figure: Alpha Centauri B, 70 Ophiuchi A, Eta Cassiopeiae A, and Delta Pavonis. 82 G. Eridani (GJ 139) was also picked as a Tier 1 target star for NASA's proposed Space Interferometry Mission (SIM) mission to search for terrestrial-sized or larger planets.[18]

See also

References

  1. ^ a b c d e van Leeuwen, Floor (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752v1, Bibcode 2007A&A...474..653V, doi:10.1051/0004-6361:20078357  Note: see VizieR catalogue I/311.
  2. ^ a b c d 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, Bibcode 2009A&A...501..941H, doi:10.1051/0004-6361/200811191 
  3. ^ a b Gray, R. O. et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal 132 (1): 161–170, Bibcode 2006AJ....132..161G, doi:10.1086/504637 
  4. ^ a b c "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. ^ Staff (2007-06-08), List of the Nearest 100 Stellar Systems, Research Consortium on Nearby Stars (RECONS), Georgia State University, http://www.chara.gsu.edu/RECONS/, retrieved 2007-07-26 
  6. ^ 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, Bibcode 1983ApJS...53..643J, doi:10.1086/190905  — See the table on p. 653.
  7. ^ a b c 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, Bibcode 2008A&A...487..373S, doi:10.1051/0004-6361:200809698 
  8. ^ Based upon:
    \frac{L}{L_{sun}} = \left ( \frac{R}{R_{sun}} \right )^2 \left ( \frac{T_{eff}}{T_{sun}} \right )^4
    where L is luminosity, R is radius and Teff is the effective temperature.
  9. ^ Krimm, Hans (August 19, 1997), Luminosity, Radius and Temperature, Hampden-Sydney College, http://ceres.hsc.edu/homepages/classes/astronomy/spring99/Mathematics/sec20.html, retrieved 2007-05-16 
  10. ^ a b Schröder, C.; Reiners, A.; Schmitt, J. 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, Bibcode 2009A&A...493.1099S, doi:10.1051/0004-6361:200810377 
  11. ^ a b Hillenbrand, Lynne A. (November 2008), "Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics", The Astrophysical Journal 687 (2): 1264–1293, Bibcode 2008ApJ...687.1264M, doi:10.1086/591785 
  12. ^ 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, http://books.google.com/books?id=VhE1AQAAIAAJ&pg=PA159  Coordinates are for the 1875 equinox.
  13. ^ Hearnshaw, J. B. (1973), "The iron abundance of 82 Eridani", Astronomy and Astrophysics 29: 165–170, Bibcode 1973A&A....29..165H 
  14. ^ Frisch, P. C. (1993), "G-star astropauses - A test for interstellar pressure", Astrophysical Journal 407 (1): 198–206, Bibcode 1993ApJ...407..198F, doi:10.1086/172505 
  15. ^ 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, arXiv:1108.3447, Bibcode 2011yCat..35349058P, doi:10.1051/0004-6361/201117055 
  16. ^ Schneider, Jean, "Star: HD 20794", Extrasolar Planets Encyclopaedia (Paris Observatory), http://exoplanet.eu/star.php?st=HD+20794, retrieved 2011-12-05 
  17. ^ Dole, Stephen H. (1970), Habitable Planets for Man (2nd ed.), London: American Elsevier Pub. Co, ISBN 0-444-00092-5, http://www.rand.org/pubs/commercial_books/CB179-1/ 
  18. ^ McCarthy, Chris (2005). "SIM Planet Search Tier 1 Target Stars". San Francisco State University. http://tauceti.sfsu.edu/~chris/SIM/. Retrieved 2007-07-26. 

External links

Stars of Eridanus
Bayer
Flamsteed
Nearby