SIMBAD references

The oxygen-bearing molecular ions OH⁺, H₂O⁺, and H₃O⁺ are key species that probe the ionization rate of (partially) molecular gas that is ionized by X-rays and cosmic-rays permeating the interstellar medium. We report Herschel far-infrared and submillimeter spectroscopic observations of OH⁺ in Mrk 231, showing both ground-state P-Cygni profiles, and excited line profiles with blueshifted absorption wings extending up to ≃1000 km s^–1. In addition, OH⁺ probes an excited component peaking at central velocities, likely arising from the torus probed by the OH centimeter-wave megamaser. Four lines of H₂O⁺ are also detected at systemic velocities, but H₃O⁺ is undetected. Based on our earlier OH studies, we estimate an abundance ratio of OH/OH⁺∼5–10 for the outflowing components and ≃20 for the torus, and an OH⁺ abundance relative to H nuclei of >=10^–7. We also find high OH⁺/H₂O⁺ and OH⁺/H₃O⁺ ratios; both are >=4 in the torus and >=10-20 in the outflowing gas components. Chemical models indicate that these high OH⁺ abundances relative to OH, H₂O⁺, and H₃O⁺ are characteristic of gas with a high ionization rate per unit density, ζ/n_H∼(1–5)×10^–17 cm³ s^–1 and ∼(1-2) x 10^–16 cm³ s^–1 for the above components, respectively, an ionization rate of ζ ∼ (0.5-2) x 10^–12 s^–1, and a low molecular fraction, f_H2_∼0.25. X-rays appear to be unable to explain the inferred ionization rate, and thus we suggest that low-energy (10-400 MeV) cosmic-rays are primarily responsible for the ionization, with {dot}M_CR∼0.01M_☉ yr^–1 and {dot}E_CR∼10⁴⁴ erg s^–1; the latter corresponds to ∼1% of the luminosity of the active galactic nucleus and is similar to the energetics of the molecular outflow. We suggest that cosmic-rays accelerated in the forward shock associated with the molecular outflow are responsible for the ionization, as they diffuse through the outflowing molecular phase downstream.