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[2] mas/yr
Dec.: -52.7±0.3[2] mas/yr
Parallax (π)32.30 ± 0.36[2] mas
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. 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
  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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