18 Draconis

18 Draconis is a likely binary star[7] system in the northern circumpolar constellation of Draco. With an apparent visual magnitude of 4.84,[2] it is just bright enough to be faintly visible to the naked eye. The distance to this system, as estimated from an annual parallax shift of 4.5 mas,[1] is roughly 720 light years. It is moving closer to the Sun with a heliocentric radial velocity of −1.4 km/s,[4] and is a probable member of the Sirius stream of co-moving stars.[8]

18 Draconis

18 Draconis in optical light
Observation data Epoch J2000      Equinox J2000
Constellation	Draco
Right ascension	16^h 40^m 55.11952^s[1]
Declination	+64° 35′ 20.5824″[1]
Apparent magnitude (V)	4.84[2]
Characteristics
Spectral type	K0 III CN−0.5 CH−2 Ca1[3]
B−V color index	1.212±0.003[2]
Astrometry
Radial velocity (Rv)	−1.37±0.09[4] km/s
Proper motion (μ)	RA: −0.599[1] mas/yr Dec.: −17.436[1] mas/yr
Parallax (π)	4.5045 ± 0.1730[1] mas
Distance	720 ± 30 ly (222 ± 9 pc)
Absolute magnitude (MV)	−1.92[2]
Details[5]
Mass	3.81±0.38 M☉
Radius	46.83±1.74 R☉
Luminosity	786.7±56.3 L☉
Surface gravity (log g)	1.69±0.06 cgs
Temperature	4,471±23 K
Metallicity [Fe/H]	−0.13±0.10 dex
Age	280±80 Myr
Other designations
g Dra, 18 Dra, BD+64° 1145, FK5 3326, HD 151101, HIP 81660, HR 6223, SAO 17188[6]
Database references
SIMBAD	data

The visible component has a stellar classification of K0 III CN−0.5 CH−2 Ca1,[3] indicating it is an evolved K-type giant star with some abundance peculiarities in its atmosphere. At the age of around 280 million years, it is most likely (99% chance) on the horizontal branch.[5] It is a barium star, which suggests it may have a degenerate white dwarf companion from which it accreted materials during an earlier stage of its evolution.[9][7] 18 Dra has an estimated 3.8 times the mass of the Sun and has expanded to 47 times the Sun's radius. The star is radiating 787 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,471 K.[5]

References

1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051.
2. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015.
3. Keenan, Philip C.; McNeil, Raymond C. (1989), "The Perkins catalog of revised MK types for the cooler stars", Astrophysical Journal Supplement Series, 71: 245, Bibcode:1989ApJS...71..245K, doi:10.1086/191373.
4. de Bruijne, J. H. J.; Eilers, A.-C. (October 2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", Astronomy & Astrophysics, 546: 14, arXiv:1208.3048, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, A61.
5. Reffert, Sabine; et al. (2015), "Precise radial velocities of giant stars. VII. Occurrence rate of giant extrasolar planets as a function of mass and metallicity", Astronomy & Astrophysics, 574: A116, arXiv:1412.4634, Bibcode:2015A&A...574A.116R, doi:10.1051/0004-6361/201322360, hdl:10722/215277.
6. "g Dra". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2018-07-17.
7. Eggleton, P. P.; Tokovinin, A. A. (2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x.
8. Famaey, B.; et al. (2005), "Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters", Astronomy and Astrophysics, 430: 165–186, arXiv:astro-ph/0409579, Bibcode:2005A&A...430..165F, doi:10.1051/0004-6361:20041272.
9. Gomez, A. E.; et al. (1997), "Absolute magnitudes and kinematics of barium stars", Astronomy and Astrophysics, 319: 881, Bibcode:1997A&A...319..881G.