Octavio Roncero

Context . The Mg-bearing cations MgC ₄ H ⁺ , MgC ₆ H ⁺ , MgC ₃ N ⁺ , and MgC ₅ N ⁺ have been recently observed in the carbon-rich envelope IRC + 10216. These species are thought to form upon radiative association between Mg ⁺ and the corresponding neutral radical. Aims . This work aims to determine the radiative association rate coefficients for the cations Al ⁺ , Mg ⁺ , and Na ⁺ reacting with carbon chain radicals to estimate the relative importance of metal-bearing carbon cations for each of these metals. Methods . For this purpose we used statistical methods in combination with highly accurate ab initio calculations to obtain the radiative association rate coefficient. Moreover, anisotropic effects beyond the mono-dimensional capture model were considered, providing a more reliable description of the simulated processes. Results . The radiative association rate coefficients to form Mg-, Al- and Na-C ₆ H ⁺ , as well as MgC ₄ H ⁺ , MgC ₃ N ⁺ , and MgC ₅ N ⁺ , were calculated with the new restrictions considered in this work. These new rate coefficients are included in a chemical model of the C-rich AGB envelope IRC + 10216. Conclusions . The abundances of MgC ₄ H ⁺ , MgC ₆ H ⁺ , and MgC ₅ N ⁺ are consistent with observations, with differences no greater than one order of magnitude, corroborating radiative association as a plausible formation mechanism for these adducts. However, the calculated abundance of MgC ₃ N ⁺ is about two orders of magnitude lower than observed, which indicates that either the chemistry of this species is significantly different from the others or the calculated rate coefficient is too low. The abundances obtained for Na-C ₆ H ⁺ and Al-C ₆ H ⁺ are around two orders of magnitude lower than for Mg, making very difficult to detect the analogous metal-bearing cations in IRC + 10216.

Minimum energy paths of the eight lowest diabatic (bottom) and adiabatic (top) A′ and A″ electronic states of CH ₂ ⁺ for non-adiabatic dynamics.

Cyclopentadiene (c-C ₅ H ₆ ) is considered a key molecule in the formation of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). The synthesis of PAHs from simpler precursors is known as the “bottom-up” theory, which, so far, has been dominated by reactions between organic radicals. However, this mechanism struggles to account for the origin of the smallest cycles themselves. However, it struggles to account for the origin of the smallest cycles themselves. Ion-molecule reactions emerge as promising alternative pathways to explain the formation of these molecules. We investigated the reaction network of the main ionic precursor of cyclopentadiene, c-C ₅ H ₇ ⁺ . To this end, we established an integrated protocol that combines astrochemical modelling to identify viable formation routes under cold ISM conditions, automated reaction path searches, and kinetic simulations to obtain accurate descriptions of the reaction pathways and reliable rate constants. In particular, we examined the reaction between ethylene (C ₂ H ₄ ) and the linear propargyl cation (CH ₂ CCH ⁺ ). Our results reveal that the formation of c-C ₅ H ₇ ⁺ by radiative association is inefficient, contrary to our initial expectations. Instead, the system predominantly evolves through bimolecular channels yielding c-C ₅ H ₅ ⁺ and CH ₃ CCH ₂ ⁺ , with the formation of c-C ₅ H ₅ ⁺ ; this offers new insights into the reactivity that supports molecular growth in the ISM.

The likely JWST detection of vibrationally excited H ₃ ⁺ emission in Orion’s irradiated disk system d203-506 raises the important question of whether cosmic-ray ionization is enhanced in disks within clustered star-forming regions, or whether alternative mechanisms contribute to H ₃ ⁺ formation and excitation. We present a detailed model of the photodissociation region (PDR) component of a protoplanetary disk – comprising the outer disk surface and the photoevaporative wind – exposed to strong external far-ultraviolet (FUV) radiation. We investigate key gas-phase reactions involving excited H ₂ that lead to the formation of H ₃ ⁺ in the PDR, including detailed state-to-state dynamical calculations of reactions H ₂ ( v ≥ 0) + HOC ⁺ → H ₃ ⁺ + CO and H ₂ ( v ≥ 0) + H ⁺ → H ₂ ⁺ + H. We also consider the effects of photoionization of vibrationally excited H ₂ ( v ≥ 4), a process not previously included in PDR or disk models. We find that these FUV-driven reactions dominate the formation of H ₃ ⁺ in the PDR of strongly irradiated disks, largely independently of cosmic-ray ionization. The predicted H ₃ ⁺ abundance in the disk PDR peaks at x (H ₃ ⁺ ) ≳ 10 ⁻⁸ , coinciding with regions of enhanced HOC ⁺ and water vapor abundances, and is linked to the strength of the external FUV field ( G ₀ ). The predicted H ₃ ⁺ column density (≲10 ¹³ cm ⁻² ) agrees with the presence of H ₃ ⁺ in the PDR of d203-506. We also find that formation pumping, resulting from exoergic reactions between excited H ₂ and HOC ⁺ , drives the vibrational excitation of H ₃ ⁺ in these regions. We expect this photochemistry to be highly active in disks where G ₀ > 10 ³ . The H ₃ ⁺ formation pathways studied here may also be relevant in the inner disk region (near the host star), in exoplanetary ionospheres, and in the early Universe.

Most protoplanetary disks experience a phase in which they are subjected to strong ultraviolet radiation from nearby massive stars. This UV radiation can substantially alter their chemistry by producing numerous radicals and molecular ions. In this Letter we present a detailed analysis of the JWST-NIRSpec spectrum of the d203-506 obtained as part of the PDRs4All Early Release Science program. Using state-of-the-art spectroscopic data, we searched for species using a multi-molecule fitting tool, PAHTATMOL, which we developed for this purpose. Based on this analysis, we report the clear detection of ro-vibrational emission of the CH radical and the likely detection of the H₃⁺ molecular ion, with estimated abundances of a few times 10⁻⁷ and approximately 10⁻⁸, respectively. The presence of CH is predicted by gas-phase models and is well explained by hydrocarbon photochemistry. Interstellar H₃⁺ is usually formed through reactions of H₂ with H₃⁺ originating from cosmic ray ionization of H₂. However, recent theoretical studies suggest that H₃⁺ also forms through far-UV (FUV)-driven chemistry in strongly irradiated (G₀ > 10³), dense (n_H > 10⁶ cm⁻³) gas. The latter is favored as an explanation for the presence of hot H₃⁺ (T_ex ≳ 1000 K) in the outer disk layers of d203-506, coinciding with the emission of FUV-pumped H₂ and other photodissociation region (PDR) species, such as CH⁺, CH₃⁺, and OH. Our detection of infrared emission from vibrationally excited H₃⁺ and CH raises questions about their excitation mechanisms and underscores that FUV radiation can have a profound impact on the chemistry of planet-forming disks. They also demonstrate the power of JWST to push the limit of the detection of elusive species in protoplanetary disks.

Carbon hydrides play a crucial role in the formation of complex organic molecules in highly UV illuminated regions of the interstellar medium (ISM).

Context. In diffuse interstellar clouds, the excitation temperature derived from the lowest levels of H₃⁺ is systematically lower than that derived from H₂. The differences may be attributed to the lack of state-specific formation and destruction rates of H₃⁺, which are needed to thermalize the two species. Aims. In this work, we aim to investigate the possible influence of rotational excitation collisions of H₃⁺ with atomic hydrogen on its excitation temperature. Methods. We used a time-independent close-coupling method to calculate the state-to-state rate coefficients, incorporating a very accurate and full-dimensional potential energy surface recently developed for H₄⁺. We take a symmetric top approach to describe a frozen H₃⁺ as an equilateral triangle. Results. We derive rotational excitation collision rate coefficients of H₃⁺ with atomic hydrogen in a temperature range corresponding to diffuse interstellar conditions up to (J, K, ±) = (7, 6, +) and (J, K, ±) = (6, 4, +) for its ortho and para forms. This allows us to obtain a consistent set of collisional excitation rate coefficients and to improve on a previous study that included speculations regarding these contributions. Conclusions. The new state-specific inelastic H₃⁺ + H rate coefficients yield differences of up to 20% in the excitation temperature, and their impact increases with decreasing molecular fraction. We also confirm the impact of chemical state-to-state destruction reactions on the excitation balance of H₃⁺, and that reactive H + H₃⁺ collisions are also needed to account for possible further ortho to para transitions.

We present MADWAVE3, a FORTRAN90 code designed for quantum time dependent wave packet propagation in triatomic systems. This program allows the calculation of state-to-state probabilities for inelastic and reactive collisions, as well as photodissociation processes, over one or multiple coupled diabatic electronic states. The code is highly parallelized using MPI and OpenMP. The execution requires the potential energy surfaces of the different electronic states involved, as well as the transition dipole moments for photodissociation processes. The formalism underlying the code is presented in section 2, together with the modular structure of the code. This is followed by the installation procedures and a comprehensive list and explanation of the parameters that control the code, organized within their respective namelists. Finally, a case study is presented, focusing on the prototypical reactive collision H+DH(v,j) -> H2(v',j') + D. Both the potential energy surface and the input files required to reproduce the calculation are provided and are available on the repository main page. This example is used to study the parallelization speedup of the code.