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ρ Ophiuchi
	Location of ρ Ophiuchi (circled red)
Observation data; Epoch J2000.0 Equinox J2000.0
Constellation	Ophiuchus
Right ascension	16h 25m 35.11766s
Declination	−23° 26′ 49.8150″
Apparent magnitude (V)	4.63
Characteristics
ρ Oph AB
Spectral type	B2/3V + B2V
U−B color index	-0.56
B−V color index	+0.24
Astrometry
ρ Oph AB
Radial velocity (Rv)	-11.40 ± 3 km/s
Proper motion (μ)	RA: -5.53 mas/yr ; Dec.: -21.74 mas/yr
Parallax (π)	9.03 ± 0.90 mas
Distance	360 ± 40 ly ; (110 ± 10 pc)
Absolute magnitude (MV)	-2.5 ± 0.3
Orbit
Primary	ρ Oph A
Companion	ρ Oph B
Period (P)	2398 ± 326 yr
Semi-major axis (a)	4.25 ± 0.79″
Eccentricity (e)	0.675 ± 0.322
Inclination (i)	135.3 ± 6.0°
Longitude of the node (Ω)	77.5 ± 13.5°
Periastron epoch (T)	B 2327 ± 343
Argument of periastron (ω); (secondary)	226.1 ± 15.3°
Orbit
Primary	ρ Oph D
Companion	ρ Oph E
Period (P)	675.5 ± 32.5 yr
Semi-major axis (a)	1.01 ± 0.15″
Eccentricity (e)	0.707 ± 0.112
Inclination (i)	134.8 ± 2.7°
Longitude of the node (Ω)	152.7 ± 4.6°
Periastron epoch (T)	B 2008.6 ± 34.2
Argument of periastron (ω); (secondary)	260.4 ± 1.1°
Details
ρ Oph A
Mass	9.6 M☉
Radius	8.5 ± 1 R☉
Luminosity	13000 L☉
Temperature	21900 K
Rotation	1.205 d
Rotational velocity (v sin i)	240 ± 10 km/s
ρ Oph B
Mass	8 M☉
ρ Oph C
Mass	5 M☉
ρ Oph D
Mass	3.06 M☉
ρ Oph E
Mass	1.97 M☉
Other designations
	ρ Oph, 5 Oph, WDS J16256-2327
	ρ Oph AB: CD−23°12861, HIP 80473
	ρ Oph A: HD 147933, HR 6112, SAO 184382
	ρ Oph B: HD 147934, HR 6113, SAO 184381
	ρ Oph C: CD−23°12862, HIP 80474, SAO 184383
Database references
SIMBAD	ρ Oph
	ρ Oph A
	ρ Oph B
	ρ Oph C
	ρ Oph D/E

Rho Ophiuchi (ρ Ophiuchi) is a multiple star system in the constellation Ophiuchus. The central system has an apparent magnitude of 4.63.^[2] Based on the central system's parallax of 9.03 mas,^[1] it is located about 360 light-years (110 parsecs) away.^[1] The other stars in the system are slightly farther away.^[11]

System

The central pair is known as Rho Ophiuchi AB. It consists of at least two blue-colored subgiants or main-sequence stars, designated Rho Ophiuchi A and B, respectively.^[11] Rho Ophiuchi AB is a visual binary, and the sky-projected distance between the two stars appears to be 3.1″, corresponding to a separation of at least 344 astronomical units (au).^[11] However, the actual separation is larger, and the two take about 2,400 years to complete an orbit.^[7] The two stars dominate the radiation field around the Rho Ophiuchi cloud complex.^[9]

Rho Ophiuchi A emits X-rays, and exhibits strong variability in emission over periods of about 1.2 days, corresponding to its rotation period. The exact origin of its X-ray variability is unknown: it could be an magnetically active spot on its surface, or it could be a small low-mass companion.^[12] Related to this is its extremely strong magnetic field; its dipole strength is at least B_d = 1.9 ± 0.2 kG.^[9]

Several other stars are located close to Rho Ophiuchi AB. HD 147932 is located 2.5 arcminutes away (at least 17,000 au), and is known as Rho Ophiuchi C.^[11] HD 147888 is located 2.82 arcminutes away (at least 19,000 au), and is known as Rho Ophiuchi DE.^[11] Stars C and D are both B-type main-sequence stars,^[11] and D itself is another binary with an orbital period of around 680 years.^[7]

Cloud complex

Rho Ophiuchi is the namesake of the Rho Ophiuchi cloud complex. It is a nebula of gas and dust, which the Rho Ophiuchi system is embedded in. It is one of the easiest star forming regions to observe, as it is one of the nearest, and it is visible from both hemispheres.^[13]

The interstellar extinction (A_V) of Rho Ophiuchi is measured to be 1.45 magnitudes, meaning the dust and gas in front of Rho Ophiuchi absorbs light from the system, making it appear 1.45 magnitudes dimmer than it would be if there were no dust or gas.^[14] Additionally, gas and dust also scatters more higher-frequency light, leaving the light appearing more reddish. The interstellar reddening (E_B−V) of Rho Ophiuchi has been measured to be 0.47 magnitudes.^[6]

References

^ ^a ^b ^c ^d ^e ^f ^g van Leeuwen, F.; et al. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
^ ^a ^b Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". VizieR Online Data Catalog. 2237. Bibcode:2002yCat.2237....0D.
^ Houk, N.; Smith-Moore, M. (1988). "Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Volume 4, Declinations −26°.0 to −12°.0". Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Volume 4. Bibcode:1988mcts.book.....H.
^ ^a ^b Nicolet, B. (1964). "Catalogue of homogeneous data in the UBV photoelectric photometric system". Astronomy and Astrophysics Supplement Series. 34: 1–49. Bibcode:1978A&AS...34....1N.
^ Gontcharov, G. A. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters. 32 (11): 759–771. arXiv:1606.08053. Bibcode:2006AstL...32..759G. doi:10.1134/S1063773706110065. S2CID 119231169.
^ ^a ^b Wegner, W. (2003). "The total-to-selective extinction ratio determined from near IR photometry of OB stars". Astronomische Nachrichten. 324 (3): 219–237. Bibcode:2003AN....324..219W. doi:10.1002/asna.200310081.
^ ^a ^b ^c ^d ^e ^f Novaković, B. (2007). "Orbits of Five Visual Binary Stars". Baltic Astronomy. 16: 435–442. arXiv:0712.4242. Bibcode:2007BaltA..16..435N.
^ ^a ^b Hernández, Jesús; Calvet, Nuria; Hartmann, Lee; Briceño, César; Sicilia-Aguilar, Aurora; Berlind, Perry (2005). "Herbig Ae/Be Stars in nearby OB Associations". The Astronomical Journal. 129 (2): 856–871. arXiv:astro-ph/0410494. Bibcode:2005AJ....129..856H. doi:10.1086/426918. S2CID 17672346.
^ ^a ^b ^c ^d ^e ^f Hubrig, S.; Schöller, M.; Järvinen, S. P.; Küker, M.; Kholtygin, A. F.; Steinbrunner, P. (2018). "Detection of a centrifugal magnetosphere in one of the most massive stars in the ρ Oph star-forming cloud". Astronomische Nachrichten. 339 (1): 72–77. arXiv:1712.05939. Bibcode:2018AN....339...72H. doi:10.1002/asna.201713457. S2CID 119084902.
^ ^a ^b Allen, Christine; Ruelas-Mayorga, Alex; Sánchez, Leonardo J.; Costero, Rafael (2018). "The dynamical evolution of multiple systems of trapezium type". Monthly Notices of the Royal Astronomical Society. arXiv:1809.03537. doi:10.1093/mnras/sty2502.
^ ^a ^b ^c ^d ^e ^f Cordiner, M. A.; Fossey, S. J.; Smith, A. M.; Sarre, P. J. (2013). "Cordiner, M. A., Fossey, S. J., Smith, A. M. and Sarre, P. J. 2013, ApJ, 764, L10". The Astrophysical Journal Letters. 764 (1): L10. arXiv:1301.6167. Bibcode:2013ApJ...764L..10C. doi:10.1088/2041-8205/764/1/L10. S2CID 119204101.
^ Pillitteri, Ignazio; Wolk, Scott J.; Reale, Fabio; Oskinova, Lida (2017). "The early B-type star Rho Ophiuchi a is an X-ray lighthouse". Astronomy & Astrophysics. 602: A92. arXiv:1703.04686. Bibcode:2017A&A...602A..92P. doi:10.1051/0004-6361/201630070. S2CID 119431278.
^ Wilking, B. A.; Gagné, M.; Allen, L. E. (2008). "Star Formation in the ρ Ophiuchi Molecular Cloud". Handbook of Star Forming Regions. arXiv:0811.0005. Bibcode:2008hsf2.book..351W.
^ Shatsky, N.; Tokovinin, A. (2002). "The mass ratio distribution of B-type visual binaries in the Sco OB2 association". Astronomy and Astrophysics. 382: 92–103. arXiv:astro-ph/0109456. Bibcode:2002A&A...382...92S. doi:10.1051/0004-6361:20011542. S2CID 16697655.