HD 81799
{{Short description|Star and suspected binary system in the constellation Hydra}}
{{Starbox begin
| name = HD 81799
}}
{{Starbox observe
| epoch = J2000
| constell = Hydra
| dec = {{DEC|−22|20|37.4967}}
}}
{{Starbox character
| type = red clump
}}
{{Starbox astrometry
| radial_v = {{Val|29.05|0.28}}
| parallax = 19.8887
| p_error = 0.3128
}}
{{Starbox detail
| source =
| mass =
| rotational_velocity =
| age_gyr =
}}
{{Starbox catalog
| names={{odlist | B=G Hya | BD=−21°2802 | HD=81799 | HIP=46371 | HR=3749 | SAO=177469 | LTT=3479 }}
}}
{{Starbox reference
| Simbad = HD+81799
}}
{{Starbox end}}
HD 81799 (G Hydrae) is a suspected astrometric binary star system in the equatorial constellation of Hydra. It is visible to the naked eye with an apparent visual magnitude of 4.69. The distance to this system, as determined from an annual parallax shift of {{Val|19.9|ul=mas}}, is 164 light years. It is moving further away from the Earth with a heliocentric radial velocity of 29 km/s. The system has a relatively high rate of proper motion, traversing the celestial sphere at the rate of {{Val|233|19|u=mas/yr}} along a position angle of 136°.
The stellar classification of the visible component is K2+ IIIb, which matches an evolved K-type giant star. It is a red clump star, which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. The interferometry-measured angular diameter of the primary, after correcting for limb darkening, is {{Val|1.96|0.03|ul=mas}}, which, at its estimated distance, equates to a physical radius of about 10.6 times the radius of the Sun. It is radiating 42 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,490 K.
The system is a likely (99.4% chance) source of the X-ray emission coming from these coordinates.
References
{{reflist|refs=
{{cite simbad | title=HD 81799 | access-date=2018-07-24 }}
| first1=P. | last1=Keenan | first2=R. | last2=McNeil
| title = The Perkins catalog of revised MK types for the cooler stars
| journal=Astrophysical Journal Supplement Series
| volume=71 | pages=245–266 | date=October 1989
| bibcode=1989ApJS...71..245K | doi=10.1086/191373 }}
| title=A catalogue of multiplicity among bright stellar systems
| last1=Eggleton | first1=P. P. | last2=Tokovinin | first2=A. A.
| journal=Monthly Notices of the Royal Astronomical Society
| volume=389 | issue=2 | pages=869 | date=2008
| doi=10.1111/j.1365-2966.2008.13596.x | doi-access=free | postscript=.
| bibcode=2008MNRAS.389..869E | arxiv=0806.2878 | s2cid=14878976 }}
| last1=Soubiran | first1=C. | last2=Le Campion | first2=J.-F.
| last3=Cayrel de Strobel | first3=G. | last4=Caillo | first4=A.
| title=The PASTEL catalogue of stellar parameters
| journal=Astronomy and Astrophysics
| volume=515 | page=A111 | date=June 2010
| doi=10.1051/0004-6361/201014247 | bibcode=2010A&A...515A.111S
| postscript=. | arxiv=1004.1069 | s2cid=118362423 }}
| last1=Haakonsen | first1=Christian Bernt | last2=Rutledge | first2=Robert E.
| title=XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
| journal=The Astrophysical Journal Supplement
|volume=184 | issue=1 | pages=138–151 | date=September 2009
| doi=10.1088/0067-0049/184/1/138 | bibcode=2009ApJS..184..138H
| arxiv=0910.3229 | s2cid=119267456 | postscript=. }}
| display-authors=1 | last1=Richichi | first1=A.
| last2=Percheron | first2=I. | last3=Khristoforova | first3=M.
| title=CHARM2: An updated Catalog of High Angular Resolution Measurements
| journal=Astronomy and Astrophysics
| volume=431 | pages=773–777 | date=February 2005
| issue=2 | doi=10.1051/0004-6361:20042039 | bibcode=2005A&A...431..773R | doi-access=free }}
| first1=Kenneth R. | last1=Lang
| title=Astrophysical formulae | volume=1
| series=Astronomy and astrophysics library
| edition=3rd | publisher=Birkhäuser
| date=2006 | isbn=3-540-29692-1
| url=https://books.google.com/books?id=OvTjLcQ4MCQC&pg=PA41
}}. The radius (R*) is given by:
:
& = \frac{(10^{-3}\cdot 50.3\cdot 1.96)\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\
& \approx 21.2\cdot R_{\bigodot}
\end{align}
| last1=Alves | first1=David R.
| title=K-Band Calibration of the Red Clump Luminosity
| journal=The Astrophysical Journal
| volume=539 | issue=2 | pages=732–741 | date=August 2000
| doi=10.1086/309278 | bibcode=2000ApJ...539..732A
| arxiv=astro-ph/0003329 | s2cid=16673121
| postscript=. }}
| title=New proper motion determination of Luyten catalogue stars (LTT) with declination between -5° and -30° and right ascension between 0h and 13h 30m
| last1=Wroblewski | first1=H. | last2=Torres | first2=C.
| journal=Astronomy and Astrophysics Supplement
| volume=128 | pages=457–458 | date=March 1998
| doi=10.1051/aas:1998157 | bibcode=1998A&AS..128..457W | doi-access=free }}
}}
{{Stars of Hydra}}
{{DEFAULTSORT:HD 81799}}
Category:Horizontal-branch stars
Category:Hydra (constellation)