Phi Draconis

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]

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]
Distance	300 ± 10 ly (93 ± 3 pc)
Absolute magnitude (MV)	−0.74 (−0.48/+0.94)[4]
Orbit[4]
Primary	A
Companion	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
Companion	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]