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Context. The Mid-InfraRed Instrument (MIRI) on board the James Webb Space Telescope (JWST) gives unique access to the physical and chemical structure of inner disks (<10 au), where the majority of the planets are forming. However, the interpretation of mid-infrared (mid-IR) spectra requires detailed thermo-chemical models able to provide synthetic spectra readily comparable to spectroscopic observations. This is particularly important for OH, which can be excited by a number of processes. Aims. Our goal is to explore the potential of mid-IR emission of OH to probe H2O photodissociation, and thus implicitly the far-ultraviolet (FUV) radiation field in the inner disks. Methods. We include in the DALI disk model prompt emission of OH following photodissociation of H2O in its B~ electronic state by photons at λ < 144 nm. Compared with previous modeling work, we also take into account the propensity of forming OH in the A′ symmetric states. This model allows us to compute in a self-consistent manner the thermal and chemical structure of the disk and the resulting mid-IR line intensities of OH and H2O. Results. The OH line intensities in the 9–13 μm range are proportional to the total amount of water photodissociated in the disk. As such, these OH lines are a sensitive tracer of the amount of H2O exposed to the FUV field, which depends on the temperature, density, and strength of the FUV field reaching the upper molecular layers. In particular, we show that the OH line fluxes primarily scale with the FUV field emitted by the central star in contrast with H2O lines in the 10–20 μm range which scale with the bolometric luminosity. OH is therefore an important diagnostic to probe the effect of Lyα and constrain the dust FUV opacity in upper molecular layers. A strong asymmetry between the A′ and A″ components of each rotational quadruplet is predicted. Conclusions. OH mid-IR emission is a powerful tool to probe H2O photodissociation and infer the physical conditions in disk atmospheres. As such, the inclusion of OH mid-IR lines in the analysis of JWST-MIRI spectra will be crucial for robustly inferring the chemical composition of planet-forming disks. The interpretation of less excited OH lines in the MIRI-MRS range requires additional quantum calculations of the formation pumping of OH (ro-)vibrational levels by O+H2 and the collisional rate coefficients. [ABSTRACT FROM AUTHOR]

The diverse Lyman-alpha (Ly  |$\alpha$|⁠) line profiles are essential probes of gas in and around galaxies. While isotropic models can successfully reproduce a range of  Ly  |$\alpha$| observables, the correspondence between the model and actual physical parameters remains uncertain. We investigate the effect of anisotropies on Ly  |$\alpha$| escape using a simplified setup: an empty hole (fractional size |$\tilde{s}$|⁠) within a semi-infinite slab with constant column density. Due to the slab's high line-centre optical depth (⁠|$\tau _0\gtrsim 10^{5-6}$|⁠), most photons should escape through the empty channel. However, our numerical findings indicate that only a fraction |${\sim} \tilde{s}$| of photons exit through this channel. To explain this puzzle, we developed an analytical model describing the scattering and transmission behaviour of  Ly  |$\alpha$| photons in an externally illuminated slab. Our findings show that the number of scatterings per reflection follows a Lévy distribution (⁠|${\propto} N^{-3/2}$|⁠). This means that the mean number of scatterings is orders of magnitude greater than expectations, facilitating a shift in frequency and the subsequent photon escape. Our results imply that  Ly  |$\alpha$| photons are more prone to traverse high-density gas and are surprisingly less biased to the 'path of least resistance'. Hence,  Ly  |$\alpha$| can trace an average hydrogen distribution rather than only low-column density channels. [ABSTRACT FROM AUTHOR]

The present-day abundance of beryllium in the solar atmosphere provides clues about mixing mechanisms within stellar interiors. However, abundance determinations based on the Be II313.107 nm line are prone to systematic errors due to imperfect model spectra. These errors arise from missing continuous opacity in the UV, a significant unidentified blend at 313.102 nm, departures from local thermodynamic equilibrium (LTE), and microturbulence and macroturbulence fudge parameters associated with one-dimensional (1D) hydrostatic model atmospheres. Although these factors have been discussed in the literature, no study has yet accounted for all of them simultaneously. To address this, we present 3D non-LTE calculations for neutral and ionised beryllium in the Sun. We used these models to derive the present-day solar beryllium abundance, calibrating the missing opacity on high resolution solar irradiance data and the unidentified blend on the centre-to-limb variation. We find a surface abundance of 1.21 ± 0.05 dex, which is significantly lower than the value of 1.38 dex that has been commonly adopted since 2004. Taking the initial abundance via CI chondrites, our result implies that beryllium has been depleted from the surface by an extra 0.11 ± 0.06 dex, or 22 ± 11%, on top of any effects of atomic diffusion. This is in tension with standard solar models, which predict negligible depletion, as well as with contemporary solar models that have extra mixing calibrated on the abundances of helium and lithium, which predict excessive depletion. These discrepancies highlight the need for further improvements to the physics in solar and stellar models. [ABSTRACT FROM AUTHOR]

Context. Recent spectra of protoplanetary disks around very low-mass stars (VLMS), captured by the Mid-InfraRed Instrument (MIRI) on board the James Webb Space Telescope (JWST), reveal a rich carbon chemistry. Current interpretations of these spectra are based on 0D slab models and provide valuable estimates for molecular emission temperatures and column densities in the innermost disk (radius ≲ 1 au). However, the established fitting procedures and simplified models are challenged by the many overlapping gas features. Aims. We aim to simultaneously determine the molecular and the dust composition of the disk around the VLMS Sz 28 in a Bayesian way. Methods. We modelled the JWST/MIRI spectrum of Sz 28 up to 17 μm using the Dust Continuum Kit with Line emission from Gas (DuCKLinG). Systematically excluding different molecules from the Bayesian analysis allowed for an evidence determination of all investigated molecules and isotopologues. We continued by examining the emission conditions and locations of all molecules, analysing the differences to previous 0D slab fitting, and analysing the dust composition. Results. We find very strong Bayesian evidence for the presence of C2H2, HCN, C6H6, CO2, HC3N, C2H6, C3H4, C4H2, and CH4 in the JWST/MIRI spectrum of Sz 28. Additionally, we identify CH3 and find tentative indications for NH3. There is no evidence for water in the spectrum. However, we show that column densities of up to 2 × 1017 cm−2 could be hidden in the observational noise if assuming similar emission conditions of water as the detected hydrocarbons. Contrary to previous 0D slab results, a C4H2 quasi-continuum is robustly identified. We confirm previous conclusions that the dust in Sz 28 is highly evolved, with large grains (5 μm) and a high crystallinity fraction being retrieved. We expect some of the stated differences to previous 0D slab fitting results to arise from an updated data reduction of the spectrum, but also due to the different modelling process. The latter reason underpins the need for more advanced models and fitting procedures. [ABSTRACT FROM AUTHOR]

In the present paper we consider the full nonlocal thermodynamic equilibrium (non-LTE) radiation transfer problem. This formalism allows us to account for deviation from equilibrium distribution of both the radiation field and the massive particles. In the present study, two-level atoms with broadened upper level represent the massive particles. In the absence of velocity-changing collisions, we demonstrate the analytic equivalence of the full non-LTE source function with the corresponding standard non-LTE partial frequency redistribution (PFR) model. We present an iterative method based on operator splitting techniques that can be used to numerically solve the problem at hand. We benchmark it against the standard non-LTE transfer problem for a two-level atom with PFR. We illustrate the deviation of the velocity distribution function of excited atoms from the equilibrium distribution. We also discuss the dependence of the emission profile and the velocity distribution function on elastic collisions and velocity-changing collisions. [ABSTRACT FROM AUTHOR]

The chemical compositions of stars encode the history of the universe and are thus fundamental for advancing our knowledge of astrophysics and cosmology. However, measurements of elemental abundance ratios, and our interpretations of them, strongly depend on the physical assumptions that dictate the generation of synthetic stellar spectra. Three-dimensional radiation-hydrodynamic (3D RHD) box-in-a-star simulations of stellar atmospheres offer a more realistic representation of surface convection occurring in late-type stars than do traditional one-dimensional (1D) hydrostatic models. As evident from a multitude of observational tests, the coupling of 3D RHD models with line formation in nonlocal thermodynamic equilibrium (non-LTE) today provides a solid foundation for abundance analysis for many elements. This review describes the ongoing and transformational work to advance the state of the art and replace 1D LTE spectrum synthesis with its 3D non-LTE counterpart. In summary: 3D and non-LTE effects are intricately coupled, and consistent modeling thereof is necessary for high-precision abundances; such modeling is currently feasible for individual elements in large surveys. Mean 3D (〈3D〉) models are not adequate as substitutes. The solar abundance debate is presently dominated by choices and systematic uncertainties that are not specific to 3D non-LTE modeling. 3D non-LTE abundance corrections have a profound impact on our understanding of FGK-type stars, exoplanets, and the nucleosynthetic origins of the elements. [ABSTRACT FROM AUTHOR]

The Mg ii resonance doublet at 2796 Å and 2803 Å is an increasingly important tool to study cold, |$T \sim 10^{4}\,$|  K, gas – an observational-driven development requiring theoretical support. We develop a new Monte Carlo radiative transfer code to systematically study the joined Mg ii and Ly α escape through homogeneous and 'clumpy' multiphase gas with dust in arbitrary three-dimensional geometries. Our main findings are (i) the Mg ii spectrum differs from Ly α due to the large difference in column densities, even though the atomic physics of the two lines are similar. (ii) The Mg ii escape fraction is generally higher than that of Ly α because of lower dust optical depths and path lengths – but large variations due to differences in dust models and the clumpiness of the cold medium exist. (iii) Clumpy media possess a 'critical covering factor' above which Mg ii radiative transfer matches a homogeneous medium. The critical covering factors for Mg ii and Ly α differ, allowing constraints on the cold gas structure. (iv) The Mg ii doublet ratio |$R_{\rm MgII}$| varies for strong outflows/inflows (⁠|$\gtrsim 700 \,\mathrm{km\, s}^{-1}$|⁠), in particular, |$R_{\rm MgII}\lt 1$| being an unambiguous tracer for powerful galactic winds. (v) Scattering of stellar continuum photons can decrease |$R_{\rm MgII}$| from two to one, allowing constraints on the scattering medium. Notably, we introduce a novel probe of the cold gas column density – the halo doublet ratio – which we show to be a powerful indicator of ionizing photon escape. We discuss our results in the context of interpreting and modelling observations as well as their implications for other resonant doublets. [ABSTRACT FROM AUTHOR]

Context. The MIRI instrument on board the James Webb Space Telescope probes the chemistry and dust mineralogy of the inner regions of protoplanetary disks. The observed spectra are unprecedented in their detail and reveal a rich chemistry with strong diversity between objects. This complicates interpretations that are mainly based on manual continuum subtraction and 0D slab models. Aims. We investigate the physical conditions under which the gas emits in protoplanetary disks. Based on MIRI spectra, we apply a full Bayesian analysis that provides the posterior distributions of dust and molecular properties, such as column densities and emission temperatures. Methods. To do so, we introduced the Dust Continuum Kit with Line emission from Gas (DuCKLinG), a Python-based model simultaneously describing the molecular line emission and the dust continuum of protoplanetary disks without large computational cost. The model describes the dust continuum emission by dust models with precomputed dust opacities. The molecular emission is based on LTE slab models but from extended radial ranges with gradients in column densities and emission temperatures. We compare the model to observations using Bayesian analysis with linear regression techniques to reduce the dimension of the parameter space. We benchmarked this model to a complex thermo-chemical ProDiMo model of AATau and fit the MIRI spectrum of GW Lup. The latter allowed for a comparison to the previous results obtained with single slab models and hand-fitted continuum. Results. We successfully decrease the computational time of the fitting method by a factor of 80 by eliminating linear parameters, such as the emission areas, from the Bayesian run. This approach does not significantly change the retrieved molecular parameters, and only the calculated errors on the optically thin dust masses slightly decrease. For an AA Tau ProDiMo mock observation, we find that the retrieved molecular conditions from DuCKLinG (column densities from 3 × 1018 cm−2 to 4 × 1020 cm−2, radial range from 0.2 au to 1.2 au, and temperature range from about 200 K to 400 K) fall within the true values from ProDiMo (column densities between 4 × 1017 cm-2 to 5 × 1020 cm−2, radial extent 0.1 au to 6.6 au, and temperature range from about 120 to 1000 K). The smaller DuCKLinG ranges can be explained by the relative flux contributions of the different parts of ProDiMo. The parameter posterior of GW Lup reinforces previously found results. The previously determined column densities fall within the retrieved ranges in this study for all examined molecules (CO2, H2O, HCN, and C2H2). Similar overlap is found for the temperatures with only the temperature range of HCN (from 570−60+60 to 750−70+90 K) not including the previously found value (875 K). This discrepancy may be due to the simultaneous fitting of all molecules compared to the step-by-step fitting of the previous study. There is statistically significant evidence for radial temperature and column density gradients for H2O and CO2 compared to the constant temperature and column density assumed in the 0D slab models. Additionally, HCN and C2H2 emit from a small region with near constant conditions. Due to the small selected wavelength range 13.6–16.3 µm, the dust properties are not well constrained for GW Lup. DuCKL inG can become an important tool to analyse the molecular emission and dust mineralogy of large samples based on JWST /MIRI spectra in an automated way. [ABSTRACT FROM AUTHOR]

Context. Rapidly growing datasets from stellar spectroscopic surveys are providing unprecedented opportunities to analyse the chemical evolution history of our Galaxy. However, spectral analysis requires accurate modelling of synthetic stellar spectra for late-type stars, for which the assumption of local thermodynamic equilibrium (LTE) has been shown to be insufficient in many cases. Errors associated with LTE can be particularly large for Ti I, which is susceptible to over-ionisation, particularly in metal-poor stars. Aims. The aims of this work are to study and quantify the 1D non-LTE effects on titanium abundances across the Hertzsprung-Russell diagram for a large sample of stars. Methods. A large grid of departure coefficients, βν, were computed on standard MARCS model atmospheres. The grid extends from 3000 K to 8000 K in Teff, −0.5 dex to +5.5 dex in log g, and −5.0 to +1.0 in [Fe/H], with non-LTE effects in this grid reaching up to 0.4 dex. This was used to compute abundance corrections that were subsequently applied to the LTE abundances of over 70 000 stars selected from the GALAH survey in addition to a smaller sample of literature Keck data for metal-poor dwarfs. Results. The non-LTE effects grow towards lower [Fe/H], lower log g, and higher Teff, with a minimum and maximum ΔA(Ti)Ti I of 0.02 and 0.19 in the GALAH sample. For metal-poor giants, the non-LTE modelling reduces the average ionisation imbalance (ΔI−II) from −0.11 dex to −0.01 dex at [Fe/H] = −1.7, and the enhancement in titanium abundances from Ti I lines results in a [Ti/Fe] versus [Fe/H] trend that more closely resembles the behaviour of Ti II at low metallicities. At higher metallicities, the results are limited by the precision of the GALAH DR3 LTE abundances and the effects are within the errors. For the most metal-poor dwarfs from the Keck sample, the average ionisation imbalance increases from −0.1 dex to +0.2 dex, a shortcoming that is consistent with previous 1D non-LTE studies and which we speculate could be related to 3D effects. Conclusions. Non-LTE effects on titanium abundances are significant. Neglecting them may alter our understanding of Galactic chemical evolution. We have made our grid of departure coefficients publicly available, with the caveat that the Ti abundances of metal-poor dwarfs need further study in 3D non-LTE. [ABSTRACT FROM AUTHOR]

Context. Amongst several spectral lines, some of the strongest chromospheric diagnostics are offered by the Ca II H & K lines. These lines can be used to gauge the temperature stratification of the atmosphere since the line core and wings are formed in different regions of the solar atmosphere. Furthermore, the Ca II lines act as tracers for the magnetic structure of the solar atmosphere, as the line cores are formed in the upper chromosphere even though they are formed in non-local thermodynamic equilibrium (NLTE). In contrast, the formation of millimetre (mm) continuum radiation occurs under local thermodynamic equilibrium (LTE) conditions. As a result, the brightness temperatures obtained from observations with the Atacama Large Millimetre/Submillimetre Array (ALMA) offer a complementary perspective on the activity and thermal structure of stellar atmospheres. Aims. The overall aim is to establish more robust solar/stellar activity indicators using ALMA observations in comparison with classical diagnostics, such as the s index and infrared triplet (IRT) index. Methods. We employed the 1.5D radiative transfer codes RH1.5D and advanced radiative transfer (ART) to compute the synthetic spectra for the Ca II lines and the millimetre (mm) continua, respectively. These calculations were performed using an enhanced network atmosphere model, which incorporates non-equilibrium hydrogen ionisation generated by the state-of-the-art 3D radiation magnetohydrodynamics (rMHD) Bifrost code. To account for the limited spatial resolution of ALMA, we simulated the effect using a Gaussian point spread function (PSF). Additionally, we analysed the correlations and slopes of scatter plots between the Ca II indices and mm continuum for the original and degraded resolutions, focusing on the entire simulation box, quiet Sun regions, and enhanced network patches separately. The activity indices generated from these lines could further be used to compare the spectra of Sun-like stars with the solar spectrum. Results. We present a comparative study between synthetic continuum brightness temperature maps at mm wavelengths (0.3 mm–8.5 mm) and the Ca II activity indices; namely, the s index and infrared triplet (IRT) index. The Ca II activity indices and mm brightness temperatures are weakly correlated at the high resolution, with the highest correlation observed at a wavelength of 0.3 mm, corresponding to ALMA band 10. As the resolution decreases, the correlation consistently increases. Conversely, the slopes exhibit a decreasing trend with increasing wavelength, while the degradation of resolution does not noticeably affect the calculated slopes. Conclusions. As the spatial resolution decreases, the standard deviations of the Ca II activity indices and brightness temperatures decrease, while the correlations between them increase. However, the slopes do not exhibit significant changes. Consequently, these relationships could be valuable for calibrating the mm continuum maps obtained through ALMA observations. [ABSTRACT FROM AUTHOR]

Fourteen models are calculated with the shock velocity ranging from 200 to 330 km s |$^{-1}$| and pre-shock hydrogen nucleon density ranging from |$2.5\times 10^{12}$| to |$4\times 10^{13}$| cm |$^{-3}$|⁠. Among them the summed emergent flux of all spectral lines accounts for about 0.1–0.3 of the total veiling flux. The hydrogen Balmer continuum accounts for 0.17–0.1, while a nearly constant fraction close to 0.5 comes from emission produced by the stellar atmosphere. The main results derived from the veiling continuum energy distributions are two strong correlations: (1) the Balmer jump (BJ) increases as F K, the shock kinetic energy flux, decreases; and (2) at a fixed fraction of surface coverage by accretion shocks |$r_{\lambda }$|⁠ , the ratio of veiling to photospheric continuum flux at wavelength λ decreases as F K decreases. Using the BJ– F K and |$r_{4500\,\mathring{\rm A}}$| – F K relations, the observed excess continua of 10 T Tauri stars are modelled. For BP Tau and 3 Orion stars our accretion luminosities are higher than published values by a factor of a few. For the six Chamaeleon I stars, our observed accretion luminosities are about 27–78 per cent higher than corresponding published values. Comparison of model results on the He  i λ5876 Å flux with observed data indicates that, while those stars with dominant λ5876 Å narrow components can be readily accounted for by the calculated models, those with much stronger broad components cannot, and suggests that for the latter objects the bulk of their excess continua at 5876 Å do not originate from accretion shocks. [ABSTRACT FROM AUTHOR]

The optical depth parameterization is typically used to study the 21-cm signals associated with the properties of the neutral hydrogen (H  i) gas and the ionization morphology during the Epoch of Reionization (EoR), without solving the radiative transfer equation. To assess the uncertainties resulting from this simplification, we conduct explicit radiative transfer calculations using the cosmological 21-cm line radiative transfer (C21LRT) code and examine the imprints of ionization structures on the 21-cm spectrum. We consider a globally averaged reionization history and implement fully ionized cavities (H  ii bubbles) of diameters d ranging from 0.01 to 10 Mpc at epochs within the emission and the absorption regimes of the 21-cm global signal. The single-ray C21LRT calculations show that the shape of the imprinted spectral features are primarily determined by d and the 21-cm line profile, which is parametrized by the turbulent velocity of the H  i gas. It reveals the spectral features tied to the transition from ionized to neutral regions that calculations based on the optical depth parametrization were unable to capture. We also present analytical approximations of the calculated spectral features of the H  ii bubbles. The multiple-ray calculations show that the apparent shape of an H  ii bubble (of d  = 5 Mpc at z  = 8), because of the finite speed of light, differs depending on whether the bubble's ionization front is stationary or expanding. Our study shows the necessity of properly accounting for the effects of line-continuum interaction, line broadening, and cosmological expansion to correctly predict the EoR 21-cm signals. [ABSTRACT FROM AUTHOR]

The 21-cm hyperfine line of neutral hydrogen is a useful tool to probe the conditions of the Universe during the Dark Ages, Cosmic Dawn, and the Epoch of Reionization. In most of the current calculations, the 21-cm line signals at given frequencies are computed, using an integrated line-of-sight line opacity, with the correction for cosmological expansion. These calculations have not fully captured the line and continuum interactions in the radiative transfer, in response to evolution of the radiation field and the variations of thermal and dynamic properties of the line-of-sight medium. We construct a covariant formulation for the radiative transfer of the 21-cm line and derive the cosmological 21-cm line radiative transfer (C21LRT) equation. The formulation properly accounts for local emission and absorption processes and the interaction between the line and continuum when the radiation propagates across the expanding Universe to the present observer. Our C21LRT calculations show that methods simply summing the line optical depth could lead to error of 5  per cent in the 21-cm signals for redshift z ∼ 12–35 and of |$\gt 10~{{\ \rm per\ cent}}$| for redshift z ≲ 8. Proper covariant radiative transfer is therefore necessary for producing correct theoretical templates for extracting information of the structural evolution of the Universe through the Epoch of Reionization from the 21-cm tomographic data. [ABSTRACT FROM AUTHOR]

Context. Magnetic neutron stars (NSs) often exhibit a cyclotron resonant scattering feature (CRSF) in their X-ray spectra. Accretion onto their magnetic poles is responsible for the emergence of X-rays, but the site of the CRSF formation is still a puzzle. A promising candidate for high-luminosity sources has always been the radiative shock in the accretion column. Nevertheless, no quantitative calculations of spectral formation at the radiative shock have been performed so far. Aims. It is well accepted that, in the accretion column of a high-luminosity, accreting magnetic NS, a radiative shock is formed. Here, we aim to explore the scenario where the shock is the site of the cyclotron-line formation. We studied spectral formation at the radiative shock and the emergent spectral shape across a wide range of the parameter space and determined which parameters hold an important role in shaping a prominent CRSF. Methods. We developed a Monte Carlo (MC) code based on the forced first collision numerical scheme to conduct radiation transfer simulations at the radiative shock. The seed photons were due to bremsstrahlung and were emitted in the post-shock region. We properly treated bulk-motion Comptonization in the pre-shock region, thermal Comptonization in the post-shock region, and resonant Compton scattering in both regions. We adopted a fully relativistic scheme for the interaction between radiation and electrons, employing an appropriate polarization-averaged differential cross-section. As a result, we calculated the angle- and energy-dependent emergent X-ray spectrum from the radiative shock, focusing on both the CRSF and the X-ray continuum, under diverse conditions. The accretion column was characterized by cylindrical symmetry, and the radiative shock was treated as a mathematical discontinuity. Results. We find that a power law, hard X-ray continuum, and a CRSF are naturally produced by the first-order Fermi energization as the photons criss-cross the shock. The depth and the width of the CRSF depend mainly on the transverse optical depth and the post-shock temperature. We show that the cyclotron-line energy centroid is shifted by ∼(20 − 30)% to lower energies compared to the classical cyclotron energy; this is due to the Doppler boosting between the shock reference frame and the bulk-motion frame. We demonstrate that a "bump" feature arises in the right wing of the CRSF due to the up-scattering of photons by the accreting plasma and extends to higher energies for larger optical depths and post-shock temperatures. Conclusions. We conclude that resonant Compton scattering of photons by electrons in a radiative shock is efficient in producing a power-law X-ray continuum with a high-energy cutoff accompanied by a prominent CRSF. The implications of the Doppler effect on the centroid of the emergent absorption feature must be considered if an accurate determination of the magnetic field strength is desired. [ABSTRACT FROM AUTHOR]

Over the last two decades there have been considerable advances in modelling the spectra of massive stars and supernovae (SNe). Despite this progress, there are still numerous uncertainties that affect the accuracy of models. For massive stars, convection, instabilities, clumping, and our inability to model stellar winds self-consistently likely introduce systematic errors into our analyses. For SNe, and particularly for core-collapse SNe, departures from spherical symmetry strongly affect observed spectra and need to be taken into account. There are also issues with clumping, and mixing processes (both in the progenitor and the SN explosion) that need to be resolved. For both massive stars and SNe, the accuracy and availability of atomic data continues to be an ongoing issue influencing analyses. [ABSTRACT FROM AUTHOR]

We present the first theoretical line profile calculations of the ultraviolet spectral lines of carbon perturbed by helium using a semiclassical collision approach and high-quality ab initio potentials and electronic transition dipole moments. The temperature range is from 5000 to 8000 K. These results are important for astrophysical modelling of spectra in atmospheres of white dwarf stars showing atomic carbon in an helium atmosphere. Beyond the conventional symmetrical Lorentzian core at low He density, these lines exhibit a blue asymmetric behaviour. This blue asymmetry is a consequence of low maxima in the corresponding C–He potential energy difference curves at short internuclear distances. The collisional profiles are carefully examined and their perturber density dependence allow to understand the various line shapes of the observed carbon spectral lines in helium-rich white dwarf photosphere where the He perturber densities reach several 1021 cm−3. [ABSTRACT FROM AUTHOR]

We present a new semi-analytical formalism for modelling metal absorption lines that emerge from a clumpy galactic environment, ALPACA. We predict the "down-the-barrel" (DTB) metal absorption line profiles and the equivalent width (EW) of absorption at different impact parameters (b) as a function of the clump properties, including clump kinematics, clump volume filling factor, clump number density profile, and clump ion column densities. With ALPACA , we jointly model the stacked DTB C  ii  λ1334 spectrum of a sample of z ∼ 3 Lyman break galaxies and the EW versus b profile of a sample of z ∼ 2 star-forming galaxy–galaxy pairs. ALPACA successfully reproduced two data sets simultaneously, and the best fit prefers a low clump volume filling factor (∼3 × 10−3). The radial velocities of the clumps are a superposition of a rapidly accelerated outflow with a maximum velocity of |$\sim 400 \, {\mathrm{km}\, \mathrm{s}^{-1}}$| and a velocity dispersion of |$\sigma \sim 120 \, {\mathrm{km}\, \mathrm{s}^{-1}}$|⁠. The joint modelling reveals a physical scenario where the absorption observed at a particular velocity is contributed by the clumps distributed over a fairly broad range of radii. We also find that the commonly adopted Sobolev approximation is at best only applicable within a narrow range of radii where the clumps are undergoing rapid acceleration in a non-volume-filling clumpy medium. Lastly, we find that the clump radial velocity profile may not be fully constrained by the joint modelling and spatially resolved Ly α emission modelling may help break the degeneracy. [ABSTRACT FROM AUTHOR]

Context. Transmission spectroscopy is one of the most powerful techniques used to characterize transiting exoplanets, since it allows for the abundance of the atomic and molecular species in the planetary atmosphere to be measured. However, stellar lines may bias the determination of such abundances if their center-to-limb variations (CLVs) are not properly accounted for. Aims. This paper aims to show that three-dimensional (3D) radiation hydrodynamic models and the assumption of non-local ther-modynamic equilibrium (non-LTE) line formation are required for an accurate modeling of the stellar CLV of the Na I D1 and K I resonance lines on transmission spectra. Methods. We modeled the CLV of the Na I D1 and K I resonance lines in the Sun with 3D non-LTE radiative transfer. The synthetic spectra were compared to solar observations with high spatial and spectral resolution, including new data collected with the CRISP instrument at the Swedish 1-m Solar Telescope between µ = 0.1 and µ = 1.0. Results. Our 3D non-LTE modeling of the Na I D1 resonance line at 5896 Å and the K I 7699 Å resonance line in the Sun is in good agreement with the observed CLV in the solar spectrum. Moreover, the simulated CLV curve for a Jupiter-Sun system inferred with a 3D non-LTE analysis shows significant differences from the one obtained from a 1D atmosphere. The latter does indeed tend to overestimate the amplitude of the transmission curve by a factor that is on the same order of magnitude as a planetary absorption depth (i.e., up to 0.2%). Conclusions. This work highlights the fact that to correctly characterize exoplanetary atmospheres, 3D non-LTE synthetic spectra ought to be used to estimate the stellar CLV effect in transmission spectra of solar-like planet hosts. Moreover, since different spectral lines show different CLV curves for the same geometry of the planet-star system, it is fundamental to model the CLV individually for each line of interest. The work will be extended to other lines and FGK-type stars, allowing for synthetic high-resolution spectra to mitigate the stellar contamination of low-resolution planetary spectra, for example, those drawn from JWST. [ABSTRACT FROM AUTHOR]

Main-sequence K-dwarfs possess strong emission in the form of the H  i Ly α line. There is a close coincidence between the energy corresponding to the transitions H  i 1s-2p and Fe  ii (5D)5s 4D5/2–(5D)5p 4D5/2. Singly ionized iron has been confirmed being pumped by photo-excitation by accidental resonance (PAR) in planetary nebulae, symbiotic stars, K-giants, and active galactic nebulae. I investigate in this work whether PAR can occur in the atmospheres of main-sequence K-dwarfs, which do not possess the large extended atmospheres of the late-type K-giants. Specifically a search for possible Fe  ii fluorescence lines is conducted. For the case when I can confirm PAR, I estimate the total flux leaving the stars in the form of Fe  ii fluorescence. I search for emission lines from the Fe  ii (5D)5p 4D5/2 level. Since those of these lines with the largest branching fractions correspond to lines at wavelengths covered by the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, a search for archival FUSE spectra from K-dwarfs within 20 ly from the sun is conducted. I retrieve and analyse FUSE spectra for four of these K-dwarfs. In each case I can confirm PAR, I fit the H  i Ly α line in Hubble Space Telescope spectra recorded with the Space Telescope Imaging Spectrograph, in order to estimate the efficiency of the PAR mechanism. I can now confirm Fe  ii fluorescence in the two closest K-dwarfs, Alpha Centauri B, and Epsilon Eridani. The total power leaving as Fe  ii fluorescence are 4.9 × 1017 and 1.30 × 1018 W respectively. [ABSTRACT FROM AUTHOR]

We developed a comprehensive model atom of Sc  ii –Sc  iii. Abundances of scandium for a sample of eight unevolved A9–B3-type stars with well-determined atmospheric parameters were determined based on the non-local thermodynamical equilibrium (NLTE) line formation for Sc  ii –Sc  iii and high-resolution observed spectra. For the Sc  ii lines, the abundance differences between NLTE and local thermodynamical equilibrium (LTE) grow rapidly with increasing effective temperature (T eff), from slightly negative at T eff = 7250 K to positive ones of up to 0.6 dex at T eff = 10 400 K. For Sc  iii in ι Her, NLTE reduces the line-to-line scatter substantially compared to the LTE case. The NLTE abundances of Sc in our five superficially normal stars are consistent within the error bars with the Solar system Sc abundance, while the LTE abundances of the late B-type stars are greatly subsolar. NLTE reduces but does not remove a deficiency of Sc in the Am stars HD 72660 and Sirius. Based on our own and the literature data, the Ca/Sc abundance ratios of the sample of 16 Am stars were found to be close together, with [Ca/Sc] = 0.6–0.7. We propose the Ca/Sc abundance ratio, but not the abundances of individual Ca and Sc elements to be used for classifying a star as Am and for testing the diffusion models. We provide the NLTE abundance corrections for 10 lines of Sc  ii in a grid of model atmospheres appropriate for A to late B-type stars. [ABSTRACT FROM AUTHOR]

Context. The inner few AU of disks around young stars, where terrestrial planets are thought to form, are best probed in the infrared. The James Webb Space Telescope is now starting to characterize the chemistry of these regions in unprecedented detail, building on earlier results of the Spitzer Space Telescope that the planet-forming zone of disks contain a rich chemistry. One peculiar subset of sources characterized by Spitzer are the so-called CO2-only sources, in which only a strong 15 μm CO2 feature was detected in the spectrum. Aims. One scenario that could explain the weak or even non-detections of molecular emission from H2O is the presence of a small, inner cavity in the disk. If this cavity were to extend past the H2O snowline, but not past the CO2 snowline, this could strongly suppress the H2O line flux with respect to that of CO2. For this work, we aimed to test the validity of this statement. Methods. Using the thermo-chemical code Dust And LInes (DALI), we created a grid of T Tauri disk models with an inner cavity, meaning we fully depleted the inner region of the disk in gas and dust starting from the dust sublimation radius and ranging until a certain cavity radius. Cavity radii varying in size from 0.1 to 10 AU were explored for this work. We extended this analysis to test the influence of cooling through H2O ro-vibrational lines and the luminosity of the central star on the CO2 /H2O flux ratio. Results. We present the evolution of the CO2 and H2O spectra of a disk with inner cavity size. The line fluxes show an initial increase as a result of an increasing emitting area, followed by a sharp decrease. As such, when a large-enough cavity is introduced, a spectrum that was initially dominated by H2O lines can become CO2-dominated instead. However, the cavity size needed for this is around 4–5 AU, exceeding the nominal position of the CO2 snowline in a full disk, which is located at 2 AU in our fiducial, L* = 1.4 L⊙ model. The cause of this is most likely the alteration of the thermal structure by the cavity, which pushes the snowlines outward. In contrast, our models show that global temperature fluctuations, for example due to changes in stellar luminosity, impact the fluxes of H2O and CO2 roughly equally, thus not impacting their ratio much. Alternative explanations for bright CO2 emission are also briefly discussed. Conclusions. Our modeling work shows that it is possible for the presence of a small inner cavity to explain strong CO2 emission in a spectrum. However, the cavity needed to do so is larger than what was initially expected. As such, this scenario will be easier to test with sufficiently high angular resolution (millimeter) observations. [ABSTRACT FROM AUTHOR]

Context. γ Cas stars are early-type Be stars that exhibit an unusually hard and bright thermal X-ray emission. One of the proposed scenarios to explain these properties postulates the existence of a neutron star companion in the propeller stage, during which the magnetosphere of a rapidly rotating neutron star repels infalling material. Aims. To test this model, we examined the fluorescent Fe Kα emission line at ~6.4keV in the X-ray spectra of γ Cas stars, which offers a powerful diagnostic of both the primary source of hard X-rays and the reprocessing material. Methods. We computed synthetic line profiles of the fluorescent Fe Kα emission line in the framework of the propelling neutron star scenario. Two reservoirs of material contribute to the fluorescence in this case: the Be circumstellar decretion disk and a shell of cool material that surrounds the shell of X-ray-emitting plasma around the putative propelling neutron star. Results. We analysed the synthetic line profiles and expected equivalent widths of the lines for three well-studied γ Cas stars. The predicted line strengths fall short of the observed values by at least an order of magnitude. Pushing the model parameters to reproduce the observed line strengths led to column densities towards the primary X-ray source that exceed the observationally determined values by typically a factor of 20, and would further imply a higher X-ray luminosity than observed. Conclusions. The strengths of the observed Fe Kα fluorescent emission lines in γ Cas stars are inconsistent with the expected properties of a propeller scenario as proposed in the literature. [ABSTRACT FROM AUTHOR]

Context. Stellar flares cannot be spatially resolved, which complicates ascertaining the physical processes behind particular spectral signatures. Due to their proximity to Earth, solar flares can serve as a stepping stone for understanding their stellar counterparts, especially when using a Sun-as-a-star instrument and in combination with spatially resolved observations. Aims. We aim to understand the disk-integrated spectral behaviors of a confined X2.2 flare and its eruptive X9.3 successor, which had energies of 2.2 × 1031 erg and 9.3 × 1031 erg, respectively, as measured by Sun-as-a-star observations with the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N). Methods. The behavior of multiple photospheric (Na D1 & D2, Mg I at 5173 Å, Fe I at 6173 Å, and Mn I at 4031 Å) and chromospheric (Ca II H & K, Hα, Hβ, and He ID3) spectral lines were investigated by means of activity indices and contrast profiles. A number of different photospheric lines were also investigated by means of equivalent widths, and radial velocity measures, which were then related to physical processes directly observed in high-resolution observations made with the Swedish 1-m Solar Telescope (SST) and the Atmospheric Imaging Assembly (AIA) on board of the Solar Dynamics Observatory (SDO). Results. Our findings suggest a relationship between the evolving shapes of contrast profile time and the flare locations, which assists in constraining flare locations in disk-integrated observations. In addition, an upward bias was found in flare statistics based on activity indices derived from the Ca II H & K lines. In this case, much smaller flares cause a similar increase in the activity index as that produced by larger flares. Hα-based activity indices do not show this bias and are therefore less susceptible to activity jitter. Sodium line profiles show a strongly asymmetric response during flare activity, which is best captured with a newly defined asymmetrical sodium activity index. A strong flare response was detected in Mn I line profiles, which is unexpected and calls for further exploration. Intensity increases in Hα, Hβ, and certain spectral windows of AIA before the flare onset suggest their potential use as short-term flare predictors. [ABSTRACT FROM AUTHOR]

Context. Synthetic spectra from 3D models of the solar atmosphere have become increasingly successful at reproducing observations, but there are still some outstanding discrepancies for chromospheric spectral lines, such as Ca II and Mg II, particularly regarding the width of the line cores. It has been demonstrated that using sufficiently high spatial resolution in the simulations significantly diminishes the differences in width between the mean spectra in observations and simulations, but a detailed investigation into how this impacts subgroups of individual profiles is currently lacking. Aims. We compare and contrast the typical shapes of synthetic Ca II 854.2 nm spectra found in Bifrost simulations having different magnetic activity with the spectral shapes found in a quiet-Sun observation from the Swedish 1-m Solar Telescope (SST). Methods. We used clustering techniques to extract the typical Ca II 854.2 nm profile shapes synthesized from Bifrost simulations with varying amounts of magnetic activity. We degraded the synthetic profiles to observational conditions and repeated the clustering, and we compared our synthetic results with actual observations. Subsequently, we examined the atmospheric structures in our models for some select sets of clusters, with the intention of uncovering why they do or do not resemble actual observations. Results. While the mean spectra for our high resolution simulations compare reasonably well with the observations, we find that there are considerable differences between the clusters of observed and synthetic intensity profiles, even after the synthetic profiles have been degraded to match observational conditions. The typical absorption profiles from the simulations are both narrower and display a steeper transition from the inner wings to the line core. Furthermore, even in our most quiescent simulation, we find a far larger fraction of profiles with local emission around the core, or other exotic profile shapes, than in the quiet-Sun observations. Looking into the atmospheric structure for a selected set of synthetic clusters, we find distinct differences in the temperature stratification for the clusters most and least similar to the observations. The narrow and steep profiles are associated with either weak gradients in temperature or temperatures rising to a local maximum in the line wing forming region before sinking to a minimum in the line core forming region. The profiles that display less steep transitions show extended temperature gradients that are steeper in the range−3 ≲ log τ5000 ≲ −1. [ABSTRACT FROM AUTHOR]

The bulk of solar flare energy is deposited in the chromosphere. Flare ribbons and footpoints in the chromosphere therefore offer great diagnostic potential of flare energy release and transport processes. High-quality observations from the Interface Region Imaging Spectrograph (IRIS) spacecraft have transformed our view of the Sun's atmospheric response to flares. Since most of the chromospheric lines observed by IRIS are optically thick, forward modelling is required to fully appreciate and extract the information they carry. Reproducing certain aspects of the Mg  ii lines remain frustratingly out of reach in state-of-the-art flare models, which are unable to satisfactorily reproduce the very broad-line profiles. A commonly proposed resolution to this is to assert that very large values of 'microturbulence' is present. We assess the validity of that approach by analysing optically thin lines in the flare chromosphere from the X-class flare SOL2014-10-25T17:08:00, using the derived value of non-thermal width as a constraint to our numerical models. A non-thermal width of the order 10 km s−1 was found within the short-lived red wing components of three spectral lines, with relatively narrow stationary components. Simulations of this flare were produced, and in the post-processing spectral synthesis we include within the downflows a microturbulence of 10 km s−1. While we can reproduce the O  i 1355.598 Å line rather well, and we can capture the general shape and properties of the Mg  ii line, the synthetic lines are still too narrow. [ABSTRACT FROM AUTHOR]

We present cross-sections and rate coefficients for rotational transitions in the water isotopologues D2O, H |$_2^{18}$| O, and HDO induced by collisions with para-H2(j 2 = 0) and ortho-H2(j 2 = 1). Quantum scattering calculations are performed at the full close-coupling level with the isotopic variants of an accurate full-dimensional H2O−H2 interaction potential. The D2O, H |$_2^{18}$| O, and HDO cross-sections are compared to the corresponding cross-sections for H2O. Large isotopic effects are observed in the case of D2O and HDO, in particular for collisions with p-H2(j 2 = 0), while the 18O isotopic substitution is found to be negligible. Rate coefficients are provided for rotational transitions among all para-D2O, ortho-D2O, and HDO levels with internal energy below 300 cm−1 and for kinetic temperatures in the range 5–300 K. Non-LTE radiative transfer calculations show that the HDO 225.9 GHz and H |$_2^{18}$| O 203.4 GHz transitions recently detected with ALMA in the young proto-planetary disc V883 Ori should be inverted (weak masers) in a large fraction of the disc. [ABSTRACT FROM AUTHOR]

In a stellar atmosphere, the resonance line polarization arises from scattering of limb-darkened radiation field by atoms. This spectral line polarization gets affected particularly in the wings, when the line photons suffer scattering on electrons in thermal motion. Scattering of line photons by atoms and electrons are, respectively, described by the atomic and Thomson electron scattering redistribution functions, which in general depend on both the frequencies and directions of incident and scattered photons. In this paper, we consider the polarized spectral line formation in spherically symmetric extended and expanding media accounting for the angle-dependent partial frequency redistribution (AD-PRD) in scattering on both atoms and electrons. We solve this computationally demanding polarized transfer problem using an accelerated lambda iteration method and a method based on orders of scattering approach. In the case of expanding spherical medium, the concerned transfer problem is solved in the comoving frame. Because of the computational limitations, we consider optically thin isothermal spherically symmetric media of different extensions for the static case as well as when the velocity fields are present. For the considered model, we show that the AD-PRD effects on the linear polarization profiles are significant and have to be accounted for. [ABSTRACT FROM AUTHOR]

We report the serendipitous discovery of a very metal-poor (VMP) Li-rich giant star (T eff = 4690 ± 80 K, log g = 1.34 ± 0.13, [Fe/H] = −2.43 ± 0.07). We analyse the Li  i 6103 and 6707 Å lines accounting for departures from local thermodynamic equilibrium (NLTE) and correcting for 3D effects using literature data, which yields a lithium abundance log ε Li  = 3.42 ± 0.07. Comparing lithium abundances from the two lines, in 1D NLTE we measure the isotope ratio 6Li/7Li = 1.64 |$^{+1.49}_{-1.08}$| per cent. When correcting for 3D effects, we detect the fragile 6Li isotope at 2-sigma level and the ratio 6Li/7Li = 5.65 |$^{+5.05}_{-2.51}$| per cent. To our knowledge, this is the first 6Li/7Li measurement in an extremely Li-rich VMP star. The Cameron–Fowler mechanism, which is proposed to produce Li-rich stars, does not imply 6Li production and is therefore inconsistent with our measurement when applying 3D corrections. We also derive NLTE abundances for 16 elements, most of which show similar abundances to those found in VMP stars. Sodium is an exception: [Na/Fe]NLTE1D = 0.07 ± 0.03, which is 0.5 dex higher than what is typical for VMP stars. This star joins the sample of rare Li-rich VMP stars, and we offer a novel way to constrain the source of lithium in such stars through isotope ratio measurements. [ABSTRACT FROM AUTHOR]

Blueshifts – or, more accurately, blue asymmetries – in broad emission lines such as C  iv λ1550 are common in luminous quasars and correlate with fundamental properties such as Eddington ratio and broad absorption line (BAL) characteristics. However, the formation of these blueshifts is still not understood, and neither is their physical connection to the BAL phenomenon or accretion disc. In this work, we present Monte Carlo radiative transfer and photoionization simulations using parametrized biconical disc-wind models. We take advantage of the azimuthal symmetry of a quasar and show that we can reproduce C  iv blueshifts provided that (i) the disc-mid-plane is optically thick out to radii beyond the line formation region, so that the receding wind bicone is obscured; and (ii) the system is viewed from relatively low (that is, more face-on) inclinations (≲40°). We show that C  iv emission-line blueshifts and BALs can form in the same wind structure. The velocity profile of the wind has a significant impact on the location of the line formation region and the resulting line profile, suggesting that the shape of the emission lines can be used as a probe of wind-driving physics. While we are successful at producing blueshifts/blue asymmetries in outflows, we struggle to match the detailed shape or skew of the observed emission-line profiles. In addition, our models produce redshifted emission-line asymmetries for certain viewing angles. We discuss our work in the context of the C  iv λ1550 emission blueshift versus equivalent-width space and explore the implications for quasar disc wind physics. [ABSTRACT FROM AUTHOR]

Electron-impact single ionization from levels of the ground configuration of the N+ ion is presented. Study analyses direct and indirect processes of the ionization. Convergence of the excitation-autoionization cross-sections is ensured by taking excitations up to shells with the principal quantum numbers n ≤ 10. The potential of the ionizing ion is used in the study. The scaled distorted wave cross-sections show a fairly good agreement with measurements. The study shows a presence of metastable states in the ion beams of experiments. [ABSTRACT FROM AUTHOR]

In order to derive sodium abundances and investigate the effects of non-local thermodynamic equilibrium (NLTE) on the formation of H-band Na I lines, we update the sodium atomic model by incorporating collision rates with hydrogen from new quantum-mechanical calculations. The differential Na abundances for 13 sample stars are obtained by analyzing high-resolution H-band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and optical spectra under both local thermodynamic equilibrium (LTE) and NLTE conditions. Consistent abundances from both bands suggest that our updated atomic model is valid for studying the formation of H-band Na I lines. Our calculations show that, in our stellar parameter space, NLTE effects are negative and can result in corrections larger than −0.4 dex on optical lines. The corrections on H-band Na I lines are typically small, within about 0.05 dex, but not negligible if accurate sodium abundance is desired. We note that the [Na/Fe] ratios favor the theoretical galactic chemical model. [ABSTRACT FROM AUTHOR]

Yttrium (Y), a key s-process element, is commonly used in nucleosynthesis studies and as a Galactic chemical clock when combined with magnesium (Mg). We study the applicability of the previously assumed local thermal equilibrium (LTE) line formation assumption in Y abundance studies of main-sequence and red giant stars, and probe the impact of NLTE (non-LTE) effects on the [Y/Mg] ratio, a proposed stellar age indicator. We derive stellar parameters, ages, and NLTE abundances of Fe, Mg, and Y for 48 solar analogue stars from high-resolution spectra acquired within the Gaia -ESO survey. For Y, we present a new NLTE atomic model. We determine a solar NLTE abundance of A(Y)NLTE = 2.12 ± 0.04 dex, 0.04 dex higher than LTE. NLTE effects on Y abundance are modest for optical Y  ii lines, which are frequently used in Sun-like stars diagnostics. NLTE has a small impact on the [Y/Mg] ratio in such stars. For metal-poor red giants, NLTE effects on Y  ii lines are substantial, potentially exceeding +0.5 dex. For the Gaia /4MOST/WEAVE benchmark star, HD 122563, we find the NLTE abundance ratio of [Y/Fe]NLTE = −0.55 ± 0.04 dex with consistent abundances obtained from different Y  ii lines. NLTE has a differential effect on Y abundance diagnostics in late-type stars. They notably affect Y  ii lines in red giants and very metal-poor stars, which are typical Galactic enrichment tracers of neutron-capture elements. For main-sequence stars, NLTE effects on optical diagnostic Y  ii lines remain minimal across metallicities. This affirms the [Y/Mg] ratio's reliability as a cosmochronometer for Sun-like stars. [ABSTRACT FROM AUTHOR]

We present high-resolution VLT/UVES spectroscopy and a detailed analysis of the unique broad absorption-line system towards the quasar SDSS J 165252.67+265001.96. This system exhibits low-ionization metal absorption lines from the ground states and excited energy levels of Fe  ii and Mn  ii , and the meta-stable |$2\, ^3S$| excited state of He  i. The extended kinematics of the absorber encompasses three main clumps with velocity offsets of −5680, −4550, and −1770 km s−1 from the quasar emission redshift, z  = 0.3509 ± 0.0003, derived from [O  ii ] emission. Each clump shows moderate partial covering of the background continuum source, Cf ≈ [0.53; 0.24; 0.81]. We discuss the excitation mechanisms at play in the gas, which we use to constrain the distance of the clouds from the active galactic nucleus (AGN) as well as the density, temperature, and typical sizes of the clouds. The number density is found to be n H ∼ 104 cm−3 and the temperature T e ∼ 104 K, with longitudinal cloudlet sizes of ≳0.01 pc. cloudy photoionization modelling of He  i ⋆, which is also produced at the interface between the neutral and ionized phases, assuming the number densities derived from Fe  ii , constrains the ionization parameter to be log  U ∼ −3. This corresponds to distances of a few 100 pc from the AGN. We discuss these results in the more general context of associated absorption-line systems and propose a connection between FeLoBALs and the recently identified molecular-rich intrinsic absorbers. Studies of significant samples of FeLoBALs, even though rare per se, will soon be possible due to large dedicated surveys paired with high-resolution spectroscopic follow-ups. [ABSTRACT FROM AUTHOR]

The double detonation is a widely discussed mechanism to explain Type Ia supernovae from explosions of sub-Chandrasekhar mass white dwarfs. In this scenario, a helium detonation is ignited in a surface helium shell on a carbon/oxygen white dwarf, which leads to a secondary carbon detonation. Explosion simulations predict high abundances of unburnt helium in the ejecta, however, radiative transfer simulations have not been able to fully address whether helium spectral features would form. This is because helium can not be sufficiently excited to form spectral features by thermal processes, but can be excited by collisions with non-thermal electrons, which most studies have neglected. We carry out a full non-local thermodynamic equilibrium radiative transfer simulation for an instance of a double detonation explosion model, and include a non-thermal treatment of fast electrons. We find a clear He  i λ10830 feature which is strongest in the first few days after explosion and becomes weaker with time. Initially this feature is blended with the Mg  ii λ10927 feature but over time separates to form a secondary feature to the blue wing of the Mg  ii λ10927 feature. We compare our simulation to observations of iPTF13ebh, which showed a similar feature to the blue wing of the Mg  ii λ10927 feature, previously identified as C  i. Our simulation shows a good match to the evolution of this feature and we identify it as high velocity He  i λ10830. This suggests that He  i λ10830 could be a signature of the double detonation scenario. [ABSTRACT FROM AUTHOR]

Detailed chemical abundances of very metal-poor (VMP; [Fe/H] < −2) stars are important for better understanding the first stars, early star formation, and chemical enrichment of galaxies. Big on-going and coming high-resolution spectroscopic surveys provide a wealth of material that needs to be carefully analysed. For VMP stars, their elemental abundances should be derived based on the non-local thermodynamic equilibrium (non-LTE = NLTE) line formation because low metal abundances and low electron number density in the atmosphere produce the physical conditions favourable for the departures from LTE. The galactic archaeology research requires homogeneous determinations of chemical abundances. For this purpose, we present grids of the 1D-NLTE abundance corrections for lines of Na  i , Mg  i , Ca  i , Ca  ii , Ti  ii , Fe  i , Zn  i , Zn  ii , Sr  ii , and Ba  ii in the range of atmospheric parameters that represent VMP stars on various evolutionary stages and cover effective temperatures from 4000 to 6500 K, surface gravities from |$\rm log g$|  = 0.5 to 5.0, and metallicities −5.0 ≤ [Fe/H] ≤ −2.0. The data is publicly available, and we provide the tools for interpolating in the grids online. [ABSTRACT FROM AUTHOR]

Based on observations obtained with the Nordic Optical Telescope (NOT) we investigate the spectral variability of the Herbig Ae star RR Tau. This star belongs to the UX Ori family, characterized by very deep fadings caused by the screening of the star with opaque fragments (clouds) of the protoplanetary discs. At the moments of such minima one observes strong spectral variability due to the fact that the dust cloud occults, for an observer, not only the star but also a part of the region where the emission spectrum originates. We calculated a series of obscuration models to interpret the observed variability of the H α line parameters. We consider two main obscuration scenarios: (1) the dust screen rises vertically above the circumstellar disc, and (2) the screen intersects the line-of-sight moving azimuthally with the disc. In both cases, the model of the emission region consists of a compact magnetosphere and a magnetocentrifugal disc wind. Comparison with observations shows that the first scenario explains well the variability of the radiation flux, the equivalent width, as well as the asymmetry of the H α line during eclipses, while the second scenario explains them only partly. This permits us to suggest that in the case of RR Tau, the main causes of the eclipses are either a structured disc wind, or the charged dust lifted along the field lines of the poloidal component of the magnetic field of the circumstellar disc. [ABSTRACT FROM AUTHOR]

We study the ionized and highly ionized gas phases in the Seyfert 2 galaxy IC 5063 by means of the Very Large Telescope (VLT)/Multi-Unit Spectroscopic Explorer (MUSE) integral field spectroscopy. Our analysis allowed us to detect a high-ionization gas outflow traced by the coronal lines [Fe  vii ] λ6087 and [Fe  x ] λ6375. Both emissions are found to be extended. The former up to 1.2 kpc and 700 pc north-west (NW) and south-east (SE) from the nucleus, respectively. The latter reaches 700 pc NW of the nucleus. This is the first time that [Fe  x ] emission is observed at such distances from the central engine in an active galactic nucleus. The [Fe  vii ] λ6087 emission peaks at the nucleus, with two secondary peaks at the position of the NW and SE radio lobes. The gas kinematics is complex, with the coronal emission displaying split line profiles along the radio jet and line widths of several hundred km s−1. Velocity shifts of up to 600 km s−1 in excess of the systemic velocity of the galaxy are found very close to the radio lobes and along the jet propagation. The extended coronal gas is characterized by temperatures reaching 20 000 K and electron densities >102 cm−3, with the larger values associated with the regions of larger turbulence, likely due to the passage of the radio jet. This hypothesis is supported by photoionization models that combine the effects of the central engine and shocks. Our work highlights the strong relationship between extended coronal emission and the radio jet, with the former suitably tracing the latter, which in the case of IC 5063, propagates very close to the galaxy disc. [ABSTRACT FROM AUTHOR]

Context. In the solar spectrum, the Balmer series line Hε is a weak blend on the wing of Ca II H. Recent high-resolution Hε spectroheliograms reveal a reversed granulation pattern and in some cases, even unique structures. It is apparent that Hε could potentially be a useful diagnostic tool for the lower solar atmosphere. Aims. Our aim is to understand how Hε is formed in the quiet Sun. In particular, we consider the particular physical mechanism that sets its source function and extinction, how it is formed in different solar structures, and why it is sometimes observed in emission. Methods. We used a 3D radiative magnetohydrodynamic (MHD) simulation that accounts for non-equilibrium hydrogen ionization, run with the Bifrost code. To synthesize Hε and Ca II H spectra, we made use of the RH code, which was modified to take into account the non-equilibrium hydrogen ionization. To determine the dominant terms in the Hε source function, we adopted a multi-level description of the source function. Making use of the synthetic spectra and simulation, we studied the contribution function to the relative line absorption or emission and compared it with atmospheric quantities at different locations. Results. Our multi-level source function description suggests that the Hε source function is dominated by interlocking, with the dominant interlocking transition being through the ground level, populating the upper level of Hε via the Lyman series. This makes the Hε source function partly sensitive to temperature. The Hε extinction is set by Lyman-α. In some cases, this temperature dependence gives rise to Hε emission, indicating heating. The typical absorption profiles show reversed granulation and the Hε line core reflects mostly the Ca II H background radiation. Conclusions. Synthetic Hε spectra can reproduce quiet Sun observations quite well. High-resolution observations reveal that Hε is not just a weak absorption line. Regions with Hε in emission are especially interesting to detect small-scale heating events in the lower solar atmosphere, such as Ellerman bombs. Thus, Hε can be an important new diagnostic tool for studies of heating in the solar atmosphere, augmenting the diagnostic potential of Ca II H when observed simultaneously. [ABSTRACT FROM AUTHOR]

We present an extended grid of multi-epoch 1D nonlocal thermodynamic equilibrium radiative transfer calculations for nebular-phase Type Ibc supernovae (SNe) from He-star explosions. Compared to our previous work, which was focused on a post-explosion epoch of 200 days, here we study the spectral evolution from 100 to about 450 days. We also augment the model set with progenitors that evolved without wind mass loss. Models with the same final, pre-SN mass have similar yields and produce essentially the same emergent spectra. Hence, the uncertain progenitor mass loss history compromises the inference of the initial, main sequence mass. This shortcoming does not affect Type IIb SNe in which mass-loss has left a small residual H-rich envelope in the progenitor star at core collapse and, hence, an intact He core. However, our 1D models with a different pre-SN mass tend to yield widely different spectra, as seen through variations in the strong emission lines due to [N II] λλ 6548, 6583, [O I] λλ 6300, 6364, [Ca II] λλ 7291, 7323, [Ni II] λ 7378, and the forest of Fe II lines below 5500 Å. At the lower mass end, the ejecta are He-rich, and at 100 days, they cool through He I, N II, Ca II, and Fe II lines, with N II and Fe II dominating at 450 days. These models, associated with He giants, stand in conflict to observed SNe Ib, which typically lack strong N II emission. Instead, they may lead to SNe Ibn or, because of additional stripping by a companion star, ultra-stripped SNe Ic. In contrast, for higher pre-SN masses, the ejecta are progressively He poor and cool at 100 days through O I, Ca II, and Fe II lines, with O I and Ca II dominating at 450 days. Non-uniform, aspherical, large-scale mixing is more likely to determine the SN type at intermediate pre-SN masses, rather than any compositional differences. Variations in clumping and mixing, as well as departures from spherical symmetry would increase the spectral diversity, but also introduce additional degeneracies. More robust predictions from spectral modeling thus require that careful attention be paid to the initial conditions by incorporating the salient features of physically consistent 3D explosion models. [ABSTRACT FROM AUTHOR]

Three-dimensional radiation-hydrodynamics (3D RHD) simulations of stellar surface convection provide valuable insights into many problems in solar and stellar physics. However, almost all 3D near-surface convection simulations to date are based on solar-scaled chemical compositions, which limits their relevance when applied to stars with peculiar abundance patterns. To overcome this difficulty, we implement the robust and widely used FreeEOS equation of state and our Blue opacity package into the Stagger 3D radiation-magnetohydrodynamics code. We present a new 3D RHD model of the solar atmosphere, and demonstrate that the mean stratification as well as the distributions of key physical quantities are in good agreement with those of the latest Stagger solar model atmosphere. The new model is further validated by comparisons with solar observations. The new model atmospheres reproduce the observed flux spectrum, continuum centre-to-limb variation, and hydrogen line profiles at a satisfactory level, thereby confirming the realism of the model and the underlying input physics. These implementations open the prospect for studying other stars with different α-element abundance, carbon-enhanced metal-poor stars, and population II stars with peculiar chemical compositions using 3D Stagger model atmospheres. [ABSTRACT FROM AUTHOR]

Accretion disc coronae (ADC) sources are very high inclination neutron star or black hole binaries, where the outer accretion flow blocks a direct view of the central source. The weak observed X-ray emission is instead produced mainly by scattering of the intrinsic radiation from highly ionized gas surrounding the source, the ADC. However, the origin of this scattering material is still under debate. We use the ADC source 2S 0921-630 (V395 Car) to test whether it is consistent with a thermal-radiative wind produced by the central X-ray source illuminating and puffing up the outer disc. This wind is clearly visible in blueshifted absorption lines in less highly inclined systems, where the source is seen directly through this material. Using the phenomenological photoionized plasma model, we first characterize the parameter that drives emission lines observed in 2S0921 in XMM–Newton and Chandra data. Following this, we run the Monte Carlo radiation transfer simulation to get scattered/reprocessed emissions in the wind, with the density and velocity structure obtained from the previous work. Our model agrees with all the wind emission lines in the Chandra high and medium energy grating spectra for an intrinsic source luminosity of L > 0.2  L Edd. This result strongly favours thermal-radiative winds as the origin of the ADC. We also show how high-resolution spectra via microcalorimeters can provide a definitive test by detecting blueshifted absorption lines. [ABSTRACT FROM AUTHOR]

We use the line ratios of resolved X-ray resonance and satellite line pairs from Si  xiii and Mg  xi ions to test the widely held assumption that the shocked plasma in the wind of hot stars is in collisional ionization equilibrium. We specifically apply this test to the prototypical O supergiant ζ Puppis, as its line ratios are well resolved in its total 813 ks Chandra observation. This satellite-to-resonance line ratio is known to be a reliable diagnostic of plasma temperature but has yet to be applied to hot stars due to needing a long exposure time to see both lines above the continuum. In testing the assumed plasma state, we also provide evidence against significant ionization and excitation from proton collisions. Since dielectronic recombination (DR) allows somewhat lower energy free electrons to excite resonance lines than by the usual collisional pathways, we also use the DR flux ratios to extend diagnostics of the ionization fraction of the parent ion to lower temperatures than any other method can achieve. Also, the DR ratio supports a concept of an average free electron temperature, for each target ion used, which can constrain heating models. [ABSTRACT FROM AUTHOR]

Engine-driven explosions with continuous energy input from the central system have been suggested for supernovae (SNe) associated with a Gamma-Ray Burst (GRB), superluminous SNe (SLSNe), and at least a fraction of broad-lined SNe Ic (SNe Ic-BL) even without an associated GRB. In the present work, we investigate observational consequences in this scenario, focusing on the case where the energy injection is sufficiently brief, which has been suggested for GRB-SNe. We construct a simplified, spherical ejecta model sequence taking into account the major effects of the central engine; composition mixing, density structure, and the outermost ejecta velocity. Unlike most of the previous works for GRB-SNe, we solve the formation of the photosphere self-consistently, with which we can predict the photometric and spectroscopic observables. We find that these ejecta properties strongly affect their observational appearance in the initial phase (≲ a week since the explosion), highlighted by blended lines suffering from higher-velocity absorptions for the flatter density distribution and/or higher outermost ejeca velocity. This behaviour also affects the multiband light curves in a non-monotonic way. Prompt follow-up observations starting immediately after the explosion thus provides key diagnostics to unveil the nature of the central engine behind GRB-SNe and SNe Ic-BL. For SN 2017iuk associated with GRB 171205A these diagnosing observational data are available, and we show that the expected structure from the engine-driven explosion, i.e. a flat power-law density structure extending up to ≳100 000 km s−1, can explain the observed spectral evolution reasonably well. [ABSTRACT FROM AUTHOR]

Context. Fabry–Pérot interferometers (FPIs) have become very popular in solar observations because they offer a balance between cadence, spatial resolution, and spectral resolution through a careful design of the spectral sampling scheme according to the observational requirements of a given target. However, an efficient balance requires knowledge of the expected target conditions, the properties of the chosen spectral line, and the instrumental characteristics. Aims. Our aim is to find a method that allows the optimal spectral sampling of FPI observations in a given spectral region to be found. The selected line positions must maximize the information content in the observation with a minimal number of points. Methods. In this study, we propose a technique based on a sequential selection approach in which a neural network is used to predict the spectrum (or physical quantities, if the model is known) from the information at a few points. Only those points that contain relevant information and improve the model prediction are included in the sampling scheme. Results. We have quantified the performance of the new sampling schemes by showing the lower errors in the model parameter reconstructions. The method adapts the separation of the points according to the spectral resolution of the instrument, the typical broadening of the spectral shape, and the typical Doppler velocities. The experiments that use the Ca II 8542 Å line show that the resulting wavelength scheme naturally places more points in the core than in the wings (by almost a factor of 4), consistent with the sensitivity of the spectral line at each wavelength interval. As a result, observations focused on magnetic field analysis should prioritize a denser grid near the core, while those focused on thermodynamic properties would benefit from a larger coverage. The method can also be used as an accurate interpolator to improve the inference of the magnetic field when using the weak-field approximation. Conclusions. Overall, this method offers an objective approach for designing new instrumentation or observing proposals with customized configurations for specific targets. This is particularly relevant when studying highly dynamic events in the solar atmosphere with a cadence that preserves spectral coherence without sacrificing much information. [ABSTRACT FROM AUTHOR]

The use of the coronal approximation to model line emission from the solar transition region has led to discrepancies with observations over many years, particularly for Li- and Na-like ions. Studies have shown that a number of atomic processes are required to improve the modelling for this region, including the effects of high densities, solar radiation, and charge transfer on ion formation. Other non-equilibrium processes, such as time-dependent ionization and radiative transfer, are also expected to play a role. A set of models which include the three relevant atomic processes listed above in ionization equilibrium has recently been built. These new results cover the main elements observed in the transition region. To assess the effectiveness of the results, this work predicts spectral line intensities using differential emission measure modelling. Although limited in some respects, this differential emission measure modelling does give a good indication of the impact of the new atomic calculations. The results are compared to predictions of the coronal approximation and to observations of the average, quiet Sun from published literature. Significant improvements are seen for the line emission from Li- and Na-like ions, intercombination lines, and many other lines. From this study, an assessment is made of how far down into the solar atmosphere the coronal approximation can be applied, and the range over which the new atomic models are valid. [ABSTRACT FROM AUTHOR]

Context. The mid-infrared (MIR) emission of molecules such as H2O, HCN, OH, CO2, and C2H2, has been identified in the Spitzer Infrared Spectrograph (IRS) spectra of many protoplanetary disks. According to the modelling results, the blend strengths are affected by different disk properties such as the gas mass and dust content in the disks. An observational correlation between HCN and water blend fluxes has been noted, specifically related to a changing disk gas mass. Aims. We aim to find out whether the explanation for the observed flux correlation between HCN and water in the MIR could also be attributed to other properties and processes taking place in disks, such as the evolution of dust grains. We also consider what the consequences of these results would be in relation to the disk evolution. Methods. We used pre-existing ProDiMo radiation thermal-chemical disk models exploring a range of properties such as the disk gas mass, disk inner radius, dust size power law distribution, and, finally, time-dependent dust evolution. From these models, we computed the MIR fluxes of HCN and H2O blends. Simultaneously, we derived the spectral indices from the simulated spectral energy distributions (SEDs) in the Spitzer IRS regime. Finally, we compared these quantities with the observed data. Results. The MIR blend fluxes correlation between HCN and water can be explained as a consequence of dust evolution, namely, changes in the dust MIR opacity. Other disk properties, such as the disk inner radius and the disk flaring angle, can only partially cover the dynamic range of the HCN and water blend observations. At the same time, the dynamic range of the MIR SED slopes is better reproduced by the disk structure (e.g. inner radius, flaring) than by the dust evolution. Our model series do not reproduce the observed trend between continuum flux at 850 µm and the MIR HCN/H2O blend ratio. However, our models show that this continuum flux is not a unique indicator of disk mass and it should therefore be used jointly with complementary observational data for optimal results. Conclusions. The presence of an anti-correlation between MIR H2O blend fluxes and the MIR SED is consistent with a scenario where dust evolves in disks, producing lower opacity and stronger features in the Spitzer spectral regime, while the gas eventually becomes depleted at a later stage, leaving behind an inner cavity in the disk. [ABSTRACT FROM AUTHOR]

Context. An asteroid spun up to its critical limit has unique surface mechanical properties that its gravity and the centrifugal force largely balance, creating a relaxation environment where low-energy events such as mass shedding may trigger subsequent long complex motion of an asteroid's regolith grains. Exploring such an evolution process may provide key clues for understanding the early formation of multi-asteroid systems. Aims. This paper investigates the complex evolution process of loose particles becoming triggered by shedding events and the dependency of their dynamical propagation on the contact mechanical properties of the asteroid surface. Methods. We present a numerical model for tracking the trajectory of a shed particle that considers the collision between the particle and the surface of an asteroid. Monte Carlo simulations are performed to reflect the statistical behavior of shed particles. We also introduce zero-velocity surfaces to our data-based analysis in order to reveal the intrinsic invariance of the evolutionary processes. We used the average mechanical energy of the particle cloud to check the connection between contact property and the temporal-spatial distribution of the shed particles. Results. We sketch a common evolutionary path of the particle in the vicinity of a fast-rotating asteroid, that is, particles dislodged from the unstable region will eventually enter, through several collisions with the surface, non-return orbits that launch from the minimum geopotential area of the unstable region. The common trend is independent of any particular asteroid morphology, and all shed particles (no matter where they originate from) enter the same evolutionary path. We also find that the orbital energy of the particle cloud is statistically independent of the surface contact property, meaning that the collision coefficient of restitution is a nonsensitive parameter in the outward spreading process of the shed particles. [ABSTRACT FROM AUTHOR]

Context. Observations of center-to-limb variations (CLVs) of spectral lines and continua provide a good test for the accuracy of models with a solar and stellar atmospheric structure and spectral line formation. They are also widely used to constrain elemental abundances, and are becoming increasingly more important in atmospheric studies of exoplanets. However, only a few such data sets exist for chromospheric lines. Aims. We aim to create a set of standard profiles by means of mosaics made with the CRISP and CHROMIS instruments of the Swedish 1-m Solar Telescope (SST), as well as to explore the robustness of said profiles obtained using this method. Methods. For each spectral line, we used a mosaic that ranges from the center to the limb. Each of these mosaics were averaged down to 50 individual spectral profiles and spaced by 0.02 in the μ scale. These profiles were corrected for p-mode oscillations, and their line parameters (equivalent width, line shift, full-width at half-maximum, and line depth) were then compared against literature values whenever possible. Results. We present a set of 50 average profiles that are spaced equidistantly along the cosine of the heliocentric angle (μ) by steps of 0.02 for five continuum points between 4001 and 7772 Å, as well as ten of the most commonly observed spectral lines at the SST (Ca II H & K, Ηβ, Mg I 5173 Å, C I 5380 Å, Fe I 6173 Å, Fe I 6301 Å, Ha, O I 7772 Å, and Ca II 8542 Å). Conclusions. The CLV of line profiles and continua are shared in the CDS as machine readable tables, providing a quantitative constraint on theoretical models that aim to model stellar atmospheres. [ABSTRACT FROM AUTHOR]

Solar wind (SW) charge exchange (CX) is an important mechanism responsible for X-ray emissions in comets' coma and many other astrophysical environments. By incorporating the experimentally derived total cross-sections and the tested state-selectivity models into the Kronos program, we generate X-ray spectra in H-like and fully stripped C, N, O ion CX collision with atomic H. Using these data, we revisit the fitting of Chandra X-ray data of Comet C/1999 S4 by considering the typical SW collision energies of 0.8 and 3 keV/u, respectively. Our results are in an excellent agreement with the astrophysical observation. We find that charge exchange between C5+,6+, N6+,7+, O7+,8+ and H significantly contributes to the comet's spectrum ranging from 300 to 800 eV for both slow and fast SW ions, where N7+ abundance is underestimated in previous studies. [ABSTRACT FROM AUTHOR]

The Accreting Millisecond X-ray Pulsar IGR J17591–2342 is a Low Mass X-ray Binary (LMXB) system that went in outburst on 2018 August and it was monitored by the NICER observatory and partially by other facilities. We aim to study how the spectral emission of this source evolved during the outburst by exploiting the whole X-ray data repository of simultaneous observations. The continuum emission of the combined broad-band spectra is on average well described by an absorbed Comptonization component scattering blackbody-distributed photons peaking at (0.8 ± 0.5) keV by a moderately optically thick corona (τ = 2.3 ± 0.5) with temperature of (34 ± 9) keV. A blackbody component with temperature and radial size of (0.8 ± 0.2) keV and (3.3 ± 1.5) km, respectively, is required by some of the spectra and suggests that part of the central emission, possibly a fraction of the neutron star surface, is not efficiently scattered by the corona. The continuum at low energies is characterized by significant residuals suggesting the presence of an absorption edge of O  viii and of emission lines of Ne  ix ions. Moreover, broad Fe  i and Fe  xxv Kα emission lines are detected at different times of the outburst, suggesting the presence of reflection in the system. [ABSTRACT FROM AUTHOR]