Imai (star)

Imai /ˈiːmaɪ/, Delta Crucis (Latinised from δ Crucis, abbreviated Delta Cru, δ Cru), is a star in the southern constellation of Crux, and is the faintest of the four bright stars that form the prominent asterism known as the Southern Cross. This star has an apparent magnitude of 2.79, and its proper name was adopted by the astronomical community on 10 August 2018.[12] Imai is a massive, hot and rapidly rotating star that is in the process of evolving into a giant, and is located at a distance of about 345 light-years (106 parsecs) from the Sun.

Delta Crucis
Location of δ Crucis (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Crux
Right ascension 12h 15m 08.71673s[1]
Declination –58° 44′ 56.1369″[1]
Apparent magnitude (V) 2.79[2]
Characteristics
Spectral type B2 IV[3]
U−B color index –0.921[4]
B−V color index –0.235[4]
Variable type β Cep[5]
Astrometry
Radial velocity (Rv)+22.2[2] km/s
Proper motion (μ) RA: -35.81[1] mas/yr
Dec.: -10.36[1] mas/yr
Parallax (π)9.45 ± 0.15[1] mas
Distance345 ± 5 ly
(106 ± 2 pc)
Absolute magnitude (MV)–3.2[6]
Details
Mass8.9 ± 0.1[7] M☉
Radius8.0[8] R☉
Luminosity10,000[6] L☉
Surface gravity (log g)3.88[6] cgs
Temperature22,570 ± 1,840[9] K
Rotational velocity (v sin i)210[10] km/s
Age18.1 ± 3.2[7] Myr
Other designations
Imai, CD-58 4466, FK5 455, HD 106490, HIP 59747, HR 4656, SAO 239791.[11]
Database references
SIMBADdata

Nomenclature

δ Crucis (Latinised to Delta Crucis) is the star's Bayer designation. It is sometimes called Pálida (Pale [one]) in Portuguese.[13]

Imai is the name for the star designated Delta Crucis by the Mursi people of modern-day Ethiopia. The star Imai has some significance as when it "ceases to appear in the evening sky at dusk (around the end of August), it is said that the Omo [river] rises high enough to flatten the imai grass that grows along its banks, and then subsides." The Mursi use a series of southern stars to mark their calendar to track seasonal flooding of the Omo river.[14] In 2016, the IAU organized a Working Group on Star Names (WGSN)[15] to catalog and standardize proper names for stars. The WGSN approved the name Imai for this star on 10 August 2018 and it is now so included in the List of IAU-approved Star Names.[12]

Properties

This star has a stellar classification of B2 IV,[3] making it a subgiant star that is in the process of evolving away from the main sequence It is now developing into a red giant and will in the distant future end as a white dwarf. Presently it is radiating around 10,000 times the luminosity of the Sun from its outer atmosphere at an effective temperature of 22,570 K,[6] causing it to glow with a blue-white hue.[16] Delta Crucis is a strong candidate Beta Cephei variable.[5] Its rotation is very fast, with a projected rotational velocity of 210 km s−1.[10]

Delta Crucis is a member of the Lower Centaurus Crux (LCC) component of the Scorpius-Centaurus Association, which is an OB association of massive stars that share a common origin and motion through space.[6] This is the nearest OB association to the Sun, with the LCC component having an age in the range of 16–20 million years.[17]

In culture

In Chinese, 十字架 (Shí Zì Jià), meaning Cross, refers to an asterism consisting of δ Crucis, γ Crucis, α Crucis and β Crucis.[18] Consequently, δ Crucis itself is known as 十字架四 (Shí Zì Jià sì, English: the Fourth Star of Cross.).[19]

The Aranda and Luritja people around Hermannsburg, Central Australia named Iritjinga, "The Eagle-hawk", a quadrangular arrangement comprising this star, γ Cru (Gacrux), γ Cen (Muhilfain) and δ Cen (Ma Wei).[20]

δ Cru is represented in the flags of Australia, New Zealand, Samoa, and Papua New Guinea as one of the stars comprising the Southern Cross. It is also featured in the flag of Brazil, along with 26 other stars, each of which represents a state. δ Cru represents the State of Minas Gerais.[21]

References
  1. van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600
  2. Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veroeffentlichungen des Astronomischen Rechen-Instituts Heidelberg, Astronomisches Rechen-Institut Heidelberg, 35 (35): 1, Bibcode:1999VeARI..35....1W
  3. Houk, Nancy; Cowley, A. P. (1978), Catalogue of two-dimensional spectral types for the HD stars: Declinations -90 to -53, 1, Ann Arbor: Dept. of Astronomy, University of Michigan, Bibcode:1975mcts.book.....H
  4. Gutierrez-Moreno, Adelina; Moreno, Hugo (June 1968), "A photometric investigation of the Scorpio-Centaurus association", Astrophysical Journal Supplement, 15: 459, Bibcode:1968ApJS...15..459G, doi:10.1086/190168
  5. Telting, J. H.; et al. (June 2006), "A high-resolution spectroscopy survey of β Cephei pulsations in bright stars", Astronomy and Astrophysics, 452 (3): 945–953, Bibcode:2006A&A...452..945T, doi:10.1051/0004-6361:20054730
  6. de Geus, E. J.; de Zeeuw, P. T.; Lub, J. (June 1989), "Physical parameters of stars in the Scorpio-Centaurus OB association", Astronomy and Astrophysics, 216 (1–2): 44–61, Bibcode:1989A&A...216...44D
  7. Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (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
  8. 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
  9. Sokolov, N. A. (May 1995), "The determination of T_eff_ of B, A and F main sequence stars from the continuum between 3200 A and 3600 A", Astronomy and Astrophysics Supplement, 110: 553, Bibcode:1995A&AS..110..553S
  10. Bernacca, P. L.; Perinotto, M. (1970). "A catalogue of stellar rotational velocities". Contributi Osservatorio Astronomico di Padova in Asiago. 239 (1): 1. Bibcode:1970CoAsi.239....1B.
  11. "HD 106490 – Variable Star of beta Cep type", SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg, retrieved 2005-11-05
  12. "IAU Catalog of Star Names". International Astronomical Union. Retrieved 2018-09-17.
  13. da Silva Oliveira, R., "Crux Australis: o Cruzeiro do Sul" Archived 2013-12-06 at the Wayback Machine, Artigos: Planetario Movel Inflavel AsterDomus.
  14. "IAU Working Group on Star Names (WGSN)" (Press release). IAU.org.
  15. "IAU Working Group on Star Names (WGSN)". Retrieved 17 September 2018.
  16. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-10, retrieved 2012-01-16
  17. Jilinski, E.; et al. (March 2006), "Radial velocity measurements of B stars in the Scorpius-Centaurus association", Astronomy and Astrophysics, 448 (3): 1001–1006, arXiv:astro-ph/0601643, Bibcode:2006A&A...448.1001J, doi:10.1051/0004-6361:20041614, S2CID 17818058
  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 7 月 29 日
  20. Raymond Haynes; Roslynn D. Haynes; David Malin; Richard McGee (1996). Explorers of the Southern Sky: A History of Australian Astronomy. Cambridge University Press. p. 8. ISBN 978-0-521-36575-8.
  21. "Astronomy of the Brazilian Flag". FOTW Flags Of The World website.
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