27 Hydrae

27 Hydrae is a member of a triple star system[5] system in the equatorial constellation of Hydra,[9] located 222 light years away from the Sun.[1] It is visible to the naked eye as a faint, orange-hued star with a combined apparent visual magnitude of 4.82.[2] The system is moving further from the Earth with a heliocentric radial velocity of +25.6 km/s.[6]

27 Hydrae
Observation data
Epoch J2000      Equinox J2000
Constellation Hydra
Right ascension 09h 20m 29.01857s[1]
Declination −09° 33′ 20.5054″[1]
Apparent magnitude (V) 4.818[2] (4.91 + 7.03 + 10.99)[3]
Characteristics
Spectral type K0III[4] + F4V + K2V[5]
Astrometry
Radial velocity (Rv)+25.60±0.13[6] km/s
Proper motion (μ) RA: −12.48[1] mas/yr
Dec.: −27.37[1] mas/yr
Parallax (π)14.66 ± 0.31 mas[1]
Distance222 ± 5 ly
(68 ± 1 pc)
Details
A
Mass2.17[2] M☉
Radius11[6] R☉
Luminosity57.5[2] L☉
Surface gravity (log g)2.9[6] cgs
Temperature4,965±26[2] K
Metallicity [Fe/H]−0.07[6] dex
Rotational velocity (v sin i)3.3[6] km/s
Age1.91[2] Gyr
B
Radius1.82[7] R☉
Luminosity5.885[7] L☉
Temperature6,664[7] K
C
Radius0.72[8] R☉
Luminosity0.227[8] L☉
Temperature4,685[8] K
Other designations
CCDM J09204-0934, WDS J09204-0934[9]
A: 27 Hya, BD−08°2643, HD 80586, HIP 45811, HR 3709, SAO 136768
B: BD−09°2801, HD 80550, HIP 45802, SAO 136767
C: TYC 5463-1518-1
Database references
SIMBADdata
B
C

The magnitude 4.91[5] primary, component A, is an aging giant star with a stellar classification of K0 III.[4] It is a red clump giant,[10] which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. The star is 1.9[2] billion years old with 2.17[2] times the mass of the Sun. It has swelled to 11[6] times the Sun's radius and is radiating 57.5[2] times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,965 K.[2] The star is suspected to host a low-mass companion.[11]

The stellar companions to this star, designated components B and C, lie at an angular separation of 229.10″ from the primary, and form a binary pair with a separation of 9.20″ as of 2015.[3] The brighter member of the pair, component B, is a seventh magnitude F-type main-sequence star with a class of F4 V, while its companion is an eleventh magnitude K-type main-sequence star with a class of K2 V.[5]

Substellar companion

The Okayama Planet Search team published a paper in late 2008 reporting investigations into radial velocity variations observed for a set of evolved stars, showing hints of a substellar companion orbiting the primary member of the wide binary system 27 Hydrae.[11] Its orbital period is estimated at 9.3 years, but no planet has been confirmed yet.

The 27 Hydrae planetary system
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b (unconfirmed) ≥10 MJ ≈5.9 3,400 — — —

References
  1. 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.
  2. Luck, R. Earle (2015). "Abundances in the Local Region. I. G and K Giants". Astronomical Journal. 150 (3). 88. arXiv:1507.01466. Bibcode:2015AJ....150...88L. doi:10.1088/0004-6256/150/3/88. S2CID 118505114.
  3. Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122 (6): 3466, Bibcode:2001AJ....122.3466M, doi:10.1086/323920, retrieved 2015-07-22
  4. Houk, N.; Swift, C. (1999). "Michigan catalogue of two-dimensional spectral types for the HD Stars". Michigan Spectral Survey. 5. Bibcode:1999MSS...C05....0H.
  5. Eggleton, P. P.; Tokovinin, A. A. (September 2008). "A catalogue of multiplicity among bright stellar systems". Monthly Notices of the Royal Astronomical Society. 389 (2): 869–879. arXiv:0806.2878. Bibcode:2008MNRAS.389..869E. doi:10.1111/j.1365-2966.2008.13596.x. S2CID 14878976.
  6. Massarotti, Alessandro; et al. (January 2008). "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity". The Astronomical Journal. 135 (1): 209–231. Bibcode:2008AJ....135..209M. doi:10.1088/0004-6256/135/1/209. S2CID 121883397.
  7. 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.
  8. 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.
  9. "27 Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved May 3, 2019.
  10. Alves, David R. (August 2000), "K-Band Calibration of the Red Clump Luminosity", The Astrophysical Journal, 539 (2): 732–741, arXiv:astro-ph/0003329, Bibcode:2000ApJ...539..732A, doi:10.1086/309278, S2CID 16673121
  11. Toyota, Eri; et al. (2008). "Radial Velocity Search for Extrasolar Planets in Visual Binary Systems". Publications of the Astronomical Society of Japan. 61 (1): 19–28. Bibcode:2009PASJ...61...19T. doi:10.1093/pasj/61.1.19.
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