BY Draconis

BY Draconis is a multiple star system in the constellation Draco, consisting of at least three components. Components A and B are main sequence stars,[4] and form a close binary star system with a short orbital period of only 5.98 days. Their individual spectroscopic classifications are dK5e and dK7e.[6] They form the prototype of a class of variable stars known as BY Draconis variables.[5]

BY Draconis AB
Observation data
Epoch J2000      Equinox J2000
Constellation Draco
Right ascension 18h 33m 55.7728s[1]
Declination +51° 43′ 08.905″[1]
Apparent magnitude (V) 8.07[1]
Characteristics
Spectral type K6Ve[1]
U−B color index +0.99[2]
B−V color index +1.19[2]
Variable type BY Dra + UV Cet[3]
Astrometry
Radial velocity (Rv)−25.484±0.046[4] km/s
Proper motion (μ) RA: 185.759[1] mas/yr
Dec.: −325.590[1] mas/yr
Parallax (π)60.59 ± 0.28[1] mas
Distance53.8 ± 0.2 ly
(16.50 ± 0.08 pc)
Absolute magnitude (MV)7.48 / 8.63[5]
Orbit[4]
PrimaryBY Dra A
CompanionBY Dra B
Period (P)5.9751139 ± 0.0000046 d
Semi-major axis (a)4.4472 ± 0.0091 mas
Eccentricity (e)0.30014 ± 0.00062
Inclination (i)154.41± 0.29°
Longitude of the node (Ω)152.3 ± 0.1°
Periastron epoch (T)JD 2453999.2144 ± 0.0021
Argument of periastron (ω)
(secondary)		230.33 ± 0.17°
Semi-amplitude (K1)
(primary)		28.394 ± 0.060 km/s
Semi-amplitude (K2)
(secondary)		32.284 ± 0.061 km/s
Details[4]
BY Dra A
Mass0.792 ± 0.026 M☉
BY Dra B
Mass0.697 ± 0.023 M☉
Other designations
BY Dra, GJ 719, BD+51 2402, HD 234677, LTT 15477, SAO 31048, HIP 91009.
Database references
SIMBADBY Dra AB
BY Dra C
ARICNSdata

The third component (C) is, by comparison, widely separated from the A-B pair by an angular distance of 17 arcseconds, which corresponds to 260 AU at the estimated distance of this star system—where an AU is the average distance from the Earth to the Sun. Component C is an M5 class red dwarf star. There may be a fourth component to the system, orbiting with a ≤1000‑day period, responsible for the eccentricity of the 5.98-day orbit, but this has not been visually confirmed.[4]

The variability of BY Draconis is caused by activity in the stellar photosphere called starspots, which are comparable to sunspots on the Sun, in combination with rapid rotation that changes the viewing angle of the activity relative to the observer. This variation has an average periodicity of 3.8285 days, but the brightness also changes over the course of several years—depending on the level of surface activity. Most observers believe that the primary star (A) is responsible for the variability as the secondary produces only a third of the total luminosity from the system. However, the spots may occur on both stars. Unlike the Sun, these spots may occur in the polar regions of the stars.[6]

References
  1. "V* BY Dra". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2008-04-18.
  2. Epps, E. A. (1972). "UBV photoelectric observations. I. Stars within 25 parasecs of the Sun. II. Stars in quasar, galaxy fields. III. Stars in Kapteyn selected areas. IV. Miscellaneous stars". Royal Observatory Bulletin. 176: 77–115. Bibcode:1972RGOB..176..127E.
  3. Samus', N. N; Kazarovets, E. V; Durlevich, O. V; Kireeva, N. N; Pastukhova, E. N (2017). "General catalogue of variable stars: Version GCVS 5.1". Astronomy Reports. 61 (1): 80. Bibcode:2017ARep...61...80S. doi:10.1134/S1063772917010085. S2CID 125853869.
  4. Hełminiak, K.G. (2012). "New high-precision orbital and physical parameters of the double-lined low-mass spectroscopic binary BY Draconis". Monthly Notices of the Royal Astronomical Society. 419 (2): 1285–1293. arXiv:1109.5059. Bibcode:2012MNRAS.419.1285H. doi:10.1111/j.1365-2966.2011.19785.x. S2CID 19002972.
  5. Boden, A. F.; Lane, B. F. (2001). "A Preliminary Visual Orbit of BY Draconis". The Astrophysical Journal. 547 (2): 1071–1076. arXiv:astro-ph/0001138. Bibcode:2001ApJ...547.1071B. doi:10.1086/318394. S2CID 10704476.
  6. Pettersen, B. R.; Olah, K.; Sandmann, W. H. (1992). "Longterm behaviour of starspots. II - A decade of new starspot photometry of BY Draconis and EV Lacertae". Astronomy and Astrophysics Supplement Series. 96 (3): 497–504. Bibcode:1992A&AS...96..497P.
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