HD 203857

HD 203857 is a double star in the constellation Cygnus. It is near the lower limit of visibility to the naked eye, having a combined apparent visual magnitude of 6.46.[2] The distance to the primary component is approsimatly 1,230 light years based on parallax, and it has an absolute magnitude of −0.75.[2] The star is drifting closer to the Sun with a radial velocity of −6.3 km/s.[1] It has a stellar classification of K5[3] and is known to be evolved. The star likely hosts an extrasolar planet, though yet unconfirmed.

HD 203857
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cygnus
Right ascension 21h 23m 48.38864s[1]
Declination +37° 21′ 05.3177″[1]
Apparent magnitude (V) 6.46[2]
Characteristics
Spectral type K5[3]
B−V color index 1.472±0.006[2]
Astrometry
Radial velocity (Rv)−6.26±0.16[1] km/s
Proper motion (μ) RA: −5.999[1] mas/yr
Dec.: −8.364[1] mas/yr
Parallax (π)2.6601 ± 0.0417 mas[1]
Distance1,230 ± 20 ly
(376 ± 6 pc)
Absolute magnitude (MV)−0.75[2]
Details[4]
Mass1.7 M☉
Radius46-50 R☉
Luminosity513 L☉
Temperature4,080 K
Other designations
BD+36°4543, HD 203857, HIP 105637, HR 8193, SAO 71280, CCDM J21237+3722A[5]
Database references
SIMBADdata

HD 203857 is listed in the Washington Double Star Catalogue as having five visual companions. It is separated by six arc-minutes from HD 203784, an F-type subgiant, though it is likely they are actually not gravitationally–bound.[4] HD 203784 is thought to be closer to us and less luminous than HD 203857.[6] There are also fainter stars at 23″ and 178″. HD 203784 has a 13th-magnitude star and a 14th-magnitude star within 20″.[7]

Substellar companion

Okayama Planet Search team has published a paper in late 2008 reporting investigations on radial velocity variations observed for a set of evolved stars, announcing possible detection of a substellar companion orbiting the giant star HD 203857.[4] Orbital period is estimated 2.3 years, but planet has still to be confirmed.

The HD 203857 planetary system
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b (unconfirmed) ≥8.1 MJ 2.1 836.8 0.3 — —

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. 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.
  3. Hoffleit, Dorrit; Jaschek, Carlos (1991), The Bright star catalogue (5th Revised ed.), New Haven, Conn.: Yale University Observatory, Bibcode:1991bsc..book.....H.
  4. Toyota, Eri; et al. (February 2009), "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.
  5. "HD 203857". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2020-03-09.
  6. 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.
  7. Finzi, A.; Wolf, R. A. (1967), "Type I Supernovae", The Astrophysical Journal, 150: 115, Bibcode:1967ApJ...150..115F, doi:10.1086/149317.
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