Astronomy:HD 181720

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Short description: Star in the constellation Sagittarius
HD 181720 / Sika
Observation data
Equinox J2000.0]] (ICRS)
Constellation Sagittarius[1]
Right ascension  19h 22m 52.985s[2]
Declination –32° 55′ 08.59″[2]
Apparent magnitude (V) 7.84[3]
Characteristics
Evolutionary stage main sequence
Spectral type G1V[3]
Apparent magnitude (B) 8.44[1]
Apparent magnitude (J) 6.652±0.019[1]
Apparent magnitude (H) 6.346±0.029[1]
Apparent magnitude (K) 6.294±0.034[1]
B−V color index 0.599±0.019[1]
V−R color index 0.599[3]
Astrometry
Radial velocity (Rv)−45.404±0.0006[4] km/s
Proper motion (μ) RA: 88.036[2] mas/yr
Dec.: −415.197[2] mas/yr
Parallax (π)16.6766 ± 0.0273[2] mas
Distance195.6 ± 0.3 ly
(59.96 ± 0.10 pc)
Absolute magnitude (MV)4.10[3]
Details
Mass0.87±0.01[5] M☉
Radius1.42±0.02[5] R☉
Luminosity2.112±0.003[5] L☉
Surface gravity (log g)4.06±0.02[5] cgs
Temperature5,781±18[3] K
Metallicity [Fe/H]−0.53±0.02[3] dex
Rotation47 days[citation needed]
Rotational velocity (v sin i)1.5[3] km/s
Age12.4±0.5[5] Gyr
Other designations
CD–33°14164, Gaia DR2 6745589980571162752, HIP 95262, SAO 211218, PPM 298918, LTT 7666, NLTT 47718, 2MASS J19225298-3255079[6]
Database references
SIMBADdata
Exoplanet Archivedata
Extrasolar Planets
Encyclopaedia		data

HD 181720 is star with an orbiting substellar companion in the southern constellation of Sagittarius. It is located at a distance of 196 light years from the Sun based on parallax measurements,[2] but is drifting closer with a radial velocity of −45.4 km/s.[4] The star has an absolute magnitude of 4.10,[3] but at that distance it has an apparent visual magnitude of 7.84,[3] which is too faint to be seen with the naked eye. It shows a high proper motion, traversing the celestial sphere at an angular rate of 0.444 arcsec yr−1.[7]

The spectrum of HD 181720 presents as an ordinary G-type main-sequence star with a stellar classification of G1V.[3] It is an older star with an estimated age of roughly 12 billion years[5] and a minimal level of magnetic activity in its chromosphere.[3] The star has 87% of the mass of the Sun but the radius is now 42% larger than the Sun's.[5] Its metallicity content, as measured by the abundance of iron, is three-tenths as much as in the Sun. HD 181720 is radiating more than double the luminosity of the Sun from its photosphere at an effective temperature of 5,781 K.[3]

The star HD 181720 is named Sika. The name was selected in the NameExoWorlds campaign by Ghana, during the 100th anniversary of the IAU. Sika means gold in the Ewe language.[8][9]

Planetary system

In 2009, a gas giant planet was found in orbit around the star.[3] It was named "Toge" in 2019.[8][9] The planets around such metal-poor stars are rare (only two known similar cases are HD 111232 and HD 22781, as of 2019).[10] The upper bound on the mass of this component is 32 times the mass of Jupiter, which would place it in the brown dwarf range.[11]

The HD 181720 planetary system[3][12]
Companion
(in order from star) 		Mass Semimajor axis
(AU) 		Orbital period
(days) 		Eccentricity Inclination Radius
b (Toge) ≥ 0.37 MJ 1.78 956 ± 14 0.26 ± 0.06 — —

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters 38 (5): 331. doi:10.1134/S1063773712050015. Bibcode: 2012AstL...38..331A. 
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940  Gaia DR3 record for this source at VizieR.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 Santos, Nuno C. et al. (2010). "The HARPS Search for Southern Extra-solar Planets XXI. Three New Giant Planets Orbiting the Metal-poor Stars HD 5388, HD 181720, and HD 190984". Astronomy and Astrophysics 512 (A47): A47. doi:10.1051/0004-6361/200913489. Bibcode: 2010A&A...512A..47S. 
  4. ↑ 4.0 4.1 Soubiran, C. et al. (2018). "Gaia Data Release 2. The catalogue of radial velocity standard stars". Astronomy and Astrophysics 616: A7. doi:10.1051/0004-6361/201832795. Bibcode: 2018A&A...616A...7S. 
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Bonfanti, A. et al. (2016). "Age consistency between exoplanet hosts and field stars". Astronomy & Astrophysics 585: 14. doi:10.1051/0004-6361/201527297. A5. Bibcode: 2016A&A...585A...5B. 
  6. ↑ "HD 181720". SIMBAD. Centre de données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HD+181720. 
  7. ↑ Luyten, W. J. (June 1995). "NLTT Catalogue (Luyten, 1979)". VizieR Online Data Catalog. Bibcode: 1995yCat.1098....0L. 
  8. ↑ 8.0 8.1 "Approved names" (in en). http://www.nameexoworlds.iau.org/final-results. 
  9. ↑ 9.0 9.1 "International Astronomical Union | IAU". https://www.iau.org/news/pressreleases/detail/iau1912/. 
  10. ↑ Adibekyan, Vardan (2019). "Heavy Metal Rules. I. Exoplanet Incidence and Metallicity". Geosciences 9 (3): 105. doi:10.3390/geosciences9030105. Bibcode: 2019Geosc...9..105A. 
  11. ↑ Kiefer, F. et al. (January 2021). "Determining the true mass of radial-velocity exoplanets with Gaia. Nine planet candidates in the brown dwarf or stellar regime and 27 confirmed planets". Astronomy & Astrophysics 645. doi:10.1051/0004-6361/202039168. Bibcode: 2021A&A...645A...7K. 
  12. ↑ Santos, Nuno C. et al. (2011). "The HARPS search for southern extrasolar planets XXV. Results from the metal-poor sample". Astronomy and Astrophysics 526: A112. doi:10.1051/0004-6361/201015494. Bibcode: 2011A&A...526A.112S. 

Coordinates: Sky map 19h 22m 52.9859s, −32° 55′ 08.595″



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