Eta2 Hydri

Eta2 Hydri, Latinized from η2 Hydri and often catalogued as HD 11977, is a star in the constellation of Hydrus. It is visible to the naked eye with an apparent visual magnitude of +4.68,[2] and is one of the least variable stars known. The distance to Eta2 Hydri is approximately 222 light-years based on parallax measurements, but it is drifting closer to the Sun with a radial velocity of −18 km/s.[1]

For other stars with this Bayer designation, see η Hydri.
Eta2 Hydri
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Hydrus
Right ascension 01h 54m 56.13205s[1]
Declination –67° 38′ 50.2946″[1]
Apparent magnitude (V) +4.68[2]
Characteristics
Evolutionary stage Horizontal branch
Spectral type G8IIIb[3]
B−V color index 0.931±0.036[2]
Variable type Constant[4]
Astrometry
Radial velocity (Rv)−17.75±0.15[1] km/s
Proper motion (μ) RA: +75.680[1] mas/yr
Dec.: +73.609[1] mas/yr
Parallax (π)14.6943 ± 0.1523 mas[1]
Distance222 ± 2 ly
(68.1 ± 0.7 pc)
Absolute magnitude (MV)0.57±0.07[5]
Details[4]
Mass1.91±0.21 M☉
Radius10.09±0.32 R☉
Luminosity68.96±0.86[1] L☉
Surface gravity (log g)2.66±0.10[5] cgs
Temperature4,970±70 K
Metallicity [Fe/H]−0.21±0.10 dex
Rotational velocity (v sin i)2.4±1.0 km/s
Age1.30±0.48[5] Gyr
Other designations
η2 Hyi, CPD−68 101, HD 11977, HIP 8928, HR 570, SAO 248460[6]
Database references
SIMBADdata

The stellar classification of Eta2 Hydri is G8IIIb,[3] which indicates this is a evolved giant star. It is located in the clump zone of the HR diagram, which indicates it has already ascended the red giant branch and is now generating energy by core helium fusion surrounded by a hydrogen fusing shell. Based on its mass, it was probably a class A star (similar to Vega or Fomalhaut) when it was on the main sequence.[4] It is estimated to be around 1.3[5] billion years old and has expanded to 10 times the Sun's diameter, though is only around 1.9 times as massive as the Sun. The star has a leisurely rotation, with a period of no more than 230–270 days.[4] As of 2005, an extrasolar planet was confirmed to be orbiting the star.

Planetary system

In 2005, the giant planet Eta2 Hydri b was found in orbit around Eta2 Hydri by measuring radial velocity variations.[4] This object has at least 6.3[7] times the mass of the Sun and is orbiting with a period of 1.95 years. It is the first giant planet to be found in orbit around an intermediate-mass giant star.[4]

The Eta2 Hydri planetary system[4]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b >6.29±0.07[7] MJ 1.89±0.11[7] 711±8 0.40±0.07 — —

See also

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. Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters. 38 (5): 331. arXiv:1108.4971. Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. S2CID 119257644. Retrieved 2021-07-11.
  3. Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
  4. Setiawan, J.; et al. (2005). "A substellar companion around the intermediate-mass giant star HD 11977". Astronomy & Astrophysics. 437 (2): L31–L34. arXiv:astro-ph/0505510. Bibcode:2005A&A...437L..31S. doi:10.1051/0004-6361:200500133. S2CID 6362483.
  5. da Silva, L.; et al. (November 2006), "Basic physical parameters of a selected sample of evolved stars", Astronomy and Astrophysics, 458 (2): 609–623, arXiv:astro-ph/0608160, Bibcode:2006A&A...458..609D, doi:10.1051/0004-6361:20065105, S2CID 9341088
  6. "eta02 Hyi". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2021-07-11.{{cite web}}: CS1 maint: postscript (link)
  7. Kunitomo, M.; et al. (July 2005). "Planet Engulfment by ~1.5-3 Msun Red Giants". Astronomy and Astrophysics. 437 (2): L31–L34. arXiv:astro-ph/0505510. Bibcode:2005A&A...437L..31S. doi:10.1051/0004-6361:200500133. S2CID 6362483.
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