Molecular ions in the O-rich evolved star OH231.8+4.2: HCO+, H13CO+ and first detection of SO+, N2H+, and H3O+

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 (HCO+)-C-13 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 30 m radio telescope and with Herschel/HIFI. Except for H3O+, all the molecular ions detected in this work display emission lines with broad profiles (FWHM similar to 50-90 km s(-1)), which indicates that these ions are abundant in the fast bipolar outflow of OH 231.8. The narrow profile (FWHM similar to 14 km s(-1)) and high critical densities (>10(6) cm(-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 similar to 10-20 K for all ions except for H3O+, which is most consistent with T-ex approximate to 100 K. 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 approximate to [1-8] x 10(13) cm(-2), leading to beam-averaged fractional abundances relative to H-2 of X(HCO+) approximate to 10(-8), X((HCO+)-C-13) approximate to 2 x 10(-9), X(SO+) approximate to 4 x 10(-9), X(N2H+) approximate to 2 x 10(-9), and X(H3O+) approximate to 7 x 10(-9) cm(-2). We have performed chemical kinetics models to investigate the formation of these ions in OH 231.8 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 similar to[3-8] x 10(16) cm); H3O+ would also form quite abundantly in regions closer to the center (X(H3O+) similar to 10(-9) at similar to 10(16) cm). 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 (approximate to 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 OH 231. 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 similar to 800 yr ago.