Gamma Coronae Borealis

Gamma Coronae Borealis, Latinized from γ Coronae Borealis, is a binary star system in the northern constellation of Corona Borealis. It is visible to the naked eye with an apparent visual magnitude of 3.83.[2] Based upon an annual parallax shift of 22.33[1] mas as seen from Earth, it is located about 146 light years from the Sun. The system is moving closer to the Sun with a radial velocity of about −15 km/s.[5]

Gamma Coronae Borealis
Observation data
Epoch J2000      Equinox J2000
Constellation Corona Borealis
Right ascension  15h 42m 44.56551s[1]
Declination +26° 17′ 44.2847″[1]
Apparent magnitude (V) 3.83[2] (4.04 + 5.60)[3]
Characteristics
Spectral type B9 V + A3 V[4]
U−B color index −0.03[2]
B−V color index +0.00[2]
Astrometry
Radial velocity (Rv)−15.26±4.54[5] km/s
Proper motion (μ) RA: −111.65[1] mas/yr
Dec.: +49.52[1] mas/yr
Parallax (π)22.33 ± 0.50[1] mas
Distance146 ± 3 ly
(45 ± 1 pc)
Absolute magnitude (MV)+0.56[6]
Orbit[7]
Period (P)91.2±0.4 yr
Semi-major axis (a)0.729±0.006″
Eccentricity (e)0.48±0.01
Inclination (i)94.45±0.07°
Longitude of the node (Ω)111.75±0.09°
Periastron epoch (T)1931.6±0.3
Argument of periastron (ω)
(secondary)		103.8±0.6°
Details
γ CrB A
Mass2.51±0.04[8] M☉
Luminosity59[8] L☉
Temperature7,649±22[9] K
Rotational velocity (v sin i)182±12[5] km/s
Age400[10] Myr
Other designations
γ CrB, 8 CrB, BD+26° 2722, HD 140436, HIP 76952, HR 5849, SAO 83958, ADS 9757, WDS J15427+2618AB[11]
Database references
SIMBADdata

Binary system

The components of this system have been optically separated, with the secondary lying about one arc second away from the primary. They have an orbital period of 91.2 years and an eccentricity of 0.48.[7] The semimajor axis is 0.73 arc seconds − roughly the distance from the Sun to Neptune[12] − and the inclination of the orbital plane is 94.5°.[7] Both components are main sequence stars: the visual magnitude 4.04[3] primary has a stellar classification of B9 V,[13] while the secondary is of class A3 V[4] and magnitude 5.60.[3]

Variability

In 1969, γ Coronae Borealis was confirmed to be variable with an amplitude of 0.05 magnitudes. A year later, it was "confirmed" to be a δ Scuti variable with the earliest known spectral type in the class. The observed variations were not strictly periodic, but showed a characteristic timescale of 0.03 days (43 minutes).[14] γ Coronae Borealis also showed anomalous behaviour not seen in other δ Scuti stars, such as periods without variation.[15]

When the pulsation mechanisms of the δ Scuti stars and the similar but hotter Slowly pulsating B-type (SPB) stars were identified, it became apparent that γ Coronae Borealis and a handful of similar low amplitude variable stars such as γ Ursae Minoris and Maia fell between those two classes in a region with no known driver for pulsations. The Maia variable class was named for these stars. Ironically, Maia itself has been shown not to be variable, and a number of other reported members of the class have been shown to be variables of other types, so the Maia variables appear to be very rare if they are even a real variable class.[16] However, satellite observations have discovered a number of small-amplitude variables between the SPB and δ Scuti instability regions as well as a number of candidates in the open clusters NGC 3766 and NGC 1893.[17]

The mechanism of variability for γ Coronae Borealis is not known. Spectroscopic variations with a period of 0.9 days have been observed, which matches the likely rotation period of the star, and radial velocity changes are also seen with a possible period of 0.45 days.[16] Short-period non-radial pulsations may be carried around by the rotation of the star,[18] but the driver of such pulsations in a star of this temperature is unknown.[17]

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References
  1. van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357.
  2. Fernie, J. D. (June 1969), "UBV observations of miscellaneous stars", Journal of the Royal Astronomical Society of Canada, 63: 133–135, Bibcode:1969JRASC..63..133F.
  3. Cvetkovic, Z.; Ninkovic, S. (2010), "On the Component Masses of Visual Binaries", Serbian Astronomical Journal, 180 (180): 71–80, Bibcode:2010SerAJ.180...71C, doi:10.2298/SAJ1080071C.
  4. Malkov, O. Yu.; et al. (2012), "Dynamical masses of a selected sample of orbital binaries", Astronomy & Astrophysics, 546: 5, Bibcode:2012A&A...546A..69M, doi:10.1051/0004-6361/201219774, A69.
  5. Becker, Juliette C.; et al. (April 2015), "Extracting Radial Velocities of A- and B-type Stars from Echelle Spectrograph Calibration Spectra", The Astrophysical Journal Supplement Series, 217 (2): 13, arXiv:1503.03874, Bibcode:2015ApJS..217...29B, doi:10.1088/0067-0049/217/2/29, 29.
  6. 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.
  7. Hartkopf, W. I.; et al. (June 30, 2006), Sixth Catalog of Orbits of Visual Binary Stars, United States Naval Observatory, retrieved 2017-06-02.
  8. Zorec, J.; Royer, F. (January 2012), "Rotational velocities of A-type stars. IV. Evolution of rotational velocities", Astronomy & Astrophysics, 537: A120, arXiv:1201.2052, Bibcode:2012A&A...537A.120Z, doi:10.1051/0004-6361/201117691.
  9. Muñoz Bermejo, J.; et al. (May 2013), "A PCA approach to stellar effective temperatures", Astronomy and Astrophysics, 453: A95, arXiv:1303.7218, Bibcode:2013A&A...553A..95M, doi:10.1051/0004-6361/201220961.
  10. Vican, Laura (June 2012), "Age Determination for 346 Nearby Stars in the Herschel DEBRIS Survey", The Astronomical Journal, 143 (6): 135, arXiv:1203.1966, Bibcode:2012AJ....143..135V, doi:10.1088/0004-6256/143/6/135.
  11. "gam CrB". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-10-20.
  12. Kaler, James B. (July 25, 2008), "Gamma Coronae Borealis", Stars, University of Illinois, retrieved 18 November 2014.
  13. Abt, Helmut A.; Morrell, Nidia I. (July 1995), "The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars", Astrophysical Journal Supplement, 99: 135, Bibcode:1995ApJS...99..135A, doi:10.1086/192182.
  14. Percy, John R. (February 1970), "The Light Variation of γ Coronae Borealis", Publications of the Astronomical Society of the Pacific, 82: 126, Bibcode:1970PASP...82..126P, doi:10.1086/128893.
  15. Veto, B.; Kovacs, G. (October 1981), "Standstill of gamma CrB", Information Bulletin on Variable Stars, 2030 (2030): 1, Bibcode:1981IBVS.2030....1V.
  16. Percy, John R.; Wilson, Joseph B. (June 2000), "Another Search for Maia Variable Stars", The Publications of the Astronomical Society of the Pacific, 112 (772): 846–851, Bibcode:2000PASP..112..846P, doi:10.1086/316577.
  17. Balona, L. A; Engelbrecht, C. A; Joshi, Y. C; Joshi, S; Sharma, K; Semenko, E; Pandey, G; Chakradhari, N. K; Mkrtichian, David; Hema, B. P; Nemec, J. M (2016). "The hot γ Doradus and Maia stars". Monthly Notices of the Royal Astronomical Society. 460 (2): 1318. arXiv:1606.06426. Bibcode:2016MNRAS.460.1318B. doi:10.1093/mnras/stw1038.
  18. Lehmann, H.; et al. (November 1997), "The RV-variations of γ Coronae Borealis - a frequency analysis", Astronomy and Astrophysics, 327: 167–172, Bibcode:1997A&A...327..167L.
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