Epsilon Scorpii

Epsilon Scorpii is a binary star^[14] in the southern zodiac constellation of Scorpius. The primary component has the proper name Larawag /ˈlærəwæɡ/.^[15] It has an apparent visual magnitude of around +2.3,^[6] making it the fifth-brightest member of the constellation. Parallax measurements made during the Hipparcos mission provide a measured distance of 63.7 light-years (19.5 parsecs).^[1]

ε Scorpii

Location of ε Sco (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Scorpius
ε Sco A
Right ascension	16h 50m 09.8s[1]
Declination	−34° 17′ 36″[1]
Apparent magnitude (V)	2.24 - 2.35[2]
ε Sco B
Right ascension	16h 50m 10.7s[3]
Declination	−34° 17′ 44″[3]
Characteristics
ε Sco A
Evolutionary stage	Horizontal branch[4]
Spectral type	K1 III[5]
U−B color index	+1.147[6]
B−V color index	+1.150[6]
Variable type	suspected[2]
Astrometry
ε Sco A
Radial velocity (Rv)	−2.5[7] km/s
Proper motion (μ)	RA: −614.85 mas/yr[1] Dec.: −255.98 mas/yr[1]
Parallax (π)	51.19±0.22 mas[1]
Distance	63.7 ± 0.3 ly (19.54 ± 0.08 pc)
Absolute magnitude (MV)	0.78±0.04[8]
ε Sco B
Proper motion (μ)	RA: −611.794 mas/yr[3] Dec.: −261.515 mas/yr[3]
Parallax (π)	50.4231±0.0326 mas[3]
Distance	64.68 ± 0.04 ly (19.83 ± 0.01 pc)
Details
ε Sco A
Mass	1.31+0.13 −0.24[9] M☉
Radius	12.65[9][a] R☉
Luminosity	60[9] L☉
Surface gravity (log g)	2.34[9] cgs
Temperature	4,522±28[9] K
Metallicity [Fe/H]	−0.17[10] dex
Rotational velocity (v sin i)	2.6±0.5[11] km/s
Age	3.92+0.55 −0.54[9] Gyr
ε Sco B
Mass	0.262±0.021[12] M☉
Radius	0.288±0.019[12] R☉
Luminosity	(9.30±0.25)×10−3[12] L☉
Temperature	3,359±112[12] K
Other designations
Wei, Larawag, 26 Scorpii, CD−34 11285, FK5 628, GCTP 3823.00, Gl 639.1, GJ 9579, HD 151680, HIP 82396, HR 6241, LHS 3244, SAO 208078[13]
Database references
SIMBAD	A
	B

Characteristics

Larawag has a stellar classification of K1 III,^[5] which indicates it has exhausted the supply of hydrogen at its core and evolved into a giant star. Its mass is estimated at 1.3 times the mass of the Sun, and its radius is 12.65 times that of the Sun.^[9] Presently it is generating energy through the nuclear fusion of helium at its core, which, considering the star's composition, places it along an evolutionary branch termed the horizontal branch. In the color-magnitude diagram, the star is located in the red clump.^[4] The star's outer atmosphere has an effective temperature of 4,522 K,^[9] giving it the orange hue of a cool K-type star.^[16]

It is classified as a suspected variable star with a magnitude range of 2.23 to 2.35,^[2] although a study of Hipparcos photometry showed a variation of no more than 0.01–0.02 magnitudes.^[4] It is an X-ray source with a luminosity of (1.5–1.6)×10²⁷ erg/s.^[8][17]

In 2018 it was found that Epsilon Scorpii A has a proper motion companion separated by 14.4", making it a binary star sytstem.^[14] The companion, B, is much smaller and fainter than the primary, with 0.26% of the mass, 0.29% the radius, and less than 1% of the Sun's luminosity.^[12]

Nomenclature

ε Scorpii, Latinised to Epsilon Scorpii, is the star's Bayer designation.

The star bore the traditional name Larawag in the culture of the Wardaman people of the Northern territory of Australia,^[18] meaning clear sighting.^[19] In 2016, the IAU organized a Working Group on Star Names (WGSN)^[20] to catalog and standardize proper names for stars. The WGSN approved the name Larawag for Epsilon Scorpii A on 19 November 2017 and it is now so included in the List of IAU-approved Star Names.^[15]

Patrick Moore introduced the name Wei as Chinese name for this star.^[21] However, this seems to be a misreading, as Chinese 尾宿 (Wěi Xiù, English Tail) refers to an asterism (i.e. Chinese constellation) consisting of Epsilon Scorpii, Mu¹ Scorpii, Zeta Scorpii, Eta Scorpii, Theta Scorpii, Iota Scorpii, Kappa Scorpii, Lambda Scorpii and Upsilon Scorpii.^[22] Consequently, the name for Epsilon Scorpii itself is 尾宿二 (Wěi Xiù èr), which means "the Second Star of Tail".^[23][24]

In culture

Epsilon Scorpii appears on the flag of Brazil, symbolising the state of Ceará.^[25]

Notes

1. Calculated, using the Stefan-Boltzmann law and the star's effective temperature and luminosity, with respect to the solar nominal effective temperature of 5,772 K:$${\displaystyle {\sqrt {{\biggl (}{\frac {5,772}{4,522}}{\biggr )}^{4}\cdot 10^{1.78}}}=12.65\ R_{\odot }.}$$