HR 7031
Binary star in the constellation Corona Australis
From Wikipedia, the free encyclopedia
HD 172991, also known as HR 7031 or rarely 20 G. Coronae Australis, is a binary star[14] located in the southern constellation Corona Australis. It has a combined apparent magnitude of 5.44,[2] making it faintly visible to the naked eye. The system is located relatively far at a distance of 1,220 light years based on Gaia DR3 parallax measurements[1] but is receding with a heliocentric radial velocity of −17.4 km/s.[5] At its current distance HD 172991's brightness is diminished by magnitudes due to interstellar dust and it has an absolute magnitude of −2.56.[6]
 | |
| Observation data Epoch J2000.0 Equinox ICRS | |
|---|---|
| Constellation | Corona Australis |
| Right ascension | 18h 44m 57.15228s[1] |
| Declination | −39° 41′ 10.2781″[1] |
| Apparent magnitude (V) | 5.44±0.01[2] |
| Characteristics | |
| A | |
| Spectral type | K1/2III[3] |
| B−V color index | +0.87[4] |
| B | |
| Spectral type | B9/A1V[3] |
| Astrometry | |
| Radial velocity (Rv) | −17.4±0.8[5] km/s |
| Proper motion (μ) | RA: +9.815 mas/yr[1] Dec.: −6.261 mas/yr[1] |
| Parallax (π) | 2.6806±0.1453 mas[1] |
| Distance | 1,220 ± 70 ly (370 ± 20 pc) |
| Absolute magnitude (MV) | −2.56[6] |
| Details | |
| A | |
| Mass | 4.98±0.05[7] M☉ |
| Radius | 53.9±3.0[8] R☉ |
| Luminosity | 1,750+188 −154[1] L☉ |
| Surface gravity (log g) | 1.03[9] cgs |
| Temperature | 4,990±123[10] K |
| Metallicity [Fe/H] | −0.41[9] dex |
| Rotational velocity (v sin i) | 3.9±2[11] km/s |
| Other designations | |
| 20 G. Coronae Australis[12], CD−39°12864, CPD−39°8163, GC 25628, HD 172991, 172992, HIP 91989, HR 7031, SAO 210518[13] | |
| Database references | |
| SIMBAD | data |
HD 172991 has a stellar classification of K1/2 III,[3] indicating that it is an evolved red giant. The companion, HD 172992, is a B-type star with a stellar classification of B9/A1 V.[3] Although the two components can be resolved in the spectrum, they cannot be observed in telescopes, making observation difficult.[15]
HD 172991 has 4.98 times the mass of the Sun[7] but it has expanded to 53.9 times the Sun's radius.[8] It radiates 1,750 times the luminosity of the Sun[1] from its enlarged photosphere at an effective temperature of 4,990 K,[10] giving it a yellowish-orange hue. It is metal deficient with an iron abundance 39% of the Sun's ([Fe/H] = −0.41)[9] and spins modestly with a projected rotational velocity of 3.9 km/s.[11]