Chi Ophiuchi

Chi Ophiuchi, Latinized from χ Ophiuchi, is a variable star in the equatorial constellation of Ophiuchus.^[13] It has a blue-white hue and is faintly visible to the naked eye with an apparent visual magnitude that fluctuates around 4.22.^[3] The distance to this object, as determined from parallax measurements, is approximately 500 light years,^[2] but it is moving closer to the Sun with a radial velocity of −19 km/s.^[8] This star is a proper motion member of the Upper Scorpius sub-group in the Scorpius–Centaurus OB association; the nearest such co-moving association of massive stars to the Sun.^[14]

Chi Ophiuchi

A blue-light light curve for Chi Ophiuchi, adapted from Cuypers et al. (1989)[1]
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Ophiuchus
Right ascension	16h 27m 01.43557s[2]
Declination	−18° 27′ 22.4500″[2]
Apparent magnitude (V)	4.22[3] (4.18 to 5.0)[4]
Characteristics
Evolutionary stage	main sequence
Spectral type	B2Vne[5]
U−B color index	−0.75[6]
B−V color index	+0.28[6]
R−I color index	+0.22[6]
Variable type	GCAS[4] + LERI[7]
Astrometry
Radial velocity (Rv)	−19.0±2.1[8] km/s
Proper motion (μ)	RA: −5.378[2] mas/yr Dec.: −22.308[2] mas/yr
Parallax (π)	6.5402±0.1961 mas[2]
Distance	500 ± 10 ly (153 ± 5 pc)
Absolute magnitude (MV)	−2.27[9]
Details
Mass	10.1±0.7[10] M☉
Radius	4.44±0.09[11] R☉
Luminosity	55,847[12] L☉
Surface gravity (log g)	3.53±0.04[11] cgs
Temperature	30,000±300[11] K
Rotational velocity (v sin i)	150±3[11] km/s
Age	22.5±2.6[10] Myr
Other designations
χ Oph, 7 Ophiuchi, BD−18°4282, FK5 3298, GC 22117, HD 148184, HIP 80569, HR 6118, SAO 159918, PPM 231703[13]
Database references
SIMBAD	data

This is a massive Be star with a stellar classification of B2Vne,^[5] where the 'n' indicates "nebulous" lines due to rapid rotation. It is spinning with a projected rotational velocity of 144 km/s. As the critical velocity for the star is 477 km/s, the inclination angle of its poles must be small; estimated as ~20°.^[15] It is a Gamma Cassiopeiae variable with an amplitude of 0.15 magnitude.^[16] The brightness has been measured varying from magnitude 4.18 down to 5.0.^[4]

Chi Ophiuchi is 22.5 million years old with 10.1 times the mass of the Sun.^[10] It has four times the Sun's radius and is radiating 56,000 times the luminosity of the Sun from its photosphere at an effective temperature of about 30,000 K. A weak magnetic field has been detected in the chromosphere of this star.^[17] It is being orbited by a symmetrical disk of ejected gas extending out to 0.52 AU (112 R_☉),^[15] and excess radio and infrared emission has been detected from this structure.^[18]

Abt and Levy (1978) cataloged Chi Ophiuchi as a single-lined spectroscopic binary with a period of 138.8 days and an eccentricity of 0.44, although the orbital elements are considered marginal.^[19] Eggleton and Tokovinin (2008) list it as a single star.^[20]