Non-local Thermodynamic Equilibrium Model Research Articles

NLTE modelling of water-rich exoplanet atmospheres. Cooling and heating rates

The hydrogen and water molecules respond very differently to the collisional-radiative processes taking place in planetary atmospheres. Naturally, the question arises whether H2O-rich atmospheres are more (or less) resilient to long-term mass loss than H2-dominated ones if they radiate away the incident stellar energy more (or less) efficiently. If confirmed, the finding would have implications on our understanding of the evolution of sub-Neptune exoplanets. As a key step towards answering this question, we present a non-local thermodynamic equilibrium (NLTE) model of H2O for the atmospheric region where the gas accelerates to escape the planet and conditions relevant to close-in sub-Neptunes. Our exploratory calculations for isothermal gas composed of H2, H2O and e− reveal that: (1) In the pressure region ∼10−2–10−4 dyn cm−2 where the stellar extreme-ultraviolet (XUV) photons are typically deposited in the atmosphere, H2O is in rotational LTE but vibrational NLTE. Vibrational LTE is facilitated by high H2O abundances and fractional ionizations, and we report critical densities for the LTE-NLTE transition; (2) Vibrational cooling may locally dominate over rotational cooling, partly because of the comparatively small opacities of ro-vibrational lines; (3) Even low H2O abundances notably enhance the cooling, foreseeably offsetting some of the stellar heating; (4) Heating due to the deposition of stellar infrared (IR) photons is significant at pressures ≳0.1 dyn cm−2. We estimate the contribution of H2O excitation to the internal energy of the gas and speculate on the photodissociation from the excited vibrational states. Ultimately, our findings motivate the consideration of NLTE in the mass loss rate calculations of H2O-rich atmospheres.

Searching for NLTE effects in the high-resolution transmission spectrum of WASP-121 b with cloudy for exoplanets

ABSTRACT Ultrahot Jupiters (UHJs) undergo intense irradiation by their host stars and are expected to experience non-local thermodynamic equilibrium (NLTE) effects in their atmospheres. Such effects are computationally intensive to model but, at the low pressures probed by high-resolution cross-correlation spectroscopy (HRCCS), can significantly impact the formation of spectral lines. The UHJ WASP-121 b exhibits a highly inflated atmosphere, making it ideal for investigating the impact of NLTE effects on its transmission spectrum. Here, we formally introduce cloudy for exoplanets, a cloudy-based modelling code, and use it to generate 1D LTE and NLTE atmospheric models and spectra to analyse archival HARPS WASP-121 b transmission spectra. We assessed the models using two HRCCS methods: (i) Pearson cross-correlation, and (ii) a method that aims to match the average observed line depth for given atmospheric species. All models result in strong detections of Fe i (7.5 < S/N < 10.5). However, the highest S/N model (LTE) does not agree with the best-matching model of the average line depth (NLTE). We also find degeneracy, such that increasing the isothermal temperature and metallicity of the LTE models can produce average line depths similar to cooler, less metal rich NLTE models. Thus, we are unable to conclusively remark on the presence of NLTE effects in the atmosphere of WASP-121 b. We instead highlight the need for standardized metrics in HRCCS that enable robust statistical assessment of complex physical models, e.g. NLTE or 3D effects, that are currently too computationally intensive to include in HRCCS atmospheric retrievals.

The photospheres of the hottest fastest stars in the Galaxy

We perform nonlocal thermodynamic equilibrium (NLTE) model atmosphere analyses of the three hottest hypervelocity stars (space velocities between ≈1500–2800 km s−1) known to date, which were recently discovered spectroscopically and identified as runaways from Type Ia supernovae. The hottest of the three (J0546+0836, effective temperature Teff = 95 000 ± 15 000 K, surface gravity log 𝑔 = 5.5 ± 0.5) has an oxygen-dominated atmosphere with a significant amount of carbon (C = 0.10 ± 0.05, O = 0.90 ± 0.05, mass fractions). Its mixed absorption+emission line spectrum exhibits photospheric absorption lines from O v and O vi as well as O iii and O iv emission lines that are formed in a radiation-driven wind with a mass-loss rate of the order of Ṁ= 10−8 M⊙ yr−1. Spectroscopically, J0546+0836 is a [WC]–PG1159 transition-type pre-white dwarf. The second object (J0927–6335) is a PG1159-type white dwarf with a pure absorption-line spectrum dominated by C III/C IV and O III/O IV. We find Teff = 60 000 ± 5000 K, log 𝑔 = 7.0 ± 0.5, and a carbon-and oxygen-dominated atmosphere with C = 0.47 ± 0.25, O = 0.48 ± 0.25, and possibly a minute amount of helium (He = 0.05 ± 0.05). Comparison with post-AGB evolutionary tracks suggests a mass of M ≈ 0.5 M⊙ for both objects, if such tracks can safely be applied to these stars. We find the third object (J1332−3541) to be a relatively massive (M = 0.89 M⊙) hydrogen-rich (DAO) white dwarf with Teff = 65 657 ± 2390 K, log 𝑔 = 8.38 ± 0.08, and abundances H = 0.65 ± 0.04 and He = 0.35 ± 0.04. We discuss our results in the context of the “dynamically driven double-degenerate double-detonation” (D6) scenario proposed for the origin of these stars.

Rate coefficients for the rotational de-excitation of OCS by collision with He

Context. In typical molecular clouds, analyzing the physicochemical conditions with nonlocal thermodynamic equilibrium models requires knowledge of the collisional rate coefficients between the detected molecule and the most common colliders in the interstellar medium (ISM); for example H2, He, and H. The OCS molecule has been widely observed in the ISM. However, the collision data available for this species were calculated using a potential energy surface (PES) that shows differences with surfaces presented recently. Aims. The main goal of this work is to report a new set of rate coefficients for the collision of OCS with He based on a new PES computed at a high level of theory. Methods. We developed an analytical PES using a large set of ab initio energies calculated using the coupled cluster with single, double, and perturbative triple excitations (CCSD(T)) method at the completed basis set limit for the OCS+He complex. We used this surface in close-coupling calculations, and computed a new set of collisional rate coefficients for OCS and He. Results. We compare the de-excitation rate coefficients with previously available data. Furthermore, we observe a |Δj| = 1 propensity rule. Finally, we report a set of rate coefficients for the lower 39 rotational states of OCS, which is the largest set determined to date for this collision.

Theoretical study of inelastic processes in collisions of Y and Y+ with hydrogen atom

ABSTRACT Utilizing a simplified quantum model approach, the low-energy inelastic collision processes between yttrium atoms (ions) and hydrogen atoms have been studied. Rate coefficients corresponding to the mutual neutralization, ion-pair formation, excitation, and de-excitation processes for the above collision systems have been provided in the temperature range of 1000–10 000 K. Three ionic states and 73 covalent states are considered in calculations for the collisions of yttrium atoms with hydrogen atoms, which include six molecular symmetries and 4074 partial inelastic reaction processes. For the collisions of yttrium ions with hydrogen atoms, one ionic state and 116 covalent states are included, which are related to three molecular symmetries and 13 572 partial inelastic collision processes. It is found that the rate coefficients for the mutual neutralization process have a maximum at T = 6000 K, which is an order of magnitude higher than those of other processes. Notably, the positions of optimal windows for the collisions of yttrium atoms and ions with hydrogen atoms are found near electronic binding energy −2 eV (Y) and −4.4 eV (Y+), respectively. The scattering channels located in or near these optimal windows have intermediate-to-large rate coefficients (greater than 10−12 cm3 s−1). The reported data should be useful in the study of non-local thermodynamic equilibrium modelling.

Observational constraints on the origin of the elements

Aims. We constrain the role of different Type Ia supernova (SN Ia) channels in the chemical enrichment of the Galaxy by studying the abundances of nickel in Galactic stars. We investigated four different SN Ia sub-classes, including the classical single-degenerate near-Chandrasekhar mass (Mch) SN Ia, the fainter SN Iax systems associated with He accretion from the companion, as well as two sub-Chandrasekhar mass (sub-Mch) SN Ia channels. The latter include the double detonation of a white dwarf accreting helium-rich matter and violent white dwarf mergers. Methods. The chemical abundances in Galactic stars were determined using Gaia eDR3 astrometry and photometry and high-resolution optical spectra. Non-local thermodynamic equilibrium (NLTE) models of Fe and Ni were used in the abundance analysis. We included new delay-time distributions arising from the different SN Ia channels in models of the Galactic chemical evolution, as well as recent yields for core-collapse supernovae and asymptotic giant branch stars. The data-model comparison was performed using a Markov chain Monte Carlo framework that allowed us to explore the entire parameter space allowed by the diversity of explosion mechanisms and the Galactic SN Ia rate, taking the uncertainties of the observed data into account. Results. We show that NLTE effects have a non-negligible impact on the observed [Ni/Fe] ratios in the Galactic stars. The NLTE corrections to Ni abundances are not large, but strictly positive, lifting the [Ni/Fe] ratios by ∼ + 0.15 dex at [Fe/H] −2. We find that the distributions of [Ni/Fe] in LTE and in NLTE are very tight, with a scatter of ≲0.1 dex at all metallicities. This supports earlier work. In LTE, most stars have scaled solar Ni abundances, [Ni/Fe] ≈ 0, with a slight tendency for sub-solar [Ni/Fe] ratios at lower [Fe/H]. In NLTE, however, we find a mild anti-correlation between [Ni/Fe] and metallicity, and slightly elevated [Ni/Fe] ratios at [Fe/H] ≲ −1.0. The NLTE data can be explained by models of the Galactic chemical evolution that are calculated with a substantial fraction, ∼75%, of sub-Mch SN Ia.

The bright blue side of the night sky: Spectroscopic survey of bright and hot (pre-) white dwarfs

We report on the spectroscopic confirmation of 68 new bright (G = 13.5–17.2 mag) and blue (pre-)white dwarfs (WDs). This finding has allowed us to almost double the number of the hottest (Teff ≥ 60 kK) known WDs brighter than G = 16 mag. We increased the number of known ultra-high excitation (UHE) WDs by 20%, found one unambiguous close binary system consisting of one DA WD with an irradiated low-mass companion, one DAO, and one DOA WD that are likely in their transformation phase of becoming pure DA WDs, one rare, naked O(H) star, two DA and two DAO WDs with Teff possibly in excess of 100 kK, three new DOZ WDs, and three of our targets are central stars of (possible) planetary nebulae. Using non-local thermodynamic equilibrium models, we derived the atmospheric parameters of these stars and by fitting their spectral energy distribution we derived their radii, luminosities, and gravity masses. In addition, we derived their masses in the Kiel and Hertzsprung-Russell diagram (HRD). We find that Kiel, HRD, and gravity mass agree only in half of the cases. This is not unexpected and we attribute this to the neglect of metal opacities, possibly stratified atmospheres, as well as possible uncertainties of the parallax zero point determination. Furthermore, we carried out a search for photometric variability in our targets using archival data, finding that 26% of our targets are variable. This includes 15 new variable stars, with only one of them being clearly an irradiation effect system. Strikingly, the majority of the variable stars exhibit non-sinusoidal light-curve shapes, which are unlikely explained in terms of close binary systems. We propose that a significant fraction of all (not just UHE) WDs develop spots when entering the WD cooling phase. We suggest that this could be related to the on-set of weak magnetic fields and possibly diffusion.

Discovery of an extended G giant chromosphere in the 2019 eclipse of γ Per

The November 2019 eclipse of γ Per was a rare opportunity to seek evidence for a chromosphere of the G8 giant, hitherto suspected but not detected. Twenty-nine years after the only other observed eclipse, we aim to find chromospheric absorption in the strong Ca II H&K lines, and to determine its column densities and scale height. Using the Telescopio Internacional de Guanajuato Robótico-Espectroscópico (TIGRE) in Guanajuato (central Mexico) before, during and after the 8 days of total eclipse, we obtained good S/N spectra of the G8 giant alone and composite spectra of the partial phases, near eclipse and far from eclipse. In the near UV of the Ca II H&K and Hϵ lines, the G giant spectrum that was adequately scaled was subtracted from the composite spectra in partial phases, near and far from eclipse, to obtain the A3 companion spectra with and without traces of chromospheric absorption. In addition, we used PHOENIX full non-local thermodynamic equilibrium model atmospheres on the blue A star spectrum, iSpec spectral analysis of the red G giant spectrum, and evolution tracks to study both components of γ Per. For the first time, we present evidence for this rare type of a not very extended G giant chromosphere, reaching out about half of an A-star radius (~1.5 Gm) with a scale height of only 0.17 Gm. By its location in the Hertzsprung-Russell diagram, the γ Per G8 giant is very close to the onset of more extended chromospheres. Furthermore, we show that this giant has a rather inactive chromosphere, and a recent 5 ksec XMM pointing reveals only a very faint, low-energy corona. While the γ Per primary has a mass of ~3.6 M⊙, and its A3 companion has one of ~2.4 M⊙, the latter is too cool (8400 ± 300 K), which is too evolved on the main sequence to be the same age as the primary. The high eccentricity of the 5329.08 days long-period orbit may therefore be reminiscent of a rare capture event. Using the eclipse method, we resolve a pivotal case of a G giant chromosphere, which seems to represent a low-gravity analogue of the inactive Sun. A systematic change of giant chromospheric extent by Hertzsprung-Russell diagram position is confirmed. Compared to the solar chromosphere, the density scale height increases with gravity by ∝ ɡ−1.5.

EPIC 206197016: A very hot white dwarf orbited by a strongly irradiated red dwarf

Context. Very precise satellite photometry has revealed a large number of variable stars whose variability is caused either by surface spots or by binarity. Detailed studies of such variables provide insights into the physics of these objects. Aims. We study the nature of the periodic light variability of the white dwarf EPIC 206197016 that was observed by the K2 mission. Methods. We obtain phase-resolved medium-resolution spectroscopy of EPIC 206197016 using X-shooter spectrograph at VLT to understand the nature of the white dwarf variability. We use non-local thermodynamical equilibrium model atmospheres to determine stellar parameters at individual phases. Results. EPIC 206197016 is a hot DA white dwarf with Teff = 78 kK. The analysis of the spectra reveals periodic radial velocity variations that can result from gravitational interaction with an invisible secondary whose mass corresponds to a red dwarf. The close proximity of the two stars where the semimajor axis is about 3 R⊙ results in the irradiation of the companion with temperatures more than twice as high on the illuminated side compared to the nonilluminated hemisphere. This effect can explain the observed light variations. The spectra of the white dwarf show a particular feature of the Balmer lines called the Balmer line problem, where the observed cores of the lower Balmer lines are deeper than predicted. This can be attributed to either weak pollution of hydrogen in the white dwarf atmosphere by heavy elements or to the presence of a circumstellar cloud or disk.

Solar oxygen abundance using SST/CRISP center-to-limb observations of the O I 7772 Å line

Solar oxygen abundance measurements based on the O I near-infrared triplet have been a much debated subject for several decades, since non-local thermodynamic equilibrium (NLTE) calculations with 3D radiation-hydrodynamics model atmospheres introduced a large change to the 1D LTE modeling. In this work, we aim to test solar line formation across the solar disk using new observations obtained with the SST/CRISP instrument. The observed data set is based on a spectroscopic mosaic that stretches from disk center to the solar limb. By comparing the state-of-the-art 3D NLTE models with the data, we find that the 3D NLTE models provide an excellent description of the line formation across the disk. We obtain an abundance value of A(O)=(8.73 ± 0.03) dex, with a very small angular dispersion across the disk. We conclude that spectroscopic mosaics are excellent probes for geometric and physical properties of hydrodynamics models and NLTE line formation.

New insight into the massive eccentric binary HD 165052: self-consistent orbital solution, apsidal motion, and fundamental parameters

ABSTRACT HD 165052 is a short-period massive eccentric binary system that undergoes apsidal motion. As the rate of apsidal motion is directly related to the internal structure constants of the binary components, its study allows getting insight into the internal structure of the stars. We use medium- and high-resolution spectroscopic observations of HD 165052 to provide constraints on the fundamental properties of the binary system and the evolutionary state of its components. We apply a spectral disentangling code to reconstruct artefact-free spectra of the individual stars and derive the radial velocities (RVs) at the times of the observations. We perform the first analysis of the disentangled spectra with the non-local thermodynamic equilibrium model atmosphere code CMFGEN to determine the stellar properties. We derive the first self-consistent orbital solution of all existing RV data, including those reported in the literature, accounting for apsidal motion. We build, for the very first time, dedicated stellar evolution tracks with the Clés code requesting the theoretical effective temperatures and luminosities to match those obtained from our spectroscopic analysis. The binary system HD 165052, consisting of an O6.5 V((f)) primary ($T_\text{eff,P}=37\, 500 \pm 1000$ K) and an O7 V((f)) secondary ($T_\text{eff,S}=36\, 000 \pm 1000$ K), displays apsidal motion at a rate of $(11.30^{+0.64}_{-0.49})^\circ$ yr−1. Evolutionary masses are compared to minimum dynamical masses to constrain the orbital inclination. Evolutionary masses $M_\text{ev,P}=24.8\pm 1.0\, \mathrm{M}_\odot$ and $M_\text{ev,S}=20.9\pm 1.0\, \mathrm{M}_\odot$ and radii $R_\text{ev,P}=7.0^{+0.5}_{-0.4}\, \mathrm{R}_\odot$ and $R_\text{ev,S}=6.2^{+0.4}_{-0.3}\, \mathrm{R}_\odot$ are derived, and the inclination is constrained to 22.1° ≤ i ≤ 23.3°. Theoretical apsidal motion rates, derived assuming an age of 2.0 ± 0.5 Myr for the binary, are in agreement with the observational determination. The agreement with theoretical apsidal motion rates enforces the inferred values of the evolutionary stellar masses and radii.

Atomic data on inelastic processes in boron–hydrogen collisions with accounting for fine structure

ABSTRACT This paper reports the results of investigations of inelastic processes in B + H and B+ + H− collisions with accounting for the fine structure. The calculations of the cross-sections and rate coefficients for excitation, de-excitation, ion-pair formation, and mutual neutralization processes (240 in total) are performed by using the quantum asymptotic method, which is modified in this paper for accounting of the fine structure, and by the multichannel approach within the framework of the Born–Oppenheimer formalism. It is found that accounting for the fine structure may change the rate coefficients significantly, and the rates calculated with accounting for the fine structure cannot be obtained by a simple redistribution of the non-relativistic rates. The processes important for non-local thermodynamic equilibrium modelling of stellar atmospheres are designated.

Rotational State-to-State Rate Coefficients of HeHNe+ by Collision with He at Low Temperatures

The HeHNe+ molecule is expected to have, after NeH+, the strongest neon bond found so far. Furthermore, formation mechanisms in the interstellar medium (ISM) have been proposed for this molecule. If HeHNe+ is detected, the collisional rate coefficients of this species with the most abundant constituents of the ISM will be critical for employing nonlocal thermodynamic equilibrium (non-LTE) models. Therefore, the main goals of this work are to study the interaction of HeHNe+ with He and to report a set of rotational rate coefficients for this collision at low temperatures. For this purpose, the first potential energy surface for the HeHNe+ + He complex is developed from ab initio calculations. The close-coupling method is employed for studying the dynamics of the system. A |Δj| = 2 propensity rule is found for transitions starting from the rotational state j = 5. Finally, the rate coefficients for the lower 20 rotational states of HeHNe+ are reported.

TUNER-compliant error estimation for MIPAS: methodology

Abstract. This paper describes the error estimation for temperature and trace gas mixing ratios retrieved from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) limb emission spectra. The following error sources are taken into account: measurement noise, propagated temperature and pointing noise, uncertainties in the abundances of spectrally interfering species, instrument line shape errors, and spectroscopic data uncertainties in terms of line intensities and broadening coefficients. Furthermore, both the direct impact of volatile and persistent gain calibration uncertainties, offset calibration, and spectral calibration uncertainties, as well as their impact through propagated calibration-related temperature and pointing uncertainties, are considered. An error source specific to the MIPAS upper atmospheric observation mode is the propagation of the smoothing error crosstalk of the combined NO and temperature retrieval. Whenever non-local thermodynamic equilibrium modelling is used in the retrieval, related kinetic constants and mixing ratios of species involved in the modelling of populations of excitational states also contribute to the error budget. Both generalized Gaussian error propagation and perturbation studies are used to estimate the error components. Error correlations are taken into account. Estimated uncertainties are provided for a multitude of atmospheric conditions. Some error sources were found to contribute both to the random and the systematic component of the total estimated error. The sequential nature of the MIPAS retrievals gives rise to entangled errors. These are caused by error sources that affect the uncertainty in the final data product via multiple pathways, i.e., on the one hand, directly, and, on the other hand, via errors caused in a preceding retrieval step. These errors tend to partly compensate for each other. The hard-to-quantify effect of the horizontally non-homogeneous atmosphere and unknown error correlations of spectroscopic data are considered to be the major limitations of the MIPAS error estimation.

Living on the edge of the Milky Way's central molecular zone

Context. The 1°.3 (G1.3) and 1°.6 (G1.6) cloud complexes in the central molecular zone (CMZ) of our Galaxy have been proposed to possibly reside at the intersection region of the X1 and X2 orbits for several reasons. This includes the detection of co-spatial low- and high-velocity clouds, high velocity dispersion, high fractional molecular abundances of shock-tracing molecules, and kinetic temperatures that are higher than for usual CMZ clouds. Aims. By investigating the morphology and deriving physical properties as well as chemical composition, we want to find the origin of the turbulent gas and, in particular, whether evidence of an interaction between clouds can be identified. Methods. We mapped both cloud complexes in molecular lines in the frequency range from 85 to 117 GHz with the IRAM 30 m telescope. The APEX 12m telescope was used to observe higher frequency transitions between 210 and 475 GHz from selected molecules that are emitted from higher energy levels. We performed non-local thermodynamic equilibrium (non-LTE) modelling of the emission of an ensemble of CH3CN lines to derive kinetic temperatures and H2 volume densities. These were used as starting points for non-LTE modelling of other molecules, for which column densities and abundances were determined and compared with values found for other sources in the CMZ. Results. The kinematic structure of G1.3 reveals an ‘emission bridge’ at intermediate velocities (~150 km s−1) connecting low-velocity (~100 km s−1) and high-velocity (~180 km s−1) gas and an overall fluffy shell-like structure. These may represent observational evidence of cloud-cloud interactions. Low- and high-velocity gas components in G1.6 do not show this type of evidence of an interaction, suggesting that they are spatially separated. We selected three positions in each cloud complex for further analysis. Each position reveals several gas components at various peak velocities and of various line widths. We derived kinetic temperatures of 60–100 K and H2 volume densities of 104–105 cm−3 in both complexes. Molecular abundances relative to H2 suggest a similar chemistry of the two clouds, which is moreover similar to that of other GC clouds and, especially, agrees well with that of G+0.693 and G−0.11. Conclusions. We conclude that G1.3 may indeed exhibit signs of cloud-cloud interactions. In particular, we propose an interaction of gas that is accreted from the near-side dust lane to the CMZ, with gas pre-existing at this location. Low- and high-velocity components in G1.6 are rather coincidentally observed along the same line of sight. They may be associated with either overshot decelerated gas from the far-side dust line or actual CMZ gas and high-velocity gas moving on a dust lane. These scenarios would be in agreement with numerical simulations.

Atomic radiative data for oxygen and nitrogen for solar photospheric studies

Aims. Our recent reanalysis of the solar photospheric spectra with non-local thermodynamic equilibrium (non-LTE) models resulted in higher metal abundances compared to previous works. When applying the new chemical abundances to standard solar model calculations, the new composition resolves the long-standing discrepancies with independent constraints on the solar structure from helioseismology. Methods. Critical to the determination of chemical abundances is the accuracy of the atomic data, especially the f values, used in the radiative transfer models. Here we describe, in detail, the calculations of f values for neutral oxygen and nitrogen used in our non-LTE models. Results. Our calculations of f values are based on a multi-method, multi-code approach and they are the most detailed and extensive of its kind for the spectral lines of interest. We also report in this paper the details of an extensive R-matrix calculation of photoionization cross sections for oxygen. Conclusions. Our calculation resulted in reliable f values with well-constrained uncertainties. We compare our results with previous theoretical and experimental determinations of atomic data. We also quantify the influence of adopted photoionization cross sections on the spectroscopic estimate of the solar O abundance, using data from different sources. We confirm that our 3D non-LTE value is robust and unaffected by the choice of photoionization data, contrary to the recent claim made by Nahar.

The effect of water and electron collisions in the rotational excitation of HF in comets

ABSTRACT We present the first set of rate coefficients for the rotational excitation of the 7 lowest levels of hydrogen fluoride (HF) induced by collision with water molecules, the dominant collider in cometary comas, in the 5–150 K temperature range. The calculations are performed with a quantum statistical approach from an accurate rigid rotor ab initio interaction potential. Rate coefficients for excitation of HF by electron-impact are also computed, within the Born approximation, in the 10–10 000 K temperature range. These rate coefficients are then used in a simple non-local thermodynamic equilibrium (non-LTE) model of a cometary coma that also includes solar radiative pumping and radiative decay. We investigate the range of H2O densities that lead to non-LTE populations of the rotational levels of HF. We show that to describe the excitation of HF in comets, considering collisions with both water molecules and electrons is needed as a result of the large dipole of HF.

First high radiant energy xenon-pipe-based x-ray source on LMJ

We report on the performance of the highest radiant energy x-ray source at the LaserMegaJoule facility (LMJ) for testing material properties under irradiation. This radiation source was produced by the L-shell emission of xenon contained in a gas-pipe. The target was an epoxy ortho-cylinder tube with a diameter of 3 mm and a 50-μm thickness wall containing xenon gas at 1.2 atm. For this proof of principle shot, the LaserMegaJoule facility delivered a total energy of around 90 kJ of 351 nm laser and a total power of 30 TW. Thirty-two beams, divided into eight beamlets each, are arranged into two cones. The laser pulses were nearly flat, with a duration of 3 ns and a power range of 3.5–5 TW for each quadruplet. Two broadband spectrometers, DMX (DP4) and miniDMX (DP11), have been used to characterize the x-ray emission up to 10 keV. Radiant energy up to 0.5 kJ/sr has been recorded in the xenon L-shell band between 3 and 6 keV. This was the highest energy ever delivered for creating an x-ray source on the LMJ. Even though the target was axisymmetric, the strong asymmetric laser irradiation (nor axi or bottom/top symmetric) required full 3D radiation-hydrodynamic calculations to retrieve the radiant energy measured by broad band spectrometers. The main purpose of this work was to benchmark our 3D hydrorad code and the non-local thermodynamic equilibrium model by using newly developed x-ray spectrometers.

The CH4 abundance in Jupiter’s upper atmosphere

Hydrocarbon species, and in particular CH4, play a key role in the stratosphere-thermosphere boundary of Jupiter, which occurs around the μ-bar pressure level. Previous analyses of solar occultation, He and Ly-α airglow, and ISO/SWS measurements of the radiance around 3.3 μm have inferred significantly different methane concentrations. Here we aim to accurately model the CH4 radiance at 3.3 μm measured by ISO/SWS by using a comprehensive non-local thermodynamic equilibrium model and the most recent collisional rates measured in the laboratory for CH4 to shed new light onto the methane concentration in the upper atmosphere of Jupiter. These emission bands have been shown to present a peak contribution precisely at the μ-bar level, hence directly probing the region of interest. We find that a high CH4 concentration is necessary to explain the data, in contrast with the most recent analyses, and that the observations favour the lower limit of the latest laboratory measurements of the CH4 collisional relaxation rates. Our results provide precise constraints on the composition and dynamics of the lower atmosphere of Jupiter.

NLTE effects on kilonova expansion opacities

ABSTRACT A binary neutron star merger produces a rapidly evolving transient known as a kilonova (KN), which peaks a few days after merger. Modelling of KNe has often been approached assuming local thermodynamic equilibrium (LTE) conditions in the ejecta. We present the first analysis of non-local thermodynamic equilibrium (NLTE) level populations, using the spectral synthesis code sumo, and compare these to LTE values. We investigate the importance of the radiation field by conducting NLTE excitation calculations with and without radiative transfer. Level populations, in particular higher lying ones, start to show deviations from LTE several days after merger. Excitation is lower in NLTE for the majority of ions and states, and this tends to give lower expansion opacities. While the difference is small for the first few days, it grows to factors 2–10 after this. Our results are important both for demonstrating validity of LTE expansion opacities for an initial phase (less than a week), while highlighting the need for NLTE modelling during later phases. Considering also NLTE ionization, our results indicate that NLTE can give both higher or lower opacities, depending on composition and wavelength, sometimes by orders of magnitudes.