Post-AGB star

A post-AGB star (pAGB, abbreviation of post-asymptotic giant branch) is a type of luminous supergiant star of intermediate mass in a very late phase of stellar evolution. The post-AGB stage occurs after the asymptotic giant branch (AGB or second-ascent red giant) has ended. The stage sees the dying star, initially very cool and large, shrink and heat up.[1] The duration of the post-AGB stage varies based on the star's initial mass, and can range from 100,000 years for a solar-mass star to just over 1,000 years for more massive stars. The timescale gets slightly shorter with lower metallicity.[2]

Towards the end of this stage, post-AGB stars also tend to produce protoplanetary nebulae as they shed their outer layers, and this creates a large infrared excess and obscures the stars in visible light. After reaching an effective temperature of about 30,000 K, the star is able to ionise its surrounding nebula, producing a true planetary nebula.

Properties

Post-AGB stars span a large range of temperatures, as they are in the process of heating up from very cool temperatures (3,000 K or less) up to about 30,000 K. Technically, the post-AGB stage only ends when the star reaches its maximum temperature of 100-200,000 K,[2] but beyond 30,000 K, the star ionises the surrounding gas and would be considered a central star of a planetary nebula more often than a post-AGB star.

On the other hand, the luminosity of post-AGB stars is usually constant throughout the post-AGB stage, and slightly dependent on the star’s core mass, and getting slightly brighter with lower metallicity.[3][2]

Examples

Due to the dust usually obscuring them, many post-AGB stars are visually relatively dim. However there are still some post-AGB stars visible to the naked eye, the brightest of which is 89 Herculis.

Other examples include:

References
  1. Habing, Harm (1997). Paresce, Francesco (ed.). AGB and POST-AGB STARS: an Overview. Science with the VLT Interferometer. Science with the VLT Interferometer. ESO Astrophysics Symposia. Berlin, Heidelberg: Springer. pp. 183–191. doi:10.1007/978-3-540-69398-7_23. ISBN 978-3-540-69398-7.
  2. Bertolami, Marcelo Miguel Miller (2016-04-01). "New models for the evolution of post-asymptotic giant branch stars and central stars of planetary nebulae". Astronomy & Astrophysics. 588: A25. arXiv:1410.1679. Bibcode:2016A&A...588A..25M. doi:10.1051/0004-6361/201526577. ISSN 0004-6361.
  3. Vickers, Shane B.; Frew, David J.; Owers, Matt S.; Parker, Quentin A.; Bojičič, Ivan S. (2016). "Improving the distances of post-AGB objects in the Milky Way". Journal of Physics: Conference Series. 728 (7): 072013. Bibcode:2016JPhCS.728g2013V. doi:10.1088/1742-6596/728/7/072013. S2CID 126383516.
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