Phi Draconis

φ Draconis
Location of φ Draconis (circled)
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]
Distance300 ± 10 ly
(93 ± 3 pc)
Absolute magnitude (MV)−0.74 (−0.48/+0.94)[4]
Orbit[4]
PrimaryA
CompanionB
Period (P)307.8 yr
Semi-major axis (a)0.965"
(89.60 au)
Eccentricity (e)0.752
Inclination (i)95.6°
Orbit[6]
PrimaryAa
CompanionAb
Period (P)127.914 d
Eccentricity (e)0.6725
Inclination (i)48°
Semi-amplitude (K1)
(primary)		33.87 km/s
Details
Aa
Mass3.3[4] M☉
Radius2.7±0.2[7] R☉
Luminosity107[7] L☉
Surface gravity (log g)4.21±0.08[7] cgs
Temperature11,429[7] K
Rotation1.7165[6]
Rotational velocity (v sin i)81.5[6] km/s
Age245[4] Myr
Ab
Mass1.36[8] M☉
B
Mass2.25[8] M☉
Age330[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
SIMBADdata

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.

A light curve for Phi Draconis, plotted from TESS data[9]

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]

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  11. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 10 日