9 Hydrae

9 Hydrae is a single[9] star in the equatorial constellation of Hydra,[8] located 205 light years away from the Sun.[1] It is visible to the naked eye as a faint, yellow-orange hued star with an apparent visual magnitude of 4.87.[2] This body is moving closer to the Sun with a heliocentric radial velocity of −2 km/s.[1]

9 Hydrae
Observation data
Epoch J2000      Equinox J2000
Constellation Hydra
Right ascension  08h 41m 43.33591s[1]
Declination −15° 56′ 36.1727″[1]
Apparent magnitude (V) 4.866[2]
Characteristics
Spectral type K0 III CNII[3]
U−B color index +0.92[4]
B−V color index +1.07[4]
Astrometry
Radial velocity (Rv)−1.98±0.12[1] km/s
Proper motion (μ) RA: 3.676[1] mas/yr
Dec.: -97.743[1] mas/yr
Parallax (π)15.9089 ± 0.1297[1] mas
Distance205 ± 2 ly
(62.9 ± 0.5 pc)
Absolute magnitude (MV)+0.81[5]
Details[6]
Mass1.72±0.46[7] M☉
Radius11.10+0.15
−0.69[1] R☉
Luminosity54.445+0.536
−0.457[1] L☉
Surface gravity (log g)2.8 cgs
Temperature4,688±5 K
Metallicity [Fe/H]−0.01 dex
Rotational velocity (v sin i)1.7 km/s
Other designations
9 Hya, BD−15°2554, FK5 2684, HD 74137, HIP 42662, HR 3441, SAO 154552[8]
Database references
SIMBADdata

This is an aging giant star with a stellar classification of K0 III CNII,[3] where the suffix notation indicates an overabundance of cyanogen in the spectrum. 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 has 1.7[7] times the mass of the Sun but, as a consequence of evolving away from the main sequence, its envelope has swollen to 11[1] times the Sun's radius. It is radiating 54 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,688 K.[6]

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. Høg, E.; et al. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27–L30. Bibcode:2000A&A...355L..27H.
  3. Houk, N.; Smith-Moore, M. (1988). "Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Volume 4, Declinations -26°.0 to -12°.0". Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. 4. Bibcode:1988mcts.book.....H.
  4. Johnson, H. L. (1966). "UBVRIJKL Photometry of the Bright Stars". Communications of the Lunar and Planetary Laboratory. 4: 99. Bibcode:1966CoLPL...4...99J.
  5. 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.
  6. Massarotti, Alessandro; Latham, David W.; Stefanik, Robert P.; Fogel, Jeffrey (2008). "Rotational and Radial Velocities for a Sample of 761 Hipparcos Giants and the Role of Binarity". The Astronomical Journal. 135: 209. Bibcode:2008AJ....135..209M. doi:10.1088/0004-6256/135/1/209.
  7. Allende Prieto, C.; Lambert, D. L. (1999). "Fundamental parameters of nearby stars from the comparison with evolutionary calculations: masses, radii and effective temperatures". Astronomy and Astrophysics. 352: 555–562. arXiv:astro-ph/9911002. Bibcode:1999A&A...352..555A.
  8. "9 Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved April 26, 2019.
  9. 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.
  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.
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