Sigma Aquarii

Sigma Aquarii is a double star in the equatorial constellation of Aquarius, positioned about 1.3° to the south of the ecliptic.^[10] Its name is a Bayer designation that is Latinized from σ Aquarii, and abbreviated Sigma Aqr or σ Aqr. Due to its proximity to the ecliptic, this star is subject to occultation by the Moon.^[11] It has a white hue and is visible to the naked eye with an apparent visual magnitude of 4.81.^[2] Based upon parallax measurements, the distance to this star is approximately 206 light-years (63 pc).^[1] It is drifting further away with a radial velocity of +11 km/s.^[12]

Sigma Aquarii

Location of σ Aquarii (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Aquarius
Right ascension	22h 30m 38.821s[1]
Declination	−10° 40′ 40.51″[1]
Apparent magnitude (V)	4.81[2]
Characteristics
Spectral type	A0 IVs[3]
U−B color index	−0.14[2]
B−V color index	−0.08[2]
Astrometry
Radial velocity (Rv)	+11.7±0.6[4] km/s
Proper motion (μ)	RA: −4.095 mas/yr[1] Dec.: −38.019 mas/yr[1]
Parallax (π)	15.8247±0.4146 mas[1]
Distance	206 ± 5 ly (63 ± 2 pc)
Absolute magnitude (MV)	0.29±0.23[5]
Details
Mass	2.87[6] M☉
Radius	2.87[5] R☉
Luminosity	105[6] L☉
Surface gravity (log g)	4.07[7] cgs
Temperature	10,115[6] K
Metallicity [Fe/H]	+0.44[7] dex
Rotational velocity (v sin i)	21[8] km/s
Age	30[5] Myr
Other designations
σ Aqr, 57 Aquarii, BD–11°5850, FK5 1591, GC 31440, HD 213320, HIP 111123, HR 8573, SAO 165134, PPM 240380[9]
Database references
SIMBAD	data

The stellar classification of Sigma Aquarii is A0 IVs,^[3] suggesting that it is a subgiant star. The s qualifier means that its absorption lines are sharp (narrow) in comparison with standard stars, caused by a relatively slow rotation. This star is an estimated 30^[5] million years old and is spinning with a projected rotational velocity of 21 km/s.^[8] It has 2.87^[6] times the mass and 2.87 times the radius of the Sun.^[5] Sigma Aquarii is radiating 105^[6] times the luminosity of the Sun from its photosphere at an effective temperature of 10,115 K.^[6]

Sigma Aquarii has been categorized as a hot Am star,^[13] meaning that it is a chemically peculiar, although this is now considered doubtful.^[14] The spectrum displays at least double the normal abundances of elements like magnesium, aluminum and silicon, while helium and scandium are under-abundant. Calcium, normally deficient in Am stars, has near-normal abundance.^[13]

The Hipparcos catalogue identified Sigma Aquarii as a possible astrometric binary with an orbital period of 654 days.^[15]