Javier Alcolea
(Observatorio Astronómico Nacional (IGN/CNIG, Spain))
Stellar evolution and nucleosynthesis models are fundamental tools to derive parameters of stars from elemental atomic and isotopic abundance ratios. For evolved stars, the C/O/N elemental ratios and the C/C and O/O/O isotopic ratios provide very significant constraints to the otherwise elusive initial mass of the stars. In the case of post-AGB sources, the chemical composition of their expanding circumstellar envelopes can be derived from sensitive observations of rotational lines of several molecular species in the millimetre and sub-millimetre radio domains. The initial mass of post-AGB sources is a fundamental parameter to establish correlations with the main properties of their post-AGB envelopes (mass, momentum, kinetic energy, shape) and progress in our knowledge of their formation and evolution. In these sources, the C/O ratio can be easily constrained from the detection of C-bearing and O-bearing molecules other than CO, while the O/O ratio is also straightforwardly determined from the relative strength of the optically thin rotational lines of CO and CO. However, the results obtained up to date are far from being clear.
In this presentation, we will review the status of the question, including new accurate O/O ratio measurements for 13 targets, totalling 25 studied post-AGB envelopes: 15 O-rich sources (including the eight water fountains presented by Khouri et al. 2021) and 10 C-rich ones. Comparing the O/O ratios and the C-rich/O-rich chemical composition with models for sources that have completed the AGB evolution, we find that for about 50% of the cases, observational data align with model predictions: these canonical sources include 50% of both O-rich and C-rich sources in the sample. As for the non-canonical sources, the O-rich ones, which present O/O ratios above those expected for C/O < 1, can be explained by a premature interruption of their AGB evolution as a consequence of a quasi-explosive ejection of a large fraction of the initial mass. This hypothesis agrees with the suggestion that these envelopes form in the merging of common-envelope (or similar) events. The non-canonical C-rich sources, on the contrary, display O/O ratios below the predictions for C/O > 1. We discuss possible explanations for this enigmatic behaviour, including the possibility that these sources are extrinsic C-rich stars, whose high C abundances result from previous mass accretion from a former higher mass C-rich AGB donor companion.
Javier Alcolea
(Observatorio Astronómico Nacional (IGN/CNIG, Spain))
Elisa Masa Andrés
(Observatorio Astronómico Nacional (OAN-IGN, Spain))
Theo Khouri
(Chalmers University of Technology)
Miguel Santander-García
(Observatorio Astronómico Nacional (IGN/CNIG, Spain))
Dr
Iván Gallardo Cava
(Observatorio Astronómico Nacional (IGN/CNIG, Spain))
Hans Olofsson
(Space, Earth and Environment, Chalmers Univ. of Tech., Sweden)
Carmen Sanchez Contreras
(Centro de Astrobiología (CSIC-INTA, Spain))
Valentín Bujarrabal
(Observatorio Astronómico Nacional (IGN/CNIG, Spain))
Wouter Vlemmings
(Chalmers University of Technology)
Dr
Daniel Tafoya
(Chalmers University of Technology)
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