HR 3750

HR 3750 is a binary star system in the equatorial constellation of Hydra at a distance of 101 light years. This object is visible to the naked eye as a dim, white star with an apparent visual magnitude of 5.4. It is receding from the Earth with a heliocentric radial velocity of 57.9±0.2 km/s.[2] This binary is unusual because its eruptions do not seem to conform to the Waldmeier effect—i.e. the strongest eruptions of HR 3750 are not the ones characterized by the fast eruption onset.[5] Kinematically, the binary belongs to the thick disk of the Milky Way galaxy - a population of ancient, metal-poor stars.[3]

HR 3750
Observation data
Epoch J2000      Equinox J2000
Constellation Hydra
Right ascension 09h 27m 46.7799s[1]
Declination –06° 04′ 16.2822″[1]
Apparent magnitude (V) 5.40
Characteristics
Spectral type G1.5
Apparent magnitude (G) 5.1625[2]
Astrometry
Radial velocity (Rv)57.9±0.2[2] km/s
Proper motion (μ) RA: -242.6±0.4 mas/yr[2]
Dec.: -52.7±0.3 mas/yr[2]
Parallax (π)32.30 ± 0.36 mas[2]
Distance101 ± 1 ly
(31.0 ± 0.3 pc)
Absolute magnitude (MV)A:2.76; B:4.28[3]
Details[3]
HD 81809 A
Mass1.39±0.09 M☉
Radius2.58±0.14 R☉
Luminosity (bolometric)5.8±0.3[4] L☉
Surface gravity (log g)3.74±0.10 cgs
Temperature5620±80 K
Metallicity [Fe/H]-0.37±0.08 dex
Rotation40.2 d[4]
Rotational velocity (v sin i)2.0±1.5 km/s
Age12±1 Gyr
HD 81809 B
Mass0.95±0.05 M☉
Radius1.23±0.07 R☉
Luminosity (bolometric)1.025±0.055[4] L☉
Surface gravity (log g)4.24±0.10 cgs
Temperature5730±100 K
Other designations
BD−05 2802, GJ 344, HD 81809, HIP 46404, HR 3750, SAO 136872, LTT 3482, 2MASS J09274680-0604164[1]
Database references
SIMBADdata
ARICNSdata

The star system is a spectroscopic binary with a 32 year, nearly edge-on orbit. The primary, HD 81809 A's visual magnitude is 5.610±0.005 while the secondary, HD 81809 B's visual magnitude is 7.115±0.015 The larger star, HD 81809 A, is unusually inflated for its age and composition, possibly due to engulfment of a 0.36M☉ red dwarf star 1-3 billion years ago.[3]

HD 81809 A[4] has a well defined chromospheric activity cycle with a period of 7.3±1.5 years.[6]

References
  1. "HD 81809". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved June 21, 2021.
  2. Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
  3. Fuhrmann, Klaus; Chini, Rolf (2018), "Fossil Merger of a Population II Star", The Astrophysical Journal, 858 (2): 103, Bibcode:2018ApJ...858..103F, doi:10.3847/1538-4357/aabaff, S2CID 125357563
  4. Egeland, Ricky (2018), "Deconvolving the HD 81809 Binary: Rotational and Activity Evidence for a Subgiant with a Sun-like Cycle", The Astrophysical Journal, 866 (2): 80, arXiv:1807.10870, Bibcode:2018ApJ...866...80E, doi:10.3847/1538-4357/aadf86, S2CID 119537525
  5. Garg, Suyog; Karak, Bidya Binay; Egeland, Ricky; Soon, Willie; Baliunas, Sallie (2019), "Waldmeier Effect in Stellar Cycles", The Astrophysical Journal, 886 (2): 132, arXiv:1909.12148, Bibcode:2019ApJ...886..132G, doi:10.3847/1538-4357/ab4a17, S2CID 202888617
  6. Orlando, S.; Favata, F.; Micela, G.; Sciortino, S.; Maggio, A.; Schmitt, J. H. M. M.; Robrade, J.; Mittag, M. (2017), "Fifteen years in the high-energy life of the solar-type star HD 81809", Astronomy & Astrophysics, 605: A19, arXiv:1707.06437, doi:10.1051/0004-6361/201731301, S2CID 118843788
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