Pi Aquarii

π Aquarii
Location of π Aquarii (circled)
Observation data
Epoch J2000      Equinox ICRS
Constellation Aquarius
Right ascension 22h 25m 16.623s
Declination +01° 22′ 38.63″
Apparent magnitude (V) 4.42 - 4.87
Characteristics
Spectral type B1III-IVe
U−B color index −0.98
B−V color index −0.03
Variable type γ Cas
Astrometry
Radial velocity (Rv)+4.0 km/s
Proper motion (μ) RA: +17.518 mas/yr
Dec.: +2.563 mas/yr
Parallax (π)2.9761±0.1129 mas
Distance1,100 ± 40 ly
(340 ± 10 pc)
Absolute magnitude (MV)−2.09
Orbit
Period (P)84.07±0.02 d
Semi-major axis (a)≥ 0.96 AU
Eccentricity (e)0.0 (fixed)
Inclination (i)70±10°
Longitude of the node (Ω)76.7±0.1°
Periastron epoch (T)2,450,318.5±13.2 JD
Argument of periastron (ω)
(secondary)		0.0 (fixed)°
Semi-amplitude (K1)
(primary)		16.7±0.2 km/s
Semi-amplitude (K2)
(secondary)		101.4±0.2 km/s
Details
A
Mass11.0±0.9 M☉
Radius5.75±0.99 (equatorial)
5.00±0.81 (polar) R☉
Luminosity8,300+1,700
−1,400 L☉
Surface gravity (log g)4.08±0.10 (equatorial)
3.96±0.11 (polar) cgs
Temperature22,400±1,400 (equatorial)
25,000±1,000 (polar) K
Metallicity [Fe/H]−0.02 dex
Rotational velocity (v sin i)271±13 km/s
Age10.0±5.0 Myr
B
Mass0.5–0.8 M☉
Other designations
π Aqr, 52 Aquarii, BD+00 4872, FK5 1585, HD 212571, HIP 110672, HR 8539, SAO 127520
Database references
SIMBADdata

Pi Aquarii is a binary star system in the equatorial constellation of Aquarius. Its name is a Bayer designation that is Latinized from π Aquarii, and abbreviated Pi Aqr or π Aqr. This system has a combined apparent visual magnitude of +4.57, which is bright enough to be faintly visible to the naked eye. Based upon parallax measurements, it is located at a distance of approximately 1,100 light-years (340 parsecs) from the Sun. It is drifting further away with a radial velocity of +4 km/s.

Properties

A white-light light curve for Pi Aquarii, adapted from Nazé et al. (2020)

This is a binary star system with a period of 84.1 days in a circular orbit. The primary component is a B1 giant or subgiant star. This is a massive star with 11 times the mass of the Sun, and is luminous with 8,300 times the Sun's luminosity. It is spinning rapidly with a projected rotational velocity of 271 km/s. The fast rotation make the star ellipsoidal rather than spherical, its radius at equator is 5.8 times that of the Sun, while its polar radius is 5.0 times solar. Gravity darkening make the temperature at the equator to be about 2,600 K colder than the temperature at the poles.

The secondary is likely a white dwarf star with a strong magnetic field and a mass of 0.5 to 0.8 M☉. It is an X-ray source which is probably coming from accretion onto this object, making this an intermediate polar system. The accretion rate is (4–7)×10−11 M☉·yr−1.

Pi Aquarii is notable for having undergone a transition from a Be star (showing hydrogen emission lines) into an ordinary B-type star. It is classified as a Gamma Cassiopeiae type variable star and its brightness varies from magnitude +4.45 to +4.71; a range of 0.28. The dominant variability period, 83.8±0.8 days, is nearly the same as the orbital period. Pi Aquarii has a reasonable chance of becoming a supernova some day.

False-color image of the emission nebula (green and blue) surrounding π Aqr (image center), obtained by the Northern Sky Narrowband Survey.

Infrared images from WISE at a wavelength of 22 μm reveal a bow shock located approximately 100 arcseconds from the star (51,000 astronomical units at 340 pc) in the direction of its proper motion. In optical range, a weakly ionized emission nebula with a diameter of about 5° is visible (green to blue in the image). There also appears to be some interaction with the surrounding interstellar medium (red in the image). Notably, the rectangular shape of the nebula is not aligned with the current orientation of the circumstellar disk (see the infobox).

In culture

Pi Aquarii was called Seat /ˈsiːæt/ by Grotius in the 17th century, but the name has rarely been used since.

In Chinese, 墳墓 (Fén Mù), meaning Tomb, refers to an asterism consisting of π Aquarii, γ Aquarii, ζ Aquarii, η Aquarii. Consequently, the Chinese name for π Aquarii itself is 墳墓四 (Fén Mù sì, English: the Fourth Star of Tomb.)

In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Wasat al Achbiya (وسط الأخبية - wasath al ahbiyah), which was translated into Latin as Media Tabernaculorum, meaning the middle of luck of the homes (tents). This star, along with γ Aqr (Sadachbia), ζ Aqr and η Aqr (Fenmu), were al Aḣbiyah (الأخبية), the Tent.

References

  1. 1 2 3 Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
  2. 1 2 3 4 5 6 Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  3. 1 2 3 "pi Aqr", General Catalogue of Variable Stars, Sternberg Astronomical Institute, retrieved 2012-07-03. Note: type = GCAS.
  4. 1 2 Slettebak, A. (1982), "Spectral types and rotational velocities of the brighter Be stars and A-F type shell stars", Astrophysical Journal Supplement Series, 50: 55, Bibcode:1982ApJS...50...55S, doi:10.1086/190820.
  5. 1 2 3 Nicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", Astronomy and Astrophysics Supplement Series, 34: 1–49, Bibcode:1978A&AS...34....1N.
  6. 1 2 Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astronomisches Rechen-Institut Heidelberg, 35 (35), Astronomisches Rechen-Institut Heidelberg: 1, Bibcode:1999VeARI..35....1W.
  7. ↑ Bjorkman, Karen S.; et al. (July 2002), "A Study of π Aquarii during a Quasi-normal Star Phase: Refined Fundamental Parameters and Evidence for Binarity", The Astrophysical Journal, 573 (2): 812–824, arXiv:astro-ph/0203357, Bibcode:2002ApJ...573..812B, doi:10.1086/340751, S2CID 14685744.
  8. 1 2 Concha, D.; Arcos, C.; Turis-Gallo, D.; Souza, T. B.; Curé, M.; Levenhagen, R. S.; Araya, I. (2025). "New insight into the variability of the Be star π Aquarii: Determination of stellar and disk parameters". Astronomy & Astrophysics. 697: A209. arXiv:2505.13700. Bibcode:2025A&A...697A.209C. doi:10.1051/0004-6361/202452708.
  9. ↑ Wisniewski, John P.; Draper, Zachary H.; Bjorkman, Karen S.; Meade, Marilyn R.; Bjorkman, Jon E.; Kowalski, Adam F. (2010-02-01). "DISK-LOSS AND DISK-RENEWAL PHASES IN CLASSICAL Be STARS. I. ANALYSIS OF LONG-TERM SPECTROPOLARIMETRIC DATA". The Astrophysical Journal. 709 (2): 1306–1320. arXiv:0912.1504. Bibcode:2010ApJ...709.1306W. doi:10.1088/0004-637X/709/2/1306. ISSN 0004-637X.
  10. ↑ Tetzlaff, N.; et al. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x, S2CID 118629873.
  11. 1 2 Huenemoerder, David P.; et al. (May 2024), "Chandra HETG X-Ray Spectra and Variability of π Aqr, a γ Cas-type Be Star", The Astrophysical Journal Letters, 966 (2): L23, arXiv:2404.16977, Bibcode:2024ApJ...966L..23H, doi:10.3847/2041-8213/ad4095, ISSN 2041-8205.
  12. ↑ "pi. Aqr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-07-03.
  13. ↑ Nazé, Yaël; et al. (May 4, 2020), "Let there be more variability in two γ Cas stars", Monthly Notices of the Royal Astronomical Society, 494 (1): 958–974, arXiv:2002.12656, Bibcode:2020MNRAS.494..958N, doi:10.1093/mnras/staa617, retrieved 2 August 2022.
  14. 1 2 3 Kaler, James, "Seat", Stars, retrieved 2017-08-18.
  15. 1 2 3 Pollmann, Ernst (May 2012), "Period analysis of the Halpha line profile variation of the Be binary star pi Aqr", Information Bulletin on Variable Stars, 6023: 1, Bibcode:2012IBVS.6023....1P.
  16. ↑ Underhill, A. B.; et al. (November 1979), "Effective temperatures, angular diameters, distances and linear radii for 160 O and B stars", Monthly Notices of the Royal Astronomical Society, 189 (3): 601–605, Bibcode:1979MNRAS.189..601U, doi:10.1093/mnras/189.3.601.
  17. ↑ Mayer, A.; Deschamps, R.; Jorissen, A. (2016). "Search for systemic mass loss in Algols with bow shocks". Astronomy & Astrophysics. 587: A30. arXiv:1601.00218. Bibcode:2016A&A...587A..30M. doi:10.1051/0004-6361/201526623. ISSN 0004-6361.
  18. ↑ (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  19. ↑ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 16 日 Archived 2011-05-22 at the Wayback Machine
  20. ↑ Knobel, E. B. (June 1895), "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket", Monthly Notices of the Royal Astronomical Society, 55 (8): 429, Bibcode:1895MNRAS..55..429K, doi:10.1093/mnras/55.8.429.
  21. ↑ Davis Jr., G. A. (October 1944), "The Pronunciations, Derivations, and Meanings of a Selected List of Star Names", Popular Astronomy, 52 (3): 12, Bibcode:1944PA.....52....8D.
  22. ↑ Allen, Richard Hinckley (1936) [1899]. Star Names and Their Meanings. New York: G. E. Stechert. p. 52. OCLC 9825499.
  23. ↑ γ Aqr as Aoul al Achbiya or Prima Tabernaculorum (the first of luck of the homes or tents), π Aqr as Wasat al Achbiya or Media Tabernaculorum (the middle of luck of the homes or tents) and ζ Aqr as Achr al Achbiya or Postrema Tabernaculorum (the end of luck of the homes or tents).