Phi Draconis
Star in the constellation Draco
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Phi Draconis (φ Dra / φ Draconis) is a fourth-magnitude variable star in the constellation Draco. It has the Flamsteed designation 43 Draconis. It is also a triple star system where the brightest component is a chemically peculiar Ap star.
  | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Draco |
| Right ascension | 18h 20m 45.43224s[1] |
| Declination | +71° 20′ 16.1499″[1] |
| Apparent magnitude (V) | 4.22[2] |
| Characteristics | |
| Spectral type | B8VpSi[3] + A4V + B9V[4] |
| U−B color index | −0.38[5] |
| B−V color index | −0.10[5] |
| Variable type | α2 CVn[2] |
| Astrometry | |
| Radial velocity (Rv) | −16.44[6] km/s |
| Proper motion (μ) | RA: −5.03[1] mas/yr Dec.: +37.86[1] mas/yr |
| Parallax (π) | 10.77±0.38 mas[1] |
| Distance | 300 ± 10 ly (93 ± 3 pc) |
| Absolute magnitude (MV) | −0.74 (−0.48/+0.94)[4] |
| Orbit[4] | |
| Primary | A |
| Name | B |
| Period (P) | 307.8 yr |
| Semi-major axis (a) | 0.965" (89.60 au) |
| Eccentricity (e) | 0.752 |
| Inclination (i) | 95.6° |
| Orbit[6] | |
| Primary | Aa |
| Name | Ab |
| Period (P) | 127.914 d |
| Eccentricity (e) | 0.6725 |
| Inclination (i) | 48° |
| Semi-amplitude (K1) (primary) | 33.87 km/s |
| Details | |
| Aa | |
| Mass | 3.3[4] M☉ |
| Radius | 2.7±0.2[7] R☉ |
| Luminosity | 107[7] L☉ |
| Surface gravity (log g) | 4.21±0.08[7] cgs |
| Temperature | 11,429[7] K |
| Rotation | 1.7165[6] |
| Rotational velocity (v sin i) | 81.5[6] km/s |
| Age | 245[4] Myr |
| Ab | |
| Mass | 1.36[8] M☉ |
| B | |
| Mass | 2.25[8] M☉ |
| Age | 330[4] Myr |
| Other designations | |
| φ Dra, 43 Dra, BD+71°889, CCDM J18208+7120AB, GC 25114, HD 170000, HIP 89908, HR 6920, SAO 9084 | |
| Database references | |
| SIMBAD | data |
The brightness of φ Draconis varies by about 0.04 of a magnitude every 1.7 days. This is due to very strong magnetic fields at the surface of the star, and it is classified as an α2 Canum Venaticorum variable.[2]
φ Draconis is a multiple star system containing three stars. The inner pair form a single-lined spectroscopic binary in an eccentric 128-day orbit. The outermost star orbits the inner pair every 308 years. The outer pair can be resolved visually and have a semi-major axis of 0.752″. A fourth component, C, is also listed in multiple star catalogues, but is only a chance alignment with the triple system.[4]
Phi Draconis Aa is a main-sequence Ap star with a spectral class of B8. The main abundance excess is silicon, although iron and chromium and also notably elevated.[10]
In Chinese astronomy, φ Draconis is called 柱史 (Pinyin: Zhùshǐ), meaning Official of Royal Archives, because this star is marking itself and stand alone in Official of Royal Archives asterism, Purple Forbidden enclosure (see: Chinese constellations).[11]