2015A&A...577A..52S

OH231.8+4.2, a bipolar outflow around a Mira-type variable star, displays a unique molecular richness amongst circumstellar envelopes (CSEs) around O-rich AGB and post-AGB stars. We report line observations of the HCO⁺ and H¹³CO⁺ molecular ions and the first detection of SO⁺, N₂H⁺, and (tentatively) H₃O⁺ in this source. SO⁺ and H₃O⁺ have not been detected before in CSEs around evolved stars. These data have been obtained as part of a full mm-wave and far-IR spectral line survey carried out with the IRAM 30m radio telescope and with Herschel/HIFI. Except for H₃O⁺, all the molecular ions detected in this work display emission lines with broad profiles (FWHM∼50-90km/s), which indicates that these ions are abundant in the fast bipolar outflow of OH231.8. The narrow profile (FWHM∼14km/s) and high critical densities (>10⁶cm^–3) of the H₃O⁺ transitions observed are consistent with this ion arising from denser, inner (and presumably warmer) layers of the fossil remnant of the slow AGB CSE at the core of the nebula. From rotational diagram analysis, we deduce excitation temperatures of T_ex∼10-20K for all ions except for H₃O⁺, which is most consistent with T_ex≃100K. Although uncertain, the higher excitation temperature suspected for H₃O⁺ is similar to that recently found for H₂O and a few other molecules, which selectively trace a previously unidentified, warm nebular component. The column densities of the molecular ions reported here are in the range N_tot≃[1-8]x10¹³cm^–2, leading to beam-averaged fractional abundances relative to H₂ of X(HCO⁺)≃10^–8, X(H¹³CO⁺)≃2x10^–9, X(SO⁺)≃4x10^–9, X(N₂H⁺)≃2x10^–9, and X(H₃O⁺)≃7x10^–9cm^–2. We have performed chemical kinetics models to investigate the formation of these ions in OH231.8 as the result of standard gas phase reactions initiated by cosmic-ray and UV-photon ionization. The model predicts that HCO⁺, SO⁺, and H₃O⁺ can form with abundances comparable to the observed average values in the external layers of the slow central core (at ~[3-8]x10¹⁶cm); H₃O⁺ would also form quite abundantly in regions closer to the center (X(H₃O⁺)∼10^–9 at ∼10¹⁶cm). For N₂H⁺, the model abundance is lower than the observed value by more than two orders of magnitude. The model fails to reproduce the abundance enrichment of HCO⁺, SO⁺, and N₂H⁺ in the lobes, which is directly inferred from the broad emission profiles of these ions. Also, in disagreement with the narrow H₃O⁺ spectra, the model predicts that this ion should form in relatively large, detectable amounts (≃10^–9) in the external layers of the slow central core and in the high-velocity lobes. Some of the model-data discrepancies are reduced, but not suppressed, by lowering the water content and enhancing the elemental nitrogen abundance in the envelope. The remarkable chemistry of OH231.8 probably reflects the molecular regeneration process within its envelope after the passage of fast shocks that accelerated and dissociated molecules in the AGB wind ∼800yr ago.