56 Pegasi
Star system in the constellation Pegasus
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56 Pegasi is a triple star[7] system in the northern constellation of Pegasus. It is visible to the naked eye with a combined apparent visual magnitude of 4.74.[2] The system is approximately 630 light years away from the Sun based on parallax,[15] but is drifting closer with a radial velocity of −28 km/s.[5] It is listed as a member of the Wolf 630 moving group.[16]
 | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Pegasus |
| Right ascension | 23h 07m 06.74189s[1] |
| Declination | 25° 28′ 05.7739″[1] |
| Apparent magnitude (V) | 4.74[2] |
| Characteristics | |
| Evolutionary stage | red giant branch[3][1] |
| Spectral type | K0.5II:Ba1CN-2CH-0.5[4] |
| U−B color index | +1.14[2] |
| B−V color index | +1.32[2] |
| Astrometry | |
| Radial velocity (Rv) | −27.55[5] km/s |
| Proper motion (μ) | RA: −2.747[1] mas/yr Dec.: −31.682[1] mas/yr |
| Parallax (π) | 5.1778±0.1118 mas[1] |
| Distance | 630 ± 10 ly (193 ± 4 pc) |
| Absolute magnitude (MV) | −1.32[6] |
| Orbit[7] | |
| Primary | 56 Peg A |
| Name | 56 Peg B |
| Period (P) | 111.15±0.03 d |
| Semi-major axis (a) | 0.79+0.10 −0.08 AU |
| Eccentricity (e) | 0.072+0.048 −0.045 |
| Inclination (i) | 90+42 −41° |
| Longitude of the node (Ω) | 90+60 −42° |
| Periastron epoch (T) | 2455289+15 −85 HJD |
| Argument of periastron (ω) (secondary) | 55+270 −37° |
| Semi-amplitude (K1) (primary) | 1.47±0.04[8] km/s |
| Orbit[7] | |
| Primary | 56 Peg AB |
| Name | 56 Peg C |
| Period (P) | 15,200+2,600 −1,600 d |
| Semi-major axis (a) | 22.1+3.6 −2.8 AU |
| Eccentricity (e) | 0.39+0.13 −0.12 |
| Inclination (i) | 157+4 −5° |
| Longitude of the node (Ω) | 153+14 −17° |
| Periastron epoch (T) | 2469014±2800 HJD |
| Argument of periastron (ω) (secondary) | 73+21 −24° |
| Details | |
| 56 Peg A | |
| Mass | 4.3±1.1[7] M☉ |
| Radius | 41[9] R☉ |
| Luminosity | 680[10] L☉ |
| Surface gravity (log g) | 1.41[11] cgs |
| Temperature | 4,185±85[9] K |
| Metallicity [Fe/H] | −0.38[11] dex |
| Rotational velocity (v sin i) | 4.4[12] km/s |
| Age | 229[13] Myr |
| 56 Peg B | |
| Mass | 0.13+0.06 −0.03[7] M☉ |
| 56 Peg C | |
| Mass | 0.85+0.25 −0.18[7] M☉ |
| Other designations | |
| 56 Peg, NSV 14429, BD+24°4716, FK5 3848, GC 32201, HD 218356, HIP 114155, HR 8796, SAO 91019, 2MASS J23070675+2528055[14] | |
| Database references | |
| SIMBAD | data |
Characteristics
The variable radial velocity of this star was announced in 1911 by W. W. Campbell.[8] The inner system, made up by the primary and secondary components, is a single-lined spectroscopic binary in a nearly circular orbit with a period of 111.15 days. The average separation between components is 0.79 astronomical units.[7]
The primary component is a peculiar bright giant with a stellar classification of K0.5 II: Ba1 CN-2 CH-0.5.[4] This notation indicates it is a K-type giant with some uncertainty about the classification, along with an overabundance of barium and underabundances of the CN and CH radicals. It is an active star,[6] roughly 229 million years old,[13] with 4.3 times the Sun's mass.[7] The star has expanded to 41 times the radius of the Sun[9] and is radiating 680 times the Sun's luminosity[10] from its enlarged photosphere at an effective temperature of 4,185 K.[9]
The secondary has a mass 0.13 times the mass of the Sun.[7]
The tertiary component is a white dwarf with 0.85 times the mass of the Sun. This companion lost mass when it was an AGB star, causing s-process elements, produced by nucleosynthesis, to be transferred to the primary star, resulting in its current unusual abundances. This star has an orbital period of 41.6 years, a moderate eccentricity, and an average separation of 22 AU.[7]
The system displays an excess of ultraviolet radiation that must be coming from the secondary. Simon et al. (1982) classified this object as a subdwarf O star.[17] Alternatively, it may be a white dwarf companion with an accretion disk.[18][8] Several puzzling features in the evolutionary history of this pair may be explained if the primary is a fast rotator being seen nearly pole-on. The star may have been spun up during a mass transfer episode with the secondary.[6]
Nomenclature
It is the 56th star numbered by Flamsteed in order of right ascension. In the 1795 French-language Fortin-Flamsteed edition of the Atlas Coelestis, 56 Pegasi is labelled with the letter "h".[19]