HD 221420

HD 221420 (HR 8935; Gliese 4340) is a likely binary star system[7] in the southern circumpolar constellation Octans. It has an apparent magnitude of 5.81, allowing it to be faintly seen with the naked eye. The object is relatively close at a distance of 102 light years but is receding with a heliocentric radial velocity of 26.5 km/s.

HD 221420
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Octans
Right ascension 23h 33m 19.5789s[1]
Declination −77° 23′ 07.194″[1]
Apparent magnitude (V) 5.81±0.01[2]
Characteristics
Spectral type G2 IV-V[3]
U−B color index +0.31[4]
B−V color index +0.68[4]
Astrometry
Radial velocity (Rv)26.48±0.02[5] km/s
Proper motion (μ) RA: +16.306 mas/yr[1]
Dec.: +0.736 mas/yr[1]
Parallax (π)32.1023 ± 0.0325 mas[1]
Distance101.6 ± 0.1 ly
(31.15 ± 0.03 pc)
Absolute magnitude (MV)+3.33[6]
Details
Mass1.35±0.01[7] M☉
Radius1.95±0.01[7] R☉
Luminosity4.01±1[8] L☉
Surface gravity (log g)4.03±0.03[7] cgs
Temperature5,830±44[9] K
Metallicity [Fe/H]+0.34±0.07[10] dex
Rotational velocity (v sin i)2.8±0.5[11] km/s
Age3.65±0.23[7] Gyr
Other designations
83 G. Octantis, CPD−78°1473, FK5 3887, GC 32742, GJ 4340, HD 221420, HIP 116250, HR 8935, SAO 258154[12][13]
Database references
SIMBADdata
Exoplanet Archivedata
Extrasolar Planets
Encyclopaedia		data

HD 221420 has a stellar classification of G2 IV-V,[3] indicating a solar analogue with a luminosity class intermediate between a subgiant and a main sequence star. The object is also extremely chromospherically inactive.[3] It has a comparable mass to the Sun and a diameter of 1.95 R☉.[7] It shines with a luminosity of 4 L☉[8] from its photosphere at an effective temperature of 5,830 K,[9] giving a yellow glow. HD 221420 is younger than the Sun at 3.65 billion years.[7] Despite this, the star is already beginning to evolve off the main sequence. Like most planetary hosts, HD 221420 has a metallicity over twice of that of the Sun[7] and spins modestly with a projected rotational velocity 2.8 km/s.[11]

There is a mid-M-dwarf star with a similar proper motion and parallax to HD 221420, which is likely gravitationally bound to it. The two stars are separated by 698 arcseconds, corresponding to a distance of 21,756 AU.[7]

Planetary system

In a 2019 doppler spectroscopy survey, an exoplanet was discovered orbiting the star. The planet was originally thought to be a super Jupiter, having a minimum mass of 9.7 MJ.[9] However, later observations using Hipparcos and Gaia astrometry found it to be a brown dwarf with a high-inclination orbit,[7][10][14] revealing a true mass of 23 MJ.[7]

The HD 221420 A planetary system[7]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(years)		 Eccentricity Inclination Radius
b 22.9±2.2 MJ 10.15+0.59
−0.38		 27.62+2.45
−1.54		 0.14+0.04
−0.03		 164.0+1.9
−2.6°		 —

References
  1. 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.
  2. Høg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (March 2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27–L30. Bibcode:2000A&A...355L..27H. ISSN 0004-6361.
  3. Gray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Bubar, E. J.; McGahee, C. E.; O'Donoghue, A. A.; Knox, E. R. (2 June 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. arXiv:astro-ph/0603770. Bibcode:2006AJ....132..161G. doi:10.1086/504637. eISSN 1538-3881. ISSN 0004-6256.
  4. Johnson, H. L.; Mitchell, R. I.; Iriarte, B.; Wisniewski, W. Z. (1966). "UBVRIJKL Photometry of the Bright Stars". Communications of the Lunar and Planetary Laboratory. 4: 99–110. Bibcode:1966CoLPL...4...99J.
  5. Jofré, E.; Petrucci, R.; Saffe, C.; Saker, L.; Artur de la Villarmois, E.; Chavero, C.; Gómez, M.; Mauas, P. J. D. (26 January 2015). "Stellar parameters and chemical abundances of 223 evolved stars with and without planets". Astronomy & Astrophysics. 574: A50. arXiv:1410.6422. Bibcode:2015A&A...574A..50J. doi:10.1051/0004-6361/201424474. eISSN 1432-0746. ISSN 0004-6361.
  6. Anderson, E.; Francis, Ch. (May 2012). "XHIP: An extended hipparcos compilation". Astronomy Letters. 38 (5): 331–346. arXiv:1108.4971. Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. ISSN 1063-7737. S2CID 119257644.
  7. Venner, Alexander; Vanderburg, Andrew; Pearce, Logan A. (11 June 2021). "True Masses of the Long-period Companions to HD 92987 and HD 221420 from Hipparcos–Gaia Astrometry". The Astronomical Journal. 162 (1): 12. arXiv:2104.13941. Bibcode:2021AJ....162...12V. doi:10.3847/1538-3881/abf932. eISSN 1538-3881. ISSN 0004-6256.
  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. Kane, Stephen R.; Dalba, Paul A.; Li, Zhexing; Horch, Elliott P.; Hirsch, Lea A.; Horner, Jonathan; Wittenmyer, Robert A.; Howell, Steve B.; Everett, Mark E.; Butler, R. Paul; Tinney, Christopher G.; Carter, Brad D.; Wright, Duncan J.; Jones, Hugh R. A.; Bailey, Jeremy; O'Toole, Simon J. (5 June 2019). "Detection of Planetary and Stellar Companions to Neighboring Stars via a Combination of Radial Velocity and Direct Imaging Techniques". The Astronomical Journal. 157 (6): 252. arXiv:1904.12931. Bibcode:2019AJ....157..252K. doi:10.3847/1538-3881/ab1ddf. eISSN 1538-3881.
  10. Li, Yiting; Brandt, Timothy D.; Brandt, G. Mirek; Dupuy, Trent J.; Michalik, Daniel; Jensen-Clem, Rebecca; Zeng, Yunlin; Faherty, Jacqueline; Mitra, Elena L. (26 November 2021). "Precise Masses and Orbits for Nine Radial-velocity Exoplanets". The Astronomical Journal. 162 (6): 266. arXiv:2109.10422. Bibcode:2021AJ....162..266L. doi:10.3847/1538-3881/ac27ab. eISSN 1538-3881. ISSN 0004-6256.
  11. Valenti, Jeff A.; Fischer, Debra A. (July 2005). "Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs". The Astrophysical Journal Supplement Series. 159 (1): 141–166. Bibcode:2005ApJS..159..141V. doi:10.1086/430500. eISSN 1538-4365. ISSN 0067-0049.
  12. Gould, Benjamin Apthorp (1878). "Uranometria Argentina : brillantez y posicion de las estrellas fijas, hasta la septima magnitud, comprendidas dentro de cien grados del polo austral : con atlas". Resultados del Observatorio Nacional Argentino. 1. Bibcode:1879RNAO....1.....G.
  13. "HR 8935". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved July 15, 2022.
  14. Feng, Fabo; Butler, R. Paul; et al. (August 2022). "3D Selection of 167 Substellar Companions to Nearby Stars". The Astrophysical Journal Supplement Series. 262 (21): 21. arXiv:2208.12720. Bibcode:2022ApJS..262...21F. doi:10.3847/1538-4365/ac7e57. S2CID 251864022.
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