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]
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

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]

References
  1. van Leeuwen, F. (November 2007). "Hipparcos, the New Reduction" (VizieR). Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600. Retrieved 2010-11-22.
  2. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  3. Gray, R. O.; Corbally, C. J. (2014). "An Expert Computer Program for Classifying Stars on the MK Spectral Classification System". The Astronomical Journal. 147 (4): 80. Bibcode:2014AJ....147...80G. doi:10.1088/0004-6256/147/4/80. S2CID 28584130.
  4. Liška, J. (2016). "Analysis of the multiple system with chemically peculiar component φ Draconis". Monthly Notices of the Royal Astronomical Society. 461 (1): 939–947. arXiv:1606.01333. Bibcode:2016MNRAS.461..939L. doi:10.1093/mnras/stw1355.
  5. Mermilliod, J.-C. (1986). "Compilation of Eggen's UBV data, transformed to UBV (unpublished)". Catalogue of Eggen's UBV Data. (PDF) origin: SIMBAD: 0. Bibcode:1986EgUBV........0M.
  6. Kochukhov, O.; Papakonstantinou, N.; Neiner, C. (2022). "Magnetic field topology, chemical spot distributions, and photometric variability of the Ap star φ Draconis". Monthly Notices of the Royal Astronomical Society. 510 (4): 5821. arXiv:2201.02554. Bibcode:2022MNRAS.510.5821K. doi:10.1093/mnras/stac066.
  7. North, P. (June 1998). "Do SI stars undergo any rotational braking?". Astronomy and Astrophysics. 334: 181–187. arXiv:astro-ph/9802286. Bibcode:1998A&A...334..181N.
  8. Tokovinin, Andrei (2018-03-01). "The Updated Multiple Star Catalog". The Astrophysical Journal Supplement Series. 235 (1): 6. arXiv:1712.04750. Bibcode:2018ApJS..235....6T. doi:10.3847/1538-4365/aaa1a5. ISSN 0067-0049. S2CID 119047709.
  9. "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 8 December 2021.
  10. Prvák, M.; Liška, J.; Krtička, J.; Mikulášek, Z.; Lüftinger, T. (2015). "Modelling of variability of the chemically peculiar star ϕ Draconis". Astronomy and Astrophysics. 584: A17. arXiv:1510.01192. Bibcode:2015A&A...584A..17P. doi:10.1051/0004-6361/201526647.
  11. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 10 日
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