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Eta2 Hydri - Wikipedia

Coordinates:

01^h 54^m 56^s, −67° 38′ 50″

From Wikipedia, the free encyclopedia

Eta² Hydri
Observation data Epoch J2000.0 Equinox J2000.0
Constellation	Hydrus
Right ascension	01^h 54^m 56.131^s^[1]
Declination	–67° 38′ 50.29″^[1]
Apparent magnitude (V)	+4.68^[2]
Characteristics
Evolutionary stage	Red clump^[3]
Spectral type	G8IIIb^[4]
B−V color index	0.931±0.036^[2]
Variable type	Constant^[5]
Astrometry

Radial velocity (R_v)	−17.75±0.15^[6] km/s
Proper motion (μ)	RA: +75.927 mas/yr^[1] Dec.: +73.401 mas/yr^[1]
Parallax (π)	14.7444 ± 0.0878 mas^[1]
Distance	221 ± 1 ly (67.8 ± 0.4 pc)
Absolute magnitude (M_V)	0.57±0.07^[7]

Details^[3]

Mass	2.08±0.04 `M`_☉
Radius	11.17±0.07 `R`_☉
Luminosity	65.92±0.86 `L`_☉
Surface gravity (log g)	2.71±0.27 cgs
Temperature	4,932±20 K
Metallicity [Fe/H]	−0.24±0.05 dex
Rotational velocity (v sin i)	2.4±1.0^[5] km/s
Age	0.87±0.05 Gyr

Other designations
η² Hyi, CPD−68 101, HD 11977, HIP 8928, HR 570, SAO 248460^[8]
Database references
SIMBAD	data

Eta² Hydri is a star in the southern constellation of Hydrus. The name is Latinized from η² Hydri and often catalogued as HD 11977. 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 221 light-years based on parallax measurements, but it is drifting closer to the Sun with a radial velocity of −18 km/s.^[6]

The stellar classification of Eta² Hydri is G8IIIb,^[4] which indicates this is a evolved giant star. It is located in the clump zone of the HR diagram, which means 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.^[5] It is estimated to be around 870 million years old and has expanded to 11 times the Sun's diameter, though is only around two times as massive as the Sun.^[3] The star has a leisurely rotation, with a period of no more than 230–270 days.^[5] 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.^[5] This object has at least 6.3^[9] times the mass of Jupiter 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.^[5]

The Eta² Hydri planetary system^[5]
Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	>6.29±0.07^[9] $M$ _J	1.89±0.11^[9]	711±8	0.40±0.07	—	—

References

^ ^a ^b ^c ^d Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
^ ^a ^b ^c 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.
^ ^a ^b ^c Gallenne, A.; Pietrzyński, G.; Graczyk, D.; Nardetto, N.; Mérand, A.; Kervella, P.; Gieren, W.; Villanova, S.; Mennickent, R. E.; Pilecki, B. (2018-08-01), "Fundamental properties of red-clump stars from long-baseline H-band interferometry", Astronomy & Astrophysics, 616: A68, arXiv:1806.09572, doi:10.1051/0004-6361/201833341, ISSN 0004-6361
^ ^a ^b 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.
^ ^a ^b ^c ^d ^e ^f ^g 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.
^ ^a ^b 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.
^ 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
^ "eta02 Hyi". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2021-07-11.{{cite web}}: CS1 maint: postscript (link)
^ ^a ^b ^c Kunitomo, M.; et al. (July 2005). "Planet Engulfment by ~1.5-3 M_sun 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.

External links

"Notes for star HD 11977". Extrasolar Planets Encyclopaedia. Archived from the original on July 2, 2007. Retrieved 2008-07-03.

Constellation of Hydrus

Stars

Bayer	α β γ δ ε ζ η¹ η² θ ι κ λ μ ν π¹ π² σ τ¹ τ²
Variable	VW
HD	1237 10180 20003
Other	AB8 SMSS J0313−6708 WISEPC J014807.25−720258.7

Exoplanets	η² Hydri b GJ 3021 b HD 10180 c

Star clusters	NGC 602 NGC 643 NGC 1466

Galaxies

NGC	646 802 813
Other	KKs 3 PGC 6240 Small Magellanic Cloud (mostly in Tucana) Large Magellanic Cloud (mostly in Dorado, partly in Mensa)

Planetary system

See also

References

External links