V923 Aquilae

V923 Aquilae is a variable binary star^[9] system in the equatorial constellation of Aquila. It has the designation HD 183656 from the Henry Draper Catalogue; V932 Aql is the variable star designation. The system is dimly visible to the naked eye with an apparent visual magnitude that fluctuates around 6.06.^[3] It is located at a distance of approximately 890 light years from the Sun based on parallax,^[2] but is drifting closer with a radial velocity of around −26 km/s.^[7]

V923 Aquilae

A photoelectric (yellow) light curve for V923 Aquilae, plotted from data presented by Lynds (1960).[1] The length of a full phase cycle is 0.8518 days.
Observation data Epoch J2000      Equinox J2000
Constellation	Aquila
Right ascension	19h 30m 33.12199s[2]
Declination	+03° 26′ 39.8583″[2]
Apparent magnitude (V)	6.06[3] 5.98 to 6.18[4]
Characteristics
Evolutionary stage	Main sequence[5]
Spectral type	B7III[6]
U−B color index	−0.31[3]
B−V color index	−0.019±0.008[7]
Variable type	Be star[4]
Astrometry
Radial velocity (Rv)	−26.0±7.4[7] km/s
Proper motion (μ)	RA: 11.717[2] mas/yr Dec.: 3.089[2] mas/yr
Parallax (π)	3.6705±0.0607 mas[2]
Distance	890 ± 10 ly (272 ± 5 pc)
Absolute magnitude (MV)	−0.87[7]
Details[5]
Primary
Mass	6.2±0.3 M☉
Luminosity	1517+158 −143 L☉
Surface gravity (log g)	3.64±0.21 cgs
Temperature	16,580±400 K
Rotational velocity (v sin i)	275±17 km/s
Secondary
Mass	~0.5 M☉
Other designations
V923 Aql, BD+03°4043, HD 183656, HIP 95929, HR 7415, SAO 0124704[8]
Database references
SIMBAD	data

This system was first identified as a likely spectroscopic binary by W. E. Harper in 1937, who noted it showed "narrow intense lines of peculiar spectrum".^[10] P. W. Merrill and C. G. Burwell identified it as a shell star in 1949.^[11][12] Merrill and A. L. Lowen showed in 1953 that the shell displayed large radial velocity variations.^[13] A photometric study by C. R. Lynds in 1960 showed the system varied in brightness with an amplitude of more than 0.1 in magnitude and a characteristic period of 0.85 days, although it does not behave periodically over long time intervals.^[1]

A more thorough investigation by P. Koubský and associates in 1989 using long-term radial velocity measurements determined this is a spectroscopic binary with an orbital period of 214.75 days. There is also an overlaying long-term cyclical variation of changing amplitude and period. The modelled binary system shows a primary with a class of around B5–7e and a low mass secondary separated by around 250 times the radius of the Sun (250 R_☉). They hypothesized that the long-term variation was due to an envelope created by a mass transfer from the secondary component to the primary.^[14] However, the mass transfer concept was later brought into question and remains unverified as of 2004.^[9]