56 Pegasi

56 Pegasi is a binary star system in the northern constellation of Pegasus. It is visible to the naked eye with a combined apparent visual magnitude of 4.74.[2] The system is approximately 590 light years away from the Sun based on parallax,[14] but is drifting closer with a radial velocity of −28 km/s.[6] It is listed as a member of the Wolf 630 moving group.[15]

56 Pegasi
Location of 56 Pegasi (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Pegasus
Right ascension  23h 07m 06.74364s[1]
Declination 25° 28′ 05.788″[1]
Apparent magnitude (V) 4.74[2]
Characteristics
Spectral type K0.5II:Ba1CN-2CH-0.5[3] + sdO[4][5]
U−B color index +1.14[2]
B−V color index +1.32[2]
Astrometry
Radial velocity (Rv)−27.55[6] km/s
Proper motion (μ) RA: −4.288[1] mas/yr
Dec.: −32.639[1] mas/yr
Parallax (π)6.1131 ± 0.4108[1] mas
Distance530 ± 40 ly
(160 ± 10 pc)
Absolute magnitude (MV)−1.32[7]
Orbit[5]
Period (P)111.140±0.14 d
Eccentricity (e)0
Periastron epoch (T)51738.8±0.5 MJD
Semi-amplitude (K1)
(primary)		1.47±0.04 km/s
Details
56 Peg A
Mass5.4[8] M☉
Radius41[9] R☉
Luminosity18.2 - 18.9[9] L☉
Surface gravity (log g)0.50[10] cgs
Temperature4,185±85[9] K
Metallicity [Fe/H]−0.21[11] dex
Rotational velocity (v sin i)4.4[12] km/s
Age100±100[8] Myr
Other designations
56 Peg, NSV 14429, BD+24°4716, FK5 3848, GC 32201, HD 218356, HIP 114155, HR 8796, SAO 91019[13]
Database references
SIMBADdata

The variable radial velocity of this star was announced in 1911 by W. W. Campbell. It is a single-lined spectroscopic binary in an assumed circular orbit with a period of 111.1 days. The a sin i value for this system is 0.01511 ± 0.00040 AU (2.26 ± 0.06 Gm), where a is the semimajor axis and i is the (unknown) orbital inclination. This value provides a lower bound on the true semimajor axis, which in this case is their actual separation.[5]

The primary component is a peculiar bright giant with a stellar classification of K0.5 II: Ba1 CN-2 CH-0.5.[3] This notation indicates it is a K-type giant with some uncertainty about the classification, along with an overabundance of barium and underabundances of the CN and CH radicals. It is an active star,[16] roughly 100 million years old, with 5.4 times the Sun's mass.[8] The star has expanded to 40[17] times the radius of the Sun and is radiating 680[11] times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,416 K.[18]

The system displays an excess of ultraviolet radiation that must be coming from the secondary companion. Simon et al. (1982) classified this object as a subdwarf O star.[4] Alternatively, it may be a white dwarf companion with an accretion disk.[19][5] Several puzzling features in the evolutionary history of this pair may be explained if the primary is a fast rotator being seen nearly pole-on. The star may have been spun up during a mass transfer episode with the secondary.[16]

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. Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D.
  3. Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
  4. Simon, T.; et al. (1982). "On the reality of a boundary in the H-R diagram between late-type stars with and without high temperature outer atmospheres". Astrophysical Journal. 257: 225. Bibcode:1982ApJ...257..225S. doi:10.1086/159981.
  5. Griffin, R. F. (2006). "Spectroscopic binary orbits from photoelectric radial velocities - Paper 186: 56 Pegasi". The Observatory. 126: 1. Bibcode:2006Obs...126....1G.
  6. Famaey, B.; et al. (2005). "Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data". Astronomy & Astrophysics. 430: 165. arXiv:astro-ph/0409579. Bibcode:2005A&A...430..165F. doi:10.1051/0004-6361:20041272.
  7. Frankowski, A.; Jorissen, A. (2006). "The puzzling case of 56 Pegasi: A fast rotator seen nearly pole-on". The Observatory. 126: 25. arXiv:astro-ph/0512036. Bibcode:2006Obs...126...25F.
  8. Tetzlaff, N.; et al. (2011). "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun". Monthly Notices of the Royal Astronomical Society. 410: 190. arXiv:1007.4883. Bibcode:2011MNRAS.410..190T. doi:10.1111/j.1365-2966.2010.17434.x. Vizier catalog entry
  9. Messineo, M.; Brown, A. G. A. (2019). "A Catalog of Known Galactic K-M Stars of Class I Candidate Red Supergiants in Gaia DR2". The Astronomical Journal. 158: 20. arXiv:1905.03744. Bibcode:2019AJ....158...20M. doi:10.3847/1538-3881/ab1cbd.
  10. Soubiran, Caroline; et al. (2016). "The PASTEL catalogue: 2016 version". Astronomy & Astrophysics. 591: A118. arXiv:1605.07384. Bibcode:2016A&A...591A.118S. doi:10.1051/0004-6361/201628497.
  11. 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. Vizier catalog entry
  12. De Medeiros, J. R.; Mayor, M. (1999). "A catalog of rotational and radial velocities for evolved stars". Astronomy and Astrophysics Supplement Series. 139 (3): 433. arXiv:astro-ph/0608248. Bibcode:1999A&AS..139..433D. doi:10.1051/aas:1999401. Vizier catalog entry
  13. "56 Peg". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-07-01.
  14. Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. Vizier catalog entry
  15. McDonald, A. R. E.; Hearnshaw, J. B. (August 1983). "The Wolf 630 moving group of stars". Monthly Notices of the Royal Astronomical Society. 204: 841–852. Bibcode:1983MNRAS.204..841M. doi:10.1093/mnras/204.3.841.
  16. Frankowski, A.; Jorissen, A. (February 2006). "The puzzling case of 56 Pegasi: a fast rotator seen nearly pole-on". The Observatory. 126: 25–37. arXiv:astro-ph/0512036. Bibcode:2006Obs...126...25F.
  17. van Belle, G. T.; et al. (2009). "Supergiant temperatures and linear radii from near-infrared interferometry". Monthly Notices of the Royal Astronomical Society. 394 (4): 1925. arXiv:0811.4239. Bibcode:2009MNRAS.394.1925V. doi:10.1111/j.1365-2966.2008.14146.x.
  18. Martínez, M. Isabel Pérez; et al. (2011). "The basal chromospheric Mg ii h+k flux of evolved stars: Probing the energy dissipation of giant chromospheres". Monthly Notices of the Royal Astronomical Society. 414: 418. arXiv:1102.4832. Bibcode:2011MNRAS.414..418P. doi:10.1111/j.1365-2966.2011.18421.x. Vizier catalog entry
  19. Schindler, M.; et al. (December 1982). "Ultraviolet and X-ray detection of the 56 Pegasi system /K0 IIp + WD/ - Evidence for accretion of a cool stellar wind onto a white dwarf". Astrophysical Journal, Part 1. 263: 269–276. Bibcode:1982ApJ...263..269S. doi:10.1086/160501.
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