Lambda Aquarii

Lambda Aquarii, informally known as Hydor (/ˈhaɪdɔːr/),^[12][13] is a variable star in the equatorial constellation of Aquarius. The name is Latinized from the Bayer designation λ Aquarii, and abbreviated Lambda Aqr or λ Aqr. The apparent visual magnitude of this star ranges from 3.57 down to 3.80,^[3] which is bright enough to be visible with the naked eye. The star is eclipsed by the sun from about 1-4 March;^[14] thus the star can be viewed the whole night, crossing the sky, in early September, in the current epoch. Lambda Aquarii is located at a distance of 365 light-years (112 pc) from the Sun based on parallax,^[1] but is drifting closer with a radial velocity of −10.5 km/s.^[6]

Lambda Aquarii

Location of λ Aquarii (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Aquarius
Right ascension	22h 52m 36.87404s[1]
Declination	−07° 34′ 46.5489″[1]
Apparent magnitude (V)	3.722[2] (3.57 to 3.80)[3]
Characteristics
Evolutionary stage	AGB[4]
Spectral type	M2.5 IIIa Fe–1[5]
U−B color index	+1.721[2]
B−V color index	+1.641[2]
Variable type	Lb[3]
Astrometry
Radial velocity (Rv)	−10.46±0.53[6] km/s
Proper motion (μ)	RA: +14.964 mas/yr[1] Dec.: +32.742 mas/yr[1]
Parallax (π)	8.9360±0.2356 mas[1]
Distance	365 ± 10 ly (112 ± 3 pc)
Absolute magnitude (MV)	−1.5[7]
Details
Mass	2.96±0.5[8] M☉
Radius	100.17+2.67 −2.81[9] R☉
Luminosity	1,716[10] L☉
Temperature	3,702[10] K
Metallicity [Fe/H]	−2.25[7] dex
Other designations
λ Aqr, 73 Aquarii, BD−08°968, FK5 864, HD 216386, HIP 112961, HR 8698, SAO 146362[11]
Database references
SIMBAD	data

This star lies just 0.39 degrees south of the ecliptic^[15] and so is subject to lunar and planetary occultations.^[16][17][18] On 16 April 2014, it was occulted by Venus as viewed from Australia, New Zealand and the West Pacific.^[19]

Properties

A visual band light curve for Lambda Aquarii, plotted from data published by Tabur et al. (2009).^[20] The inset plot shows the same data folded with a period of 24.5 days, after a linear trend had been removed.

The stellar classification of Lambda Aquarii is M2.5 IIIa Fe–1,^[5] indicating this is an aging red giant star with an underabundance of iron showing in its spectrum. This star is on the asymptotic giant branch and is generating energy through the nuclear fusion of hydrogen and helium along concentric shells surrounding an inert core of carbon and oxygen.^[4] With 3.6^[21] times the mass of the Sun, it has expanded to 100^[9] times the Sun's radius.

Lambda Aquarii has a magnetic field with an effective strength measured at 220±60 Gauss.^[22] It is classified as slow irregular variable and pulsation periods of 24.5, 32.0, and 49.5 days have been identified.^[20] On average, it is radiating nearly 1,600 times the luminosity of the Sun from the photosphere at an effective temperature of 3,702 K.^[10]

Naming

Hydor is from Greek Ὕδωρ "water", a name given by Proclus, according to Richard Hinckley Allen. Another Greek name for the star is Ekkhysis, from εκχυσις "outpouring".^[13]

In Chinese, 壘壁陣 (Lěi Bì Zhèn), meaning Line of Ramparts, refers to an asterism consisting of λ Aquarii, κ Capricorni, ε Capricorni, γ Capricorni, δ Capricorni, ι Aquarii, σ Aquarii, φ Aquarii, 27 Piscium, 29 Piscium, 33 Piscium and 30 Piscium.^[23] Consequently, λ Aquarii itself is 壘壁陣七 (Lěi Bì Zhèn qī, English: the Seventh Star of Line of Ramparts.)^[24]