HR 3750 is abinary star system in theequatorialconstellation ofHydra at a distance of 101 light years. This object is visible to the naked eye as a dim, white star with anapparent visual magnitude of 5.4. It is receding from the Earth with a heliocentricradial velocity of 57.9±0.2 km/s.[4] This binary is unusual because its eruptions[clarification needed] do not seem to conform to theWaldmeier effect—i.e. the strongest eruptions of HR 3750 are not the ones characterized by the fast eruption onset.[7] Kinematically, the binary belongs to thethick disk of theMilky Way galaxy—a population of ancient, metal-poor stars.[5]
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 the engulfment of a 0.36 M☉red dwarf star 1–3 billion years ago.[5]
HD 81809 A[6] has a well defined chromospheric activity cycle with a period of 7.3±1.5 years.[8]
^Keenan, Philip C.; McNeil, Raymond C. (October 1989). "The Perkins Catalog of Revised MK Types for the Cooler Stars".Astrophysical Journal Supplement.71: 245.Bibcode:1989ApJS...71..245K.doi:10.1086/191373.
^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,S2CID118843788
