31 Vulpeculae

31 Vulpeculae is a binary star^[8] system in the northern constellation of Vulpecula. It is visible to the naked eye as a faint, yellow-hued point of light with an apparent visual magnitude of 4.56.^[2] The system is located approximately 228 light years away from the Sun based on parallax,^[1] and it is drifting further away with a radial velocity of +2.25 km/s.^[6]

31 Vulpeculae

Location of 31 Vulpeculae (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Vulpecula
Right ascension	20h 52m 07.68352s[1]
Declination	Tautvaisiene201027° 05′ 48.9951″[1]
Apparent magnitude (V)	4.56[2]
Characteristics
Evolutionary stage	red clump[3]
Spectral type	G7IIIa Fe-1 Ba[2]
U−B color index	+0.46[4]
B−V color index	+0.82[4]
Variable type	suspected[5]
Astrometry
Radial velocity (Rv)	+2.25[6] km/s
Proper motion (μ)	RA: −70.902[1] mas/yr Dec.: −56.360[1] mas/yr
Parallax (π)	14.2796±0.1864 mas[1]
Distance	228 ± 3 ly (70.0 ± 0.9 pc)
Absolute magnitude (MV)	0.77[7]
Orbit[8]
Period (P)	1,860.6±1.3 d
Semi-major axis (a)	≥ 103.0 Gm
Eccentricity (e)	0.375±0.009
Periastron epoch (T)	52,316±6 MJD
Argument of periastron (ω) (secondary)	15.1±1.4°
Semi-amplitude (K1) (primary)	4.34±0.04 km/s
Details[9]
31 Vul A
Mass	2.40±0.05 M☉
Radius	8.01±0.30 R☉
Luminosity	52.53[2] L☉
Surface gravity (log g)	2.97±0.05 cgs
Temperature	5,261±15 K
Metallicity [Fe/H]	−0.05±0.02 dex
Rotational velocity (v sin i)	5.9±0.3[8] km/s
Age	700±40 Myr
Other designations
31 Vul, BD+26°4017, GC 29112, HD 198809, HIP 103004, HR 7995, SAO 89228, 2MASS J20520768+2705491[10]
Database references
SIMBAD	data

The variable radial velocity of this system was first suspected by German Astronomer Friedrich Küstner in 1914. The system appears as a single-lined spectroscopic binary with an orbital period of 5.1 years and an eccentricity of 0.38. The a sin i value for the primary is 103.0 ± 1.1 Gm (0.69 ± 0.01 AU),^[8] where a is the semimajor axis and i is the (unknown) orbital inclination. This value provides a lower bound on the actual semimajor axis for the orbit.

The primary component is an aging giant star with a stellar classification of G7IIIa Fe-1 Ba,^[2] indicating a mild barium star with an underabundance of iron.^[8] Having exhausted the supply of hydrogen at its core, it has expanded to eight times the Sun's radius.^[9] It is a red clump giant,^[3] which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. This is a suspected variable star with a magnitude that varies from 3.77 to 4.08 in the I passband.^[5] It is about 700 million years old with 2.4 times the mass of the Sun. The star is radiating 53^[2] times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 5,261 K.^[9] The companion may be a degenerate white dwarf with about 0.4 M_☉.^[8]