Non-local Thermodynamic Equilibrium Effects Research Articles

Differential abundances of open clusters and their tidal tails: Chemical tagging and chemical homogeneity

Context. Well studied open clusters (OCs) of the solar neighborhood are frequently used as reference objects to test galactic and stellar theories. For that purpose, their chemical composition needs to be known with a high level of confidence. It is also important to clarify if each OC is chemically homogeneous and if it has a unique chemical signature. Aims. The aims of this work are (1) to determine accurate and precise abundances of 22 chemical species (from Na to Eu) in the Hyades, Praesepe, and Rupecht 147 by using a large number of stars at different evolutionary states, (2) to evaluate the level of chemical homogeneity of these OCs, and (3) to compare their chemical signatures. Methods. We gathered ∼800 high resolution and high signal-to-noise spectra of ∼100 members in the three clusters, which were obtained with the latest memberships based on Gaia DR2 data. We built a pipeline, which computes atmospheric parameters and strictly line-by-line differential abundances among twin stars in our sample. With this method, we were able to reach a very high precision in the abundances (0.01–0.02 dex in most of the elements). Results. We find large differences in the absolute abundances in some elements, which can be attributed to diffusion, non-local thermodynamic equilibrium (non-LTE) effects, or systematics in the analysis. For the three OCs, we find strong correlations in the differential abundances between different pairs of elements. According to our experiment with synthetic data, this can be explained by some level of chemical inhomogeneity. We compare differential abundances of several stars from the Hyades and Praesepe tails: The stars that differ more in chemical abundances also have distinct kinematics, even though they have been identified as members of the tail. Conclusions. It is possible to obtain high precision abundances using a differential analysis even when mixing spectra from different instruments. With this technique, we find that the Hyades and Preasepe have the same chemical signature when G dwarfs and K giants are considered. Despite a certain level of inhomogeneity in each cluster, it is still possible to clearly distinguish the chemical signature of the older cluster Ruprecht 147 when compared to the Hyades and Praesepe.

Effective temperatures of red supergiants estimated from line-depth ratios of iron lines in the YJ bands, 0.97-1.32μm

ABSTRACT Determining the effective temperatures (Teff) of red supergiants (RSGs) observationally is important in many fields of stellar physics and galactic astronomy, yet some significant difficulties remain as a result of the model uncertainty, which originates mainly in the extended atmospheres of RSGs. Here we propose a line-depth ratio (LDR) method in which we use only Fe i lines. As opposed to the conventional LDR method, in which lines of multiple species are involved, this LDR method is insensitive to surface gravity effects and is expected to circumvent the uncertainty originating in the upper atmosphere of RSGs. Therefore, the LDR–Teff relationships that we calibrated empirically with red giants can be directly applied to RSGs, although various differences between the two groups of objects, for example caused by 3D non-local thermodynamic equilibrium (non-LTE) effects, need to be kept in mind. Using the near-infrared YJ-band spectra of nine well-known solar-metallicity red giants observed with the WINERED high-resolution spectrograph, we selected 12 pairs of Fe i lines that were the least contaminated with other lines. After applying their LDR–Teff relation to 10 nearby RSGs, the resulting Teff with an internal precision of $30\!-\!70\, \mathrm{K}$ shows good agreement with previous observational results assuming 1D LTE and with the Geneva stellar evolution model. We found no evidence of significant systematic bias caused by various differences, including those in the size of the non-LTE effects, between red giants and RSGs, except for one line pair that we rejected because the non-LTE effects may be as large as $\sim \! 250\, \mathrm{K}$. Nevertheless, it is difficult to evaluate the systematic bias, and further study is required, for example including the 3D non-LTE calculations of all the lines involved.

KCARTA: a fast pseudo line-by-line radiative transfer algorithm with analytic Jacobians, fluxes, nonlocal thermodynamic equilibrium, and scattering for the infrared

Abstract. A fast pseudo-monochromatic radiative transfer package using a singular value decomposition (SVD) compressed atmospheric optical depth database has been developed, primarily for simulating radiances from hyperspectral sounding instruments (resolution ≥0.1 cm−1). The package has been tested extensively for clear-sky radiative transfer cases, using field campaign data and satellite instrument data. The current database uses HITRAN 2016 line parameters and is primed for use in the spectral region spanning 605 to 2830 cm−1. Optical depths for other spectral regions (15–605 and 2830–45 000 cm−1) can also be generated for use by kCARTA. The clear-sky radiative transfer model computes the background thermal radiation quickly and accurately using a layer-varying diffusivity angle at each spectral point; it takes less than 30 s (on a 2.8 GHz core using four threads) to complete a radiance calculation spanning the infrared. The code can also compute non-local thermodynamic equilibrium effects for the 4 µm CO2 region, as well as analytic temperature, gas and surface Jacobians. The package also includes flux and heating rate calculations and an interface to an infrared scattering model.

Capabilities of bisector analysis of the Si I 10 827 Å line for estimating line-of-sight velocities in the quiet Sun

We examine the capabilities of a fast and simple method to infer line-of-sight (LOS) velocities from observations of the photospheric Si I 10 827 Å line. This spectral line is routinely observed together with the chromospheric He I 10 830 Å triplet as it helps to constrain the atmospheric parameters. We study the accuracy of bisector analysis and a line core fit of Si I 10 827 Å. We employ synthetic profiles starting from the Bifrost enhanced network simulation. The profiles are computed solving the radiative transfer equation, including non-local thermodynamic equilibrium effects on the determination of the atomic level populations of Si I. We found a good correlation between the inferred velocities from bisectors taken at different line profile intensities and the original simulation velocity at given optical depths. This good correlation means that we can associate bisectors taken at different line-profile percentages with atmospheric layers that linearly increase as we scan lower spectral line intensities. We also determined that a fit to the line-core intensity is robust and reliable, providing information about atmospheric layers that are above those accessible through bisectors. Therefore, by combining both methods on the Si I 10 827 Å line, we can seamlessly trace the quiet-Sun LOS velocity stratification from the deep photosphere to higher layers until around logτ = −3.5 in a fast and straightforward way. This method is ideal for generating quick-look reference images for future missions like the Daniel K. Inoue Solar Telescope and the European Solar Telescope, for example.

The influence of NLTE effects in Fe I lines on an inverted atmosphere

Context. Ultraviolet overionisation of iron atoms in the solar atmosphere leads to deviations in their level populations based on Saha-Boltzmann statistics. This causes their line profiles to form in non-local thermodynamic equilibrium (NLTE) conditions. When inverting such profiles to determine atmospheric parameters, the NLTE effects are often neglected and other quantities are tweaked to compensate for deviations from the LTE. Aims. We investigate how the routinely employed LTE inversion of iron lines formed in NLTE underestimates or overestimates atmospheric quantities, such as temperature (T), line-of-sight velocity (vLOS), magnetic field strength (B), and inclination (γ) while the earlier papers have focused mainly on T. Our findings has wide-ranging consequences since many results derived in solar physics are based on inversions of Fe I lines carried out in LTE. Methods. We synthesized the Stokes profiles of Fe I 6301.5 Å and 6302.5 Å lines in both LTE and NLTE using a snapshot of a 3D magnetohydrodynamic simulation. The profiles were then inverted in LTE. We considered the atmosphere inferred from the inversion of LTE profiles as the fiducial model and compared it to the atmosphere resulting from the inversion of NLTE profiles. The observed differences have been attributed to NLTE effects. Results. Neglecting the NLTE effects introduces errors in the inverted atmosphere. While the errors in T can go up to 13%, in vLOS and B, the errors can go as high as 50% or above. We find these errors to be present at all three inversion nodes. Importantly, they survive degradation from the spatial averaging of the profiles. Conclusions. We provide an overview of how neglecting NLTE effects influences the values of T, vLOS, B, and γ that are determined by inverting the Fe I 6300 Å line pair, as observed, for example, by Hinode/SOT/SP. Errors are found at the sites of granules, intergranular lanes, magnetic elements, and basically in every region susceptible to NLTE effects. For an accurate determination of the atmospheric quantities and their stratification, it is, therefore, important to take the NLTE effects into account.

The Space Weather Atmosphere Models and Indices (SWAMI) project: Overview and first results

Space weather driven atmospheric density variations affect low Earth orbit (LEO) satellites during all phases of their operational lifetime. Rocket launches, re-entry events and space debris are also similarly affected. A better understanding of space weather processes and their impact on atmospheric density is thus critical for satellite operations as well as for safety issues. The Horizon 2020 project Space Weather Atmosphere Model and Indices (SWAMI) project, which started in January 2018, aims to enhance this understanding by:Developing improved neutral atmosphere and thermosphere models, and combining these models to produce a new whole atmosphere model.Developing new geomagnetic activity indices with higher time cadence to enable better representation of thermospheric variability in the models, and improving the forecast of these indices.The project stands out by providing an integrated approach to the satellite neutral environment, in which the main space weather drivers are addressed together with model improvement. The outcomes of SWAMI will provide a pathway to improved space weather services as the project will not only address the science issues, but also the transition of models into operational services.The project aims to develop a unique new whole atmosphere model, by extending and blending the Unified Model (UM), which is the Met Office weather and climate model, and the Drag Temperature Model (DTM), which is a semi-empirical model which covers the 120–1500 km altitude range. A user-focused operational tool for satellite applications shall be developed based on this. In addition, improved geomagnetic indices shall be developed and shall be used in the UM and DTM for enhanced nowcast and forecast capability.In this paper, we report on progress with SWAMI to date. The UM has been extended from its original upper boundary of 85 km to run stably and accurately with a 135 km lid. Developments to the UM radiation scheme to enable accurate performance in the mesosphere and lower thermosphere are described. These include addition of non-local thermodynamic equilibrium effects and extension to include the far ultraviolet and extreme ultraviolet. DTM has been re-developed using a more accurate neutral density observation database than has been used in the past. In addition, we describe an algorithm to develop a new version of DTM driven by geomagnetic indices with a 60 minute cadence (denoted Hp60) rather than 3-hourlyKpindices (and corresponding ap indices). The development of the Hp60 index, and the Hp30 and Hp90 indices, which are similar to Hp60 but with 30 minute and 90 minute cadences, respectively, is described, as is the development and testing of neural network and other machine learning methods applied to the forecast of geomagnetic indices.

On the statistical properties of a hydrogenic atom broadened by linear Stark effect

Spectroscopy based on Stark broadening is a very efficient and successful method for diagnosing the physical conditions in laboratory or astrophysical plasmas. The studies are often focused on the lines of hydrogen or hydrogen-like ions due to the simple atomic structure. However, the Stark modeling may be difficult even for one-electron systems, in particular when radiative transitions with highly-degenerate Rydberg states are considered. Besides, the computations may become challenging for the modeling of line shapes in an optically thick plasma and accounting for non-local thermodynamical equilibrium (non-LTE) effects on the state populations. The statistical approach is a powerful technique which can be used to simplify complex coupled problems by replacing the numerous levels or lines by a single continuous distribution characterized by a reduced number of exact moments: area, center-of-gravity, variance, etc. In this paper, we study various statistical properties of a hydrogenic atom broadened by Stark effect. First, we derive the moments of level energies accounting for linear and quadratic Stark effects, electric-quadrupole and fine structure corrections. The resulting formulas allows one to estimate the relative importance of these corrections on the energies. We then derive the moments of the strength-weighted line energies for the Lyman and Balmer series in the framework of Linear-Stark effect. The moments are then used in a Gram–Charlier expansion series to represent the line shape, the resolution of the spectrum being controlled by the order at which the series is truncated. We show that the line shape of the transition can be expressed using universal functions of n and , independent of the atomic number and of the field intensity. This allows for fast Stark broadening calculations which can be used in more complex simulations (in opacity or emissivity codes, for radiative transfer simulations or non-LTE models).

Evaluation of the Recent Atomic Data for Fe i Lines Based on Solar and Stellar Spectra

We performed an extensive comparative analysis of the recent experimental data on Fe i transition probabilities (TP) based on the observed solar and stellar spectra. This work is part of the Vienna Atomic Line database (VALD) activities. Our motivation is to keep the VALD line list as complete as possible and provide VALD users with substantiated recommendations for the most accurate atomic data. For assessment of data quality we choose the “normal” (as opposed to peculiar and/or magnetic) Main Sequence sharp-line stars of different effective temperatures with accurately known atmospheric parameters: Sun, Procyon, HD 32115, and 21 Peg. Theoretical spectra of these stars were synthesized for 1D plane-parallel model atmospheres accounting for non-local thermodynamic equilibrium (NLTE) effects, and then they were compared with the observations. For all of the stars we derived atmospheric abundances based on the new and previous experimental TP. When using the new TPs the best agreement for iron is achieved with the TP of the Hannover group (Bard, Kock, and Kock, Astron. Astrophys.248, 315, 1991; Bard and Kock, Astron. Astrophys.282, 1014, 1994). The Hannover set is recommended as the primary source of transition probabilities in stellar abundance analysis. The new TP data significantly increase the number of spectral lines of Fe i in the optical and red spectral regions available for accurate abundance analysis of stars in a wide range of temperatures and metallicities. Comparison with the observed stellar spectra invalidates some new experimental data despite the small given uncertainty of the laboratory measurements. Finally, we note the importance of accurate line-broadening data.

Photoionization Calculations of the Radiation Force Due To Spectral Lines in AGNs

Abstract One of the main mechanisms that could drive mass outflows in active galactic nuclei (AGNs) is radiation pressure due to spectral lines. Although straightforward to understand, the actual magnitude of the radiation force is challenging to compute because the force depends on the physical conditions in the gas, as well as the strength, spectral energy distribution (SED), and geometry of the radiation field. We present results from our photoionization and radiation transfer calculations of the force multiplier, M(ξ, t), using the same radiation field to compute the gas photoionization and thermal balance. We assume low gas density (n = 104 cm−3) and column density (N ≤ 1017 cm−2), a Boltzmann distribution for the level populations, and the Sobolev approximation. Here, we describe results for two SEDs corresponding to an unobscured and obscured AGN in NGC 5548. Our main results are the following: (1) although M(ξ, t) starts to decrease with ξ for ξ ≳ 1 as shown by others, this decrease in our calculations is relatively gradual and could be nonmonotonic as M(ξ, t) can increase by a factor of a few for ξ ≈ 10–1000; (2) at these same ξ for which the multiplier is higher than in previous calculations, the gas is thermally unstable by the isobaric criterion; (3) non-local thermodynamic equilibrium effects reduce M(t, ξ) by over two orders of magnitude for ξ ≳ 100. The dynamical consequence of result (1) is that line driving can be important for ξ as high as 1000 when the LTE approximation holds, while result (2) provides a natural cloud formation mechanism that may account for the existence of narrow line regions. Result (3) suggests that line driving may not be important for ξ ≳ 100 in tenuous plasma.

Non-LTE Analyses of High-resolution H-band Spectra. III. Neutral and Singly Ionized Calcium

Abstract A calcium atomic model is updated using collisional rates from new quantum-mechanical computations. We investigated the reliability of the model atom and the non-local thermodynamic equilibrium (NLTE or non-LTE) effects on the formation of Ca i lines in both optical and H bands. NLTE and local thermodynamical equilibrium calcium abundances of 13 sample stars are derived from high-resolution and high signal-to-noise ratio optical spectra and the Apache Point Observatory Galactic Evolution Experiment (APOGEE) data. The results suggest that the updated calcium atomic model is appropriate for studying the formation of H-band calcium lines because our NLTE analyses not only reduce the line-to-line spread of derived abundances, but also lead to consistent optical-line-based and H-band-line-based abundances with a difference of −0.009 ± 0.035 dex and a calcium ionization balance between Ca i and Ca ii. In the stellar fundamental parameter space that our sample stars cover, NLTE corrections for H-band Ca i lines are small, within 0.03 dex. Even in the extreme cases of the APOGEE data, the NLTE corrections on selected H-band Ca i lines are within 0.1 dex.

Non-LTE chemical abundances in Galactic open and globular clusters

Aims. We study the effects of non-local thermodynamic equilibrium (NLTE) on the determination of stellar parameters and abundances of Fe, Mg, and Ti from the medium-resolution spectra of FGK stars. Methods. We extended the Payne fitting approach to draw on NLTE and LTE spectral models. These were used to analyse the spectra of the Gaia-ESO benchmark stars and the spectra of 742 stars in 13 open and globular clusters in the Milky Way: NGC 3532, NGC 5927, NGC 2243, NGC 104, NGC 1851, NGC 2808, NGC 362, M 2, NGC 6752, NGC 1904, NGC 4833, NGC 4372, and M15. Results. Our approach accurately recovers effective temperatures, surface gravities, and abundances of the benchmark stars and clusters members. The differences between NLTE and LTE are significant in the metal-poor regime, [Fe/H] ≲ −1. The NLTE [Fe/H] values are systematically higher, whereas the average NLTE [Mg/Fe] abundance ratios are ∼0.15 dex lower, compared to LTE. Our LTE measurements of metallicities and abundances of stars in Galactic clusters are in a good agreement with the literature. Though, for most clusters, our study yields the first estimates of NLTE abundances of Fe, Mg, and Ti. Conclusion. All clusters investigated in this work are homogeneous in Fe and Ti, with the intra-cluster abundance variations of less then 0.04 dex. NGC 2808, NGC 4833, M 2, and M 15 show significant dispersions in [Mg/Fe]. Contrary to common assumptions, the NLTE analysis changes the mean abundance ratios in the clusters, but it does not influence the intra-cluster abundance dispersions.

Formation and morphology of anomalous solar circular polarization

Context. The morphology of spectral line polarization is the most valuable observable to investigate the magnetic and dynamic solar atmosphere. However, in order to develop solar diagnosis, it is fundamental to understand the different kinds of anomalous solar signals that are routinely found in linear and circular polarization (LP,CP). Aims. We aim to explain and characterize the morphology of solar CP signals considering nonlocal thermodynamical equilibrium (NLTE) effects. Methods. An analytical two-layer model of the polarized radiative transfer equation is developed and used to solve the NLTE problem with atomic polarization in a semi-parametric way. The potential of the model for reproducing anomalous CP is shown with detailed calculations and examples. A new approach based on the zeroes of polarization signals is developed to explain their morphology. Results. We have obtained a comprehensive model that insightfully describes the formation of solar polarization with certain precision without sacrificing key physical ingredients or resorting to complex atmospheric models. The essential physical behavior of dichroism and atomic orientation has been described, introducing the concepts of dichroic inversion, neutral and reinforcing medium, critical intensity spectrum, and critical source function. We show that the zero-crossings of the CP spectrum are useful to classify its morphology and understand its formation. This led to identification and explanation of the morphology of the seven most characteristic CP signals that a single (depth-resolved) scattering layer can produce. We find that a minimal number of two magnetic layers along the line of sight is required to fully explain anomalous solar CP signals and that the morphology and polarity of Stokes V depends on magnetic, radiative, and atomic “polarities”. Some implications of these results are presented through a preliminary modeling of anomalous CP signals in the Fe I 1564.8 nm and Na I D lines.

NLTE modelling of integrated light spectra

Aims.We study the effects of non-local thermodynamic equilibrium (NLTE) on the abundance analysis of barium, magnesium, and manganese from integrated light spectroscopy, as typically applied to the analysis of extra-galactic star clusters and galaxies. In this paper, our reference object is a synthetic simple stellar population (SSP) representing a mono-metallicα-enhanced globular cluster with the metallicity [Fe/H] = −2.0 and the age of 11 Gyr.Methods.We used the MULTI2.3 program to compute LTE and NLTE equivalent widths of spectral lines of Mg I, Mn I, and Ba II ions, which are commonly used in abundance analyses of extra-galactic stellar populations. We used ATLAS12 model atmospheres for stellar parameters sampled from a model isochrone to represent individual stars in the model SSP. The NLTE and LTE equivalent widths calculated for the individual stars were combined to calculate the SSP NLTE corrections.Results.We find that the NLTE abundance corrections for the integrated light spectra of the metal-poor globular cluster are significant in many cases, and often exceed 0.1 dex. In particular, LTE abundances of Mn are consistently under-estimated by 0.3 dex for all optical lines of Mn I studied in this work. On the other hand, Ba II, and Mg I lines show a strong differential effect: the NLTE abundance corrections for the individual stars and integrated light spectra are close to zero for the low-excitation lines, but they amount to − 0.15 dex for the strong high-excitation lines. Our results emphasise the need to take NLTE effects into account in the analysis of spectra of individual stars and integrated light spectra of stellar populations.

High-precision analysis of binary stars with planets

Aims. We explore for the first time the probable chemical signature of planet formation in the remarkable binary system HD 106515. Star A hosts a massive long-period planet with ~9 MJup detected by radial velocity, while there is no planet detected at the B star. We also refine stellar and planetary parameters by using non-solar-scaled opacities when modelling the stars. Methods. We carried out a simultaneous determination of stellar parameters and abundances by applying for the first time non-solar-scaled opacities in this binary system, in order to reach the highest possible precision. We used a line-by-line strictly differential approach, using the Sun and then the A star as reference. Stellar parameters were determined by imposing an ionization and excitation balance of Fe lines, with an updated version of the FUNDPAR program, ATLAS12 model atmospheres, and the MOOG code. Opacities for an arbitrary composition were calculated through the opacity sampling method. The chemical patterns were compared with solar-twins condensation temperature Tc trends from the literature and also mutually between both stars. We take the opportunity to compare and discuss the results of the classical solar-scaled method and the high-precision procedure applied here. Results. Stars A and B in the binary system HD 106515 do not seem to be depleted in refractory elements, which is different when comparing the Sun with solar twins. The terrestrial planet formation would have been less efficient in the stars of this binary system. Together with HD 80606/7, this is the second binary system that does not seem to present a (terrestrial) signature of planet formation, when both systems host an eccentric giant planet. This is in agreement with numerical simulations, where the early dynamical evolution of eccentric giant planets clears out most of the possible terrestrial planets in the inner zone. We refined the stellar mass, radius, and age for both stars and found a notable difference of ~78% in R⋆ compared to previous works. We also refined the planet mass to mp sini = 9.08 ± 0.20 MJup, which differs by ~6% compared with the literature. In addition, we showed that the non-solar-scaled solution is not compatible with the classical solar-scaled method, and some abundance differences are comparable to non-local thermodynamic equilibrium (NLTE) or galactic chemical evolution (GCE) effects especially when using the Sun as reference. Therefore, we encourage the use of non-solar-scaled opacities in high-precision studies such as the detection of Tc trends.

NLTE Analysis of Copper Abundances in the Galactic Bulge Stars

Abstract Based on the medium-high resolution (R ∼ 20,000), modest signal-to-noise ratio (S/N ≳ 70) FLAMES-GIRAFFE spectra, we investigated the copper abundances of 129 red giant branch stars in the Galactic bulge with [Fe/H] from −1.14 to 0.46 dex. The copper abundances are derived from both local thermodynamic equilibrium (LTE) and nonlocal thermodynamic equilibrium (NLTE) with the spectral synthesis method. We find that the NLTE effects for Cu i lines show a clear dependence on metallicity, and they gradually increase with decreasing [Fe/H] for our sample stars. Our results indicate that the NLTE effects of copper are important not only for metal-poor stars but also for supersolar metal-rich ones and the LTE results underestimate the Cu abundances. We note that the [Cu/Fe] trend of the bulge stars is similar to that of the Galactic disk stars spanning the metallicity range of −1.14 < [Fe/H] < 0.0 dex, and the [Cu/Fe] ratios increase with increasing metallicity when [Fe/H] is from ∼−1.2 to ∼−0.5 dex, favoring a secondary (metallicity-dependent) production of Cu.

STiC: A multiatom non-LTE PRD inversion code for full-Stokes solar observations

The inference of the underlying state of the plasma in the solar chromosphere remains extremely challenging because of the nonlocal character of the observed radiation and plasma conditions in this layer. Inversion methods allow us to derive a model atmosphere that can reproduce the observed spectra by undertaking several physical assumptions. The most advanced approaches involve a depth-stratified model atmosphere described by temperature, line-of-sight velocity, turbulent velocity, the three components of the magntic field vector, and gas and electron pressure. The parameters of the radiative transfer equation are computed from a solid ground of physical principles. In order to apply these techniques to spectral lines that sample the chromosphere, nonlocal thermodynamical equilibrium effects must be included in the calculations. We developed a new inversion code STiC (STockholm inversion Code) to study spectral lines that sample the upper chromosphere. The code is based on the RH forward synthesis code, which we modified to make the inversions faster and more stable. For the first time, STiC facilitates the processing of lines from multiple atoms in non-LTE, also including partial redistribution effects (PRD) in angle and frequency of scattered photons. Furthermore, we include a regularization strategy that allows for model atmospheres with a complex depth stratification, without introducing artifacts in the reconstructed physical parameters, which are usually manifested in the form of oscillatory behavior. This approach takes steps toward a node-less inversion, in which the value of the physical parameters at each grid point can be considered a free parameter. In this paper we discuss the implementation of the aforementioned techniques, the description of the model atmosphere, and the optimizations that we applied to the code. We carry out some numerical experiments to show the performance of the code and the regularization techniques that we implemented. We made STiC publicly available to the community.

Exploring circumstellar effects on the lithium and calcium abundances in massive Galactic O-rich AGB stars

Context. We previously explored the circumstellar effects on Rb and Zr abundances in a sample (21) of massive Galactic O-rich asymptotic giant branch (AGB) stars. Here we are interested in clarifying the role of the extended atmosphere in the case of Li and Ca. Li is an important indicator of hot bottom burning while the total Ca abundances in these stars could be affected by neutron captures. Aims. We report new Li and Ca abundances in a larger sample (30) of massive Galactic O-rich AGB stars by using more-realistic extended model atmospheres. Li abundances had previously studied with hydrostatic models, while the Ca abundances have been determined here for the first time. Methods. We used a modified version of the spectral synthesis code Turbospectrum and consider the presence of a gaseous circumstellar envelope and radial wind in the modelling of the spectra of these massive AGB stars. The Li and Ca abundances were obtained from the 6708 Å Li I and 6463 Å Ca I resonance lines, respectively. In addition, we studied the sensitivity of the pseudo-dynamical models to variations of the stellar and wind parameters. Results. The Li abundances derived with the pseudo-dynamical models are very similar to those obtained from hydrostatic models (the average difference is 0.18 dex, σ2 = 0.02), with no difference for Ca. This indicates that the Li and Ca content in these stars is only slightly affected by the presence of a circumstellar envelope. We also found that the Li I and Ca I line profiles are not very sensitive to variations of the model wind parameters. Conclusions. The new Li abundances confirm the Li-rich (and super Li-rich, in some cases) nature of the sample stars, supporting the activation of hot bottom burning in massive Galactic AGB stars. This is in good agreement with the theoretical predictions for solar metallicity AGB models from ATON, Monash, and NuGrid/MESA but is at odds with the FRUITY database, which predicts no hot bottom burning leading to the production of Li. Most (20) sample stars display nearly solar (within the estimated errors and considering possible non-local thermodynamic equilibrium effects) Ca abundances that are consistent with the available s-process nucleosynthesis models for solar metallicity massive AGB stars, which predict overproduction of 46Ca relatively to the other Ca isotope and the creation of the radioactive isotope 41Ca (half life of 0.1 Myr) but no change in the total Ca abundance. A minority (five) of the sample stars seem to show a significant Ca depletion (by up to 1.0 dex). Possible explanations are offered to explain their apparent and unexpected Ca depletion.

Efficient Calculation of Non-local Thermodynamic Equilibrium Effects in Multithreaded Hydrodynamic Simulations of Solar Flares

Abstract Understanding the dynamics of the chromosphere is crucial to understanding energy transport across the solar atmosphere. The chromosphere is optically thick at many wavelengths and described by non-local thermodynamic equilibrium (NLTE), making it difficult to interpret observations. Furthermore, there is considerable evidence that the atmosphere is filamented, and that current instruments do not resolve small-scale features. In flares, it is likely that multithreaded models are required to describe the heating. The combination of NLTE effects and multithreaded modeling requires computationally demanding calculations, which has motivated the development of a model that can efficiently treat both. We describe the implementation of a solver in a hydrodynamic code for the hydrogen level populations that approximates the NLTE solutions. We derive an accurate electron density across the atmosphere that includes the effects of nonequilibrium ionization for helium and metals. We show the effects on hydrodynamic simulations, which are used to synthesize light curves using a postprocessing radiative transfer code. We demonstrate the utility of this model on IRIS observations of a small flare. We show that the Doppler shifts in Mg ii, C ii, and O i can be explained with a multithreaded model of loops subjected to electron beam heating, so long as NLTE effects are treated. The intensities, however, do not match the observed values very well, which is due to assumptions about the initial atmosphere. We briefly show how altering the initial atmosphere can drastically alter line profiles and derived quantities and suggest that it should be tuned to preflare observations.

NLTE Stellar Population Synthesis of Globular Clusters Using Synthetic Integrated Light Spectra. II. Expanded Photometry and Sensitivity of Near-IR Spectral Features to Cluster Age and Metallicity

Abstract We present an expanded investigation of the library of globular cluster (GC) synthetic integrated light (IL) spectra of Young & Short, focusing on the impact of non-local thermodynamic equilibrium (NLTE) modeling effects on cluster parameters derived from photometric colors and sensitivity of near-IR spectral features to cluster age and metallicity. Johnson–Cousins–Bessel UBVIJK photometric colors are produced for 910 synthetic IL spectra with two degrees of α enhancement, in both NLTE and local thermodynamic equilibrium (LTE). These color values are used to investigate the GC age–metallicity degeneracy and compare NLTE and LTE derived [M/H] values for NGC 104, NGC 5139, and NGC 6205. For a given age, derived [M/H] values are shown to increase by up to 0.05 dex when modeled in NLTE. A total of 86 spectral lines in the range λ = 12000–22000 Å, representing 14 different atomic species, were identified as sensitive to either cluster age or metallicity, 12 of which were identified as sensitive to both. Equivalent widths of the lines are measured in NLTE and LTE spectra, with NLTE effects changing the widths by up to Å depending on the atomic species.

Monte-Carlo methods for NLTE spectral synthesis of supernovae

We present JEKYLL, a new code for modelling of supernova (SN) spectra and lightcurves based on Monte-Carlo (MC) techniques for the radiative transfer. The code assumes spherical symmetry, homologous expansion and steady state for the matter, but is otherwise capable of solving the time-dependent radiative transfer problem in non-local-thermodynamic-equilibrium (NLTE). The method used was introduced in a series of papers by Lucy, but the full time-dependent NLTE capabilities of it have never been tested. Here, we have extended the method to include non-thermal excitation and ionization as well as charge-transfer and two-photon processes. Based on earlier work, the non-thermal rates are calculated by solving the Spencer-Fano equation. Using a method previously developed for the SUMO code, macroscopic mixing of the material is taken into account in a statistical sense. To save computational power a diffusion solver is used in the inner region, where the radiation field may be assumed to be thermalized. In addition, a statistical Markov-chain model is used to sample the emission frequency more efficiently, and we introduce a method to control the sampling of the radiation field, which is used to reduce the noise in the radiation field estimators. Except for a description of JEKYLL, we provide comparisons with the ARTIS, SUMO and CMFGEN codes, which show good agreement in the calculated spectra as well as the state of the gas. In particular, the comparison with CMFGEN, which is similar in terms of physics but uses a different technique, shows that the Lucy method does indeed converge in the time-dependent NLTE case. Finally, as an example of the time-dependent NLTE capabilities of JEKYLL, we present a model of a Type IIb SN, taken from a set of models presented and discussed in detail in an accompanying paper. Based on this model we investigate the effects of NLTE, in particular those arising from non-thermal excitation and ionization, and find strong effects even on the bolometric lightcurve. This highlights the need for full NLTE calculations when simulating the spectra and lightcurves of SNe.