AG Carinae
{{Short description|Luminous variable star in the constellation Carina}}
{{Starbox begin
| name = AG Carinae
}}
{{Starbox image
| image = Image:A closer look at Hubble’s 31st anniversary snapshot (51484874537).jpg
| caption = AG Carinae (Hubble Space Telescope image)
}}
{{Starbox observe
| epoch = J2000
| dec = {{DEC|−60|27|12.8107}}
| appmag_v = 6.96 {{nowrap|(5.7–9.0)}}
| constell = Carina
}}
{{Starbox character
| variable = LBV
}}
{{Starbox astrometry
| radial_v =
| parallax = 0.1925
| p_error = 0.0165
| dist_ly =
| dist_pc =
}}
{{Starbox detail
| radius = 50–552{{cite journal | url=https://ui.adsabs.harvard.edu/abs/1998A%26ARv...8..145D/abstract | bibcode=1998A&ARv...8..145D | title=The yellow hypergiants | last1=De Jager | first1=Cornelis | journal=Astronomy and Astrophysics Review | date=1998 | volume=8 | issue=3 | page=145 | doi=10.1007/s001590050009 }}
| luminosity = 600,000–900,000{{efn|name=luminosity}}
| metal_fe =
| rotational_velocity = {{val|220|50}}
| rotation = {{val|13|2}} days
| age_myr =
}}
{{Starbox catalog
| names = CD−59°3430, CPD−59°2860, HD 94910, HIP 53461, SAO 251185, WR 31b, AAVSO 1052–69
}}
{{Starbox reference
| Simbad = AG+Car
}}
{{Starbox end}}
AG Carinae (AG Car) is a star in the constellation of Carina. It is classified as a luminous blue variable (LBV) and is one of the most luminous stars in the Milky Way. The great 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.
In 1914, Harry Edwin Wood announced his discovery that this star, then called CPD−59°2860, is a variable star, based on photographic plates taken in 1911 and 1914. It was given its variable star designation, AG Carinae, in 1921.
Description
File:A Closer Look at Hubble’s 31st Anniversary Snapshot.jpg
The star is surrounded by a nebula of ejected material at 0.4–1.2 pc from the star. The nebula contains around {{solar mass|15}}, all lost from the star around 10,000 years ago. There is an 8.8-parsec-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 {{solar radius|65}} and 20,000–24,000 K, while at maximum it is {{solar radius|over 400}} and 8,000 K. The temperature varies at different minima.
One study calculated that the bolometric luminosity of AG Carinae decreases during its S Doradus-type outbursts, unlike most LBVs which remain at approximately constant luminosity. The luminosity drops from around {{solar luminosity|1.5 million}} at visual minimum to around {{solar luminosity|1 million}} 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 more massive. The initial mass of the star would have been around {{solar mass|100}} and is now thought to be {{solar mass|55–70}}.
Distance controversy
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 2,000 parsecs. 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 parsecs is based on 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 {{val|0.40|0.22|ul=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). A 2017 study argues that the 0.3 mas systematic margin of error can be ignored and that the implied distance to AG Carinae is {{val|2.50|1.41|ul=kpc}}.
In Gaia Data Release 2, the parallax is {{val|0.1532|0.0291|u=mas}}, suggesting a distance around {{val|6,500|u=pc|fmt=commas}}. A 2019 observation yields a most likely distance of {{val|4,650|fmt=commas|u=pc}}. Gaia Early Data Release 3 gives a parallax of {{val|0.1925|0.0165|u=mas}}, although with a non-trivial level of excess astrometric noise where there was none in Gaia DR2.
Light curve
File:AG-Car-Lightcurve-AAVSO.png light curve of luminous blue variable AG Car from 1 Jan 1940 to 23 Nov 2010. Up is brighter and down is fainter.|center|450x450px]]
Notes
{{notelist|refs=
{{efn | name=luminosity | Smith et al. 2019 stated that assuming the 4.65 kpc distance, the luminosity of AG Car would be 40% lower than the {{solar luminosity|1–1.5 million}} which was based on the 6 kpc distance.}}
}}
References
External links
- [http://www.ipac.caltech.edu/2mass/gallery/images_misc.html 2MASS Atlas Image Gallery: Miscellaneous Objects] includes an infrared image of AG Carinae
- [https://www.youtube.com/watch?v=ARTWCk1ieg4 Hubble Space Telescope zooms to AG Carinae] YouTube
{{Stars of Carina}}
{{DEFAULTSORT:AG Carinae}}
Category:Carina (constellation)