Phi Herculis

Phi Herculis (φ Her) is a binary star^[4] system in the northern constellation of Hercules. Based upon an annual parallax shift of 15.99 mas as seen from Earth,^[1] it is located around 204 light years from the Sun. With a combined apparent visual magnitude of 4.24,^[2] it is bright enough to be seen with the naked eye.

A light curve for Phi Herculis, assuming a 3.708 day period,^[10] plotted from TESS data^[11]

Phi Herculis

φ Herculis in optical light
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Hercules
Right ascension	16h 08m 46.17745s[1]
Declination	+44° 56′ 05.6663″[1]
Apparent magnitude (V)	4.24[2]
Characteristics
Spectral type	B9VspHgMn[3] + A8V[4]
U−B color index	−0.23[2]
B−V color index	−0.06[2]
Variable type	α2 CVn?[5]
Astrometry
Radial velocity (Rv)	−16.3±0.4[6] km/s
Proper motion (μ)	RA: −26.63[1] mas/yr Dec.: +36.76[1] mas/yr
Parallax (π)	15.99±0.45 mas[1]
Distance	204 ± 6 ly (63 ± 2 pc)
Absolute magnitude (MV)	A: 0.100 ± 0.059 B: 2.670 ± 0.074[7]
Orbit[7]
Period (P)	564.834±0.038 d
Semi-major axis (a)	32.027±0.028 mas
Eccentricity (e)	0.52614±0.00086
Inclination (i)	9.1±0.4°
Longitude of the node (Ω)	190.4±1.4°
Periastron epoch (T)	2450121.43 ± 0.20 JD
Argument of periastron (ω) (secondary)	350.3±1.4°
Semi-amplitude (K1) (primary)	2.772±0.073 km/s
Semi-amplitude (K2) (secondary)	8.1[4] km/s
Details
φ Her A
Mass	3.05±0.24[7] M☉
Luminosity	72[8] L☉
Surface gravity (log g)	4.05±0.15[4] cgs
Temperature	11,525±150[4] K
Metallicity [Fe/H]	−0.03[7] dex
Rotational velocity (v sin i)	8.0±1.0[4] km/s
Age	210[7] Myr
φ Her B
Mass	1.614±0.066[7] M☉
Surface gravity (log g)	4.30±0.15[4] cgs
Temperature	8,000±150[4] K
Rotational velocity (v sin i)	50.0±3.0[4] km/s
Other designations
φ Her, 11 Her, BD+45°2376, FK5 601, HD 145389, HIP 79101, HR 6023, SAO 45911.[9]
Database references
SIMBAD	data

This is a single-lined spectroscopic binary star system with an orbital period of 564.8 days and an eccentricity of 0.526.^[7] The primary, component A, is a B-type main sequence star with a stellar classification of B9VspHgMn.^[3] It is a chemically peculiar star of the type called a mercury-manganese star.^[4] The star is tentatively catalogued as an Alpha2 Canum Venaticorum variable, with brightness variations of just 0.01 magnitudes.^[5][12]

The secondary, component B, was first separated via interferometry in 2004.^[13] It is an A-type main sequence star of class A8V.^[4] The magnitude difference between the two components is 2.64.^[13]