Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||17h 31m 50.49153s|
|Declination||−49° 52′ 34.1220″|
|Apparent magnitude (V)||2.93 (2.76 to 2.90)|
|Spectral type||B2 Vne|
|U−B color index||−0.72|
|B−V color index||−0.16|
|R−I color index||−0.24|
|Radial velocity (Rv)||0 km/s|
|Proper motion (μ)||RA: −33.27 mas/yr
Dec.: −67.22 mas/yr
|Parallax (π)||12.20 ± 0.85 mas|
|Distance||270 ± 20 ly
(82 ± 6 pc)
|Luminosity (bolometric)||5,800 L☉|
|Surface gravity (log g)||3.99 cgs|
|Rotational velocity (v sin i)||375 km/s|
Alpha Arae (α Arae, α Ara) is the second brightest star in the southern constellation of Ara. With an average apparent visual magnitude 2.93, it is readily visible to the naked eye from the southern hemisphere. This star is close enough to the Earth that its distance can be estimated using parallax data collected during the Hipparcos mission. It is around 270 light-years (83 parsecs) away, with a 7% margin of error. The visual magnitude of the star is diminished by 0.10 magnitudes as a result of extinction from intervening gas and dust.
Alpha Arae has a stellar classification of B2 Vne, indicating that it is a massive B-type main sequence star. The 'n' notation in the suffix indicates that the absorption lines in the star's spectrum appear spread out and nebulous because of the Doppler effect from rapid rotation. The measured projected rotational velocity has been measured as high as 375 km s−1. Meilland et al. (2007) estimate the pole of the star is inclined by 55° to the line of sight, yielding an equatorial azimuthal velocity of 470 km s−1. This is close to the critical velocity where the star would start to breakup. The rapid rotation is causing a pronounced equatorial bulge of about 2.4–2.7 times the polar radius.
It is a Be star, as indicated by the 'e' notation in the star's classification. This indicates that emission lines are observed in the spectrum, which is coming from a disk of material ejected from the star because of its rapid rotation. In 2003 and 2005, Alpha Arae was observed by infrared interferometry, using the MIDI and AMBER instruments at the VLT Interferometer. The results, published in 2005 and 2007, appear to show that Alpha Arae is surrounded by a dense equatorial disk of material in Keplerian (rather than uniform) rotation, and that it is losing mass by a polar stellar wind with a terminal velocity of approximately 1,000 km/s. There is also some evidence that Alpha Arae is orbited by a companion at 0.7 AU.
This star has around 9.6 times as much mass as the Sun and an average of 4.5 times the Sun's radius. It is emitting 5,800 as much luminosity as the Sun from its outer envelope at an effective temperature of 18,044 K. This heat gives Alpha Arae the blue-white hue that is characteristic of B-type stars. It is a variable star with a magnitude that varies between 2.76m and 2.90m.
Alpha Arae has a visual companion star, CCDM J17318-4953B, located approximately 50 arcseconds away along a position angle of 168°, with an apparent visual magnitude of about 11. The two stars appear close to each other by coincidence and are not physically close in space.
With β and σ Ara it forms the Chinese asterism Choo （pinyin: chǔ, 杵）, "pestle" in traditional Chinese astronomy. It was the second star of Choo （杵二）, but R. H. Allen used the name Choo for this star only. Patrick Moore lists Choo as a proper name for this star in his star catalogue of the constellation Ara. This name is also spelt Tchou. There is another Choo in the constellation Pegasus.
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- Feinstein, A.; Marraco, H. G. (November 1979), "The photometric behavior of Be Stars", Astronomical Journal 84: 1713–1725, Bibcode:1979AJ.....84.1713F, doi:10.1086/112600
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- Meilland, A. et al. (March 2007), "First direct detection of a Keplerian rotating disk around the Be star α Arae using AMBER/VLTI", Astronomy and Astrophysics 464 (1): 59–71, arXiv:astro-ph/0606404, Bibcode:2007A&A...464...59M, doi:10.1051/0004-6361:20064848 . See Tables 1 and 4 for parameters of the star, circumstellar disk, and polar winds.
- Meilland, A.; Stee, Ph.; Chesneau, O.; Jones, C. (October 2009), "VLTI/MIDI observations of 7 classical Be stars", Astronomy and Astrophysics 505 (2): 687–693, arXiv:0908.1239, Bibcode:2009A&A...505..687M, doi:10.1051/0004-6361/200911960
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- V* alf Ara -- Eruptive variable Star, database entry, SIMBAD. Accessed on line November 26, 2008.
- Entry 17318-4953, The Washington Double Star Catalog, United States Naval Observatory. Accessed on line November 26, 2008.
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- First VLTI/MIDI observations of a Be star: Alpha Arae, O. Chesneau et al., Astronomy and Astrophysics 435, #1 (May 2005), pp. 275–287, doi:10.1051/0004-6361:20041954, Bibcode: 2005A&A...435..275C.
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- (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.
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