Epsilon Capricorni

"Kastra" redirects here. For the DJ and musical artist, see Kastra (musical artist). For the Late Roman fortified camp, see Castra ad Fluvio Frigido.

Epsilon Capricorni is a binary star system^[11] in the constellation Capricornus. It has the traditional star name Kastra, meaning "fort" or "military camp" in Latin; Epsilon Capricorni is the Bayer designation, which is Latinized from ε Capricorni and abbreviated Epsilon Cap or ε Cap. Based upon an annual parallax shift of 3.09 mas as seen from the Earth,^[2] the star is located about 1,060 light years from the Sun. It can be seen with the naked eye, having a combined apparent visual magnitude of 4.62.^[3]

Epsilon Capricorni

Location of ε Capricorni (circled)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Capricornus[1]
Right ascension	21h 37m 04.83068s[2]
Declination	−19° 27′ 57.6464″[2]
Apparent magnitude (V)	+4.62[3]
Characteristics
Spectral type	B2.5 Vpe[4]
U−B color index	−0.64[3]
B−V color index	−0.19[3]
Variable type	γ Cas[5]
Astrometry
Proper motion (μ)	RA: +12.79 mas/yr[2] Dec.: +0.28 mas/yr[2]
Parallax (π)	3.09±0.18 mas[2]
Distance	1,060 ± 60 ly (320 ± 20 pc)
Absolute magnitude (MV)	−3.03[1]
Details
ε Cap A
Mass	7.6[6] M☉
Radius	4.80[6] R☉
Luminosity	4,649[1] L☉
Surface gravity (log g)	4.0[6] cgs
Temperature	18,800[6] K
Metallicity [Fe/H]	−0.08[1] dex
Rotational velocity (v sin i)	225[7] km/s
Age	27.5±4.2[8] Myr
Other designations
Kastra[9], ε Cap, 39 Cap, BD−20°6251, FK5 3724, HD 205637, HIP 106723, HR 8260, SAO 164520, WDS J21371-1928A[10]
Database references
SIMBAD	data

In Chinese, 壘壁陣 (Lěi Bì Zhèn), meaning Line of Ramparts, refers to an asterism consisting of ε Capricorni, κ Capricorni, γ Capricorni, δ Capricorni, ι Aquarii, σ Aquarii, λ Aquarii, φ Aquarii, 27 Piscium, 29 Piscium, 33 Piscium and 30 Piscium.^[12] Consequently, the Chinese name for ε Capricorni itself is 壘壁陣二 (Lěi Bì Zhèn èr, English: the Second Star of Line of Ramparts.)^[13]

Observations

A light curve for Epsilon Capricorni, adapted from Lefèvre et al. (2009)^[14]

In 2006, this was reported as a single-lined spectroscopic binary with an orbital period of 128.5 days. It was confirmed in 2023.^[11] The primary, component Aa, is a Be star that is surrounded by ionized gas that is producing the emission lines in the spectrum. This circumstellar shell is inclined by 80° to the line of sight from the Earth.^[6] The system is undergoing both short term and long term variations in luminosity, with the short period variations showing a phase cycle of 1.03 days.^[5] It is classified as a Gamma Cassiopeiae variable with an amplitude of 0.16 in magnitude.^[5]

Epsilon Capricorni Aa is a blue-white hued B-type main sequence star with a stellar classification of B2.5 Vpe^[4] and a visual magnitude of +4.62.^[3] It has 7.6 times the mass of the Sun and 4.8 times the Sun's radius.^[6] The star is spinning rapidly, with a projected rotational velocity of 225 km/s. This is giving it an oblate shape with an equatorial bulge that is 7% larger than the polar radius.^[7]

The system has two visual companions. Component B is a visual magnitude 10.11 star at an angular separation of 65.8 arc seconds along a position angle of 46°, as of 2013. Component C with visual magnitude of 14.1 lies at an angular separation of 62.7 arc seconds along a position angle of 164°, as of 1999.^[15] Both stars are likely to be unrelated and at different distances to Epsilon Capricorni.^[16]