arXiv subjects

J. Tennyson

Publications and source records attributed to J. Tennyson.

At least 37 records · Page 2Linked to original sources

arXiv2020

ExoMol line lists -- XXXVIII. High-temperature molecular line list of silicon dioxide (SiO$_{\mathbf{2}}$)

Silicon dioxide (SiO$_2$) is expected to occur in the atmospheres of hot rocky super-Earth exoplanets but a lack of spectroscopic data is hampering its possible detection. Here, we present the first, comprehensive molecular line list for SiO$_2$. The line list, named OYT3, covers the wavenumber range 0\,--\,6000~cm$^{-1}$ (wavelengths $λ> 1.67$~$μ$m) and is suitable for temperatures up to $T=3000$~K. Almost 33 billion transitions involving 5.69 million rotation-vibration states with rotational excitation up to $J=255$ have been computed using robust first-principles methodologies. The OYT3 line list is available from the ExoMol database at http://www.exomol.com.

astro-ph.EP↗

arXiv2020

Vibrationally resolved NO dissociative excitation cross sections by electron impact

A theoretical investigation of the dissociative excitation by electron impact on the NO molecule is presented, aiming to make up for the lack of data for this process in the literature. A full set of vibrationally-resolved cross sections and corresponding rate coefficients are calculated using the Local-Complex-Potential approach and five resonant states of NO^-.

physics.atom-ph↗

arXiv2019

Low-temperature scattering with the R-matrix method: from nuclear scattering to atom-atom scattering and beyond

The R-matrix method is a fully-quantum time-independent scattering method, used to simulate nuclear, electron-atom, electron-ion, electron-molecule scattering and more. Here, a novel R-matrix method, RmatReact, is presented as applied to ultracold (sub-Kelvin) atom-atom scattering. This is in response to experimental methods which in recent decades have facilitated the routine use of ultracold atoms and small molecules in experiments. This project lays the groundwork for future codes to eventually simulate polyatom-polyatom reactive collisions. Results are presented in this paper for numerical comparisons to other established methods, and for resonances in the elastic scattering of ultracold argon atoms.

physics.chem-ph↗

arXiv2019

Identifiable Acetylene Features Predicted for Young Earth-like Exoplanets with Reducing Atmospheres undergoing Heavy Bombardment

The chemical environments of young planets are assumed to be largely influenced by impacts of bodies lingering on unstable trajectories after the dissolution of the protoplanetary disk. We explore the chemical consequences of impacts within the context of reducing planetary atmospheres dominated by carbon monoxide, methane and molecular nitrogen. A terawatt high-power laser was selected in order to simulate the airglow plasma and blast wave surrounding the impactor. The chemical results of these experiments are then applied to a theoretical atmospheric model. The impact simulation results in substantial volume mixing ratios within the reactor of 5% hydrogen cyanide (HCN), 8% acetylene (C$_2$H$_2$), 5% cyanoacetylene (HC$_3$N) and 1% ammonia (NH$_3$). These yields are combined with estimated impact rates for the Early Earth to predict surface boundary conditions for an atmospheric model. We show that impacts might have served as sources of energy that would have led to steady-state surface quantities of 0.4% C$_2$H$_2$, 400 ppm HCN and 40 ppm NH$_3$. We provide simulated transit spectra for an Earth-like exoplanet with this reducing atmosphere during and shortly after eras of intense impacts. We predict that acetylene is as observable as other molecular features on exoplanets with reducing atmospheres that have recently gone through their own `Heavy Bombardments', with prominent features at 3.05 $μ$m and 10.5 $μ$m.

astro-ph.EP↗

arXiv2018

A global potential energy surface for H$_3^+$

A globally correct potential energy surface (PES) for the \hp\ molecular ion is presented. The Born-Oppenheimer (BO) \ai\ grid points of Pavanello et. al. [\textit{J. Chem. Phys.} {\bf 136}, 184303 (2012)] are refitted as BOPES75K, which reproduces the energies below dissociation with a root mean square deviation of 0.05~\cm; points between dissociation and 75\,000 \cm\ are reproduced with the average accuracy of a few wavenumbers. The new PES75K+ potential combines BOPES75K with adiabatic, relativistic and quantum electrodynamics (QED) surfaces to provide the most accurate representation of the \hp\ global potential to date, overcoming the limitations on previous high accuracy H$_3^+$ PESs near and above dissociation. PES75K+ can be used to provide predictions of bound rovibrational energy levels with an accuracy of approaching 0.1~\cm. Calculation of rovibrational energy levels within PES75K+ suggests that the non-adiabatic correction remains a limiting factor. The PES is also constructed to give the correct asymptotic limit making it suitable for use in studies of the H$^+$\,+\,H$_2$ prototypical chemical reaction. An improved dissociation energy for H$_3^+$ is derived as $D_0\,=\,$35\,076\,$\pm\,2\,$cm$^{-1}$.

physics.chem-ph↗

arXiv2018

Theoretical study of ArH+ dissociative recombination and electron-impact vibrational excitation

Cross sections are presented for dissociative recombination and electron-impact vibrational excitation of the ArH+ molecular ion at electron energies appropriate for the interstellar environment. The R-matrix method is employed to determine the molecular structure data, i.e. the position and width of the resonance states. The cross sections and the corresponding Maxwellian rate coefficients are computed using a method based on the Multichannel Quantum Defect Theory. The main result of the paper is the very low dissociative recombination rate found at temperatures below 1000K. This is in agreement with the previous upper limit measurement in merged beams and offers a realistic explanation to the presence of ArH+ in exotic interstellar conditions.

astro-ph.GA↗

arXiv2018

ExoMol line lists - XXIX. The rotation-vibration spectrum of methyl chloride up to 1200 K

Comprehensive rotation-vibration line lists are presented for the two main isotopologues of methyl chloride, $^{12}$CH$_3{}^{35}$Cl and $^{12}$CH$_3{}^{37}$Cl. The line lists, OYT-35 and OYT-37, are suitable for temperatures up to $T=1200\,$K and consider transitions with rotational excitation up to $J=85$ in the wavenumber range $0$--$6400\,$cm$^{-1}$ (wavelengths $λ> 1.56\,μ$m). Over 166 billion transitions between 10.2 million energy levels have been calculated variationally for each line list using a new empirically refined potential energy surface, determined by refining to 739 experimentally derived energy levels up to $J=5$, and an established {\it ab initio} dipole moment surface. The OYT line lists show excellent agreement with newly measured high-temperature infrared absorption cross-sections, reproducing both strong and weak intensity features across the spectrum. The line lists are available from the ExoMol database and the CDS database.

astro-ph.EP↗

arXiv2018

Current State of Astrophysical Opacities: A White Paper

Availability of reliable atomic and molecular opacity tables is essential in a wide variety of astronomical modeling: the solar and stellar interiors, stellar and planetary atmospheres, stellar evolution, pulsating stars, and protoplanetary disks, to name a few. With the advancement of powerful research techniques such as helio-seismology and asteroseismology, solar neutrino-flux measurements, exoplanet survey satellites, three-dimensional hydrodynamic atmospheric simulations (including non-LTE and granulation effects), high-performance computing of atomic and molecular data, and innovative plasma experiments the accuracy and completeness of opacity tables is being taken to an unprecedented level. The goal of the second Workshop on Astrophysical Opacities was to gather opacity data producers and consumers from both the atomic and molecular sectors to contribute to solving outstanding problems and to develop more effective and integrated interfaces. In this review we attempt to summa- rize the discussion at the workshop and propose future directions for opacity research.

astro-ph.SR↗

arXiv2018

A population study of gaseous exoplanets

We present here the analysis of 30 gaseous extrasolar planets, with temperatures between 600 and 2400 K and radii between 0.35 and 1.9 $R_\mathrm{Jup}$. The quality of the HST/WFC3 spatially scanned data combined with our specialized analysis tools allow us to study the largest and most self-consistent sample of exoplanetary transmission spectra to date and examine the collective behavior of warm and hot gaseous planets rather than isolated case-studies. We define a new metric, the Atmospheric Detectability Index (ADI) to evaluate the statistical significance of an atmospheric detection and find statistically significant atmospheres around 16 planets out of the 30 analysed. For most of the Jupiters in our sample, we find the detectability of their atmospheres to be dependent on the planetary radius but not on the planetary mass. This indicates that planetary gravity plays a secondary role in the state of gaseous planetary atmospheres. We detect the presence of water vapour in all of the statistically detectable atmospheres, and we cannot rule out its presence in the atmospheres of the others. In addition, TiO and/or VO signatures are detected with 4$σ$ confidence in WASP-76 b, and they are most likely present in WASP-121 b. We find no correlation between expected signal-to-noise and atmospheric detectability for most targets. This has important implications for future large-scale surveys.

astro-ph.EP↗

arXiv2017

ExoMol line lists -- XXII. The rotation-vibration spectrum of silane up to 1200 K

A variationally computed $^{28}$SiH$_4$ rotation-vibration line list applicable for temperatures up to $T=1200\,$K is presented. The line list, called OY2T, considers transitions with rotational excitation up to $J=42$ in the wavenumber range $0$--$5000\,$cm$^{-1}$ (wavelengths $λ> 2\,μ$m). Just under 62.7 billion transitions have been calculated between 6.1 million energy levels. Rovibrational calculations have utilized a new `spectroscopic' potential energy surface determined by empirical refinement to 1452 experimentally derived energy levels up to $J=6$, and a previously reported \textit{ab initio} dipole moment surface. The temperature-dependent partition function of silane, the OY2T line list format, and the temperature dependence of the OY2T line list are discussed. Comparisons with the PNNL spectral library and other experimental sources indicate that the OY2T line list is robust and able to accurately reproduce weaker intensity features. The full line list is available from the ExoMol database and the CDS database.

physics.chem-ph↗

arXiv2017

Interaction of molecular nitrogen with Free-Electron-Laser radiation

We compute molecular continuum orbitals in the single center expansion scheme. We then employ these orbitals to obtain molecular Auger rates and single-photon ionization cross sections to study the interaction of N2 with Free-Electron-Laser (FEL) pulses. The nuclei are kept fixed. We formulate rate equations for the energetically allowed molecular and atomic transitions and we account for dissociation through additional terms in the rate equations. Solving these equations for different parameters of the FEL pulse, allows us to identify the most efficient parameters of the FEL pulse for obtaining the highest contribution of double core hole states (DCH) in the final atomic ion fragments. Finally we identify the contribution of DCH states in the electron spectra and show that the DCH state contribution is more easily identified in the photo-ionization rather than the Auger transitions.

physics.atom-ph↗

arXiv2017

State-to-state chemistry and rotational excitation of CH$^+$ in photon-dominated regions

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.

astro-ph.GA↗

arXiv2017

Low-energy collisions between electrons and BeH$^+$: cross sections and rate coefficients for all the vibrational states of the ion

We provide cross sections and Maxwell rate coefficients for reactive collisions of slow electrons with BeH$^+$ ions on all the eighteen vibrational levels ($X{^{1}Σ^{+}},v_{i}^{+}=0,1,2,\dots,17$) using a Multichannel Quantum Defect Theory (MQDT) - type approach. These data on dissociative recombination, vibrational excitation and vibrational de-excitation are relevant for magnetic confinement fusion edge plasma modelling and spectroscopy, in devices with beryllium based main chamber materials, such as the International Thermonuclear Experimental Reactor (ITER) and the Joint European Torus (JET). Our results are presented in graphical form and as fitted analytical functions, the parameters of which are organized in tables.

physics.plasm-ph↗

arXiv2016

Calculated low-energy electron-impact vibrational excitation cross sections for CO2 molecule

Vibrational-excitation cross sections of ground electronic state of carbon dioxide molecule by electron-impact through the CO2-(2Π) shape resonance is considered in the separation of the normal modes approximation. Resonance curves and widths are computed for each vibrational mode. The calculations assume decoupling between normal modes and employ the local complex potential model for the treatment of the nuclear dynamics, usually adopted for the electron-scattering involving diatomic molecules. Results are presented for excitation up to 10 vibrational levels in each mode and comparison with data present in the literature is discussed.

physics.chem-ph↗

arXiv2016

Uncertainty Estimates for Theoretical Atomic and Molecular Data

Sources of uncertainty are reviewed for calculated atomic and molecular data that are important for plasma modeling: atomic and molecular structure and cross sections for electron-atom, electron-molecule, and heavy particle collisions. We concentrate on model uncertainties due to approximations to the fundamental many-body quantum mechanical equations and we aim to provide guidelines to estimate uncertainties as a routine part of computations of data for structure and scattering.

physics.atom-ph↗

arXiv2016

Detection of an atmosphere around the super-Earth 55 Cancri e

We report the analysis of two new spectroscopic observations of the super-Earth 55 Cancri e, in the near infrared, obtained with the WFC3 camera onboard the HST. 55 Cancri e orbits so close to its parent star, that temperatures much higher than 2000 K are expected on its surface. Given the brightness of 55 Cancri, the observations were obtained in scanning mode, adopting a very long scanning length and a very high scanning speed. We use our specialized pipeline to take into account systematics introduced by these observational parameters when coupled with the geometrical distortions of the instrument. We measure the transit depth per wavelength channel with an average relative uncertainty of 22 ppm per visit and find modulations that depart from a straight line model with a 6$σ$ confidence level. These results suggest that 55 Cancri e is surrounded by an atmosphere, which is probably hydrogen-rich. Our fully Bayesian spectral retrieval code, T-REx, has identified HCN to be the most likely molecular candidate able to explain the features at 1.42 and 1.54 $μ$m. While additional spectroscopic observations in a broader wavelength range in the infrared will be needed to confirm the HCN detection, we discuss here the implications of such result. Our chemical model, developed with combustion specialists, indicates that relatively high mixing ratios of HCN may be caused by a high C/O ratio. This result suggests this super-Earth is a carbon-rich environment even more exotic than previously thought.

astro-ph.EP↗

arXiv2015

The 2014 KIDA network for interstellar chemistry

Chemical models used to study the chemical composition of the gas and the ices in the interstellar medium are based on a network of chemical reactions and associated rate coefficients. These reactions and rate coefficients are partially compiled from data in the literature, when available. We present in this paper kida.uva.2014, a new updated version of the kida.uva public gas-phase network first released in 2012. In addition to a description of the many specific updates, we illustrate changes in the predicted abundances of molecules for cold dense cloud conditions as compared with the results of the previous version of our network, kida.uva.2011.

astro-ph.GA↗

arXiv2015

Dissociative electron attachment and electron-impact resonant dissociation of vibrationally excited O2 molecules

State-by-state cross sections for dissociative electron attachment and electron-impact dissociation for molecular oxygen are computed using ab initio resonance curves calculated with the R-matrix method. When O2 is in its vibrational ground state, the main contribution for both processes comes from the $^2Π_u$ resonance state of $O_2^-$ but with a significant contribution from the $^4Σ$ resonant state. Vibrational excitation leads to an increased contribution from the low-lying $^2Π_{g}$ resonance, greatly increased cross sections for both processes, and the threshold moving to lower energies. These results provide important input for models of O2-containing plasmas in nonequilibrium conditions.

physics.plasm-ph↗