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]

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 mas[1]
Distance	189 ± 2 ly (57.8 ± 0.6 pc)
Absolute magnitude (MV)	+0.62[6]
Details[5]
Mass	1.44±0.16 M☉
Radius	10.44+0.24 −0.50[1] R☉
Luminosity	55.3±0.7[1] L☉
Surface gravity (log g)	2.75±0.19 cgs
Temperature	4,937±14 K
Metallicity [Fe/H]	−0.35±0.03 dex
Rotational velocity (v sin i)	0.54±0.38 km/s
Age	2.20±0.62 Gyr
Other designations
Castula, υ2 Cas, 28 Cassiopeiae, BD+58° 138, HD 5395, HIP 4422, HR 265, SAO 21855
Database references
SIMBAD	data

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