Chi Ophiuchi
Star in the constellation Ophiuchus
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Chi Ophiuchi, Latinized from χ Ophiuchi, is a variable star in the equatorial constellation of Ophiuchus.[12] It has a blue-white hue and is faintly visible to the naked eye with an apparent visual magnitude that fluctuates around 4.22.[2] The distance to this object, as determined from parallax measurements, is approximately 500 light years,[1] but it is moving closer to the Sun with a radial velocity of −19 km/s.[7] 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.[13]
  | |
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
|---|---|
| Constellation | Ophiuchus |
| Right ascension | 16h 27m 01.43557s[1] |
| Declination | −18° 27′ 22.4500″[1] |
| Apparent magnitude (V) | 4.22[2] (4.18 to 5.0)[3] |
| Characteristics | |
| Evolutionary stage | main sequence |
| Spectral type | B2Vne[4] |
| U−B color index | −0.75[5] |
| B−V color index | +0.28[5] |
| R−I color index | +0.22[5] |
| Variable type | GCAS[3] + LERI[6] |
| Astrometry | |
| Radial velocity (Rv) | −19.0±2.1[7] km/s |
| Proper motion (μ) | RA: −5.378[1] mas/yr Dec.: −22.308[1] mas/yr |
| Parallax (π) | 6.5402±0.1961 mas[1] |
| Distance | 500 ± 10 ly (153 ± 5 pc) |
| Absolute magnitude (MV) | −2.27[8] |
| Details | |
| Mass | 10.1±0.7[9] M☉ |
| Radius | 4.44±0.09[10] R☉ |
| Luminosity | 55,847[11] L☉ |
| Surface gravity (log g) | 3.53±0.04[10] cgs |
| Temperature | 30,000±300[10] K |
| Rotational velocity (v sin i) | 150±3[10] km/s |
| Age | 22.5±2.6[9] Myr |
| Other designations | |
| χ Oph, 7 Ophiuchi, BD−18°4282, FK5 3298, GC 22117, HD 148184, HIP 80569, HR 6118, SAO 159918, PPM 231703[12] | |
| Database references | |
| SIMBAD | data |
This is a massive Be star with a stellar classification of B2Vne,[4] 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.[3]
Chi Ophiuchi is 22.5 million years old with 10.1 times the mass of the Sun.[9] 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]