arXiv subjects

P. C. Stancil

Publications and source records attributed to P. C. Stancil.

At least 19 recordsLinked to original sources

arXiv2026

Variationally Compressing Quantum Circuits to Approximate Nonadiabatic Molecular Quantum Dynamics

Quantum simulation has begun to penetrate the field of quantum chemistry in hopes of efficiently calculating ground state energies and approximating real-time evolution. With modern research highlighting nonadiabatic dynamics, tunably approximating deep circuits representing potential landscapes becomes crucial for simulating real quantum systems. Variationally approximating unitaries allows for shallower circuits and accuracy tunable to hardware fidelity, so long as the observable quantities are preserved. We show the variational compression of Trotter terms preserve reaction rate coefficients via classical emulation of a hybrid quantum-classical optimization method, as well as fast-forwarded adiabatic dynamics on quantum hardware. Compressed circuits can be incorporated with product-formula-based time evolution to approximate dynamics of a particle in two coupled harmonic potentials, allowing tunability when removing high-cost qubit interactions. Approximate rate coefficients are recovered after substituting terms in a nonadiabatic dynamic process, giving proof-of-principle for observable preservation under variational optimization. Attention is paid to minimizing qubit and gate-count resources.

quant-ph↗

arXiv2024

Inelastic Triatom-Atom Quantum Close-Coupling Dynamics in Full Dimensionality: all rovibrational mode quenching of water due to H impact on a six-dimensional potential energy surface

The rovibrational level populations, and subsequent emission in various astrophysical environments, is driven by inelastic collision processes. The available rovibrational rate coefficients for water have been calculated using a number of approximations. We present a numerically exact calculation for the rovibrational quenching for all water vibrational modes due to collisions with atomic hydrogen. The scattering theory implements a quantum close-coupling (CC) method on a high level ab initio six-dimensional (6D) potential energy surface (PES). Total rovibrational quenching cross sections for excited bending levels were compared with earlier results on a 4D PES with the rigid-bender close-coupling (RBCC) approximation. General agreement between 6D-CC and 4D-RBCC calculations are found, but differences are evident including the energy and amplitude of low-energy orbiting resonances. Quenching cross sections from the symmetric and asymmetric stretch modes are provided for the first time. The current 6D-CC calculation provides accurate inelastic data needed for astrophysical modeling.

astro-ph.GA↗

arXiv2024

PDRs4All IX. Sulfur elemental abundance in the Orion Bar

One of the main problems in astrochemistry is determining the amount of sulfur in volatiles and refractories in the interstellar medium. The detection of the main sulfur reservoirs (icy H$_2$S and atomic gas) has been challenging, and estimates are based on the reliability of models to account for the abundances of species containing less than 1% of the total sulfur. The high sensitivity of the James Webb Space Telescope provides an unprecedented opportunity to estimate the sulfur abundance through the observation of the [S I] 25.249 $μ$m line. We used the [S III] 18.7 $μ$m, [S IV] 10.5 $μ$m, and [S l] 25.249 $μ$m lines to estimate the amount of sulfur in the ionized and molecular gas along the Orion Bar. For the theoretical part, we used an upgraded version of the Meudon photodissociation region (PDR) code to model the observations. New inelastic collision rates of neutral atomic sulfur with ortho- and para- molecular hydrogen were calculated to predict the line intensities. The [S III] 18.7 $μ$m and [S IV] 10.5 $μ$m lines are detected over the imaged region with a shallow increase (by a factor of 4) toward the HII region. We estimate a moderate sulfur depletion, by a factor of $\sim$2, in the ionized gas. The corrugated interface between the molecular and atomic phases gives rise to several edge-on dissociation fronts we refer to as DF1, DF2, and DF3. The [S l] 25.249 $μ$m line is only detected toward DF2 and DF3, the dissociation fronts located farthest from the HII region. The detailed modeling of DF3 using the Meudon PDR code shows that the emission of the [S l] 25.249 $μ$m line is coming from warm ($>$ 40 K) molecular gas located at A$_{\rm V}$ $\sim$ 1$-$5 mag from the ionization front. Moreover, the intensity of the [S l] 25.249 $μ$m line is only accounted for if we assume the presence of undepleted sulfur.

astro-ph.GA↗

arXiv2023

The 2017 Release of Cloudy

We describe the 2017 release of the spectral synthesis code Cloudy. A major development since the previous release has been exporting the atomic data into external data files. This greatly simplifies updates and maintenance of the data. Many large datasets have been incorporated with the result that we can now predict well over an order of magnitude more emission lines when all databases are fully used. The use of such large datasets is not realistic for most calculations due to the time and memory needs, and we describe the limited subset of data we use by default. Despite the fact that we now predict significantly more lines than the previous Cloudy release, this version is faster because of optimization of memory access patterns and other tuning. The size and use of the databases can easily be adjusted in the command-line interface. We give examples of the accuracy limits using small models, and the performance requirements of large complete models. We summarize several advances in the H- and He-like iso-electronic sequences. We use our complete collisional-radiative models of the ionization of these one and two-electron ions to establish the highest density for which the coronal or interstellar medium (ISM) approximation works, and the lowest density where Saha or local thermodynamic equilibrium can be assumed. The coronal approximation fails at surprisingly low densities for collisional ionization equilibrium but is valid to higher densities for photoionized gas clouds. Many other improvements to the physics have been made and are described. These include the treatment of isotropic continuum sources such as the cosmic microwave background (CMB) in the reported output, and the ability to follow the evolution of cooling non-equilibrium clouds.

astro-ph.GA↗

arXiv2023

A New Definition of Exoplanet Habitability: Introducing the Photosynthetic Habitable Zone

It may be possible to detect biosignatures of photosynthesis in an exoplanet's atmosphere. However, such a detection would likely require a dedicated study, occupying a large amount of telescope time. It is therefore prudent, while searching for signs of life that we may recognise, to pick the best target possible. In this work, we present a new region, the ``photosynthetic habitable zone'' \textemdash the distance from a star where both liquid water and oxygenic photosynthesis can occur. It is therefore the region where detectable biosignatures of oxygenic photosynthesis are most likely to occur. Our analysis indicates that in the most ideal conditions for life and no atmospheric effects, the photosynthetic habitable zone is almost as broad as the habitable zone. On the other hand, if conditions for life are anything less than excellent and atmospheric effects are even moderate, the photosynthetic habitable zone is concentrated at larger separations around more massive stars. Such cases are also not tidally locked to their host star, which could result in planetary rotation periods similar to the Earth's. We identify five planets, Kepler-452 b, Kepler-1638 b, Kepler-1544 b and Kepler-62 e and Kepler-62 f, that are consistently in the photosynthetic habitable zone for a variety of conditions, and we predict their day lengths to be between 9 and 11 hours. We conclude that the parameter space in which we should search for signs of life is much narrower than the standard habitable zone.

astro-ph.EP↗

arXiv2022

Relativistic Atomic Structure of Au IV and the Os Isoelectronic Sequence: opacity data for kilonova ejecta

Direct detection of gravitational waves (GW) on Aug. 17, 2017, propagating from a binary neutron star merger, opened the era of multimessenger astronomy. The ejected material from neutron star mergers, or kilonova, is a good candidate for optical and near infrared follow-up observations after the detection of GWs. The kilonova from the ejecta of GW1780817 provided the first evidence for the astrophysical site of the synthesis of heavy nuclei through the rapid neutron capture process or rprocess. Since properties of the emission are largely affected by opacities of the ejected material, enhancements in the available, but limited rprocess atomic data have been motivated recently. However, given the complexity of the electronic structure of these heavy elements, considerable efforts are still needed to converge to a reliable set of atomic structure data. The aim of this work is to alleviate this situation for low charge state elements in the Os-like isoelectronic sequence. In this regard, the general purpose relativistic atomic structure packages (GRASP0 and GRASP2K) were used to obtain energy levels and transition probabilities (E1 and M1). We provide line lists and expansion opacities for a range of r-process elements. We focus here on the Os isoelectronic sequence (Os I, Ir II, Pt III, Au IV, Hg V). The results are benchmarked against existing experimental data and prior calculations, and predictions of emission spectra relevant to kilonovae are provided. Finestructure (M1) lines in the infrared potentially observable by the James Webb Space Telescope are highlighted.

astro-ph.SR↗

arXiv2020

Prediction of a Feshbach Resonance in the Below-the-Barrier Reactive Scattering of Vibrationally Excited HD with H

Quantum reactive scattering calculations on the vibrational quenching of HD due to collisions with H were carried out employing an accurate potential energy surface. The state-to-state cross sections for the chemical reaction HD ($v=1, \ j=0$) + H $\rightarrow$ D + H$_2$ ($v'=0, \ j'$) at collision energies between 1 and 10,000 cm$^{-1}$ are presented, and a Feshbach resonance in the low-energy regime, below the reaction barrier, is observed for the first time. The resonance is attributed to coupling with the vibrationally adiabatic potential correlating to the $v=1, \ j=1$ level of the HD molecule, and it is dominated by the contribution from a single partial wave. The properties of the resonance, such as its dynamic behavior, phase behavior, and lifetime, are discussed.

physics.chem-ph↗

arXiv2020

Revisiting the Formation of HeH$^+$ in the Planetary Nebula NGC 7027

From four independent calculations using three different theoretical approaches, rate coefficients for the formation of HeH$^+$ via the radiative association of He$^+$ and H were computed. Good agreement is found between our new calculations and prior results obtained two decades ago for kinetic temperatures between $\sim$800 and 20,000 K. This finding is inconsistent with a recent claim in the literature of a wide variation in published values and establishes the robustness of our knowledge of this process for the formation of HeH$^+$. The implications of the current results to the first detection of HeH$^+$ and its modeled abundance in the planetary nebula NGC 7027 are discussed.

astro-ph.GA↗

arXiv2020

2018 NASA Laboratory Astrophysics Workshop: Scientific Organizing Committee Report

This report provides detailed findings on the critical laboratory astrophysics data needs that are required to maximize the scientific return for NASA's current and near-term planned astrophysics missions. It also provides prioritized rankings on said laboratory astrophysics data, generally by waveband. The Report is based on community input gathered at the 2018 NASA Laboratory Astrophysics Workshop (LAW) from presentations, from discussions during workshop breakout sessions, and from other solicited input deemed appropriate by the Scientific Organizing Committee (SOC) obtained prior to and after the meeting. Hence, the Report is a direct reflection of the spirit and participant make-up of LAW 2018. The Report also outlines specific opportunities and threats facing NASA's Laboratory Astrophysics Program, and articulates concrete actions by which the Agency can capitalize on the opportunities and mitigate the challenges. The Report was prepared by the SOC, with help from some invited speakers, and input and review from community members.

astro-ph.IM↗

arXiv2020

Machine-learning-corrected quantum dynamics calculations

Quantum scattering calculations for all but low-dimensional systems at low energies must rely on approximations. All approximations introduce errors. The impact of these errors is often difficult to assess because they depend on the Hamiltonian parameters and the particular observable under study. Here, we illustrate a general, system and approximation-independent, approach to improve the accuracy of quantum dynamics approximations. The method is based on a Bayesian machine learning (BML) algorithm that is trained by a small number of rigorous results and a large number of approximate calculations, resulting in ML models that accurately capture the dependence of the dynamics results on the quantum dynamics parameters. Most importantly, the present work demonstrates that the BML models can generalize quantum results to different dynamical processes. Thus, a ML model trained by a combination of approximate and rigorous results for a certain inelastic transition can make accurate predictions for different transitions without rigorous calculations. This opens the possibility of improving the accuracy of approximate calculations for quantum transitions that are out of reach of rigorous scattering calculations.

physics.chem-ph↗

arXiv2019

Electron-impact fine-structure excitation of Fe II at low temperature

Fe II emission lines are observed from nearly all classes of astronomical objects over a wide spectral range from the infrared to the ultraviolet. To meaningfully interpret these lines, reliable atomic data are necessary. In work presented here we focused on low-lying fine-structure transitions, within the ground term, due to electron impact. We provide effective collision strengths together with estimated uncertainties as functions of temperature of astrophysical importance ($10 - 100,000$ K). Due to the importance of fine-structure transitions within the ground term, the focus of this work is on obtaining accurate rate coefficients at the lower end of this temperature range, for applications in low temperature environments such as the interstellar medium. We performed three different flavours of scattering calculations: i) a intermediate coupling frame transformation (ICFT) $R$-matrix method, ii) a Breit-Pauli (BP) $R$-matrix method, and iii) a Dirac $R$-matrix method. The ICFT and BP $R$-matrix calculations involved three different AUTOSTRUCTURE target models each. The Dirac $R$-matrix calculation was based on a reliable 20 configuration, 6069 level atomic structure model. Good agreement was found with our BP and Dirac $R$-matrix collision results compared to previous $R$-matrix calculations. We present a set of recommended effective collision strengths for the low-lying forbidden transitions together with associated uncertainty estimates.

physics.atom-ph↗

arXiv2018

Full-dimensional Quantum Dynamics of SiO in Collision with H$_2$

We report the first full-dimensional potential energy surface (PES) and quantum mechanical close-coupling calculations for scattering of SiO due to H$_2$. The full-dimensional interaction potential surface was computed using the explicitly correlated coupled-cluster (CCSD(T)-F12b) method and fitted using an invariant polynomial approach. Pure rotational quenching cross sections from initial states $v_1=0$, $j_1$=1-5 of SiO in collision with H$_2$ are calculated for collision energies between 1.0 and 5000 cm$^{-1}$. State-to-state rotational rate coefficients are calculated at temperatures between 5 and 1000 K. The rotational rate coefficients of SiO with para-H$_2$ are compared with previous approximate results which were obtained using SiO-He PESs or scaled from SiO-He rate coefficients. Rovibrational state-to-state and total quenching cross sections and rate coefficients for initially excited SiO($v_1=1, j_1$=0 and 1) in collisions with para-H$_2$($v_2=0,j_2=0$) and ortho-H$_2$($v_2=0,j_2=1$) were also obtained. The application of the current collisional rate coefficients to astrophysics is briefly discussed.

physics.chem-ph↗

arXiv2017

Editorial: Special Issue on the Atomic and Molecular Processes in the Ultracold Regime, the Chemical Regime, and Astrophysics

This editorial introduces the J. Phys. B: Atomic, Molecular and Optical Physics Special Issue "Atomic and Molecular Processes in the Ultracold Regime, the Chemical Regime and Astrophysics" dedicated to Professor Alexander Dalgarno (1928-2015). After a brief biographical review, short summaries of the contributed papers and their relations to some of Prof. Dalgarno's work are given.

physics.hist-ph↗

arXiv2017

Line Ratios for Solar Wind Charge Exchange with Comets

Charge exchange (CX) has emerged in X-ray emission modeling as a significant process that must be considered in many astrophysical environment--particularly comets. Comets host an interaction between solar wind ions and cometary neutrals to promote solar wind charge exchange (SWCX). X-ray observatories provide astronomers and astrophysicists with data for many X-ray emitting comets that are impossible to accurately model without reliable CX data. Here, we utilize a streamlined set of computer programs that incorporate the multi-channel Landau-Zener theory and a cascade model for X-ray emission to generate cross sections and X-ray line ratios for a variety of bare and non-bare ion single electron capture (SEC) collisions. Namely, we consider collisions between the solar wind constituent bare and H-like ions of C, N, O, Ne, Na, Mg, Al, and Si and the cometary neutrals H2O, CO, CO2, OH, and O. To exemplify the application of this data, we model the X-ray emission of Comet C/2000 WM1 (linear) using the CX package in SPEX and find excellent agreement with observations made with the XMM-Newton RGS detector. Our analyses show that the X-ray intensity is dominated by SWCX with H, while H2O plays a secondary role. This is the first time, to our knowledge, that CX cross sections have been implemented into a X-ray spectral fitting package to determine the H to H2O ratio in cometary atmospheres. The CX data sets are incorporated into the modeling packages SPEX and Kronos.

astro-ph.HE↗

arXiv2017

Rate constants for the formation of SiO by radiative association

Accurate molecular data for the low-lying states of SiO are computed and used to calculate rate constants for radiative association of Si and O. Einstein A-coefficients are also calculated for transitions between all of the bound and quasi-bound levels for each molecular state. The radiative widths are used together with elastic tunneling widths to define effective radiative association rate constants which include both direct and indirect (inverse predissociation) formation processes. The indirect process is evaluated for two kinetic models which represent limiting cases for astrophysical environments. The first case scenario assumes an equilibrium distribution of quasi-bound states and would be applicable whenever collisional and/or radiative excitation mechanisms are able to maintain the population. The second case scenario assumes that no excitation mechanisms are available which corresponds to the limit of zero radiation temperature and zero atomic density. Rate constants for SiO formation in realistic astrophysical environments would presumably lie between these two limiting cases.

astro-ph.IM↗

arXiv2017

Charge Exchange Induced X-ray Emission of Fe XXV and Fe XXVI via a Streamlined Model

Charge exchange is an important process for the modeling of X-ray spectra obtained by the Chandra, XMM-Newton, and Suzaku X-ray observatories, as well as the anticipated Astro-H mission. The understanding of the observed X-ray spectra produced by many astrophysical environments is hindered by the current incompleteness of available atomic and molecular data -- especially for charge exchange. Here, we implement a streamlined program set that applies quantum defect methods and the Landau-Zener theory to generate total, n-resolved, and nlS-resolved cross sections for any given projectile ion/ target charge exchange collision. Using this data in a cascade model for X-ray emission, theoretical spectra for such systems can be predicted. With these techniques, Fe25+ and Fe26+ charge exchange collisions with H, He, H2, N2, H2O, and CO are studied for single electron capture. These systems have been selected as they illustrate computational difficulties for high projectile charges. Further, Fe XXV and Fe XXVI emission lines have been detected in the Galactic center and Galactic ridge. Theoretical X-ray spectra for these collision systems are compared to experimental data generated by an electron beam ion trap study. Several l- distribution models have been tested for Fe25+ and Fe26+ single electron capture. Such analysis suggests that commonly used l-distribution models struggle to accurately reflect the true distribution of electron capture as understood by more advanced theoretical methods.

astro-ph.HE↗

arXiv2016

A Comprehensive Study of Hydrogen Adsorbing to Amorphous Water-Ice: Defining Adsorption in Classical Molecular Dynamics

Gas-grain and gas-phase reactions dominate the formation of molecules in the interstellar medium (ISM). Gas-grain reactions require a substrate (e.g. a dust or ice grain) on which the reaction is able to occur. The formation of molecular hydrogen (H$_2$) in the ISM is the prototypical example of a gas-grain reaction. In these reactions, an atom of hydrogen will strike a surface, stick to it, and diffuse across it. When it encounters another adsorbed hydrogen atom, the two can react to form molecular hydrogen and then be ejected from the surface by the energy released in the reaction. We perform in-depth classical molecular dynamics (MD) simulations of hydrogen atoms interacting with an amorphous water-ice surface. This study focuses on the first step in the formation process; the sticking of the hydrogen atom to the substrate. We find that careful attention must be paid in dealing with the ambiguities in defining a sticking event. The technical definition of a sticking event will affect the computed sticking probabilities and coefficients. Here, using our new definition of a sticking event, we report sticking probabilities and sticking coefficients for nine different incident kinetic energies of hydrogen atoms [5 K - 400 K] across seven different temperatures of dust grains [10 K - 70 K]. We find that probabilities and coefficients vary both as a function of grain temperature and incident kinetic energy over the range of 0.99 - 0.22.

astro-ph.GA↗

arXiv2016

Ne X X-ray Emission due to Charge Exchange in M82

Recent X-ray observations of star-forming galaxies such as M82 have shown the Ly beta/Ly alpha line ratio of Ne X to be in excess of predictions for thermal electron impact excitation. Here we demonstrate that the observed line ratio may be due to charge exchange and can be used to constrain the ion kinetic energy to be <500 eV/u. This is accomplished by computing spectra and line ratios via a range of theoretical methods and comparing these to experiments with He over astrophysically relevant collision energies. The charge exchange emission spectra calculations were performed for Ne[10+] +H and Ne[10+] +He using widely applied approaches including the atomic orbital close coupling, classical trajectory Monte Carlo, and multichannel Landau- Zener (MCLZ) methods. A comparison of the results from these methods indicates that for the considered energy range and neutrals (H, He) the so-called "low-energy l-distribution" MCLZ method provides the most likely reliable predictions.

astro-ph.HE↗