W Aquilae

W Aquilae (W Aql) is a variable star in the constellation of Aquila. It is a type of evolved star known as an S-type star. Due to its relatively close distance of 1,200 light-years (370 pc) and equatorial location, it is easy to observe and heavily studied.^[9]

W Aquilae

W Aquilae showing the close companion Credit: Hubble Legacy Archive
Observation data Epoch J2000      Equinox J2000
Constellation	Aquila
Right ascension	19h 15m 23.357s[1]
Declination	−07° 02′ 50.33″[1]
Apparent magnitude (V)	A: 7.0 - 14.6[2] B:14.8 [3]
Characteristics
A
Evolutionary stage	asymptotic giant branch[4]
Spectral type	S6/6e[4] (S3,9e - S6,9e[2])
B−V color index	+2.58[5]
Variable type	Mira[2]
B
Evolutionary stage	main sequence[4]
Spectral type	F8/9[4]
Astrometry
Radial velocity (Rv)	−18.0[6] km/s
Proper motion (μ)	RA: 15.713[1] mas/yr Dec.: 0.103[1] mas/yr
Parallax (π)	2.6735±0.1392 mas[1]
Distance	1,220 ± 60 ly (370 ± 20 pc)
Absolute magnitude (MV)	A: −0.7 to +6.9[3] B: +7.1[3]
Details
A
Mass	1.04 - 3[4] M☉
Radius	444[7][a] R☉
Luminosity	7,500[8] L☉
Temperature	2,800[7] K
B
Mass	1.04 - 1.09[4] M☉
Temperature	5,900 - 6,170[4] K
Other designations
W Aql, IRAS 19126−0708, TYC 5142-2895-1, 2MASS J19152335−0702503, AAVSO 1910−07, Gaia DR2 420492505899016640
Database references
SIMBAD	data

Description

W Aquilae is an S-type star with a spectral type of S3,9e to S6,9e, a red giant similar to M-type stars, but in which the dominant spectrum oxides are formed by metals of the fifth period of the periodic table. W Aquilae is also rich in the element technetium. Another feature of this class of stars is the stellar mass loss, in the case of W Aquilae is estimated at 4×10⁻⁶ solar masses per year.^[10] Its effective temperature is about 2,800 K and its radius is equivalent to 440 solar radii.^[7][a] It is also a very luminous star, 7,500 times more than the sun.^[8]

Variability

In 1893, Leo Anton Carl de Ball announced that the then unnamed star was a variable star, whose brightness varied by at least one magnitude.^[11] It was listed with its variable star designation, W Aquilae, in Annie Jump Cannon's 1907 Second Catalogue of Variable Stars.^[12]

The visual band light curve of W Aquilae, from AAVSO data^[13]

W Aquilae is a variable whose brightness oscillates between magnitude +7.3 and +14.3 over a period of 490.43 days. In Mira variables (which are named after Mira, the prototype), this instability comes from pulsation in the stellar surface, causing changes in color and brightness. W Aquilae, a Mira variable, shows silicon monoxide maser emission.^[14]

Companion

A magnitude 14.8 companion has been detected 0.47" SW of W Aquilae. This is fainter than W Aquilae at minimum and corresponds to an absolute magnitude of +7.1. Although that absolute magnitude would correspond to a K4 main sequence star, a spectrum was classified as F5 or F8. The separation between the two stars is 160 AU.^[3]

Planet X

A 2014 study of W Aquilae and α Centauri with the ALMA array claimed to have accidentally detected a previously-unknown Solar System object. This received widespread press coverage as a potential discovery of planet X. The paper was withdrawn without being accepted for peer-reviewed publication.^[15]

Notes

1. Radius calculated with angular diameter and distance in Table 1