Alpha Arae
 α Arae (Choo) in the constellation Ara. |
|
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) |
|
|---|---|
| Constellation | Ara |
| Right ascension | 17h 31m 50.49153s |
| Declination | −49° 52′ 34.1220″ |
| Apparent magnitude (V) | 2.93 (2.76 to 2.90) |
| Characteristics | |
| Spectral type | B2 Vne |
| U−B color index | −0.72 |
| B−V color index | −0.16 |
| R−I color index | −0.24 |
| Variable type | BE |
| Astrometry | |
| Radial velocity (Rv) | 0 km/s |
| Proper motion (μ) |
RA: −33.27 mas/yr Dec.: −67.22 mas/yr |
| Parallax (π) | 12.20 ± 0.85mas |
| Distance | 270 ± 20 ly (82 ± 6 pc) |
| Details | |
| Mass | 9.6 M☉ |
| Radius | 4.5 R☉ |
| Luminosity (bolometric) | 5,800 L☉ |
| Surface gravity (log g) | 3.99 cgs |
| Temperature | 18,044 K |
| Rotational velocity (v sin i) | 375 km/s |
| Other designations | |
| Database references | |
| SIMBAD | data |
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 break up. 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.
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