V957 Scorpii

V957 Scorpii is a variable star in the constellation of Scorpius. It is a blue straggler in the open cluster Messier 7, a star that is unexpectedly hot compared to other members of the cluster. A 6th magnitude star, it is faintly visible to the naked eye under very good observing conditions.

V957 Scorpii

Location of V957 Scorpii (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Scorpius
Right ascension	17h 52m 13.662s[1]
Declination	−34° 47′ 57.11″[1]
Apparent magnitude (V)	5.87[2]
Characteristics
Spectral type	B5 IIIp[3]
U−B color index	−0.64[4]
B−V color index	−0.10[4]
Variable type	SX Ari[2]
Astrometry
Radial velocity (Rv)	−9.70[5] km/s
Proper motion (μ)	RA: 3.148[1] mas/yr Dec.: −5.604[1] mas/yr
Parallax (π)	3.6766±0.1521 mas[1]
Distance	890 ± 40 ly (270 ± 10 pc)
Absolute magnitude (MV)	−1.441[6]
Details
Mass	5.13[7] M☉
Radius	4.00[7] R☉
Luminosity	1,148[7] L☉
Surface gravity (log g)	3.94[7] cgs
Temperature	16,600[7] K
Metallicity [Fe/H]	+0.01[8] dex
Rotational velocity (v sin i)	30[9] km/s
Age	50[7] Myr
Other designations
V957 Sco, HR 6647, HD 162374, CD−34°12165, SAO 209383, HIP 87460
Database references
SIMBAD	data

Spectrum

Messier 7 (M7), with V957 Scorpii the bright star to the right of the central square

V957 Scorpii shows a peculiar B5 or B6 spectrum. Its luminosity class has been given as main sequence (V), subgiant (IV), giant (III), and supergiant (Ib).^[10] From its position in the H-R diagram, it is actually thought to be a main sequence star.^[11] With a helium abundance 25 times lower than that of the sun,^[12] it is classified as helium-weak. It also has a low carbon abundance and a strong magnetic field.^[7]

Messier 7

A light curve for V957 Scorpii, plotted from TESS data.^[13] The inset plot shows the same data averaged over several cycles.

Messier 7 is a naked eye open cluster. Except for one obvious orange giant star, its brightest members are mostly early A and late B main sequence stars and giants. Several of them are also chemically peculiar stars. However, two stars are hotter than the others and lie to the left of the isochrone for the cluster. These are the blue stragglers HD 162586 and V957 Scorpii. V957 Scorpii is considered 92% likely to be a member of M7. M7 has an age around 220 million years, but the apparent age of V957 Scorpii is less than 100 million years.^[12]

Variability

V957 Scorpii varies in brightness by about 0.05 magnitudes. This is thought to be due to its rotation and variations in its surface brightness. It is classified as an SX Arietis variable, also known as helium variables. Their spectral lines also vary as the stars rotate.^[2]