Upsilon2 Cassiopeiae

Upsilon2 Cassiopeiae, Latinised from υ2 Cassiopeiae, is a solitary star in the constellation of Cassiopeia, a few degrees to the south of Gamma Cassiopeiae. It has the proper name Castula /ˈkæstjʊlə/, which has been officially adopted by the IAU.[7] The star has a yellow hue and is faintly visible to the naked eye with an apparent visual magnitude of +4.62.[2] Based upon parallax measurements, it is located at a distance of approximately 189 light years from the Sun.[1] The star is drifting closer with a radial velocity of −48 km/s.[5]

For other star systems with this Bayer designation, see υ Cassiopeiae
υ2 Cassiopeiae

Map of the Bayer-designated stars in Cassiopeia. Upsilon2 Cassiopeiae is circled.
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cassiopeia
Right ascension 00h 56m 39.90413s[1]
Declination +59° 10′ 51.8006″[1]
Apparent magnitude (V) +4.62[2]
Characteristics
Evolutionary stage horizontal branch[3]
Spectral type G8 IIIb Fe−0.5[4]
U−B color index +0.68[2]
B−V color index +0.96[2]
Astrometry
Radial velocity (Rv)−47.73±0.12[5] km/s
Proper motion (μ) RA: −92.085[1] mas/yr
Dec.: −45.079[1] mas/yr
Parallax (π)17.2875 ± 0.1880[1] mas
Distance189 ± 2 ly
(57.8 ± 0.6 pc)
Absolute magnitude (MV)+0.62[6]
Details[5]
Mass1.44±0.16 M☉
Radius10.44+0.24
−0.50[1] R☉
Luminosity55.3±0.7[1] L☉
Surface gravity (log g)2.75±0.19 cgs
Temperature4,937±14 K
Metallicity [Fe/H]−0.35±0.03 dex
Rotational velocity (v sin i)0.54±0.38 km/s
Age2.20±0.62 Gyr
Other designations
Castula, υ2 Cas, 28 Cassiopeiae, BD+58° 138, HD 5395, HIP 4422, HR 265, SAO 21855
Database references
SIMBADdata

This is an aging giant star with a stellar classification of G8 IIIb Fe−0.5,[4] where the suffix notation indicates a mild underabundance of iron in the spectrum. With the supply of core hydrogen exhausted, this star has cooled and expanded off the main sequence – at present it has 10[1] times the girth of the Sun. It is a red clump giant, which means it is on the horizontal branch undergoing core helium fusion.[3]

Upsilon2 Cassiopeiae is a barium star, showing an excess of the element barium in its spectrum. This can occur from mass transfer from a more-evolved companion star that later became a white dwarf, although no companion has been detected.[6] It is 2.2 billion years old with 1.44 times the mass of the Sun.[5] The star is radiating 55[1] times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,937 K.[5]

Nomenclature

υ2 Cassiopeiae is the star's Bayer designation. In 2016, the IAU organized a Working Group on Star Names (WGSN)[8] to catalog and standardize proper names for stars. The WGSN approved the name Castula for this star on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.[7]

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. Gaia DR2 record for this source at VizieR.
  2. Argue, A. N. (1966), "UBV photometry of 550 F, G and K type stars", Monthly Notices of the Royal Astronomical Society, 133 (4): 475, Bibcode:1966MNRAS.133..475A, doi:10.1093/mnras/133.4.475.
  3. Puzeras, E.; et al. (October 2010), "High-resolution spectroscopic study of red clump stars in the Galaxy: iron-group elements", Monthly Notices of the Royal Astronomical Society, 408 (2): 1225–1232, arXiv:1006.3857, Bibcode:2010MNRAS.408.1225P, doi:10.1111/j.1365-2966.2010.17195.x, S2CID 44228180.
  4. 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.
  5. Jofré, E.; et al. (2015), "Stellar parameters and chemical abundances of 223 evolved stars with and without planets", Astronomy & Astrophysics, 574: A50, arXiv:1410.6422, Bibcode:2015A&A...574A..50J, doi:10.1051/0004-6361/201424474, S2CID 53666931.
  6. Bergeat, J.; Knapik, A. (May 1997), "The barium stars in the Hertzsprung-Russel diagram.", Astronomy and Astrophysics, 321: L9, Bibcode:1997A&A...321L...9B.
  7. Naming Stars, IAU.org, retrieved 16 December 2017
  8. IAU Working Group on Star Names (WGSN), IAU.org, retrieved 22 May 2016
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