AB Andromedae

AB Andromedae (AB And) is a binary star in the constellation Andromeda. Its maximum apparent visual magnitude is 9.49 but shows a variation in brightness down to a magnitude of 10.46 in a periodic cycle of roughly 8 hours. The observed variability is typical of W Ursae Majoris variable stars,[2] so the two stars in this system form a contact binary.

AB Andromedae
Observation data
Epoch J2000      Equinox J2000
Constellation Andromeda
Right ascension  23h 11m 32.08609s[1]
Declination +36° 53′ 35.10721″[1]
Apparent magnitude (V) 9.49 ( – 10.32) – 10.46[2]
Characteristics
Spectral type G5+G5V[2]
Apparent magnitude (B) 10.62[3]
Apparent magnitude (V) 9.675[3]
Apparent magnitude (G) 9.6953[1]
Apparent magnitude (J) 8.172[4]
Apparent magnitude (H) 7.805[4]
Apparent magnitude (K) 7.665[4]
B−V color index 0.9163[3]
Variable type EW
Astrometry
Radial velocity (Rv)−27.53±0.67[5] km/s
Proper motion (μ) RA: 107.923±0.046 [1] mas/yr
Dec.: −53.357±0.036[1] mas/yr
Parallax (π)11.7027 ± 0.0367[1] mas
Distance278.7 ± 0.9 ly
(85.5 ± 0.3 pc)
Orbit[6]
Period (P)0.3319 days
Semi-major axis (a)2.308 R☉[7]
Eccentricity (e)0.002±0.001
Argument of periastron (ω)
(secondary)		40±5°
Argument of periastron (ω)
(primary)		220±5°
Semi-amplitude (K1)
(primary)		233±1 km/s
Semi-amplitude (K2)
(secondary)		133±1 km/s
Details[8]
Primary
Mass1.04 M☉
Radius1.03 R☉
Surface gravity (log g)4.392[9] cgs
Temperature5,798 K
Age5.53±2.00[7] Gyr
Secondary
Mass0.60 M☉
Radius0.78 R☉
Surface gravity (log g)4.347[9] cgs
Temperature5,450 K
Age5.53±2.00[7] Gyr
Other designations
2MASS J23113209+3653351, BD+36 5017, HIP 114508, SAO 73069, TYC 2763-904-1
Database references
SIMBADdata

System

The observed spectral type of both stars in this system is G5, and one of them is a main sequence star very similar to the sun.[2] They are orbiting so close that their envelopes touch each other. This is a dynamically stable phase that should last until one of the two stars leaves the main sequence.

The system could also host a third body with an orbital period of 19,046 days, with a minimum mass of 0.007 M☉ and an eccentricity of 0.22, but not all data collected in time are consistent with this hypothesis.[6]

Variability

The two stars eclipse each other during their orbit, but they have an elongated shape so they show a constant variation instead of discrete eclipses. Anyway, a periodicity can be seen clearly, but it changes with time; the period shows a long-term trend and a periodic modulation of 7,000 days. The effects responsible for this behaviour could be a third body in the system, magnetic interaction between two stars,[8] mass transfer from one star to the another, mass loss of the system, and recently even an internal mechanism in the touching envelopes have been proposed.[9]

gollark: Seriously? That seems vaguely implausible. The Pi's SoC may be newer than... Core 2 Duos or something, but the A72 cores in it are *not* particularly new and as far as I know quite bad.
gollark: You should try not consuming alcohol.
gollark: The obvious solution is to consume an equal quantity of anti-ethanol molecules.
gollark: https://citrons.xyz/a/memetic-apioform-page.html
gollark: No, that was a while ago.

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. AB And, database entry, Combined General Catalog of Variable Stars (GCVS4.2, 2004 Ed.), N. N. Samus, O. V. Durlevich, et al., CDS ID II/250 Accessed on line 2018-10-22.
  3. Høg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000), "The Tycho-2 catalogue of the 2.5 million brightest stars", Astronomy & Astrophysics, 355: L27–L30, Bibcode:2000A&A...355L..27H.
  4. Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; et al. (June 2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". CDS/ADC Collection of Electronic Catalogues (2246): II/246. Bibcode:2003yCat.2246....0C.
  5. Bilir, S.; Karataș, Y.; Demircan, O.; Eker, Z. (February 2005), "Kinematics of W Ursae Majoris type binaries and evidence of the two types of formation", Monthly Notices of the Royal Astronomical Society, 357 (2): 497–517, Bibcode:2005MNRAS.357..497B, doi:10.1111/j.1365-2966.2005.08609.x.
  6. Karami, K.; Ghaderi, K.; Mohebi, R.; Sadeghi, R.; Soltanzadeh, M. M. (June 2009), "Velocity-Curve Analysis of the Spectroscopic Binary Stars V373 Cas, V2388 Oph, V401 Cyg, GM Dra, V523 Cas, AB And and HD 141929 by Artificial Neural Networks", Publications of the Astronomical Society of Australia, 26 (2): 121–127, arXiv:0907.4411, Bibcode:2009PASA...26..121K, doi:10.1071/AS09010.
  7. Yıldız, M. (2014), "Origin of W UMa-type contact binaries - age and orbital evolution", Monthly Notices of the Royal Astronomical Society, 437 (1): 185–194, arXiv:1310.5526, Bibcode:2014MNRAS.437..185Y, doi:10.1093/mnras/stt1874.
  8. Borkovits, T.; Elkhateeb, M. M.; Csizmadia, Cz.; Nuspl, J.; Bíró, I. B.; Hegedüs, T.; Csorvási, R. (2005), "Indirect evidence for short period magnetic cycles in W UMa stars. Period analysis of five overcontact systems.", Astronomy & Astrophysics, 441 (3): 1087–1097, Bibcode:2005A&A...441.1087B, doi:10.1051/0004-6361:20052805.
  9. Liu, L.; Qian, S. B.; Xiong, X. (2018), "A new mechanism of long-term period variations for W UMa-type contact binaries", Monthly Notices of the Royal Astronomical Society, 474 (4): 5199–5205, arXiv:1712.04358, Bibcode:2018MNRAS.474.5199L, doi:10.1093/mnras/stx3138.
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