Search

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]

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 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. 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 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. 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]

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]

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]

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]

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]

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]

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]

Black hole accretion discs can produce powerful outflowing plasma (disc winds), seen as blue-shifted absorption lines in stellar and supermassive systems. These winds in quasars have an essential role in controlling galaxy formation across cosmic time, but there is no consensus on how these are physically launched. A single unique observation of a stellar-mass black hole GRO J1655−40 was used to argue that magnetic driving was the only viable mechanism and motivated unified models of magnetic winds in both binaries and quasars. The alternative, X-ray heating (thermal-radiative wind), was ruled out for the low observed luminosity by the high wind density estimated from an absorption line of a metastable level of Fe  xxii. Here, we reanalyse these data using a photoionization code that includes cascades from radiative excitation as well as collisions in populating the metastable level. The cascade reduces the inferred wind density by more than an order of magnitude. The derived column is also optically thick, so the source is intrinsically more luminous than observed. We show that a thermal-radiative wind model calculated from a radiation hydrodynamic simulation matches well with the data. We revisit the previous magnetic wind solution and show that this is also optically thick, leading to a larger source luminosity. However, unlike the thermal-radiative wind, it struggles to reproduce the overall ion population at the required density. These results remove the requirement for a magnetic wind in these data and remove the basis of the self-similar unified magnetic wind models extrapolated to quasar outflows. [ABSTRACT FROM AUTHOR]

In this paper, we present the results of a spectroscopic campaign devoted to ascertaining the actual nature of a sample of five objects reported as chemically peculiar stars in the Catalogue of Ap, HgMn and Am stars (2009, A&A, 498, 961). Spectroscopic observations were obtained with Catania Astrophysical Observatory Spectropolarimeter. For each of the objects, we derive the effective temperature, gravity, rotational and radial velocities, and chemical abundances using the spectral synthesis method. All the targets were found to be SB1 systems. Our abundance analysis led us to classify HD 40788 and HD 187959 as marginal Am stars and HD 202431 as an Am star, and to confirm HD 134793 as an SrEuCr Ap star and, finally, HD 189652 as a normal star. Finally, we confirm that Am stars can possibly be used as an accurate tool to date stellar populations in galaxies. [ABSTRACT FROM AUTHOR]

H2O submillimeter emission is a powerful diagnostic of the molecular interstellar medium in a variety of sources, including low- and high-mass star-forming regions of the Milky Way, and from local to high-redshift galaxies. However, the excitation mechanism of these lines in galaxies has been debated, preventing a basic consensus on the physical information that H2O provides. Radiative pumping due to H2O absorption of far-infrared photons emitted by dust and collisional excitation in dense shocked gas have both been proposed to explain the H2O emission. Here we propose two basic diagnostics to distinguish between the two mechanisms: First, the ortho-H2O 321 − 212 75 μm and the para-H2O 220 − 111 101 μm rotational lines in shock-excited regions are expected to be in emission, while when radiative pumping dominates, the two far-infrared lines are expected to be in absorption. Second, the radiative pumping scenario predicts, based on the statistical equilibrium of H2O level populations, that the apparent isotropic net rate of far-infrared absorption in the 321 ← 212 (75 μm) and 220 ← 111 (101 μm) lines should be higher than or equal to the apparent isotropic net rate of submillimeter emission in the 321 → 312 (1163 GHz) and 220 → 211 (1229 GHz) lines, respectively. Applying both criteria to all 16 galaxies and several Galactic high-mass star-forming regions in which the H2O 75 μm and submillimeter lines have been observed with Herschel/PACS and SPIRE, we show that in most (extra)galactic sources, the H2O submillimeter line excitation is dominated by far-infrared pumping, combined in some cases with collisional excitation of the lowest energy levels. Based on this finding, we revisit the interpretation of the correlation between the luminosity of the H2O 988 GHz line and the source luminosity in the combined Galactic and extragalactic sample. [ABSTRACT FROM AUTHOR]

Radiative transfer modelling offers a powerful tool for understanding the enigmatic hydrogen emission lines from T Tauri stars. This work compares optical and near-IR spectroscopy of 29 T Tauri stars with our grid of synthetic line profiles. The archival spectra, obtained with VLT/X-Shooter, provide simultaneous coverage of many optical and infrared hydrogen lines. The observations exhibit similar morphologies of line profiles seen in other studies. We used the radiative transfer code torus to create synthetic H α, Pa β, Pa γ, and Br γ emission lines for a fiducial T Tauri model that included axisymmetric magnetospheric accretion and a polar stellar wind. The distribution of Reipurth types and line widths for the synthetic H α lines is similar to the observed results. However, the modelled infrared lines are narrower than the observations by |${\approx}80\,{~\rm km\,s}^{-1}$|⁠ , and our models predict a significantly higher proportion (≈90 per cent) of inverse P-Cygni profiles. Furthermore, our radiative transfer models suggest that the frequency of P-Cygni profiles depends on the ratio of the mass-loss to mass accretion rates and blue-shifted sub-continuum absorption was predicted for mass-loss rates as low as 10−12 M⊙ yr−1. We explore the effect of rotation, turbulence, and the contributions from red-shifted absorption in an attempt to explain the discrepancy in widths. Our findings show that, singularly, none of these effects is sufficient to explain the observed disparity. However, a combination of rotation, turbulence, and non-axisymmetric accretion may improve the fit of the models to the observed data. [ABSTRACT FROM AUTHOR]

Existing ubiquitously in the Universe with the highest luminosity, the Lyman- α (Ly α) emission line encodes abundant physical information about the gaseous medium it interacts with. Nevertheless, the resonant nature of the Ly α line complicates the radiative transfer (RT) modelling of the line profile. We revisit the problem of deciphering the Ly α emission line with RT modelling. We reveal intrinsic parameter degeneracies in the widely used shell model in the optically thick regime for both static and outflowing cases, which suggest the limitations of the model. We also explore the connection between the more physically realistic multiphase, clumpy model, and the shell model. We find that the parameters of a 'very clumpy' slab model and the shell model have the following correspondences: (1) the total column density, the effective temperature, and the average radial clump outflow velocity of the clumpy slab model are equal to the H  i column density, effective temperature, and expansion velocity of the shell model, respectively; (2) large intrinsic linewidths are required in the shell model to reproduce the wings of the clumpy slab models; (3) adding another phase of hot interclump medium increases peak separation, and the fitted shell expansion velocity lies between the outflow velocities of two phases of gas. Our results provide a viable solution to the major discrepancies associated with Ly α fitting reported in previous literature, and emphasize the importance of utilizing information from additional observations to break the intrinsic degeneracies and interpreting the model parameters in a more physically realistic context. [ABSTRACT FROM AUTHOR]

The nebular spectra of Type Ia supernovae (⪆100 d after explosion) consist mainly of emission lines from singly and doubly ionized Fe-group nuclei. However, theoretical models for many scenarios predict that non-thermal ionization leads to multiply ionized species whose recombination photons ionize and deplete Fe+, resulting in negligible [Fe  ii ] emission. We investigate a method to determine the collisional excitation conditions from [Fe  ii ] line ratios independently from the ionization state and find that it cannot be applied to highly ionized models due to the influence of recombination cascades on Fe+ level populations. When the ionization state is artificially lowered, the line ratios (and excitation conditions) are too similar to distinguish between explosion scenarios. We investigate changes to the treatment of non-thermal energy deposition as a way to reconcile overionized theoretical models with observations and find that a simple work function approximation provides closer agreement with the data for sub- M ch models than a detailed Spencer–Fano treatment with widely used cross-section data. To quantify the magnitude of additional heating processes that would be required to sufficiently reduce ionization from fast leptons, we artificially boost the rate of energy loss to free electrons. We find that the equivalent of as much as an eight times increase to the plasma loss rate would be needed to reconcile the sub- M ch model with observed spectra. Future studies could distinguish between reductions in the non-thermal ionization rates and increased recombination rates, such as by clumping. [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 test the adequacy of ultraviolet (UV) spectra for characterizing the outer structure of Type Ia supernova (SN) ejecta. For this purpose, we perform spectroscopic analysis for ASASSN-14lp, a normal SN Ia showing low continuum in the mid-UV regime. To explain the strong UV suppression, two possible origins have been investigated by mapping the chemical profiles over a significant part of their ejecta. We fit the spectral time series with mid-UV coverage obtained before and around maximum light by HST , supplemented with ground-based optical observations for the earliest epochs. The synthetic spectra are calculated with the one-dimensional MC radiative transfer code tardis from self-consistent ejecta models. Among several physical parameters, we constrain the abundance profiles of nine chemical elements. We find that a distribution of 56Ni (and other iron-group elements) that extends towards the highest velocities reproduces the observed UV flux well. The presence of radioactive material in the outer layers of the ejecta, if confirmed, implies strong constraints on the possible explosion scenarios. We investigate the impact of the inferred 56Ni distribution on the early light curves with the radiative transfer code turtls , and confront the results with the observed light curves of ASASSN-14lp. The inferred abundances are not in conflict with the observed photometry. We also test whether the UV suppression can be reproduced if the radiation at the photosphere is significantly lower in the UV regime than the pure Planck function. In this case, solar metallicity might be sufficient enough at the highest velocities to reproduce the UV suppression. [ABSTRACT FROM AUTHOR]

The winds of hot, massive stars are variable from processes happening on both large and small spatial scales. A particular case of such wind variability is 'discrete-absorption components' (DACs) that manifest themselves as outward moving density features in UV resonance line spectra. Such DACs are believed to be caused by large-scale spiral-shaped density structures in the stellar wind. We consider novel 3-D radiation-hydrodynamic models of rotating hot star winds and study the emergence of co-rotating spiral structures due to a local (pseudo-)magnetic spot on the stellar surface. Subsequently, the hydrodynamic models are used to retrieve DAC spectral signatures in synthetic UV spectra created from a 3-D short-characteristics radiative transfer code. [ABSTRACT FROM AUTHOR]

X-ray flux from the inner hot region around central compact object in a binary system illuminates the upper surface of an accretion disc and it behaves like a corona. This region can be photoionized by the illuminating radiation, and thus can emit different emission lines. We study those line spectra in black hole X-ray binaries for different accretion flow parameters including its geometry. The varying range of model parameters captures maximum possible observational features. We also put light on the routinely observed Fe-line emission properties based on different model parameters, ionization rate, and Fe abundances. We find that the Fe-line equivalent width W E decreases with increasing disc accretion rate and increases with the column density of the illuminated gas. Our estimated line properties are in agreement with observational signatures. [ABSTRACT FROM AUTHOR]

Radiative transfer out of local thermodynamic equilibrium (NLTE) has been increasingly adressed, mostly numerically, for about six decades now. However, the standard NLTE problem most often refers to the only deviation of the distribution of photons from their equilibrium, that is to say a Planckian distribution. Hereafter we revisit after Oxenius (1986, Kinetic theory of particles and Photons – Theoretical Foundations of non–LTE Plasma Spectroscopy, Springer) the so-called full NLTE problem, which considers coupling and therefore solving self–consistently for deviations from equilibrium distributions of photons as well as for massive particles constituting the atmospheric plasma. [ABSTRACT FROM AUTHOR]

Context. Hydrodynamical (or 3D) and non-local thermodynamic equilibrium (NLTE) effects are known to affect abundance analyses. However, there are very few observational abundance tests of 3D and NLTE models. Aims. We developed a new way of testing the abundance predictions of 3D and NLTE models, taking advantage of large spectroscopic survey data. Methods. We use a line-by-line analysis of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectra (H band) with the Brussels Automatic Code for Characterizing High accUracy Spectra (BACCHUS). We compute line-by-line abundances of Mg, Si, Ca, and Fe for a large number of globular cluster K giants in the APOGEE survey. We compare this line-by-line analysis against NLTE and 3D predictions. Results. While the 1D–NLTE models provide corrections in the right direction, there are quantitative discrepancies between different models. We observe a better agreement with the data for the models including reliable collisional cross-sections. The agreement between data and models is not always satisfactory when the 3D spectra are computed in LTE. However, we note that for a fair comparison, 3D corrections should be computed with self-consistently derived stellar parameters, and not on 1D models with identical stellar parameters. Finally, we focus on 3D and NLTE effects on Fe lines in the H band, where we observe a systematic difference in abundance relative to the value from the optical. Our results suggest that the metallicities obtained from the H band are more accurate in metal-poor giants. Conclusions. Current 1D–NLTE models provide reliable abundance corrections, but only when the atom data and collisional cross-sections are accurate and complete. Therefore, we call for more atomic data for NLTE calculations. In contrast, we show that 3D corrections in LTE conditions are often not accurate enough, thus confirming that 3D abundance corrections are only valid when NLTE is taken into account. Consequently, more extended self-consistent 3D–NLTE computations need to be made. The method we have developed for testing 3D and NLTE models could be extended to other lines and elements, and is particularly suited for large spectroscopic surveys. [ABSTRACT FROM AUTHOR]

In supernova (SN) spectroscopy relatively little attention has been given to the properties of optically thick spectral lines in epochs following the photosphere's recession. Most treatments and analyses of post-photospheric optical spectra of SNe assume that forbidden-line emission comprises most if not all spectral features. However, evidence exists that suggests that some spectra exhibit line profiles formed via optically thick resonance-scattering even months or years after the SN explosion. To explore this possibility, we present a geometrical approach to SN spectrum formation based on the "Elementary Supernova" model, wherein we investigate the characteristics of resonance-scattering in optically thick lines while replacing the photosphere with a transparent central core emitting non-blackbody continuum radiation, akin to the optical continuum provided by decaying 56Co formed during the explosion. We develop the mathematical framework necessary for solving the radiative transfer equation under these conditions and calculate spectra for both isolated and blended lines. Our comparisons with analogous results from the Elementary Supernova code SYNOW reveal several marked differences in line formation. Most notably, resonance lines in these conditions form P Cygni-like profiles, but the emission peaks and absorption troughs shift redward and blueward, respectively, from the line's rest wavelength by a significant amount, despite the spherically symmetric distribution of the line optical depth in the ejecta. These properties and others that we find in this work could lead to misidentification of lines or misattribution of properties of line-forming material at post-photospheric times in SN optical spectra. © 2012. The American Astronomical Society. All rights reserved.

Supernova (SN) explosions play a pivotal role in the chemical evolution of the Universe and the origin of life through the metals they release. Nebular phase spectroscopy constrains such metal yields, for example through forbidden line emission associated with O I, Ca II, Fe II, or Fe III. Fluid instabilities during the explosion produce a complex 3D ejecta structure, with considerable macroscopic, but no microscopic, mixing of elements. This structure sets a formidable challenge for detailed nonlocal thermodynamic equilibrium radiative transfer modeling, which is generally limited to 1D in grid-based codes. Here, we present a novel and simple method that allows for macroscopic mixing without any microscopic mixing, thereby capturing the essence of mixing in SN explosions. With this new technique, the macroscopically mixed ejecta are built by shuffling the shells from the unmixed coasting ejecta in mass space, or equivalently in velocity space. The method requires no change to the radiative transfer, but it necessitates high spatial resolution to resolve the rapid variation in composition with depth inherent to this shuffled-shell structure. We show the results for a few radiative-transfer simulations for a Type II SN explosion from a 15 M⊙ progenitor star. Our simulations capture the strong variations in temperature or ionization between the various shells that are rich in H, He, O, or Si. Because of nonlocal energy deposition, γ rays permeate through an extended region of the ejecta, making the details of the shell arrangement unimportant. The greater physical consistency of the method delivers spectral properties at nebular times that are more reliable, in particular in terms of individual emission line strengths, which may serve to constrain the SN yields as well as the progenitor mass for core collapse SNe. The method works for all SN types. [ABSTRACT FROM AUTHOR]

Massive sets of stellar spectroscopic observations are rapidly becoming available and these can be used to determine the chemical composition and evolution of the Galaxy with unprecedented precision. One of the major challenges in this endeavour involves constructing realistic models of stellar spectra with which to reliably determine stellar abundances. At present, large stellar surveys commonly use simplified models that assume that the stellar atmospheres are approximately in local thermodynamic equilibrium (LTE). To test and ultimately relax this assumption, we have performed non-LTE calculations for 13 different elements (H, Li, C, N, O, Na, Mg, Al, Si, K, Ca, Mn, and Ba), using recent model atoms that have physically-motivated descriptions for the inelastic collisions with neutral hydrogen, across a grid of 3756 1D MARCS model atmospheres that spans 3000 ≤ Teff∕K ≤ 8000, − 0.5 ≤log g∕cm s−2 ≤ 5.5, and − 5 ≤ [Fe/H] ≤ 1. We present the grids of departure coefficients that have been implemented into the GALAH DR3 analysis pipeline in order to complement the extant non-LTE grid for iron. We also present a detailed line-by-line re-analysis of 50 126 stars from GALAH DR3. We found that relaxing LTE can change the abundances by between − 0.7 dex and + 0.2 dex for different lines and stars. Taking departures from LTE into account can reduce the dispersion in the [A/Fe] versus [Fe/H] plane by up to 0.1 dex, and it can remove spurious differences between the dwarfs and giants by up to 0.2 dex. The resulting abundance slopes can thus be qualitatively different in non-LTE, possibly with important implications for the chemical evolution of our Galaxy. The grids of departure coefficients are publicly available and can be implemented into LTE pipelines to make the most of observational data sets from large spectroscopic surveys. [ABSTRACT FROM AUTHOR]

Nebular phase spectra of core-collapse supernovae (SNe) provide critical and unique information on the progenitor massive star and its explosion. We present a set of one-dimensional steady-state non-local thermodynamic equilibrium radiative transfer calculations of type II SNe at 300 d after explosion. Guided by the results obtained from a large set of stellar evolution simulations, we craft ejecta models for type II SNe from the explosion of a 12, 15, 20, and 25 M⊙ star. The ejecta density structure and kinetic energy, the 56Ni mass, and the level of chemical mixing are parametrized. Our model spectra are sensitive to the adopted line Doppler width, a phenomenon we associate with the overlap of Fe II and O I lines with Ly α and Ly β. Our spectra show a strong sensitivity to 56Ni mixing since it determines where decay power is absorbed. Even at 300 d after explosion, the H-rich layers reprocess the radiation from the inner metal rich layers. In a given progenitor model, variations in 56Ni mass and distribution impact the ejecta ionization, which can modulate the strength of all lines. Such ionization shifts can quench Ca II line emission. In our set of models, the [O I] λλ 6300, 6364 doublet strength is the most robust signature of progenitor mass. However, we emphasize that convective shell merging in the progenitor massive star interior can pollute the O-rich shell with Ca, which would weaken the O I doublet flux in the resulting nebular SN II spectrum. This process may occur in nature, with a greater occurrence in higher mass progenitors, and this may explain in part the preponderance of progenitor masses below 17 M⊙ that are inferred from nebular spectra. [ABSTRACT FROM AUTHOR]

We present analysis of mid-infrared (IR) spectra of the oxygen-rich Mira variable R Tri. The data were taken with the Spitzer Infrared Spectrometer (IRS) as part of a study tracking how Mira variables' regular pulsations affect circumstellar envelopes. We detected strong emission lines at 13.87, 16.18, and 17.6 |$\hbox{$\mu $m}$|⁠ , and one strong absorption feature at 14.98 |$\hbox{$\mu $m}$|⁠. The emission features at 13.87 and 16.18 |$\hbox{$\mu $m}$| are excited vibrational bands of CO2, while the absorption feature is the fundamental ν 2 band. The 17.6 |$\hbox{$\mu $m}$| emission feature has a completely different character than the molecular lines and we report its identification as Fe  i fluorescence. We used a two-slab model with the radiative transfer code radex to model the CO2 Q-branch bandheads. Our results indicate a slab of gas with T ∼600 K located at ∼3–4 R*. The cool temperature discrepancy with the radius provides observational evidence for the previously theoretical 'refrigeration zone'. [ABSTRACT FROM AUTHOR]

The origin, geometry, and kinematics of the broad-line region (BLR) gas in quasars and active galactic nuclei (AGN) are uncertain. We demonstrate that clumpy biconical disc winds illuminated by an AGN continuum can produce BLR-like spectra. We first use a simple toy model to illustrate that disc winds make quite good BLR candidates, because they are self-shielded flows and can cover a large portion of the ionizing flux-density (ϕ H - nH) plane. We then conduct Monte Carlo radiative transfer and photoionization calculations, which fully account for self-shielding and multiple scattering in a non-spherical geometry. The emergent model spectra show broad emission lines with equivalent widths and line ratios comparable to those observed in AGN, provided that the wind has a volume filling factor of fV ≲ 0.1. Similar emission line spectra are produced for a variety of wind geometries (polar or equatorial) and for launch radii that differ by an order of magnitude. The line emission arises almost exclusively from plasma travelling below the escape velocity, implying that 'failed winds' are important BLR candidates. The behaviour of a line-emitting wind (and possibly any 'smooth flow' BLR model) is similar to that of the locally optimally emitting cloud model originally proposed by Baldwin et al. (1995), except that the gradients in ionization state and temperature are large-scale and continuous, rather than within or between distinct clouds. Our models also produce UV absorption lines and X-ray absorption features, and the stratified ionization structure can partially explain the different classes of broad absorption line quasars. [ABSTRACT FROM AUTHOR]

We extend the range of validity of the artis 3D radiative transfer code up to hundreds of days after explosion, when Type Ia supernovae (SNe Ia) are in their nebular phase. To achieve this, we add a non-local thermodynamic equilibrium population and ionization solver, a new multifrequency radiation field model, and a new atomic data set with forbidden transitions. We treat collisions with non-thermal leptons resulting from nuclear decays to account for their contribution to excitation, ionization, and heating. We validate our method with a variety of tests including comparing our synthetic nebular spectra for the well-known one-dimensional W7 model with the results of other studies. As an illustrative application of the code, we present synthetic nebular spectra for the detonation of a sub-Chandrasekhar white dwarf (WD) in which the possible effects of gravitational settling of 22Ne prior to explosion have been explored. Specifically, we compare synthetic nebular spectra for a 1.06 M⊙ WD model obtained when 5.5 Gyr of very efficient settling is assumed to a similar model without settling. We find that this degree of 22Ne settling has only a modest effect on the resulting nebular spectra due to increased 58Ni abundance. Due to the high ionization in sub-Chandrasekhar models, the nebular [Ni  ii ] emission remains negligible, while the [Ni  iii ] line strengths are increased and the overall ionization balance is slightly lowered in the model with 22Ne settling. In common with previous studies of sub-Chandrasekhar models at nebular epochs, these models overproduce [Fe  iii ] emission relative to [Fe  ii ] in comparison to observations of normal SNe Ia. [ABSTRACT FROM AUTHOR]

Atomic hydrogen masers occur in recombination plasmas in sufficiently dense H  ii regions. These hydrogen recombination line (HRL) masers have been observed in a handful of objects to date and the analysis of the atomic physics involved has been rudimentary. In this work, a new model of HRL masers is presented which uses an nl -model to describe the atomic populations interacting with free–free radiation from the plasma, and an escape probability framework to deal with radiative transfer effects. The importance of including the collisions between angular momentum quantum states and the free–free emission in models of HRL masers is demonstrated. The model is used to describe the general behaviour of radiative transfer of HRLs and to investigate the conditions under which HRL masers form. The model results show good agreement with observations collected over a broad range of frequencies. Theoretical predictions are made regarding the ratio of recombination lines from the same upper quantum level for these objects. [ABSTRACT FROM AUTHOR]

Manganese (Mn) is a key Fe-group element, commonly employed in stellar population and nucleosynthesis studies to explore the role of SN Ia. We have developed a new non-local thermodynamic equilibrium (NLTE) model of Mn, including new photo-ionisation cross-sections and new transition rates caused by collisions with H and H− atoms. We applied the model in combination with one-dimensional (1D) LTE model atmospheres and 3D hydrodynamical simulations of stellar convection to quantify the impact of NLTE and convection on the line formation. We show that the effects of NLTE are present in Mn I and, to a lesser degree, in Mn II lines, and these increase with metallicity and with the effective temperature of a model. Employing 3D NLTE radiative transfer, we derive a new abundance of Mn in the Sun, A(Mn) = 5.52 ± 0.03 dex, consistent with the element abundance in C I meteorites. We also applied our methods to the analysis of three metal-poor benchmark stars. We find that 3D NLTE abundances are significantly higher than 1D LTE. For dwarfs, the differences between 1D NLTE and 3D NLTE abundances are typically within 0.15 dex, however, the effects are much larger in the atmospheres of giants owing to their more vigorous convection. We show that 3D NLTE successfully solves the ionisation and excitation balance for the RGB star HD 122563 that cannot be achieved by 1D LTE or 1D NLTE modelling. For HD 84937 and HD 140283, the ionisation balance is satisfied, however, the resonance Mn I triplet lines still show somewhat lower abundances compared to the high-excitation lines. Our results for the benchmark stars confirm that 1D LTE modelling leads to significant systematic biases in Mn abundances across the full wavelength range from the blue to the IR. We also produce a list of Mn lines that are not significantly biased by 3D and can be reliably, within the 0.1 dex uncertainty, modelled in 1D NLTE. [ABSTRACT FROM AUTHOR]

Mid-infrared molecular line emission detected with the Spitzer Space Telescope is often interpreted using slab models. However, we need to understand the mid-infrared line emission in 2D disk models, such that we gain information about from where the lines are being emitted and under which conditions, such that we gain information about number densities, temperatures, and optical depths in both the radial and vertical directions. In this paper, we introduce a series of 2D thermochemical models of a prototypical T Tauri protoplanetary disk, in order to examine how sensitive the line-emitting regions are to changes in the UV and X-ray fluxes, the disk flaring angle, dust settling, and the dust-to-gas ratio. These all affect the heating of the inner disk, and thus can affect the mid-infrared spectral lines. Using the ProDiMo and FLiTs codes, we produce a series of 2D thermochemical disk models. We find that there is often a significant difference between the gas and dust temperatures in the line emitting regions, and we illustrate that the size of the line emitting regions is relatively robust against changes in the stellar and disk parameters (namely, the UV and X-ray fluxes, the flaring angle, and dust settling). These results demonstrate the potential for localized variations in the line-emitting region to greatly affect the resulting spectra and line fluxes, and the necessity of allowing for such variations in our models. [ABSTRACT FROM AUTHOR]

Context. Magnetic fields in cool stars can be investigated by measuring Zeeman line broadening and polarization in atomic and molecular lines. Similar to the Sun, these fields are complex and height-dependent. Many molecular lines dominating M-dwarf spectra (e.g., FeH, CaH, MgH, and TiO) are temperature- and Zeeman-sensitive and form at different atmospheric heights, which makes them excellent probes of magnetic fields on M dwarfs. Aims. Our goal is to analyze the complexity of magnetic fields in M dwarfs. We investigate how magnetic fields vary with the stellar temperature and how "surface" inhomogeneities are distributed in height – the dimension that is usually neglected in stellar magnetic studies. Methods. We have determined effective temperatures of the photosphere and of magnetic features, magnetic field strengths and filling factors for nine M dwarfs (M1–M7). Our χ2 analysis is based on a comparison of observed and synthetic intensity and circular polarization profiles. Stokes profiles were calculated by solving polarized radiative transfer equations. Results. Properties of magnetic structures depend on the analyzed atomic or molecular species and their formation heights. Two types of magnetic features similar to those on the Sun have been found: a cooler (starspots) and a hotter (network) one. The magnetic field strength in both starspots and network is within 3–6 kG, on average it is 5 kG. These fields occupy a large fraction of M dwarf atmospheres at all heights, up to 100%. The plasma β is less than one, implying highly magnetized stars. Conclusions. A combination of molecular and atomic species and a simultaneous analysis of intensity and circular polarization spectra have allowed us to better decipher the complexity of magnetic fields on M dwarfs, including their dependence on the atmospheric height. This work provides an opportunity to investigate a larger sample of M dwarfs and L-type brown dwarfs. [ABSTRACT FROM AUTHOR]

Context. The extent of the gas in protoplanetary discs is observed to be universally larger than the extent of the dust. This is often attributed to radial drift and grain growth of the millimetre grains, but line optical depth produces a similar observational signature. Aims. We investigate in which parts of the disc structure parameter space dust evolution and line optical depth are the dominant drivers of the observed gas and dust size difference. Methods. Using the thermochemical model DALI with dust evolution included we ran a grid of models aimed at reproducing the observed gas and dust size dichotomy. Results. The relation between Rdust and dust evolution is non-monotonic and depends on the disc structure. The quantity Rgas is directly related to the radius where the CO column density drops below 1015 cm−2 and CO becomes photodissociated; Rgas is not affected by dust evolution but scales with the total CO content of the disc. While these cases are rare in current observations, Rgas/Rdust > 4 is a clear sign of dust evolution and radial drift in discs. For discs with a smaller Rgas/Rdust, identifying dust evolution from Rgas/Rdust requires modelling the disc structure including the total CO content. To minimize the uncertainties due to observational factors requires FWHMbeam < 1× the characteristic radius and a peak S/N > 10 on the 12CO emission moment zero map. For the dust outer radius to enclose most of the disc mass, it should be defined using a high fraction (90–95%) of the total flux. For the gas, any radius enclosing >60% of the 12CO flux contains most of the disc mass. Conclusions. To distinguish radial drift and grain growth from line optical depth effects based on size ratios requires discs to be observed at high enough angular resolution and the disc structure should to be modelled to account for the total CO content of the disc. [ABSTRACT FROM AUTHOR]

Context. Stellar spectra synthesis is essential for the characterization of potential planetary hosts. In addition, comprehensive stellar variability calculations with fast radiative transfer are needed to disentangle planetary transits from stellar magnetically driven variability. The planet-hunting space telescopes, such as CoRoT, Kepler, and TESS, bring vast quantities of data, rekindling the interest in fast calculations of the radiative transfer. Aims. We revisit the opacity distribution functions (ODF) approach routinely applied to speed up stellar spectral synthesis. To achieve a considerable speedup relative to the state of the art, we further optimize the approach and search for the best ODF configuration. Furthermore, we generalize the ODF approach for fast calculations of flux in various filters often used in stellar observations. Methods. In a parameter-sweep fashion, we generated ODF in the spectral range from UV to IR with different setups. The most accurate ODF configuration for each spectral interval was determined. We adapted the wavelength grid based on the transmission curve for calculations of the radiative fluxes through filters before performing the normal ODF procedure. Results. Our optimum ODF configuration allows for a three-fold speedup, compared to the previously used ODF configurations. The ODF generalization to calculate fluxes through filters results in a speedup of more than two orders of magnitude. [ABSTRACT FROM AUTHOR]

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