Non-local Thermodynamic Equilibrium Research Articles

Quantitative spectroscopy of late O-type main-sequence stars with a hybrid non-LTE method

Context. Late O-type stars at luminosities log L/L⊙ ≲ 5.2 show weak winds with mass-loss rates lower than 10−8 M⊙ yr−1. This implies that, unlike their more massive and more luminous siblings, their photospheric layers are not strongly affected by the stellar wind. Aims. A hybrid non-local thermodynamic equilibrium (non-LTE) approach – line-blanketed hydrostatic model atmospheres computed under the assumption of LTE in combination with non-LTE line-formation calculations – is tested for analyses of late O-type stars with masses up to ~25 M⊙. A sample of 20 mostly sharp-lined Galactic O stars of spectral types O8 to O9.7 and luminosity classes V and IV, previously studied in the literature using full non-LTE model atmospheres, is investigated. Methods. Hydrostatic and plane-parallel atmospheric structures and synthetic spectra computed with Kurucz’s ATLAS12 code together with the non-LTE line-formation codes DETAIL and SURFACE, which account for the effects of turbulent pressure on the atmosphere, were employed. High-resolution spectra were analysed for atmospheric parameters using hydrogen lines, multiple ionisation equilibria, and elemental abundances. Fundamental stellar parameters were derived by considering stellar evolution tracks and Gaia Early Data Release 3 (EDR3) parallaxes. Interstellar reddening was characterised by fitting spectral energy distributions from the UV to the mid-IR. Results. A high precision and accuracy is achieved for all derived parameters for 16 sample stars (4 objects show composite spectra). Turbulent pressure effects turn out to be significant for the quantitative analysis. Effective temperatures are determined to 1–3% uncertainty levels, surface gravities to 0.05 to 0.10 dex, masses to better than 8%, radii to better than 10%, and luminosities to better than 20% uncertainty typically. Abundances for C, N, O, Ne, Mg, Al, and Si are derived with uncertainties of 0.05–0.10 dex and for helium within 0.03–0.05 dex (1σ standard deviations) in general. Overall, results from previous studies using unified photosphere plus wind (full) non-LTE model atmospheres are reproduced, and with higher precision. The improvements are most pronounced for elemental abundances, and smaller microturbulent velocities are found. An overall good agreement is found between our spectroscopic distances and those from Gaia. Gaia EDR3-based distances to the Lac OB1b association and to the open clusters NGC 2244, IC 1805, NGC 457, and IC 1396 are determined as a byproduct. The derived N/C versus N/O abundance ratios tightly follow the predictions from stellar evolution models. Two ON stars show a very high degree of mixing of CNO-processed material and appear to stem from binary evolution.

Spectroscopic and interferometric signatures of magnetospheric accretion in young stars

Aims. We aim to assess the complementarity between spectroscopic and interferometric observations in the characterisation of the inner star-disc interaction region of young stars. Methods. We used the MCFOST code to solve the non-local thermodynamic equilibrium problem of line formation in non-axisymmetric accreting magnetospheres. We computed the Brγ line profile originating from accretion columns for models with different magnetic obliquities. We also derived monochromatic synthetic images of the Brγ line-emitting region across the line profile. This spectral line is a prime diagnostic of magnetospheric accretion in young stars and is accessible with the long baseline near-infrared interferometer GRAVITY installed at the ESO Very Large Telescope Interferometer. Results. We derive Brγ line profiles as a function of rotational phase and compute interferometric observables, visibilities, and phases, from synthetic images. The line profile shape is modulated along the rotational cycle, exhibiting inverse P Cygni profiles at the time the accretion shock faces the observer. The size of the line’s emission region decreases as the magnetic obliquity increases, which is reflected in a lower line flux. We apply interferometric models to the synthetic visibilities in order to derive the size of the line-emitting region. We find the derived interferometric size to be more compact than the actual size of the magnetosphere, ranging from 50 to 90% of the truncation radius. Additionally, we show that the rotation of the non-axisymmetric magnetosphere is recovered from the rotational modulation of the Brγ-to-continuum photo-centre shifts, as measured by the differential phase of interferometric visibilities. Conclusions. Based on the radiative transfer modelling of non-axisymmetric accreting magnetospheres, we show that simultaneous spectroscopic and interferometric measurements provide a unique diagnostic to determine the origin of the Brγ line emitted by young stellar objects and are ideal tools to probe the structure and dynamics of the star-disc interaction region.

High-mass eclipsing binaries: A testbed for models of interior structure and evolution

Aims. The surface chemical compositions of stars are affected by physical processes that bring the products of thermonuclear burning to the surface. Despite their potential in helping us understand the structure and evolution of stars, elemental abundances are available for only a few high-mass binary stars. We aim to enlarge this sample by determining the physical properties and photospheric abundances for four eclipsing binary systems that contain high-mass stars: V1034 Sco, GL Car, V573 Car, and V346 Cen. The components have masses of 8–17 M⊙, have effective temperatures from 22 500 to 32 200 K, and are all on the main sequence. Methods. We present new high-resolution and high signal-to-noise spectroscopy from the High Accuracy Radial velocity Planet Searcher (HARPS), which we analysed using spectral disentangling and non-local thermodynamic equilibrium spectral synthesis. We modelled existing light curves and new photometry from the Transiting Exoplanet Survey Satellite (TESS). Results. We measure the stellar masses to a 0.6–2.0% precision, radii to a 0.8–1.7% precision, effective temperatures to a 1.1–1.6% precision, and abundances of C, N, O, Mg, and Si. The abundances are similar to those found in our previous studies of high-mass eclipsing binaries; our sample now comprises 25 high-mass stars in 13 binary systems. We also find tidally excited pulsations in V346 Cen. Conclusions. These results reinforce our previous conclusions: interior chemical element transport is not as efficient in binary star components as in their single-star counterparts in the same mass regime and evolutionary stage, possibly due to the effects of tidal forces. Our ultimate goal is to provide a larger sample of OB-type stars in binaries to enable a thorough comparison to stellar evolutionary models, as well as to single high-mass stars.

Non-local thermodynamic equilibrium analysis of the methylidyne radical molecular lines in metal-poor stellar atmospheres

Aims. An analysis of the methylidyne (CH) radical in non-local thermodynamic equilibrium (NLTE) is performed for the physical conditions of cool stellar atmospheres typical of red giants (log ɡ = 2.0, Teff = 4500 K) and the Sun. The aim of the present work is to explore whether the G band of the CH molecule, which is commonly used in abundance diagnostics of carbon-enhanced metal-poor stars, is sensitive to NLTE effects. Methods. LTE and NLTE theoretical spectra were computed with the MULTI code. We used one-dimensional (1D) LTE hydrostatic MARCS model atmospheres with parameters representing eleven red giant stars with metallicities ranging from [Fe/H] = −4.0 to [Fe/H] = 0.0 and carbon-to-iron ratios of [C/Fe] = 0.0, +0.7, +1.5, and +3.0. The CH molecule model was represented by 1981 energy levels, 18 377 radiative bound-bound transitions, and 932 photo-dissociation reactions. The rates due to transitions caused by collisions with free electrons and hydrogen atoms were computed using classical recipes. Results. Our calculations suggest that NLTE effects in the statistical equilibrium of the CH molecule are significant and cannot be neglected for precision spectroscopic analysis of C abundances. The NLTE effects are mostly driven by radiative over-dissociation, owing to the very low dissociation threshold of the molecule and significant resonances in the photo-dissociation cross-sections. The NLTE effects in the G band increase with decreasing metallicity. When comparing the C abundances determined from the CH G band in LTE and in NLTE, we show that the C abundances are always under-estimated if LTE is assumed. The NLTE corrections to C abundance inferred from the CH feature range from +0.04 dex for the Sun to +0.21 dex for a red giant with metallicity [Fe/H] = −4.0. Conclusions. Departures from the LTE assumption in the CH molecule are non-negligible, and NLTE effects have to be taken into account in the diagnostic spectroscopy based on the CH lines. We show here that the NLTE effects in the optical CH lines are non-negligible for the Sun and red giant stars, but further calculations are warranted to investigate the effects in other regimes of stellar parameters.

Impact of opacity effects on chromospheric oscillations inferred from NLTE inversions

Context. Spectropolarimetric inversions are a fundamental tool for diagnosing the solar atmosphere. Chromospheric inferences rely on the interpretation of spectral lines that are formed under nonlocal thermodynamic equilibrium (NLTE) conditions. In the presence of oscillations, changes in the opacity impact the response height of the spectral lines and hinder the determination of the real properties of the fluctuations. Aims. We aim to explore the relationship between the chromospheric oscillations inferred by NLTE inversion codes and the intrinsic fluctuations in velocity and temperature produced by the waves. Methods. We computed numerical simulations of wave propagation in a sunspot umbra with the code MANCHA. We used the NLTE synthesis and inversion code NICOLE to compute spectropolarimetric Ca II 8542 Å line profiles for the atmospheric models obtained as the output from the simulations. We then inverted the synthetic profiles and compared the inferences from the inversions with the known atmospheres from the simulations. Results. NLTE inversions of the Ca II 8542 Å line capture low-frequency oscillations, including those in the main band of chromospheric oscillations around 6 mHz. In contrast, waves with frequencies above 9 mHz are poorly characterized by the inversion results. Velocity oscillations at those higher frequencies exhibit clear signs of opacity fluctuations; namely the power of the signal at constant optical depth greatly departs from the power of the oscillations at constant geometrical height. The main response of the line to velocity fluctuations comes from low chromospheric heights, whereas the response to temperature shows sudden jumps between the high photosphere and the low chromosphere. This strong variation in the height where the line is sensitive to temperature is revealed as a strong oscillatory power in the inferred fluctuations, which is much stronger than the actual power from the intrinsic temperature oscillations. Conclusions. Our results validate the use of NLTE inversions to study chromospheric oscillations with frequencies below ∼9 mHz. However, the interpretation of higher-frequency oscillations and the power of temperature oscillations must be addressed with care, as these exhibit signatures of opacity oscillations.

The excitation of CO in CO-dominated cometary comae

ABSTRACTAn abundance of CO significantly surpassing the abundance of H2O is observed in the comae of comets at large heliocentric distances. In these environments, CO molecules can be the most abundant species and they may be therefore the dominant projectiles inducing collisional excitation of the cometary molecules. It is thus of high interest to investigate the excitation of CO by CO. This article provides a new set of CO–CO collisional rate coefficients for temperatures up to 150 K and for CO rotational levels j1 up to 10. These data are obtained from quantum scattering calculations using the coupled states approximation. They are used in a simple radiative transfer model in order to test their impact on the excitation of cometary CO. Because mutual (de-)excitations of the target and projectile are important, the CO projectile was assumed to be thermalized at the kinetic temperature. We found that the non-local thermodynamical equilibrium regime extends for CO densities in the range 103–107 cm−3. We also observed that as soon as the CO/H2O ratio is larger than 70 per cent/30 per cent, the contribution of H2O collisions can be neglected. Similarly, the excitation of CO by CO may be ignored for relatively low CO/H2O density ratios (≤30 per cent/70 per cent). Finally, when the coma is a ∼50 per cent/50 per cent mixture of CO and H2O, the contribution of both colliders is similar and has to be considered.

BD +30°549: a young helium-weak silicon star in the NGC 1333 star-forming region

ABSTRACT We present results of the spectroscopic study of the chemically peculiar star BD +30°549, which is a bona fide member of the young NGC 1333 star-forming region. We found that the star possesses negligible rotation and a helium-weak spectroscopic pattern with strongly enhanced Si ii and Si iii lines. The fundamental parameters of the star, Teff = 13 100 K and log (L/L⊙) = 2.1, indicate that its age is about 2.7 Myr and that its position on the Hertzsprung–Russell diagram is at the end of the pre-main-sequence evolutionary track, close to the zero-age main sequence. Abundance analysis reveals the modest deficit of almost all elements, with the exception of Si , Fe , Ca and P, which are overabundant. We performed non-local thermodynamic equilibrium (non-LTE) calculations for Si ii/Si iii, Mg ii and Ca ii in order to check the influence of departures from LTE on line formation. Non-LTE calculations lead to much better reproduction of individual silicon line profiles, but do not completely remove the abundance discrepancy between Si ii and Si iii lines. We also investigate the effects of possible chemical stratification in BD +30°549. We suspect that the ‘Si ii/Si iii anomaly’ observed in the BD +30°549 spectrum arises under the combined action of the vertical and horizontal abundance gradients and non-LTE effects. We suppose that the evolutionary status and phenomena observed in BD +30°549 indicate that conditions favourable for the selective diffusion and formation of the surface chemical composition peculiarities (slow rotation and stabilization of the atmosphere) could already be built up at the pre-main-sequence phase.

Decreasing ultrafast x-ray pulse durations with saturable absorption and resonant transitions.

Saturable absorption is a nonlinear effect where a material's ability to absorb light is frustrated due to a high influx of photons and the creation of electron vacancies. Experimentally induced saturable absorption in copper revealed a reduction in the temporal duration of transmitted x-ray laser pulses, but a detailed account of changes in opacity and emergence of resonances is still missing. In this computational work, we employ nonlocal thermodynamic equilibrium plasma simulations to study the interaction of femtosecond x rays and copper. Following the onset of frustrated absorption, we find that a K-M resonant transition occurring at highly charged states turns copper opaque again. The changes in absorption generate a transient transparent window responsible for the shortened transmission signal. We also propose using fluorescence induced by the incident beam as an alternative source to achieve shorter x-ray pulses. Intense femtosecond x rays are valuable to probe the structure and dynamics of biological samples or to reach extreme states of matter. Shortened pulses could be relevant for emerging imaging techniques.

Transfer learning as a method to reproduce high-fidelity non-local thermodynamic equilibrium opacities in simulations

Simulations of high-energy density physics often need non-local thermodynamic equilibrium opacity data. These data, however, are expensive to produce at relatively low fidelity. It is even more so at high fidelity such that the opacity calculations can contribute 95 % of the total computation time. This proportion can even reach large proportions. Neural networks can be used to replace the standard calculations of low-fidelity data, and the neural networks can be trained to reproduce artificial, high-fidelity opacity spectra. In this work, it is demonstrated that a novel neural network architecture trained to reproduce high-fidelity krypton spectra through transfer learning can be used in simulations. Further, it is demonstrated that this can be done while achieving a relative per cent error of the peak radiative temperature of the hohlraum of approximately 1 % to 4 % while achieving a 19.4 $\times$ speed up.

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.

Non-LTE spectroscopy of the tetradecad region of methane recorded in a hypersonic flow

Laboratory spectroscopic data is essential for the modeling of hot exoplanet atmospheres, since molecules such as methane, a major component of hot-Jupiter-type exoplanet atmospheres, have a complex vibrational energy structure that makes computational predictions difficult at high temperatures for ro-vibrational transitions involving highly excited vibrational sates. To better inform line lists used in radiative transfer modeling, the ro-vibrational spectrum of methane has been recorded in the tetradecad region between 1.7 and 1.65 μm (5880–6060 cm−1) through non-local thermodynamic equilibrium (non-LTE) cavity ringdown spectroscopy (CRDS). Non-LTE conditions, characterized by a low rotational temperature (∼39 K) and a high vibrational temperature (up to 1130 K), have been obtained by hypersonic expansion of a pre-heated mixture of argon and methane in a contoured Laval nozzle. The high vibrational temperature increases the intensity of new hot bands, while the very low rotational temperature greatly simplifies their rotational structure, thus facilitating their identification. A close comparison of the recorded CRDS data to the TheoReTS database reveals both inefficient vibrational relaxation between polyads and efficient vibrational relaxation between vibrational states forming a polyad. These effects result in an overpopulation of the lowest vibrational energy level of each polyad, an effect not widely currently incorporated in non-LTE radiative transfer models. A series of new hot band transitions originating from the pentad and octad polyads were assigned and are provided as a line list for use in future databases.

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.

Non-LTE formation of the Fe I 6173 Å line in the solar atmosphere

The current analysis is dedicated to a detailed investigation of the non-local thermodynamic equilibrium (NLTE) effects influencing the formation of the Fe I 6173 Å line, which is widely used by many instruments, including the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) and the Polarimetric and Helioseismic Imager on board the Solar Orbiter. We synthesize the Stokes profiles in a snapshot of a three-dimensional magnetohydrodynamic simulation of the solar photosphere under both LTE and NLTE conditions. The simulation cube contains a sunspot and a plage region around it. The LTE and NLTE Stokes profiles formed in different features are compared and analysed. NLTE effects are evident in both intensity and polarization profiles. For the 6173 Å line, UV overionization is the dominant NLTE mechanism, and scattering effects are much less important. In addition to Fe, an NLTE treatment of Si, Mg, and Al is necessary to set the right photon density in the UV. This is found to further enhance the LTE departures compared to the case where Fe alone is treated in NLTE. These effects in the Stokes profiles survive even when the profiles are averaged spatially or sampled on a coarse wavelength grid such as that used by the SDO/HMI and other magnetographs. The deviations from the LTE profiles are stronger in the Fe I 6173 Å compared to the 6301 Å–6302 Å lines because in the latter case, line scattering compensates the effect of UV overionization. Based on the nature of departures from LTE, treating the 6173 Å line in LTE will likely result in an overestimation of temperature and an underestimation of the magnetic field strength.

Non-LTE radiative transfer with Turbospectrum

Physically realistic models of stellar spectra are needed in a variety of astronomical studies, from the analysis of fundamental stellar parameters, to studies of exoplanets and stellar populations in galaxies. Here we present a new version of the widely used radiative transfer code Turbospectrum, which we update so that it is able to perform spectrum synthesis for lines of multiple chemical elements in non-local thermodynamic equilibrium (NLTE). We use the code in the analysis of metallicites and abundances of the Gaia FGK benchmark stars, using 1D MARCS atmospheric models and the averages of 3D radiation-hydrodynamics simulations of stellar surface convection. We show that the new more physically realistic models offer a better description of the observed data, and we make the program and the associated microphysics data publicly available, including grids of NLTE departure coefficients for H, O, Na, Mg, Si, Ca, Ti, Mn, Fe, Co, Ni, Sr, and Ba.

Transfer Learning of High-Fidelity Opacity Spectra in Autoencoders and Surrogate Models

Simulations of high energy density physics are expensive, largely in part for the need to produce nonlocal thermodynamic equilibrium opacities. High-fidelity spectra may reveal new physics in the simulations not seen with low-fidelity spectra, but the cost of these simulations also scales with the level of fidelity of the opacities being used. Neural networks are capable of reproducing these spectra, but neural networks need data to train them, which limits the level of fidelity of the training data. This article demonstrates that it is possible to reproduce high-fidelity spectra with median errors in the realm of 3%–4% using as few as 50 samples of high-fidelity Krypton data by performing transfer learning on a neural network trained on many times more low-fidelity data.

Sustained Heating of the Chromosphere and Transition Region Over a Sunspot Light Bridge

Sunspot light bridges (LBs) exhibit a wide range of short-lived phenomena in the chromosphere and transition region. In contrast, we use here data from the Multi-Application Solar Telescope (MAST), the Interface Region Imaging Spectrograph (IRIS), Hinode, the Atmospheric Imaging Assembly (AIA), and the Helioseismic and Magnetic Imager (HMI) to analyze the sustained heating over days in an LB in a regular sunspot. Chromospheric temperatures were retrieved from the MAST Ca ii and IRIS Mg ii lines by nonlocal thermodynamic equilibrium inversions. Line widths, Doppler shifts, and intensities were derived from the IRIS lines using Gaussian fits. Coronal temperatures were estimated through the differential emission measure, while the coronal magnetic field was obtained from an extrapolation of the HMI vector field. At the photosphere, the LB exhibits a granular morphology with field strengths of about 400 G and no significant electric currents. The sunspot does not fragment, and the LB remains stable for several days. The chromospheric temperature, IRIS line intensities and widths, and AIA 171 and 211 Å intensities are all enhanced in the LB with temperatures from 8000 K to 2.5 MK. Photospheric plasma motions remain small, while the chromosphere and transition region indicate predominantly redshifts of 5–20 km s−1 with occasional supersonic downflows exceeding 100 km s−1. The excess thermal energy over the LB is about 3.2 × 1026 erg and matches the radiative losses. It could be supplied by magnetic flux loss of the sunspot (7.5 × 1027 erg), kinetic energy from the increase in the LB width (4 × 1028 erg), or freefall of mass along the coronal loops (6.3 × 1026 erg).

Origins of B-type stars at high Galactic latitudes based on abundances and kinematics

ABSTRACT The spectral analysis and kinematic studies of faint blue stars of O and B types above the mid-plane of the Milky Way can lead to a better understanding of their populations and origins. In this paper, we present an analysis of the chemical composition and kinematics of 17 B-type stars. Our sample includes 13 B-type stars with |Z| > 0.5 kpc and |b| ≥ 20°, three B-type stars with |Z| ≤ 0.5 kpc and |b| < 20°, and a hot post-asymptotic giant branch (post-AGB) star without infrared excess, in the Milky Way. Our analysis rests on medium-resolution spectra and non-local thermodynamic equilibrium tlusty model atmospheres. The ages and masses of the stars are obtained using solar-metallicity PARSEC isochrones, and we calculate their velocities and Galactic trajectories using galpy and data from the Gaia EDR3. The larger projected rotational velocities and results of abundance analysis of 16 B-type stars indicate that they are likely to be B-type stars of normal Population I. The three B-type stars with |Z| ≤ 0.5 kpc and |b| < 20° can be formed in the Galactic disc. The origins of the 13 B-type stars with |Z| > 0.5 kpc and |b| ≥ 20° are as follows. (i) Analysis of the C and Mg abundances, kinematics and orbits show that 10 of them originate in the Galactic disc at different Galactocentric distances. They may be scattered from the disc via the binary ejection mechanism or the dynamical ejection mechanism. (ii) The three stars presenting abnormal C or Mg abundances and counter-rotating Vϕ might be post-AGB stars accreted from a disrupted satellite of the Milky Way.

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.

A high-resolution spectroscopic search for multiple populations in the 2 Gyr old cluster NGC 1846

ABSTRACT We present detailed C, O, Na, Mg, Si, Ca, Ti, V, Fe, Zr, Ba, and Eu abundance measurements for 20 red giant branch (RGB) stars in the Large Magellanic Cloud (LMC) star cluster NGC 1846 ([Fe/H] = −0.59). This cluster is 1.95 Gyr old and lies just below the supposed lower age limit (2 Gyr) for the presence of multiple populations in massive star clusters. Our measurements are based on high- and low-resolution VLT/FLAMES spectra combined with photometric data from Hubble Space Telescope (HST). Corrections for non-local thermodynamic equilibrium effects are also included for O, Na, Mg, Si, Ca, Fe, and Ba. Our results show that there is no evidence for multiple populations in this cluster based on the lack of any intrinsic star-to-star spread in the abundances of Na and O: We place 95 per cent confidence limits on the intrinsic dispersion for these elements of ≤0.07 and ≤0.09 dex, respectively. However, we do detect a significant spread in the carbon abundances, indicating varying evolutionary mixing occurring on the RGB that increases with luminosity. Overall, the general abundance patterns for NGC 1846 are similar to those seen in previous studies of intermediate-age LMC star clusters and field stars.

The solar photospheric silicon abundance according to CO5BOLD

Context. In this work, we present a photospheric solar silicon abundance derived using CO5BOLD model atmospheres and the LINFOR3D spectral synthesis code. Previous works have differed in their choice of a spectral line sample and model atmosphere as well as their treatment of observational material, and the solar silicon abundance has undergone a downward revision in recent years. We additionally show the effects of the chosen line sample, broadening due to velocity fields, collisional broadening, model spatial resolution, and magnetic fields. Aims. Our main aim is to derive the photospheric solar silicon abundance using updated oscillator strengths and to mitigate model shortcomings such as over-broadening of synthetic spectra. We also aim to investigate the effects of different line samples, fitting configurations, and magnetic fields on the fitted abundance and broadening values. Methods. CO5BOLD model atmospheres for the Sun were used in conjunction with the LINFOR3D spectral synthesis code to generate model spectra, which were then fit to observations in the Hamburg solar atlas. We took pixel-to-pixel signal correlations into account by means of a correlated noise model. The choice of line sample is crucial to determining abundances, and we present a sample of 11 carefully selected lines (from an initial choice of 39 lines) in both the optical and infrared, which has been made possible with newly determined oscillator strengths for the majority of these lines. Our final sample includes seven optical Si i lines, three infrared Si i lines, and one optical Si ii line. Results. We derived a photospheric solar silicon abundance of log εSi = 7.57 ± 0.04, including a −0.01 dex correction from Non-Local Thermodynamic Equilibrium (NLTE) effects. Combining this with meteoritic abundances and previously determined photospheric abundances results in a metal mass fraction Z/X = 0.0220 ± 0.0020. We found a tendency of obtaining overly broad synthetic lines. We mitigated the impact of this by devising a de-broadening procedure. The over-broadening of synthetic lines does not substantially affect the abundance determined in the end. It is primarily the line selection that affects the final fitted abundance.