R Coronae Australis

R Coronae Australis (R CrA) is a variable binary system in the constellation Corona Australis.^[2][8] It has varied between magnitudes 10 and 14.36.^[10] A small reflection/emission nebula NGC 6729 extends from the star towards SE. It is also the brightest feature of the Coronet Cluster, therefore sometimes called R CrA Cluster.

R Coronae Australis

A colour photo of the R Coronae Australis region in the southern Milky Way.
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Corona Australis
Right ascension	19h 01m 53.6503s[1]
Declination	−36° 57′ 07.87″[1]
Apparent magnitude (V)	+11.91
Characteristics
Spectral type	B5IIIpe[2]
Variable type	INSA[3]
Astrometry
Radial velocity (Rv)	−36.0±4.9[4] km/s
Proper motion (μ)	RA: 1.582[5] mas/yr Dec.: −30.835[5] mas/yr
Parallax (π)	10.5361±0.6971 mas[5]
Distance	152.9+8.1 −7.3[6] pc
Absolute magnitude (MV)	−0.30[7]
Orbit
Period (P)	45±2[8] yr
Semi-major axis (a)	0.1968±0.0045[8]" (27-28 AU[6])
Eccentricity (e)	0.4[6]
Inclination (i)	70[6]°
Details
A
Mass	3.5[7] M☉
Radius	6.2[7] R☉
Luminosity	132[7] L☉
Temperature	9,550[7] K
Age	1+1 −0.5[6] Myr
B
Mass	0.3 - 0.55[6] M☉
Surface gravity (log g)	3.45±0.06[6] cgs
Temperature	3,650 - 3,870[6] K
Other designations
CD−37°13027, HIP 93449, Wray 15-1887
Database references
SIMBAD	data

A visual band light curve for R Coronae Australis, plotted from ASAS data^[9]

This star is moving toward the Solar System with a radial velocity of 36 km s⁻¹. It was previously believed that in roughly 222,000 years, this system could have approached within 1.77 light-years (0.54 parsecs) of the Sun. However, the estimate had a considerable margin of error in it.^[11] With the release of Gaia DR2, the star was determined to be 4 times further from the Sun than initially believed, constraining the approach to only 111 ± 31 light-years (34.0 ± 9.5 parsecs).^[5] Examination of other objects known to be in the same star-forming region gives a distance of 152.9+8.1
−7.3 pc, suggesting an error in the Gaia parallax for R CrA itself.^[6]

Johann Friedrich Julius Schmidt discovered that R Coronae Australis is a variable star, in 1865. It appeared with its variable star designation in Annie Jump Cannon's 1907 work Second Catalogue of Variable Stars.^[12]

A companion to the star was proposed in 2019 with a mass between 0.1 and 1 Solar masses, depending on the characteristics of the stellar environment, orbiting the primary in 43–47 years.^[8] The companion was later directly observed to be a red dwarf with a mass between 0.3 M_☉ and 0.55 M_☉.^[6] It has also been proposed that the primary component is itself a close binary.^[7]