AG Car (Hubble Space Telescope image)
Epoch J2000 Equinox J2000
|Right ascension||10h 56m 11.57699s|
|Declination||−60° 27′ 12.8056″|
|Apparent magnitude (V)||6.96 (5.7 – 9.0)|
|U−B color index||–0.58|
|B−V color index||+0.61|
|Proper motion (μ)|| RA: −4.701 mas/yr |
Dec.: +1.873 mas/yr
|Parallax (π)||0.1532 ± 0.0291 mas|
|Absolute magnitude (MV)||~-8 (at minimum)|
|Luminosity||1.5 × 106 L☉|
|Rotation||13 ± 2 days|
|Rotational velocity (v sin i)||220 ± 50 km/s|
AG Carinae (AG Car) is a star in the constellation Carina. It is classified as a luminous blue variable and is one of the most luminous stars in the Milky Way. The large distance (20,000 light-years) and intervening dust mean that the star is not usually visible to the naked eye; its apparent brightness varies erratically between magnitude 5.7 and 9.0.
The star is surrounded by a nebula of ejected material at 0.4–1.2pc from the star. The nebula contains around 15 M☉, all lost from the star around 10,000 years ago. There is an 8.8pc wide empty cavity in the interstellar medium around the star, presumably cleared by fast winds earlier in the star's life.
AG Carinae is apparently in a transitional phase between a massive class O blue supergiant and a Wolf–Rayet star, where it is highly unstable and suffers from erratic pulsations, occasional larger outbursts, and rare massive eruptions. The spectral type varies between WN11 at visual minimum and an early A hypergiant at maximum. At visual minimum the star is about 65 R☉ and 20,000–24,000K, while at maximum it is over 400 R☉ and 8,000K. The temperature varies at different minima.
One study calculated that the bolometric luminosity of AG Carinae decreases during the S Doradus outbursts, unlike most LBVs which remain at approximately constant luminosity. The luminosity drops from around 1.5 million L☉ at visual minimum to around 1 million L☉ at visual maximum, possibly due to the energy required to expand a considerable fraction of the star.
Evolutionary models of the star suggest that it had a low rotation rate for much of its life, but current observations show fairly rapid rotation.
Models of LBV progenitors of type IIb supernovae list AG Carinae as matching the final stellar spectrum prior to core collapse, although the models are for stars with 20 to 25 times the mass of the Sun while AG Carinae is thought to be considerably larger. The initial mass of the star would have been around 100 M☉ and is now thought to be 55-70 M☉.
Parallaxes from data release 1 (DR1) of the Gaia mission suggest a much closer distance to AG Carinae and its neighbour Hen 3-519 than previously accepted, around 2 kpc. Then both stars would be less luminous than LBVs and it is argued that they would be former red supergiants whose unusual characteristics are the result of binary evolution.
The earlier Hipparcos parallax for AG Carinae had a margin of error larger than the parallax itself and so gave little information about its distance. The distance of 6,000 pc is based assumptions about the properties of LBVs, models of interstellar extinction, and kinematical measurements. The Gaia DR1 parallax, derived from the combination of the first year of Gaia measurements with Tycho astrometry, is 0.40 ± 0.22 mas. The Gaia team recommend that a further 0.3 mas systematic error is allowed for (i.e. added to the formal margin of error). Smith et al. argue that the 0.3 mas systematic margin of error can be ignored and that the implied distance to AG Carinae is 2.50 ± 1.41 kpc.
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- Groh, J. H.; Hillier, D. J.; Damineli, A.; Whitelock, P. A.; Marang, F.; Rossi, C. (2009). "On the Nature of the Prototype Luminous Blue Variable Ag Carinae. I. Fundamental Parameters During Visual Minimum Phases and Changes in the Bolometric Luminosity During the S-Dor Cycle". The Astrophysical Journal. 698 (2): 1698. arXiv:0904.2363. Bibcode:2009ApJ...698.1698G. doi:10.1088/0004-637X/698/2/1698.
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- Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051.
- Vamvatira-Nakou, C.; Hutsemekers, D.; Royer, P.; Cox, N. L. J.; Naze, Y.; Rauw, G.; Waelkens, C.; Groenewegen, M. A. T. (2015). "The Herschel view of the nebula around the luminous blue variable star AG Carinae". Astronomy & Astrophysics. 1504: 3204. arXiv:1504.03204. Bibcode:2015A&A...578A.108V. doi:10.1051/0004-6361/201425090.
- Stahl, O.; Jankovics, I.; Kovács, J.; Wolf, B.; Schmutz, W.; Kaufer, A.; Rivinius, Th.; Szeifert, Th. (2001). "Long-term spectroscopic monitoring of the Luminous Blue Variable AG Carinae". Astronomy and Astrophysics. 375: 54–69. Bibcode:2001A&A...375...54S. doi:10.1051/0004-6361:20010824.
- Groh, J. H.; Hillier, D. J.; Damineli, A. (July 2011). "On the Nature of the Prototype Luminous Blue Variable AG Carinae. II. Witnessing a Massive Star Evolving Close to the Eddington and Bistability Limits". The Astrophysical Journal. 736 (1): 46. arXiv:1105.0814. Bibcode:2011ApJ...736...46G. doi:10.1088/0004-637X/736/1/46.
- Groh, J. H.; Damineli, A.; Hillier, D. J. (2008). P. Benaglia (ed.). "LBVs and the nature of the S Dor cycles: The case of AG Carinae". Massive Stars: Fundamental Parameters and Circumstellar Interactions. 33: 132. arXiv:astro-ph/0702612. Bibcode:2008RMxAC..33..132G.
- Groh, J. H.; Meynet, G.; Ekström, S. (2013). "Massive star evolution: luminous blue variables as unexpected supernova progenitors". Astronomy & Astrophysics. 550: 4. arXiv:1301.1519. Bibcode:2013A&A...550L...7G. doi:10.1051/0004-6361/201220741. L7.
- Smith, N.; Stassun, K. G. (2017). "The Canonical Luminous Blue Variable AG Car and Its Neighbor Hen 3-519 are Much Closer than Previously Assumed". The Astronomical Journal. 153 (3): 7. arXiv:1610.06522. Bibcode:2017AJ....153..125S. doi:10.3847/1538-3881/aa5d0c. 125.
- Gaia Collaboration (2016). "VizieR Online Data Catalog: Gaia DR1 (Gaia Collaboration, 2016)". VizieR On-line Data Catalog: I/337. Originally Published in: Astron. Astrophys. 1337. Bibcode:2016yCat.1337....0G.