{{Short description|Binary star system in the constellation Hydra}} {{Sky|09|27|46.7799|-|06|04|16.2822}} {{Starbox begin}} {{Starbox observe | epoch = J2000 | constell = [[Hydra (constellation)|Hydra]]<ref name=constellation/> | ra = {{RA|09|27|46.7799}}<ref name=SIMBAD/>{{failed verification|date=September 2025}} | dec = {{DEC|-06|04|16.282}}<ref name=SIMBAD/>{{failed verification|date=September 2025}} | appmag_v = 5.40{{cn|date=September 2025}} }} {{Starbox character | type = [[subgiant]]<ref name=Fuhrmann2018/> | class = G1.5IV-V<ref name=Keenan1989/> | appmag_1_passband = G | appmag_1 = 5.1625<ref name=DR3/> | b-v = | u-b = | variable = }} {{Starbox astrometry | radial_v = {{Val|+54.93|0.21}}<ref name="Di Mauro 2026"/> | prop_mo_ra = −242.6{{±|0.4}} | prop_mo_dec = −52.7{{±|0.3}} |pm_footnote=<ref name=DR3/> | parallax = 32.30 | p_error = 0.36 | parallax_footnote = <ref name=DR3/> | absmag_v = A:2.76; B:4.28<ref name=Fuhrmann2018/> }} {{Starbox orbit | reference = <ref name="Di Mauro 2026"/> | period_unitless = {{val|34.53|0.16}} years | eccentricity= {{val|0.366|0.022}} | k1 = {{val|5.42|0.17}} | k2 = {{Val|6.30|0.25}} | axis = {{val|0.4089|0.0079}} | inclination = {{val|85.4|0.0}} | periastron = {{val|2455083|211|fmt=commas|u=[[Julian day|JD]]}} | periarg_primary = {{val|347.5|6.7}} | node = {{val|150.42|0.39}} }} {{Starbox detail | source = <ref name="Di Mauro 2026"/> | component1=HD 81809 A | component2=HD 81809 B | mass= {{val|0.87|0.08}} | mass2= {{Val|0.83|0.01}} | radius = {{Val|1.95|0.08}} | radius2 = {{Val|1.12|0.03}} | temperature = {{Val|5619|125|fmt=commas}} | temperature2 = {{Val|5833|72|fmt=commas}} | luminosity = {{val|3.44|0.43}} | luminosity2 = {{Val|1.31|0.10}} | gravity = {{Val|3.7981|0.0054}} | gravity2 = {{Val|4.2776|0.0027}} | metal_fe = {{Val|-0.49|0.14}} | metal2_fe = {{Val|-0.23|0.06}} | age_gyr = {{Val|9.75|1.78}} | age_gyr2 = {{Val|11.1|1.5}} | rotation = 40.2<ref name=Egeland2018/> days | rotational_velocity = 2.0{{±|1.5}} | luminosity_bolometric = 5.8{{±|0.3}}<ref name=Egeland2018>{{citation |arxiv=1807.10870 |doi=10.3847/1538-4357/aadf86 |title=Deconvolving the HD 81809 Binary: Rotational and Activity Evidence for a Subgiant with a Sun-like Cycle |year=2018 |last1=Egeland |first1=Ricky |journal=The Astrophysical Journal |volume=866 |issue=2 |page=80 |bibcode=2018ApJ...866...80E |s2cid=119537525 |doi-access=free }}</ref> }} {{Starbox catalog | names = {{odlist | GJ=344 | BD=-05 2802 | HD=81809 | HIP=46404 | HR=3750 | SAO=136872| LTT=3482|2MASS=J09274680-0604164}}<ref name=SIMBAD/> }} {{Starbox reference | Simbad=HR+3750 | ARICNS=00888 }} {{Starbox end}}
'''HR 3750''' is a [[binary star]] system in the [[celestial equator|equatorial]] [[constellation]] of [[Hydra (constellation)|Hydra]] at a distance of 101 [[light year]]s. 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.<ref name=DR3/> This binary is unusual because its eruptions{{clarify|what eruptions are these?|date=September 2025}} 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.<ref>{{citation |arxiv=1909.12148 |doi=10.3847/1538-4357/ab4a17 |title=Waldmeier Effect in Stellar Cycles |year=2019 |last1=Garg |first1=Suyog |last2=Karak |first2=Bidya Binay |last3=Egeland |first3=Ricky |last4=Soon |first4=Willie |last5=Baliunas |first5=Sallie |journal=The Astrophysical Journal |volume=886 |issue=2 |page=132 |bibcode=2019ApJ...886..132G |s2cid=202888617 |doi-access=free }}</ref> Kinematically, the binary belongs to the [[thick disk]] of the [[Milky Way]] galaxy—a population of ancient, metal-poor stars.<ref name=Fuhrmann2018>{{citation |doi=10.3847/1538-4357/aabaff |title=Fossil Merger of a Population II Star |year=2018 |last1=Fuhrmann |first1=Klaus |last2=Chini |first2=Rolf |journal=The Astrophysical Journal |volume=858 |issue=2 |page=103 |bibcode=2018ApJ...858..103F |s2cid=125357563 |doi-access=free }}</ref>
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, has been hypothetised to have engulfed a {{Solar_mass|0.36|link=y}} [[red dwarf]] star 1–3 billion years ago to explain its unusually older age than expected from theorethical models,<ref name=Fuhrmann2018/> but the age discrepancy has been resolved {{As of|2026|lc=y}} with newer data, discarding the need of a stellar merger. The secondary component, however, appears to be more metal-enriched than the primary, which is unexpected for stars that presumably [[star formation|formed]] in the same envoirment. This indicates that component B had its metallicity changed after a collision with (metal-enriched) planetesimals.<ref name="Di Mauro 2026"/>
HD 81809 A<ref name=Egeland2018/> has a well defined chromospheric activity cycle with a period of 7.3{{±|1.5}} years.<ref>{{citation |arxiv=1707.06437 |doi=10.1051/0004-6361/201731301 |title=Fifteen years in the high-energy life of the solar-type star HD 81809 |year=2017 |last1=Orlando |first1=S. |last2=Favata |first2=F. |last3=Micela |first3=G. |last4=Sciortino |first4=S. |last5=Maggio |first5=A. |last6=Schmitt |first6=J. H. M. M. |last7=Robrade |first7=J. |last8=Mittag |first8=M. |journal=Astronomy & Astrophysics |volume=605 |pages=A19 |s2cid=118843788 }}</ref>
==References== {{Reflist|refs=
<ref name=DR3>{{cite Gaia DR3|5745464991235898752}}</ref>
<ref name=SIMBAD>{{cite simbad | title=HD 81809| access-date=June 21, 2021}}</ref>
<ref name=constellation>{{cite constellation|HR 3750}}</ref>
<ref name=Keenan1989>{{cite journal|title=The Perkins Catalog of Revised MK Types for the Cooler Stars|last1=Keenan|first1=Philip C.|last2=McNeil|first2=Raymond C.|volume=71|page=245|date=October 1989|journal=Astrophysical Journal Supplement|doi=10.1086/191373|bibcode=1989ApJS...71..245K}}</ref>
<ref name="Di Mauro 2026">{{Cite journal |last=Di Mauro |first=Maria Pia |last2=Pezzotti |first2=Camilla |last3=Moedas |first3=Nuno |last4=Catanzaro |first4=Giovanni |last5=Maxted |first5=Pierre F. L. |last6=Corsaro |first6=Enrico |last7=Reda |first7=Raffaele |last8=Scuflaire |first8=Richard |last9=Bonanno |first9=Alfio Maurizio |last10=Giovannelli |first10=Luca |last11=Beck |first11=Paul G. |date=2026-03-16 |title=On the Contradictory Case of the Binary System HD 81809 Hosting Two Pulsating Solar-like Stars Observed by TESS |journal=The Astrophysical Journal |volume=1000 |issue=1 |pages=92 |doi=10.3847/1538-4357/ae40ad |doi-access=free|issn=0004-637X|hdl=2108/456765 |hdl-access=free }}</ref> }}
{{Stars of Hydra}}
[[Category:G-type main-sequence stars]] [[Category:G-type subgiants]] [[Category:Binary stars]] [[Category:Hydra (constellation)]] [[Category:Durchmusterung objects|BD-05 2802]] [[Category:Gliese and GJ objects|0344]] [[Category:Henry Draper Catalogue objects|81809]] [[Category:Hipparcos objects|046404]] [[Category:Bright Star Catalogue objects|3750]]