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OH as a probe of the warm water cycle in planet-forming disks
Authors:
Marion Zannese,
Benoît Tabone,
Emilie Habart,
Javier R. Goicoechea,
Alexandre Zanchet,
Ewine F. van Dishoeck,
Marc C. van Hemert,
John H. Black,
Alexander G. G. M. Tielens,
A. Veselinova,
P. G. Jambrina,
M. Menendez,
E. Verdasco,
F. J. Aoiz,
L. Gonzalez-Sanchez,
Boris Trahin,
Emmanuel Dartois,
Olivier Berné,
Els Peeters,
Jinhua He,
Ameek Sidhu,
Ryan Chown,
Ilane Schroetter,
Dries Van De Putte,
Amélie Canin
, et al. (30 additional authors not shown)
Abstract:
Water is a key ingredient for the emergence of life as we know it. Yet, its destruction and reformation in space remains unprobed in warm gas. Here, we detect the hydroxyl radical (OH) emission from a planet-forming disk exposed to external far-ultraviolet (FUV) radiation with the James Webb Space Telescope. The observations are confronted with the results of quantum dynamical calculations. The hi…
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Water is a key ingredient for the emergence of life as we know it. Yet, its destruction and reformation in space remains unprobed in warm gas. Here, we detect the hydroxyl radical (OH) emission from a planet-forming disk exposed to external far-ultraviolet (FUV) radiation with the James Webb Space Telescope. The observations are confronted with the results of quantum dynamical calculations. The highly excited OH infrared rotational lines are the tell-tale signs of H2O destruction by FUV. The OH infrared ro-vibrational lines are attributed to chemical excitation via the key reaction O+H=OH+H which seeds the formation of water in the gas-phase. We infer that the equivalent of the Earth ocean's worth of water is destroyed per month and replenished. These results show that under warm and irradiated conditions water is destroyed and efficiently reformed via gas-phase reactions. This process, assisted by diffusive transport, could reduce the HDO/H2O ratio in the warm regions of planet-forming disks.
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Submitted 22 December, 2023; v1 submitted 21 December, 2023;
originally announced December 2023.
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Quantum study of reaction O(3P) + H2(v,j) $\rightarrow$ OH + H: OH formation in strongly UV-irradiated gas
Authors:
A. Veselinova,
M. Agundez,
J. R. Goicoechea,
M. Menendez,
A. Zanchet,
E. Verdasco,
P. G. Jambrina,
F. J. Aoiz
Abstract:
The reaction between atomic oxygen and molecular hydrogen is an important one in astrochemistry as it regulates the abundance of the hydroxyl radical and serves to open the chemistry of oxygen in diverse astronomical environments. However, the existence of a high activation barrier in the reaction with ground state oxygen atoms limits its efficiency in cold gas. In this study we calculate the depe…
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The reaction between atomic oxygen and molecular hydrogen is an important one in astrochemistry as it regulates the abundance of the hydroxyl radical and serves to open the chemistry of oxygen in diverse astronomical environments. However, the existence of a high activation barrier in the reaction with ground state oxygen atoms limits its efficiency in cold gas. In this study we calculate the dependence of the reaction rate coefficient on the rotational and vibrational state of H$_2$ and evaluate the impact on the abundance of OH in interstellar regions strongly irradiated by far-UV photons, where H2 can be efficiently pumped to excited vibrational states. We use a recently calculated potential energy surface and carry out time-independent quantum mechanical scattering calculations to compute rate coefficients for the reaction O(3P) + H2(v,j) -> OH + H, with H2 in vibrational states v = 0-7 and rotational states j = 0-10. We find that the reaction becomes significantly faster with increasing vibrational quantum number of H2, although even for high vibrational states of H2 (v = 4-5) for which the reaction is barrierless, the rate coefficient does not strictly attain the collision limit and still maintains a positive dependence with temperature. We implemented the calculated state-specific rate coefficients in the Meudon PDR code to model the Orion Bar PDR and evaluate the impact on the abundance of the OH radical. We find the fractional abundance of OH is enhanced by up to one order of magnitude in regions of the cloud corresponding to Av = 1.3-2.3, compared to the use of a thermal rate coefficient for O + H2, although the impact on the column density of OH is modest, of about 60 %. The calculated rate coefficients will be useful to model and interpret JWST observations of OH in strongly UV-illuminated environments.
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Submitted 5 March, 2021;
originally announced March 2021.
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Hyperfine excitation of SH$^+$ by H
Authors:
François Lique,
Alexandre Zanchet,
Niyazi Bulut,
Javier R. Goicoechea,
Octavio Roncero
Abstract:
SH$^+$ is a surprisingly widespread molecular ion in diffuse interstellar clouds. There, it plays an important role triggering the sulfur chemistry. In addition, SH$^+$ emission lines have been detected at the UV-illuminated edges of dense molecular clouds, \mbox{so-called} photo-dissociation regions (PDRs), and toward high-mass protostars. An accurate determination of the SH$^+$ abundance and of…
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SH$^+$ is a surprisingly widespread molecular ion in diffuse interstellar clouds. There, it plays an important role triggering the sulfur chemistry. In addition, SH$^+$ emission lines have been detected at the UV-illuminated edges of dense molecular clouds, \mbox{so-called} photo-dissociation regions (PDRs), and toward high-mass protostars. An accurate determination of the SH$^+$ abundance and of the physical conditions prevailing in these energetic environments relies on knowing the rate coefficients of inelastic collisions between SH$^+$ molecules and hydrogen atoms, hydrogen molecules, and electrons. In this paper, we derive SH$^+$--H fine and hyperfine-resolved rate coefficients from the recent quantum calculations for the SH$^+$--H collisions, including inelastic, exchange and reactive processes. The method used is based on the infinite order sudden approach. State-to-state rate coefficients between the first 31 fine levels and 61 hyperfine levels of SH$^+$ were obtained for temperatures ranging from 10 to 1000 K. Fine structure-resolved rate coefficients present a strong propensity rule in favour of $Δj = ΔN$ transitions. The $Δj = ΔF$ propensity rule is observed for the hyperfine transitions. {The new rate coefficients will help significantly in the interpretation of SH$^+$ spectra from PDRs and UV-irradiated shocks where the abundance of hydrogen atoms with respect to hydrogen molecules can be significant.
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Submitted 26 April, 2020;
originally announced April 2020.
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New Global Potential Energy Surfaces of the ground 3A' and 3A' states of the H2O system
Authors:
Alexandre Zanchet,
Marta Menéndez,
Pablo G. Jambrina,
F. Javier Aoiz
Abstract:
This paper presents two new adiabatic, global potential energy surfaces (PESs) for the two lowest $^3A'$ and $^3A''$ electronic states of the O($^3P$)+H$_2$ system. For each of these states, ab initio electronic energies were calculated for more than 5000 geometries using internally contracted multireference configuration interaction methods. The calculated points were then fitted using the ansatz…
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This paper presents two new adiabatic, global potential energy surfaces (PESs) for the two lowest $^3A'$ and $^3A''$ electronic states of the O($^3P$)+H$_2$ system. For each of these states, ab initio electronic energies were calculated for more than 5000 geometries using internally contracted multireference configuration interaction methods. The calculated points were then fitted using the ansatz by Aguado et al. [Comput. Phys. Commun. 108, 259 (1998)] leading to very accurate analytical potentials well adapted to perform reaction dynamics studies. Overall, the topographies of both PESs are in good agreement with the benchmark potentials of Rogers et al., but those presented in this work reproduce better the height and degeneracy of the two states at the saddle point. Moreover, the long range potential in the entrance channel does not require any cutoff. These features makes the new PESs particularly suitable for a comparison of the dynamics on each of them. The new set of PESs were then used to perform quantum mechanics and quasiclassical trajectory calculations to determine integral and differential cross sections, which are compared to the experimental measurements by Garton et al. [J. Chem. Phys., 118, 1585 (2003)].
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Submitted 30 May, 2019;
originally announced June 2019.
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Formation of interstellar SH$^+$ from vibrationally excited H$_2$: Quantum study of S$^+$ + H$_2$ $\rightleftarrows$ SH$^+$ + H reactions and inelastic collisions
Authors:
Alexandre Zanchet,
Francois Lique,
Octavio Roncero,
Javier R. Goicoechea,
Niyazi Bulut
Abstract:
The rate constants for the formation, destruction, and collisional excitation of SH$^+$ are calculated from quantum mechanical approaches using two new SH$_2^+$ potential energy surfaces (PESs) of $^4A''$ and $^2A''$ electronic symmetry. The PESs were developed to describe all adiabatic states correlating to the SH$^+$ ($^3Σ^-$) + H($^2S$) channel. The formation of SH$^+$ through the S$^+$ + H…
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The rate constants for the formation, destruction, and collisional excitation of SH$^+$ are calculated from quantum mechanical approaches using two new SH$_2^+$ potential energy surfaces (PESs) of $^4A''$ and $^2A''$ electronic symmetry. The PESs were developed to describe all adiabatic states correlating to the SH$^+$ ($^3Σ^-$) + H($^2S$) channel. The formation of SH$^+$ through the S$^+$ + H$_2$ reaction is endothermic by $\approx$ 9860 K, and requires at least two vibrational quanta on the H$_2$ molecule to yield significant reactivity. Quasi-classical calculations of the total formation rate constant for H$_2$($v=2$) are in very good agreement with the quantum results above 100K. Further quasi-classical calculations are then performed for $v=3$, 4, and 5 to cover all vibrationally excited H$_2$ levels significantly populated in dense photodissociation regions (PDR). The new calculated formation and destruction rate constants are two to six times larger than the previous ones and have been introduced in the Meudon PDR code to simulate the physical and illuminating conditions in the Orion bar prototypical PDR. New astrochemical models based on the new molecular data produce four times larger SH$^+$ column densities, in agreement with those inferred from recent ALMA observations of the Orion bar.
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Submitted 7 May, 2019;
originally announced May 2019.
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How reactant polarization can be used to change the effect of interference on reactive collisions
Authors:
P. G. Jambrina,
M. Menendez,
A. Zanchet,
E. Garcia,
F. J. Aoiz
Abstract:
It is common knowledge that integral and differential cross sections (DCSs) are strongly dependent on the spatial distribution of the molecular axis of the reactants. Hence, by controlling the axis distribution, it is possible to either promote or hinder the yield of products into specific final states or scattering angles. This idea has been successfully implemented in experiments by polarizing t…
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It is common knowledge that integral and differential cross sections (DCSs) are strongly dependent on the spatial distribution of the molecular axis of the reactants. Hence, by controlling the axis distribution, it is possible to either promote or hinder the yield of products into specific final states or scattering angles. This idea has been successfully implemented in experiments by polarizing the internuclear axis before the reaction takes place, either by manipulating the rotational angular distribution or by Stark effect in the presence of an orienting field. When there is a dominant reaction mechanism, characterized by a set of impact parameters and angles of attack, it is expected that a preparation that helps the system to reach the transition state associated with that mechanism will promote the reaction, whilst a different preparation would generally impair the reaction. However, when two or more competing mechanisms via interference contribute to the reaction into specific scattering angles and final states, it is not evident which would be the effect of changing the axis preparation. To address this problem, throughout this article we have simulated the effect that different experimental preparations have on the DCSs for the H + D2 reaction at relatively high energies, for which it has been shown that several competing mechanisms give rise to interference that shapes the DCS. To this aim, we have extended the formulation of the polarization dependent DCS to calculate polarization dependent generalized deflection functions of ranks greater than zero. Our results show that interference are very sensitive to changes in the internuclear axis preparation, and that the shape of the DCS can be controlled exquisitely.
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Submitted 7 November, 2018;
originally announced November 2018.
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Formation and destruction of SiS in space
Authors:
Alexandre Zanchet,
Octavio Roncero,
Marcelino Agúndez,
José Cernicharo
Abstract:
The presence of SiS in space seems to be restricted to a few selected types of astronomical environments.
It is long known to be present in circumstellar envelopes around evolved stars and it has also been detected in a handful of star-forming regions with evidence of outflows, like Sgr\,B2, Orion\,KL and more recently L1157-B1.
The kinetics of reactions involving SiS is very poorly known and…
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The presence of SiS in space seems to be restricted to a few selected types of astronomical environments.
It is long known to be present in circumstellar envelopes around evolved stars and it has also been detected in a handful of star-forming regions with evidence of outflows, like Sgr\,B2, Orion\,KL and more recently L1157-B1.
The kinetics of reactions involving SiS is very poorly known and here we revisit the chemistry of SiS in space by studying some potentially important reactions of formation and destruction of this molecule. We calculated {\it ab initio} potential energy surfaces of the SiOS system and computed rate coefficients in the temperature range 50-2500 K for the reaction of destruction of SiS, in collisions with atomic O, and of its formation, through the reaction between Si and SO. We find that both reactions are rapid, with rate coefficients of a few times 10$^{-10}$ cm$^3$ s$^{-1}$, almost independent of temperature.
In the reaction between Si and SO, SiO production is 5-7 times more efficient than SiS formation.
The reaction of SiS with O atoms can play an important role in destroying SiS in envelopes around evolved stars. We built a simple chemical model of a postshock gas to study the chemistry of SiS in protostellar outflows and we found that SiS forms with a lower abundance and later than SiO, that SiS is efficiently destroyed through reaction with O, and that the main SiS-forming reactions are Si + SO and Si + SO$_2$.
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Submitted 4 July, 2018;
originally announced July 2018.
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State-to-state chemistry and rotational excitation of CH$^+$ in photon-dominated regions
Authors:
A. Faure,
P. Halvick,
T. Stoecklin,
P. Honvault,
M. D. Epée Epée,
J. Zs. Mezei,
O. Motapon,
I. F. Schneider,
J. Tennyson,
O. Roncero,
N. Bulut,
A. Zanchet
Abstract:
We present a detailed theoretical study of the rotational excitation of CH$^+$ due to reactive and nonreactive collisions involving C$^+(^2P)$, H$_2$, CH$^+$, H and free electrons. Specifically, the formation of CH$^+$ proceeds through the reaction between C$^+(^2P)$ and H$_2(ν_{\rm H_2}=1, 2)$, while the collisional (de)excitation and destruction of CH$^+$ is due to collisions with hydrogen atoms…
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We present a detailed theoretical study of the rotational excitation of CH$^+$ due to reactive and nonreactive collisions involving C$^+(^2P)$, H$_2$, CH$^+$, H and free electrons. Specifically, the formation of CH$^+$ proceeds through the reaction between C$^+(^2P)$ and H$_2(ν_{\rm H_2}=1, 2)$, while the collisional (de)excitation and destruction of CH$^+$ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10-3000~K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH$^+$ with H atoms at kinetic temperatures below 50~K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our Non-LTE calculations confirm that the formation pumping due to vibrationally excited H$_2$ has a substantial effect on the excitation of CH$^+$ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH$^+$ toward the Orion Bar and the planetary nebula NGC~7027. Our results further suggest that the population of $ν_{\rm H_2}=2$ might be significant in the photon-dominated region of NGC~7027.
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Submitted 7 April, 2017;
originally announced April 2017.
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Product lambda-doublet ratios for the O(3P) + D2 reaction: A mechanistic imprint
Authors:
P. G. Jambrina,
A. Zanchet,
J. Aldegunde,
M. Brouard,
F. J. Aoiz
Abstract:
In the last decade, the development of theoretical methods have allowed chemists to reproduce and explain almost all of the experimental data associated with elementary atom plus diatom collisions. However, there are still a few examples where theory cannot account yet for experimental results. This is the case for the preferential population of one of the $Λ$-doublet states produced by chemical r…
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In the last decade, the development of theoretical methods have allowed chemists to reproduce and explain almost all of the experimental data associated with elementary atom plus diatom collisions. However, there are still a few examples where theory cannot account yet for experimental results. This is the case for the preferential population of one of the $Λ$-doublet states produced by chemical reactions. In particular, recent measurements of the OD($^2Π$) product of the O($^3$P) + D$_2$ reaction have shown a clear preference for the $Π(A')$ $Λ$-doublet states, in apparent contradiction with {\em ab initio} calculations, which predict a larger reactivity on the $A"$ potential energy surface. Here we present a method to calculate the $Λ$-doublet ratio when concurrent potential energy surfaces participate in the reaction. It accounts for the experimental $Λ$-doublet populations via explicit consideration of the stereodynamics of the process. Furthermore, our results demonstrate that the propensity of the $Π(A')$ state is a consequence of the different mechanisms of the reaction on the two concurrent potential energy surfaces.
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Submitted 12 August, 2016;
originally announced August 2016.
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Electron transport signature of H$_2$ dissociation on atomic gold wires
Authors:
Alexandre Zanchet,
Anaís Dorta-Urra,
Octavio Roncero,
Alfredo Aguado,
José Ignacio Martínez,
Fernando Flores,
Nicolás Lorente
Abstract:
Non-equilibrium Green's functions calculations based on density functional theory show a direct link between the initial stages of H$_2$ dissociation on a gold atomic wire and the electronic current supported by the gold wire. The simulations reveal that for biases below the stability threshold of the wire, the minimum-energy path for H$_2$ dissociation is not affected. However, the electronic cur…
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Non-equilibrium Green's functions calculations based on density functional theory show a direct link between the initial stages of H$_2$ dissociation on a gold atomic wire and the electronic current supported by the gold wire. The simulations reveal that for biases below the stability threshold of the wire, the minimum-energy path for H$_2$ dissociation is not affected. However, the electronic current presents a dramatic drop when the molecule initiates its dissociation. This current drop is traced back to quantum interference between electron paths when the molecule starts interacting with the gold wire.
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Submitted 26 December, 2013;
originally announced December 2013.