Post-AGB Stars

Abstract

▪ Abstract In this contribution, a review is presented on the ample data obtained on post-AGB stars, both on the central stars and their circumstellar material. The fast evolutionary phase is characterized by a rapid change in the properties of the objects, but the variety is so large that there is yet no clear consensus on how the detailed studies of individual objects are linked together by evolutionary channels. The absence of strong molecular veiling in the photospheres of the central stars, together with a spread in intrinsic metallicity make post-AGB stars very useful in constraining AGB chemical evolutionary models. We discuss the surprisingly wide variety of chemical signatures observed. The onset in the creation process of the panoply of structures and shapes observed in planetary nebulae occurs during the short post-AGB evolution, but the physical nature of the processes involved is still badly understood. In the rapidly growing field of circumstellar mineralogy, post-AGB stars have their story to tell and also the molecular envelope changes significantly due to dilution and hardening of the stellar radiation. The real-time evolution of some objects suffering a late thermal flash is reviewed and their possible link to other hydrogen-deficient objects is discussed. Any review on stellar evolution has a section on binaries and this contribution is no exception because binaries make up a significant fraction of the post-AGB stars known to date.

TheWISEview of RV Tauri stars

“…Also the dust component in these two objects was presumed to be C-rich. Both assertions are, however, no longer supported by more recent works, like the photospheric abundance analysis of Van Winckel et al (1998) and the dust studies of , , , Gielen et al (2007Gielen et al ( , 2009 ;Hillen et al (2015). One exception may be V453 Oph, which shows a mild s-process overabundance but it is not accompanied by C enhancement (Deroo et al 2005).…”

“…Optically bright post-AGB stars show typically two different types of SED (see Fig. 2) (Van Winckel 2003). The first group of post-AGB stars display SEDs with a broad IR excess starting in the near-IR region, which points to the presence of hot dust near the star.…”

“…These studies show that RV Tauri stars are mainly not enriched in C and s-process elements (e.g. Giridhar et al 1994Giridhar et al , 1998Giridhar et al , 2000Gonzalez et al 1997a,b;Van Winckel et al 1998;Maas et al 2002Maas et al , 2003Maas et al , 2005Deroo et al 2005). Earlier work suggested, for instance, the two prototypical RV Tauri stars AC Her and RU Cen to be C-enriched, based on their high [C/Fe] (Gehrz & Woolf 1970;Gehrz & Ney 1972).…”

“…Depleted photospheres display a similar chemical composition as the gas phase of the interstellar medium: refractory elements, which have high dust condensation temperature, are underabundant, while volatiles, which have low condensation temperature, are more abundant (e.g. Van Winckel 2003). The process to acquire the chemical anomaly is not completely understood yet, but it must be a chemical rather than a nucleosynthetic process.…”

FUSE, STIS and Keck spectroscopic analysis of the UV-bright star vZ 1128 in M3 (NGC 5272)

“…Third dredge-up brings 12 C and sprocess isotopes produced by the burning of helium into the convective envelope. As a result, the atmospheric C/O ratio quickly exceeds unity (Van Winckel 2003). We expect vZ 1128 to exhibit the effects of first dredge-up, any extra mixing that took place on the RGB, and third dredge-up, if it occurred.…”