OU Andromedae

OU Andromedae (also HR 9024) is a rotationally variable star in the constellation Andromeda. Varying between magnitudes 5.87 and 5.94, it has been classified as an FK Comae Berenices variable, but the classification is still uncertain. It has a spectral classification of G1IIIe, meaning that it is a giant star that shows emission lines in its spectrum.[2] It is also likely in its horizontal branch phase of evolution.[6]

OU Andromedae
Observation data
Epoch J2000      Equinox J2000
Constellation Andromeda
Right ascension  23h 49m 40.9598s[1]
Declination +36° 25′ 31.0082″[1]
Apparent magnitude (V) 5.87 - 5.94[2]
Characteristics
Spectral type G1 IIIe[3]
Apparent magnitude (G) 5.6187[1]
Apparent magnitude (J) 4.723[4]
Apparent magnitude (H) 4.208[4]
Apparent magnitude (K) 4.097[4]
B−V color index 0.79[3]
Variable type FK Com[2]
Astrometry
Radial velocity (Rv)−1.89±0.25[1] km/s
Proper motion (μ) RA: −0.858±0.094[1] mas/yr
Dec.: −47.402±0.078[1] mas/yr
Parallax (π)7.1692 ± 0.0677[1] mas
Distance455 ± 4 ly
(139 ± 1 pc)
Details[5]
Mass2.85 M☉
Radius9.46 R☉
Luminosity71.2 L☉
Surface gravity (log g)2.8 cgs
Temperature5,360 K
Metallicity [Fe/H]-0.07 dex
Rotation24.3 days
Rotational velocity (v sin i)21.5 km/s
Age645 – 891[6] Myr
Other designations
2MASS J23494097+3625309, FK5 3914, HIP 117503, SAO 73535, BD+35°5110, HR 9024, HD 223460.
Database references
SIMBADdata

Fast rotation

The spin rate of OU Andromedae is unusually high for an evolved star of this type, showing a projected rotational velocity of 21.5 km/s. One possible explanation is that it may have engulfed a nearby giant planet, such as a hot Jupiter, since an infrared excess has been observed.[3] Another explanation relies on its strong magnetic field; if OU Andromedae was an Ap star during its main sequence stage of evolution, it could have retained both the strong magnetic field and the fast rotation of Ap stars.[5]

X-ray source

OU Andromedae is a bright X-ray source, due to the activity of its corona; it's estimated that solar-like active regions cover 30% of the surface. This is another effect of the strong magnetic field, which produces an uninterrupted flaring activity that generates a large volume of hot plasma at coronal temperatures (~7.5×106 K).[7]

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. N. N. Samus; O. V. Durlevich; et al. "OU And database entry". Combined General Catalog of Variable Stars (2017 ed.). CDS. Retrieved 2018-11-12.
  3. Rodrigues da Silva, R.; et al. (March 2015). "On the Nature of Rapidly Rotating Single Evolved Stars". The Astrophysical Journal. 801 (1): 6. arXiv:1503.03447. Bibcode:2015ApJ...801...54R. doi:10.1088/0004-637X/801/1/54. 54.
  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. Borisova, A.; Aurière, M.; Petit, P.; Konstantinova-Antova, R.; Charbonnel, C.; Drake, N. A. (June 2016). "The different origins of magnetic fields and activity in the Hertzsprung gap stars, OU Andromedae and 31 Comae". Astronomy & Astrophysics. 591: 15. arXiv:1604.07647. Bibcode:2016A&A...591A..57B. doi:10.1051/0004-6361/201526726. A57.
  6. Stock, S.; Reffert, S.; Quirrenbach, A. (August 2018). "Precise radial velocities of giant stars. X. Bayesian stellar parameters and evolutionary stages for 372 giant stars from the Lick planet search". Astronomy & Astrophysics. 616: 15. arXiv:1805.04094. Bibcode:2018A&A...616A..33S. doi:10.1051/0004-6361/201833111. A33.
  7. Gondoin, P. (October 2003), "The corona of HD 223460 (HR 9024)", Astronomy and Astrophysics, 409: 263–274, Bibcode:2003A&A...409..263G, doi:10.1051/0004-6361:20030978
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