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

Abstract

ABSTRACT+ 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 ∼ 10−20 K for all ions except for H 3 O + , which is most consistent with T ex ≈ 100 K. Although uncertain, the higher excitation temperature suspected for H 3 O + is similar to that recently found for H 2 O 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] × 10 13 cm −2 , leading to beam-averaged fractional abundances relative to H 2 of X(HCO + ) ≈ 10 −8 , X(H 13 CO + ) ≈ 2 × 10 −9 , X(SO + ) ≈ 4 × 10 −9 , X(N 2 H + ) ≈ 2 × 10 −9 , and X(H 3 O + ) ≈ 7 × 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 H 3 O + can form with abundances comparable to the observed average values in the external layers of the slow central core (at ∼[3−8] × 10 16 cm); H 3 O + would also form quite abundantly in regions closer to the center (X(H 3 O + ) ∼ 10 −9 at ∼10 16 cm). For N 2 H + , 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 N 2 H + in the lobes, which is directly inferred from the broad emission profiles of these ions. Also, in disagreement with the narrow H 3 O + 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 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 ∼800 yr ago.

The millimeter IRAM-30 m line survey toward IK Tauri

“…The observations presented in this paper correspond to a sensitive millimeter-wavelength (~79-356 GHz) survey carried out with the IRAM-30 m telescope toward the CSEs of two O-rich evolved stars, IK Tau and OH231.8+4.2, during several observing runs from 2009 to 2014. Partial results from the survey toward OH231.8+4.2 are reported in Sánchez Contreras et al (2014, 2015) and Velilla Prieto et al (2013, 2015).…”

“…Cernicharo et al, 2000, Smith et al, 2015). However, the number of studies in the millimeter wavelength range has increased in the last years, and these studies have evidenced that O-rich CSEs do also host a rich variety of molecules (Ziurys et al, 2007, Kim et al, 2010, De Beck et al, 2013, Sánchez Contreras et al, 2015, and references therein).…”

Aλ3 mm and 1 mm line survey toward the yellow hypergiant IRC +10420

“…The formation of these two cations is strongly coupled to the cosmic-ray ionization rate of H 2 ( ζ ) via the cation H3+. Chemical modeling calculations similar to those carried out by Quintana-Lacaci et al (2013) and Sánchez Contreras et al (2015) indicate that at the low edge of the range of values of ζ in the Galaxy (10 −17 –10 −15 s −1 ; Dalgarno 2006), the formation of HCO + in O-rich envelopes is dominated by the reactions CO+ + H2 → HCO++H,C+ + H2O → HCO+ + H, where CO + is formed by the reaction between C + and OH. At high values of ζ , HCO + is mainly formed by the reaction H3+ + CO → HCO+ + H2, where H3+ is formed in the reaction between H 2 and H2+, where the latter is produced by the cosmic-ray ionization of H 2 .…”

Circumstellar ammonia in oxygen-rich evolved stars

“…Lyubimkov et al 2011). However, chemical models assuming a high 14 N-enrichment by a factor of 40 in the pre-planetary nebula OH 231.8+4.2 do not seem to reproduce the observed abundances of N-bearing molecules Sánchez Contreras et al 2015).…”

“…These O-rich sources towards which Nbearing molecules other than HCN have been detected include OH 231.8+4.2 Sánchez Contreras et al 2015), IRC +10420 (Quintana- Lacaci et al 2013Lacaci et al , 2016, and IK Tau (Velilla Prieto et al 2017). This may suggest that there could be a general enhancement of the production of 14 N from stellar nucleosynthesis (e.g.…”