2025A&A...698A.319A

Aims. Our goal is to characterise the chemistry and physical conditions of the circumstellar envelopes (CSEs) of Asymptotic Giant Branch (AGB) binary candidate stars with UV-excess and X-ray emission. In particular, our aim is to identify the effects of the internal X-ray emission in the abundance of certain key molecules.
Methods. We observed the 86.0-94.0 and 260.0-272.5 GHz spectral ranges and searched for rotational transitions of the X-ray sensitive molecule HCO⁺ in four AGB stars. Two detected in both UV and X-ray emission, and the other two detected only in UV. We derived the CSEs's physical parameters from previous CO observations and determined the molecular abundances of the detected species using radiative transfer models. We developed chemical kinetics models that account for the effects of internal X-ray emission (as well as UV radiation) and compared our predictions with observations.
Results. We report the detection of HCO⁺ in the X-ray emitting C-rich AGB T Dra, while it remains undetected in the spectra of the other three sources. In T Dra we also detected SiO, HCN, HNC, HC₃ N, SiC₂, C₂H, and SiS. For the other targets, only HCN and SiO were detected. The high fractional abundance of HCO⁺ derived for T Dra ([1.5-3.0] × 10^–8) is in good agreement with the predictions from our chemical kinetics models including the effects of internal X-ray emission, and one order of magnitude higher than the values expected for C-rich AGB stars. Additionally, we identified abundance enhancements for HNC and HC₃N alongside a depletion of CO in the innermost regions of T Dra's envelope.
Conclusions. An internal X-ray source can significantly alter molecular abundances in AGB CSEs, and enhance HCO⁺, N₂H⁺, HNC, and HC₃N while depleting parent species like CO. The UV radiation has a weaker effect unless the envelope is optically thin or porous.