Super-AGB star

A super-AGB star is a star with a mass intermediate between those that end their lives as a white dwarf and those that end with a core collapse supernova, and properties intermediate between asymptotic giant branch (AGB) stars and red supergiants. They have initial masses of 7.5–9.25 M☉ in stellar-evolutionary models, but have exhausted their core hydrogen and helium, left the main sequence, and expanded to become large, cool, and luminous.

HR diagram

Super-AGB stars occupy the top-right of the Hertzsprung–Russell diagram (HR diagram), and have cool temperatures between 3,000 and 4,000 K, which is similar to normal AGB stars and red supergiant stars (RSG stars).[1] These cool temperatures allow molecules to form in their photospheres and atmospheres.[2] Super-AGB stars emit most of their light in the infra-red spectrum because of their extremely cool temperatures.

The Chandrasekhar limit and their life

A super-AGB star's core may grow to (or past) the Chandrasekhar mass because of continued hydrogen (H) and helium (He) shell burning, ending as core-collapse supernovae.[1][3] The most massive super-AGB stars (at around 9 M☉) are theorized to end in electron capture supernovae. The error in this determination due to uncertainties in the third dredge-up efficiency and AGB mass-loss rate could lead to about a doubling of the number of electron-capture supernovae, which also supports the theory that these stars make up 66% of the supernovae detected by satellites.

These stars are at a similar stage in life to red giant stars, such as Aldebaran, Mira, and Chi Cygni, and are at a stage where they start to brighten, and their brightness tends to vary, along with their size and temperature.

These stars represent a transition to the more massive supergiant stars that undergo full fusion of elements heavier than helium. During the triple-alpha process, some elements heavier than carbon are also produced: mostly oxygen, but also some magnesium, neon, and even heavier elements, gaining an oxygen-neon (ONe) core. Super-AGB stars develop partially degenerate carbon–oxygen cores that are large enough to ignite carbon in a flash analogous to the earlier helium flash. The second dredge-up is very strong in this mass range and that keeps the core size below the level required for burning of neon as occurs in higher-mass supergiants.

List of super-AGB candidates

Candidate Right ascension Declination Location Discovery Notes Refs
HV 2112 01h 10m 03.87s −72° 36′ 52.6″ Small Magellanic Cloud 2014 Classified formerly as a supergiant TZO candidate,[4][5][6][7] as a possible foreground galactic S-type star,[8] and as an unresolved multiple star[9][8] [6][10]
MSX SMC 055 00h 50m 07.2s −73° 31′ 25.1″ Small Magellanic Cloud 2009 Considered to be a likely super-AGB star candidate[11] [11]
VX Sagittarii 18h 08m 04.04831s −22° 13′ 26.6327″ Sagittarius 1904 Commonly classified as an unusually cool red supergiant or red hypergiant.[12][13] [14]

References

  1. ^ a b Groenewegen, M. A. T.; Sloan, G. C. (2018). "Luminosities and mass-loss rates of Local Group AGB stars and red supergiants". Astronomy & Astrophysics. 609: A114. arXiv:1711.07803. Bibcode:2018A&A...609A.114G. doi:10.1051/0004-6361/201731089. S2CID 59327105.
  2. ^ Levesque, Emily M.; Massey, Philip; Olsen, K. A. G.; Plez, Bertrand; Josselin, Eric; Maeder, Andre; Meynet, Georges (2005). "The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not as Cool as We Thought". The Astrophysical Journal. 628 (2): 973–985. arXiv:astro-ph/0504337. Bibcode:2005ApJ...628..973L. doi:10.1086/430901. S2CID 15109583.
  3. ^ Poelarends, A. J. T.; Herwig, F.; Langer, N.; Heger, A. (2008). "The Supernova Channel of Super-AGB Stars". The Astrophysical Journal. 675 (1): 614–625. arXiv:0705.4643. Bibcode:2008ApJ...675..614P. doi:10.1086/520872. S2CID 18334243.
  4. ^ Levesque, Emily (1 September 2014). "Discovery of a Thorne-̇Żytkow object candidate in the Small Magellanic Cloud". Monthly Notices of the Royal Astronomical Society: Letters. 443: L94–L98. arXiv:1406.0001. Bibcode:2014MNRAS.443L..94L. doi:10.1093/mnrasl/slu080. S2CID 119192926.
  5. ^ McMillan, Paul (2018). "Gaia DR2 Confirms that Candidate Thorne-Żytkow Object HV 2112 is in the Small Magellanic Cloud". Research Notes of the American Astronomical Society. 2 (2) (published May 2018): 18. Bibcode:2018RNAAS...2...18M. doi:10.3847/2515-5172/aac0fb. S2CID 125376171.
  6. ^ a b Tout, Christopher (2014). "HV2112, a Thorne-Zytkow object or a super asymptotic giant branch star". Monthly Notices of the Royal Astronomical Society. 445 (published November 2014): L36–L40. arXiv:1406.6064. Bibcode:2014MNRAS.445L..36T. doi:10.1093/mnrasl/slu131.
  7. ^ Worley, Clare (2016). "The proper motion of HV2112: a TŻO candidate in the SMC". Monthly Notices of the Royal Astronomical Society. 459 (1) (published June 2016): L31–L35. arXiv:1602.08479. Bibcode:2016MNRAS.459L..31W. doi:10.1093/mnrasl/slw034.
  8. ^ a b MacCarone, Thomas J.; De Mink, Selma E. (2016). "Large proper motion of the Thorne-Żytkow object candidate HV 2112 reveals its likely nature as foreground Galactic S-star". Monthly Notices of the Royal Astronomical Society: Letters. 458 (1): L1. arXiv:1601.05455. Bibcode:2016MNRAS.458L...1M. doi:10.1093/mnrasl/slw004. S2CID 119306614.
  9. ^ González-Fernández, Carlos; Dorda, Ricardo; Negueruela, Ignacio; Marco, Amparo (2015). "A new survey of cool supergiants in the Magellanic Clouds". Astronomy & Astrophysics. 578: A3. arXiv:1504.00003. Bibcode:2015A&A...578A...3G. doi:10.1051/0004-6361/201425362. S2CID 55959019.
  10. ^ Goldman, Steven R.; Van Loon, Jacco Th.; Gómez, José F.; Green, James A.; Zijlstra, Albert A.; Nanni, Ambra; Imai, Hiroshi; Whitelock, Patricia A.; Groenewegen, Martin A. T.; Oliveira, Joana M. (2018). "A dearth of OH/IR stars in the Small Magellanic Cloud". Monthly Notices of the Royal Astronomical Society. 473 (3): 3835–3853. doi:10.1093/mnras/stx2601.
  11. ^ a b Groenewegen, M. A. T.; Sloan, G. C.; Soszyński, I.; Petersen, E. A. (2009). "Luminosities and mass-loss rates of SMC and LMC AGB stars and red supergiants". Astronomy & Astrophysics. 506 (3): 1277–1296. arXiv:0908.3087. Bibcode:2009A&A...506.1277G. doi:10.1051/0004-6361/200912678.
  12. ^ Lockwood, G. W.; Wing, R. F. (1982). "The light and spectrum variations of VX Sagittarii, an extremely cool supergiant". Monthly Notices of the Royal Astronomical Society. 198 (2): 385–404. Bibcode:1982MNRAS.198..385L. doi:10.1093/mnras/198.2.385. ISSN 0035-8711.
  13. ^ Xu, Shuangjing; Zhang, Bo; Reid, Mark J; Menten, Karl M; Zheng, Xingwu; Wang, Guangli (2018). "The Parallax of the Red Hypergiant VX Sgr with Accurate Tropospheric Delay Calibration". The Astrophysical Journal. 859 (1): 14. arXiv:1804.00894. Bibcode:2018ApJ...859...14X. doi:10.3847/1538-4357/aabba6. S2CID 55572194.
  14. ^ Tabernero, Hugo M.; Dorda Laforet, Ricardo; Negueruela Díez, Ignacio; Marfil, Emilio Gómez (2021). "The nature of VX Sagitarii". Astronomy & Astrophysics. 646: A98. arXiv:2011.09184. Bibcode:2021A&A...646A..98T. doi:10.1051/0004-6361/202039236. S2CID 227013580.

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