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Our aim in this study is to investigate the electronic structure of the gaseous YN molecule and provide an accurate molecular line list of spectroscopic transitions between the five lowest singlet states. The calculations were done by using large basis set for yttrium aug-cc-pVQZ-PP with relativistic effective core potentials at the spin-free level. We first used the method of complete active space self-consistent field, which was followed by the multireference singles and doubles configuration interaction. Potential energy curves and permanent and transition dipole moments were calculated at the internuclear distance range of 1.3–2.96 Å. For each state, we calculated the spectroscopic constants (T e, ω e, ω e x e, ω e y e, B e, α e, R e). The calculated values of the spectroscopic constants are in excellent agreement with the experimental constants available, with a relative difference of less than 5 cm−1. We further calculated the transition intensities of allowed transitions, and we provided the absorption spectra up to a temperature of 4000 K. The computed molecular line list contains 2.6 million transitions calculated between almost 18 186 vibro-rotational energy levels. It covers the frequency range between 0 and 30 000 cm−1. The effect of temperature on the spectra of hot YN was investigated with partition functions calculated between 298 and 4000 K. This study should help in identifying the singlet state spectrum of the hot YN molecule, which is possibly found in the atmospheres of cool stars and hot rocky super-Earths. [ABSTRACT FROM AUTHOR]

The high-precision measurements of exoplanet transit light curves that are now available contain information about the planet properties, their orbital parameters, and stellar limb darkening (LD). Recent 3D magnetohydrodynamical (MHD) simulations of stellar atmospheres have shown that LD depends on the photospheric magnetic field, and hence its precise determination can be used to estimate the field strength. Among existing LD laws, the uses of the simplest ones may lead to biased inferences, whereas the uses of complex laws typically lead to a large degeneracy among the LD parameters. We have developed a novel approach in which we use a complex LD model but with second derivative regularization during the fitting process. Regularization controls the complexity of the model appropriately and reduces the degeneracy among LD parameters, thus resulting in precise inferences. The tests on simulated data suggest that our inferences are not only precise but also accurate. This technique is used to re-analyse 43 transit light curves measured by the NASA Kepler and Transiting Exoplanet Survey Satellite missions. Comparisons of our LD inferences with the corresponding literature values show good agreement, while the precisions of our measurements are better by up to a factor of 2. We find that 1D non-magnetic model atmospheres fail to reproduce the observations while 3D MHD simulations are qualitatively consistent. The LD measurements, together with MHD simulations, confirm that Kepler-17, WASP-18, and KELT-24 have relatively high magnetic fields (⁠|$\gt 200$| G). This study paves the way for estimating the stellar surface magnetic field using the LD measurements. [ABSTRACT FROM AUTHOR]

We employed advanced ionization equilibrium models that we developed in Dufresne et al. (2024), which include charge transfer and density effects, to model UV stellar irradiances for a sample of stars. Our sample includes |$\epsilon$| Eridani (K2 V), |$\alpha$| Centauri A (G2 V), Procyon (F5 V), and Proxima Centauri (M5.5 Ve). We measured line fluxes from STIS data sets and used O  iv and O  v as density diagnostics to find the formation pressure of ions in the transition region (TR) and adopted a simple differential emission measure (DEM) modelling. Our findings indicate significant improvements in modelling spectral lines from anomalous ions such as Si  iv , C  iv , and N  v of the Li- and Na-like sequences, which produce the strongest lines in the UV. For example, the Si  iv lines were under-predicted by a factor of 5 and now are within 40 per cent the observed fluxes. The improved models allow us to obtain for the first time reliable estimates of some stellar chemical abundances in the TR. We compared our results with available photospheric abundances in the literature and found no evidence for the first ionization potential (FIP) effect in the TR of our stellar sample. Finally, we compared our results with the solar TR that can also be described by photospheric abundances. [ABSTRACT FROM AUTHOR]

Based on the high resolution and high signal-to-noise spectra, we derived the chemical abundances of 20 elements for 20 barium (Ba-) stars. For the first time, the detailed abundances of four sample stars, namely HD 92482, HD 150430, HD 151101, and HD 177304 have been analysed. Additionally, Ba element abundance has been measured using high-resolution spectra for the first time in six of the other 16 sample stars. Based on the [s/Fe] ratios, the Ba-unknown star HD 115927 can be classified as a strong Ba-star, while the Ba-likely star HD 160538 can be categorized into a mild Ba-star. Consequently, our sample comprises three strong and 17 mild Ba-stars. The light odd-Z metal elements and Fe-peak elements exhibit near-solar abundances. The [ |$\alpha$| /Fe] ratios demonstrate decreasing trends with increasing metallicity. Moreover, the abundances of neutron-capture (n-capture) elements show significant enhancements in different degrees. Using a threshold of the signed distances to the solar rapid-process (r-process) abundance pattern |$d_{\rm s}$| = 0.6, we find that all of our sample stars are normal Ba-stars, indicating that the enhancements of slow-process (s-process) elements should be attributed to material transfer from their companions. We compare the observed n-capture patterns of sample stars with the FRUITY models, and estimate the mass of the Thermally-Pulsing Asymptotic Giant Branch stars that previously contaminated the Ba-stars. The models with low masses can successfully explain the observations. From a kinematic point of view, we note that most of our sample stars are linked with the thin disc, while HD 130255 may be associated with the thick disc. [ABSTRACT FROM AUTHOR]

The construction of exact solutions for radiative transfer in a plane-parallel medium has been addressed by Hemsch and Ferziger in 1972 for a partial frequency redistribution model of the formation of spectral lines consisting in a linear combination of frequency coherent and fully incoherent scattering. The method of solution is based on an eigenfunction expansion of the radiation field, leading to two singular integral equations with Cauchy or Cauchy-type kernels, that have to be solved one after the other. We reconsider this problem, using as starting point the integral formulation of the radiative transfer equation, where the terms involving the coupling between the two scattering mechanisms are clearly displayed, as well as the primary source of photons. With an inverse Laplace transform, we recover the singular integral equations previously established and, with Hilbert transforms as in the previous work, recast them as boundary value problems in the complex plane. Their solutions are presented in detail for an infinite and a semi-infinite medium. The coupling terms are carefully analyzed and consistency with either the coherent or the incoherent limit is systematically checked. We recover the important result of the previous work that an exact solution exists for an infinite medium, whereas for a semi-infinite medium, which requires the introduction of half-space auxiliary functions, the solution is given by a Fredholm integral equation to be solved numerically. The solutions of the singular integral equations are used to construct explicit expressions providing the radiation field for an arbitrary primary source and for the Green's function. An explicit expression is given for the radiation field emerging from a semi-infinite medium. [ABSTRACT FROM AUTHOR]

New hot line lists for five isotopologues of N |$_2$| O called TYM are presented, for the parent |$^{14}$| N |$_2$| |$^{16}$| O and 4 singly substituted species |$^{14}$| N |$_2$| |$^{17}$| O, |$^{14}$| N |$_2$| |$^{18}$| O, |$^{14}$| N |$^{15}$| N |$^{16}$| O and |$^{15}$| N |$^{14}$| N |$^{16}$| O. The line lists have been computed with the variational program TROVE (Theoretical ROVibrational Energies) using a new empirical potential energy surface (PES) and an accurate ab initio dipole moment surface of N |$_2$| O Ames-1. The PES was obtained by fitting to experimentally derived energies of N |$_2$| O compiled using the well established measured active rotation vibration energy levels (MARVEL) procedure. Here we also introduce an 'artificial symmetry group' |$C_{n \text{s}}(\text{AEM})$| for an efficient construction of rotation-vibrational basis set of a linear non-symmetric triatomic molecule of the XYZ type. The line lists cover the rotational excitations up to |$J=160$| and the wavenumber range up to 20000 cm |$^{-1}$|⁠. MARVEL energies are also used to improve predicted line positions resulting in excellent agreement with the available experimental spectra, as demonstrated. An extensive comparison with existing line lists for N |$_2$| O HITRAN, HITEMP, NOSL-296, NOSD-1000, and Ames-296K is provided. The TYM line lists are freely accessible from www.exomol.com. [ABSTRACT FROM AUTHOR]

Magnetars are neutron stars that host huge, complex magnetic fields which require supporting currents to flow along the closed field lines. This makes magnetar atmospheres different from those of passively cooling neutron stars because of the heat deposited by back-flowing charges impinging on the star surface layers. This particle bombardment is expected to imprint the spectral and, even more, the polarization properties of the emitted thermal radiation. We present solutions for the radiative transfer problem for bombarded plane-parallel atmospheres in the high magnetic field regime. The temperature profile is assumed a priori, and selected in such a way to reflect the varying rate of energy deposition in the slab (from the impinging currents and/or from the cooling crust). We find that thermal X-ray emission powered entirely by the energy released in the atmosphere by the magnetospheric back bombardment is linearly polarized and X-mode dominated, but its polarization degree is significantly reduced (down to 10 per cent–50 per cent) when compared with that expected from a standard atmosphere heated only from the cooling crust below. By increasing the fraction of heat flowing in from the crust the polarization degree of the emergent radiation increases, first at higher energies (⁠|${\sim} 10\ \mathrm{keV}$|⁠) and then in the entire soft X-ray band. We use our models inside a ray-tracing code to derive the expected emission properties as measured by a distant observer and compare our results with recent IXPE observations of magnetar sources. [ABSTRACT FROM AUTHOR]

The radial velocity (RV) method of exoplanet detection requires mitigation of nuisance signals arising from stellar activity. Using analytic cool and facular spot models, we explore the use of central line moments (CLMs) for recovering and monitoring rotation induced RV variability. Different spot distribution patterns, photosphere-spot contrast ratios, and the presence or absence of the convective blueshift lead to differences in CLM signals between M and G dwarfs. Harmonics of the rotation period are often recovered with the highest power in standard periodogram analyses. By contrast, we show the true stellar rotation may be more reliably recovered with string length minimization. For solar minimum activity levels, recovery of the stellar rotation signal from CLMs is found to require unfeasibly high signal-to-noise observations. The stellar rotation period can be recovered at solar maximum activity levels from CLMs for reasonable cross-correlation function (CCF) signal-to-noise ratios >1000–5000. The CLMs can be used to recover and monitor stellar activity through their mutual correlations and correlations with RV and bisector inverse span. The skewness of a CCF, a measure of asymmetry, is described by the third CLM, |$M_3$|⁠. Our noise-free simulations indicate the linear RV versus |$M_3$| correlation is up to 10 per cent higher than the RV versus bisector inverse span correlation. We find a corresponding ∼5 per cent increase in linear correlation for CARMENES observations of the M star, AU Mic. We also assess the effectiveness of the time derivative of the second CLM, |$M_2$|⁠ , for monitoring stellar activity. [ABSTRACT FROM AUTHOR]

The elemental and isotopic abundances of volatiles like carbon, oxygen, and nitrogen may trace a planet's formation location relative to H2O, CO2, CO, NH3, and N2 "snowlines," or the distance from the star at which these volatile elements sublimate. By comparing the C/O and 12C/13C ratios measured in giant exoplanet atmospheres to complementary measurements of their host stars, we can determine whether the planet inherited stellar abundances from formation inside the volatile snowlines, or nonstellar C/O and 13C enrichment characteristic of formation beyond the snowlines. To date, there are still only a handful of exoplanet systems where we can make a direct comparison of elemental and isotopic CNO abundances between an exoplanet and its host star. Here, we present a 12C/13C abundance analysis for host star WASP-77A (whose hot Jupiter's 12C/13C abundance was recently measured). We use MARCS stellar atmosphere models and the radiative transfer code TurboSpectrum to generate synthetic stellar spectra for isotopic abundance calculations. We find a 12C/13C ratio of 51 ± 6 for WASP-77A, which is subsolar (∼91) but may still indicate 13C enrichment in its companion planet WASP-77A b (12C/13C = 26 ± 16, previously reported). Together with the inventory of carbon and oxygen abundances in both the host and companion planet, these chemical constraints point to WASP-77A b's formation beyond the H2O and CO2 snowlines and provide chemical evidence for the planet's migration to its current location ∼0.024 au from its host star. [ABSTRACT FROM AUTHOR]

In this research, we investigated the observed spectra of the hot peculiar star HD25354 with an effective temperature T eff = 12,800 K, identified the lines of radioactive chemical elements, including the elements with short decay time, and estimated the abundances of these elements. We tried to confirm or reject the existence of promethium lines and lines of other radioactive elements which were detected in previous investigations of this star and explain the physical mechanisms which are responsible for the synthesis of these elements in the stellar atmosphere. We used two high-dispersion spectra of HD25354 observed with the 2 m telescope of Terskol observatory with resolving power near R = 60,000, and a signal to noise ratio near 200. The spectrum of the star from the archive of the 1.93 m telescope of Haute-Provence observatory was also used. The observations were compared with synthetic spectra and the abundance of promethium was found using the best four lines of this element in the observed spectra: log N (Pm) = 5.84 ± 0.16 in the scale logN(H) = 12. It is comparable to the abundances of stable lanthanides in the atmosphere of this star. The abundance of thorium derived from two lines of double-ionized thorium is log N (Th) = 3.59 ± 0.15. The upper limits for technetium, radium, actinium, uranium, and americium abundances are found to be equal to 4.0, 3.0, 1.25, 3.5, and 4.0, respectively. Maybe the existence of promethium lines and lines of other unstable chemical elements in the spectra of HD25354, as well as the other stars of our Galaxy, Magellanic Clouds, and Fornax dwarf galaxy, can be explained by contamination of its atmosphere by the products of kilonova outburst and by symmetric decay of chemical elements with long decay times located at the island of stability (atomic numbers Z = 110–128) of transfermium elements. Maybe the decay of superheavy elements of the island of stability can be one of the reasons for the enhanced abundances of rare earth lanthanides in different types of stars. [ABSTRACT FROM AUTHOR]

A theoretical line list for |$^{15}$| NH |$_3$| CoYuTe-15 is presented based on the empirical potential energy and ab initio dipole moments surfaces developed and used for the production of the ExoMol line list CoYuTe for |$^{14}$| NH |$_3$|⁠. The ro-vibrational energy levels and wavefunctions are computed using the variational program trove. The line list ranges up to 10 000 cm |$^{-1}$| (⁠|$\lambda \ge 1$| μ m) and contains 929 795 249 transitions between 1 269 961 states with |$J\le 30$|⁠. The line list should be applicable for temperatures up to |$\sim$| 1000 K. To improve the accuracy of the line positions, a set of experimentally derived energy levels of |$^{15}$| NH |$_3$| is produced using the marvel (Measured Active Rotation Vibration Energy Level) procedure. To this end, 37 experimental sources of the line positions of |$^{15}$| NH |$_3$| available in the literature are collected, combined, and systematized to produce a self-consistent spectroscopic network of 21 095 |$^{15}$| NH |$_3$| transitions covering 40 vibrational bands ranging up to 6818 cm |$^{-1}$| and resulting in 2777 energy term values. These marvel energies are then used to replace the theoretical values in the CoYuTe-15 line list and also complemented by pseudo- marvel energies obtained by an isotopologue extrapolation using the previously reported marvel energies of the |$^{14}$| NH |$_3$| parent isotopologue of ammonia. A list of 53 856 high-resolution transitions between marvel states and theoretical intensities is provided in the HITRAN format. Comparison with the recent experimental spectra of |$^{15}$| NH |$_3$| illustrate the potential of the line list for detections and as an efficient assistant in spectroscopic assignments. The line list is available from www.exomol.com. [ABSTRACT FROM AUTHOR]

Context. Ultra-luminous X-ray sources (ULXs) are high-mass X-ray binaries with an X-ray luminosity above 1039 erg s−1. These ULXs can be powered by black holes that are more massive than 20 M⊙, accreting in a standard regime, or lighter compact objects accreting supercritically. There are only a few ULXs with known optical or ultraviolet (UV) counterparts, and their nature is debated. Determining whether optical/UV radiation is produced by the donor star or by the accretion disc is crucial for understanding ULX physics and testing massive binary evolution. Aims. We conduct, for the first time, a fully consistent multi-wavelength spectral analysis of a ULX and its circumstellar nebula. We aim to establish the donor star type and test the presence of strong disc winds in the prototypical ULX Holmberg II X-1 (Ho II X-1). Furthermore, we aim to obtain a realistic spectral energy distribution of the ionising source, which is needed for robust nebula analysis. We acquired new UV spectra of Ho II X-1 with the Hubble Space Telescope (HST) and complemented them with archival optical and X-ray data. We explored the spectral energy distribution of the source and analysed the spectra using the stellar atmosphere code PoWR and the photoionisation code CLOUDY. Our analysis of the X-ray, UV, and optical spectra of Ho II X-1 and its nebula consistently explains the observations. We do not find traces of disc wind signatures in the UV and the optical, rejecting previous claims of the ULX being a supercritical accretor. The optical/UV counterpart of Ho II X-1 is explained by a B-type supergiant donor star. Thus, the observations are fully compatible with Ho II X-1 being a close binary consisting of an ≳66 M⊙ black hole accreting matter from an ≃22 M⊙ B-supergiant companion. Furthermore, we propose a possible evolution scenario for the system, suggesting that Ho II X-1 is a potential gravitational wave source progenitor. [ABSTRACT FROM AUTHOR]

Context. Despite mounting evidence that extreme red supergiants (RSGs) undergo episodic mass-loss events, their role in RSG evolution remains uncertain. Critical questions remain unanswered, such as whether or not these events can strip the star, and their timescale and frequency. Aims. This study delves into [W60] B90, one of the most luminous and extreme RSGs in the Large Magellanic Cloud (LMC), with our aim being to search for evidence of episodic mass loss. Our discovery of a bar-like nebular structure at 1 pc, which is reminiscent of the bar around Betelgeuse, raised the question of whether [W60] B90 also has a bow shock, motivating the present study. Methods. We collected and analyzed proper motion data from Gaia, as well as new multi-epoch spectroscopic and imaging data, and archival time-series photometry in the optical and mid-infrared (MIR). We used MARCS models to derive the physical properties of the star from the spectra. Results. We find [W60] B90 to be a walkaway star, with a supersonic peculiar velocity in the direction of the bar. We detect shocked emission between the bar and the star, based on the [S II]/Hα > 0.4 criterion, providing strong evidence for a bow shock. The 30 yr optical light curve reveals semi-regular variability, showing three similar dimming events with ΔV ~ 1 mag, a recurrence of ~12 yr, and a rise time of 400 days. We find the MIR light curve to vary by 0.51 mag and 0.37 mag in the WISE1 and WISE2 bands, respectively, and by 0.42 mag and 0.25 mag during the last dimming event. During this event, optical spectroscopy reveals spectral variability (M3 I to M4 I), a correlation between the Teff and the brightness, increased extinction, and, after the minimum, spectral features incompatible with the models. We also find a difference of >300 K between the Teff measured from the TiO bands in the optical and the atomic lines from our J-band spectroscopy. Conclusions. [W60] B90 is a more massive analog of Betelgeuse in the LMC and therefore the first single extragalactic RSG with a suspected bow shock. Its high luminosity of log(L/L⊙) = 5.32 dex, mass-loss rate, and MIR variability compared to other RSGs in the LMC indicate that it is in an unstable evolutionary state, undergoing episodes of mass loss. Investigating other luminous and extreme RSGs in low-metallicity environments using both archival photometry and spectroscopy is crucial to understanding the mechanism driving episodic mass loss in extreme RSGs in light of the Humphreys-Davidson limit and the "RSG problem". [ABSTRACT FROM AUTHOR]

Aims. Obtaining accurate derivations of stellar atmospheric parameters is crucial in the fields of stellar and exoplanet characterization. We present the upgraded version of our computational tool ODUSSEAS with a new reference scale applied to derive Teff and [Fe/H] values for M dwarfs. Methods. The new reference dataset of ODUSSEAS consists of Teff values based on interferometry, and [Fe/H] values derived by applying updated values for the parallaxes. These reference parameters are related to the pseudo-equivalent widths (EWs) of more than 4000 stellar absorption lines. The machine learning Python "scikit learn" package creates models to determine the stellar parameters for subsequent analysis. Results. We determined Teff and [Fe/H] values for 82 planet-host stars in SWEET-Cat. We demonstrate that our new version of ODUSSEAS is capable of determining the parameters with a greater accuracy than the original by comparing our results to other methods in literature. We also compared our parameters for the same stars by measuring their spectra obtained from several instruments, showing the consistency of our determinations with standard deviation of 30 K and 0.03 dex. Finally, we examined the correlation among planetary mass and stellar metallicity, confirming prior evidence indicating that massive planets mainly form around metal-rich stars in the case of M dwarfs as well. [ABSTRACT FROM AUTHOR]

Context. The study of exoplanets at different evolutionary stages can shed light on their formation, migration, and evolution. The determination of exoplanet properties depends on the properties of their host stars. It is therefore important to characterise the host stars for accurate knowledge on their planets. Aims. Our final goal is to derive, in a homogeneous and accurate way, the stellar atmospheric parameters and elemental abundances of ten young TESS (Transiting Exoplanet Survey Satellite) transiting planet-hosting GK stars followed up with the HARPS-N (High Accuracy Radial velocity Planet Searcher for the Northern hemisphere) at TNG (Telescopio Nazionale Galileo) spectrograph within the Global Architecture of Planetary Systems (GAPS) programme. Methods. We derived stellar kinematic properties, atmospheric parameters, and abundances of 18 elements. Depending on stellar parameters and chemical elements, we used methods based on line equivalent widths and spectral synthesis. Lithium line measurements were used as approximate age estimations. We exploited chemical abundances and their ratios to derive information on planetary composition. Results. Elemental abundances and kinematic properties are consistent with the nearby Galactic thin disk. All targets show C/O<0.8 and 1.0

We report the discovery of the new accreting millisecond X-ray pulsar SRGA J144459.2−604207 using data of the SRG/ART-XC. The source was observed twice in February 2024 during the declining phase of the outburst. The timing analysis revealed a coherent signal near 447.9 Hz modulated by the Doppler effect due to the orbital motion. The derived parameters for the binary system are consistent with a circular orbit with a period of ∼5.2 h. The pulse profiles of the persistent emission, showing a sine-like part during half a period with a plateau in between, can be well modeled by emission from two circular spots that are partially eclipsed by the accretion disk. Additionally, during our observations with an exposure of 133 ks, we detected 19 thermonuclear X-ray bursts. All bursts have similar shapes and energetics, and none show any signs of an expanding photospheric radius. The burst recurrence times decreases linearly from ∼1.6 h at the beginning of observations to ∼2.2 h at the end and anticorrelate with the persistent flux. The spectral evolution during the bursts is consistent with the models of the neutron star atmospheres that are heated by accretion and implies a neutron star radius of 11–12 km and a distance to the source of 8–9 kpc. We also detected coherent pulsations during the bursts and showed that the pulse profiles differ substantially from those observed in the persistent emission. However, we could not find a simple physical model explaining the pulse profiles detected during the bursts. [ABSTRACT FROM AUTHOR]

Context. The outcome of the formation of massive stars is an important anchor point in the formation and evolution process of these stars. It provides insight into the physics of the assembly process, and sets the conditions for stellar evolution. For massive stars, the outcome of formation is rarely observed because the processes involved unfold deep down in highly extincted molecular clouds. Aims. We characterize a population of highly reddened stars in the very young massive star-forming region M17. The group of 18 O4.5 to B9 stars constitutes one of the best samples of almost zero-age main-sequence (ZAMS) high- and intermediate-mass stars. Their properties allow us to identify the empirical location of the ZAMS of massive stars, and the rotation and mass-loss rate of stars close to or at the onset of core-hydrogen burning. Methods. We performed quantitative spectroscopic modeling of a uniform set of over 100 spectral features in optical VLT/X-shooter spectra using the nonlocal thermal equilibrium stellar atmosphere code FASTWIND and a fitting approach based on a genetic algorithm, KIWI-GA. The spectral energy distributions of photometric observations were used to determine the line-of-sight extinction. From a comparison of their positions in the Hertzsprung-Russell diagram with MIST evolutionary tracks, we inferred the stellar masses and ages. Results. We find an age of 0.4−0.2+0.6 Myr for our sample, however we also identify a strong relation between the age and the mass of the stars. All sources are highly reddened, with AV ranging from 3.6 to 10.6 mag. The sample can be subdivided into two groups. Stars more massive than 10 M⊙ have reached the ZAMS. Their projected ZAMS spin rate distribution extends to 0.3 of the critical velocity; their mass-loss rates agree with those of other main-sequence O and early-B stars. Stars with a mass in the range 3 < M/M⊙ < 7 are still on the pre-main sequence (PMS), and some of them have circumstellar disks. Evolving their υ sin i to the ZAMS assuming angular momentum conservation yields values up to ~0.6 υcrit. For PMS stars without disks, we find tentative mass-loss rates up to 10−8.5 M⊙ yr−1. The total-to-selective extinction RV is higher for PMS stars with disks than for the remainder of the sample. Conclusions. We constrain the empirical location of the ZAMS for massive (10 < M/M⊙ < 50) stars and find it to agree with its location in MIST evolutionary tracks. The ZAMS rotation rates for intermediate-mass stars are twice as high as for massive stars, suggesting that the angular momentum gain processes differ between the two groups. The relation between the age and mass of the stars suggests a lag in the formation of more massive stars relative to lower mass stars. Taking the derived mass-loss rates at face value, stellar winds are already initiated in the PMS phase. The PMS-star winds are found to be substantially more powerful than indicated by predictions for line-driven outflows. [ABSTRACT FROM AUTHOR]

Multiple populations (MPs) in globular clusters (GCs) are stars that are distinct for their abundances of light elements. The MPs can be directly separated by measuring abundances of C, N, O, Na, Al, and Mg with spectroscopy or indirectly from photometric sequences created by the impact of different chemistry on band passes of particular filters, such as the HST pseudo-colours in the ultraviolet. An attempt to link HST pseudo-colour maps (PCMs) and spectroscopy was made by Marino et al. (2019, MNRAS, 487, 3815), using abundances mostly from our FLAMES survey. However, we found that an incomplete census of stars in common was used in their population tagging. We corrected the situation by building our own PCMs and matching them with our abundances in 20 GCs, plus two GCs from other sources, doubling the sample with spectroscopic abundances available. We found that the pseudo-colour (magF275W − 2 × magF336W + magF438) does not have a monotonic trend with Na abundances, enhanced by proton-capture reactions in MPs. Moreover, on average about 16% of stars with spectroscopic Na abundances show a discrepant tagging of MPs with respect to the HST photometry. Stars with second generation (SG) chemistry are mistaken for first generation (FG) objects according to HST photometry and vice versa. In general, photometric indices tend to overestimate the fraction of FG stars, in particular in low mass GCs. We offer a simple explanation for these findings. Finally, we publish all our PCMs (with more than 31 800 stars in 22 GCs) with star ID and coordinates; this is done to ensure easy verification and reproduction, as should be the case in scientific papers. [ABSTRACT FROM AUTHOR]

Context. The complex dynamics of asymptotic giant branch (AGB) stars and the resulting stellar winds have a significant impact on the measurements of stellar parameters and amplify their uncertainties. Three-dimensional (3D) radiative hydrodynamic (RHD) simulations of convection suggest that convection-related structures at the surface of AGB star affect the photocentre displacement and the parallax uncertainty measured by Gaia. Aims. We explore the impact of the convection on the photocentre variability and aim to establish analytical laws between the photo-centre displacement and stellar parameters to retrieve such parameters from the parallax uncertainty. Methods. We used a selection of 31 RHD simulations with CO5BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the Gaia G band [320–1050 nm]. From these maps, we calculated the photocentre position and temporal fluctuations. We then compared the synthetic standard deviation to the parallax uncertainty of a sample of 53 Mira stars observed with Gaia. Results. The simulations show a displacement of the photocentre across the surface ranging from 4 to 13% of the corresponding stellar radius, in agreement with previous studies. We provide an analytical law relating the pulsation period of the simulations and the photocentre displacement as well as the pulsation period and stellar parameters. By combining these laws, we retrieve the surface gravity, the effective temperature, and the radius for the stars in our sample. Conclusions. Our analysis highlights an original procedure to retrieve stellar parameters by using both state-of-the-art 3D numerical simulations of AGB stellar convection and parallax observations of AGB stars. This will help us refine our understanding of these giants. [ABSTRACT FROM AUTHOR]

Context. In this paper we present a grid of self-consistent 1D model atmospheres of cool stars, sub-stellar objects, and exoplanets in the effective temperature range 300-3000 K, including cloud formation, chemical non-equilibrium effects, and stellar irradiation. Aims. The new grid extends the classical MARCS model atmosphere grid from 2008 towards lower effective temperatures and a broader range of object types. Methods. The new model atmosphere computations, MSG, are based on a combination of three well-tested codes, the classical MARCS 1D atmospheres, the StaticWeather cloud formation code, and the GGchem chemical equilibrium code. The combined code has been updated with new and more complete molecular and atomic opacities, cloud formation, and advanced chemical equilibrium calculations, and we also added new numerical methods at low temperatures to allow for a more robust convergence. Results. The coupling between the MARCS radiative transfer and GGchem chemical equilibrium computations has effectively made it possible to reach convergence based on the electron pressure for warmer models and gas pressure for cooler models, enabling self-consistent modelling of stellar, sub-stellar, and exoplanetary objects in a very wide range of effective temperatures. We will make new cloud-free and non-irradiated models for solar metallicity and a selected variety of other chemical compositions immediately available from our home page (https://cels.nbi.ku.dk). Illustrative examples of cloudy and irradiated models as well as models based on non-equilibrium chemistry are also presented, and we will describe these in more detail and make them available upon completion at the same place for a larger range of parameter space. Conclusions. For solar metallicity models, the new additional molecular opacities only affect the structure of models cooler than Teff = 2500 K, and the effect becomes substantial for models below Teff ~1500 K. Atomic line opacities are important for models warmer than ~3000 K. The line profile of the molecular opacities may have a larger effect on the model structure than previously anticipated, particularly in the uppermost layers at low gas pressure. The qualitative changes in the relative abundances of TiO, H2O, CH4, NH3, and other molecules in our models follow the observationally defined M, L, T (and Y) sequences, but they also reveal more complex and depth-dependent abundance changes and therefore a spectral classification depending on more parameters. The self-consistent coupling to StaticWeather cloud computations allows for detailed comparison between nucleation and observed relative dimming of different spectral bands, with advanced applications for new identification methods of potential exoplanetary biology. [ABSTRACT FROM AUTHOR]

Context. Asymptotic giant branch (AGB) stars are significant contributors to the metal enrichment of the interstellar medium. They have strong dust-driven winds that have their origin in regions close to the AGB star's surface, where dense dust clouds form. Aims. In this methods paper, we adapted models from advanced radiation-hydrodynamical (RHD) simulations as input for radiative transfer software to create synthetic observables. A major goal is to describe an AGB star's non-sphericity and to simulate its effects on the surrounding dusty envelope. Methods. We developed tools in Python to translate models of an AGB star and its dust-driven wind from 3D RHD simulations with CO5BOLD into the format used for radiative transfer with RADMC-3D. We preserved the asymmetric shape of the AGB star by including the star as a 'dust species' and by using temperature data computed in CO5BOLD. The circumstellar silicate dust from the 3D RHD simulation is included using Mg2SiO4 opacity data in RADMC-3D with spatially dependent grain sizes. We compared images and spectral energy distributions (SEDs) created with RADMC-3D of a model snapshot with similar output made with a spherically symmetric stellar atmosphere from the 1D program DARWIN and with a point source star in RADMC-3D. Results. Our CO5BOLD model features substantial and clumpy dust formation just above 3.4 au from the grid centre (∼1 R⋆ above the star), and large-scale structures due to giant convection cells are visible on the stellar surface. With the properties of VLTI as a basis, we have created simple synthetic observables where the dust clouds close to the star and features on the stellar surface are resolved. The flux density and the contrast to the star are high enough that optical interferometers, such as the VLTI, should be able to detect these dust clouds. We find that it is important to include asymmetric stellar models since their irregular shapes, radiation fields, and their dusty envelopes even put their marks on spatially unresolved observables and affect the flux levels and shapes of the SEDs. The effects on flux levels can mostly be linked to the clumpiness of the circumstellar dust. In contrast, the angle-dependent illumination resulting from temperature variations on the stellar surface causes shifts in the wavelengths of the flux maximum, as shown by replacing the asymmetric star with a spherical one. Conclusions. The methods presented here are an important step towards producing realistic synthetic observables and testing predictions of advanced 3D RHD models. With the model used here, we find that optical interferometers should be able to resolve thermal emission from dense clouds in the dust-formation zone close to an AGB star. Taking the angle-dependence of SEDs as a proxy for temporal variations in unresolved data, we conclude that not all variability observed in AGB stars should be interpreted as global changes in the sense of spherical 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]

Context. The nature of mass-loss in massive stars is one of the most important and difficult to constrain processes in the evolution of massive stars. The largest observational uncertainties are related to the influence of metallicity and wind structure with optically thick clumps. Aims. We aim to constrain the wind parameters of sample of 18 O-type stars in the LMC, through analysis with stellar atmosphere and wind models including the effects of optically thick clumping. This will allow us to determine the most accurate spectroscopic mass-loss and wind structure properties of massive stars at sub-solar metallicity to date. This will allow us to gain insight into the impact of metallicity on massive stellar winds. Methods. Combining high signal to noise (S/N) ratio observations in the ultraviolet and optical wavelength ranges gives us access to diagnostics of multiple different ongoing physical processes in the stellar wind. We produce synthetic spectra using the stellar atmosphere modelling code FASTWIND, and reproduce the observed spectra using a genetic algorithm based fitting technique to optimise the input parameters. Results. We empirically constrain 15 physical parameters associated with the stellar and wind properties of O-type stars from the dwarf, giant and supergiant luminosity classes. These include temperature, surface gravity, surface abundances, rotation, macroturbulence and wind parameters. Conclusions. We find, on average, mass-loss rates a factor of 4–5 lower than those from theoretical predictions commonly used in stellar-evolution calculations, but in good agreement with more recent theoretical predictions. In the 'weak-wind' regime we find massloss rates orders of magnitude below any theoretical predictions. We find a positive correlation of clumping factors with effective temperature with an average fcl = 14 ± 8 for the full sample. It is clear that there is a difference in the porosity of the wind in velocity space, and interclump density, above and below a temperature of roughly 38 kK. Above 38 kK an average 46 ± 24% of the wind velocity span is covered by clumps and the interclump density is 10–30% of the mean wind. Below an effective temperature of roughly 38 kK there must be additional light leakage for supergiants. For dwarf stars at low temperatures there is a statistical preference for very low clump velocity spans, however it is unclear if this can be physically motivated as there are no clearly observable wind signatures in UV diagnostics. [ABSTRACT FROM AUTHOR]

A theoretical line list for |$^{15}$| NH |$_3$| CoYuTe-15 is presented based on the empirical potential energy and ab initio dipole moments surfaces developed and used for the production of the ExoMol line list CoYuTe for |$^{14}$| NH |$_3$|⁠. The ro-vibrational energy levels and wavefunctions are computed using the variational program trove. The line list ranges up to 10 000 cm |$^{-1}$| (⁠|$\lambda \ge 1$| μ m) and contains 929 795 249 transitions between 1 269 961 states with |$J\le 30$|⁠. The line list should be applicable for temperatures up to |$\sim$| 1000 K. To improve the accuracy of the line positions, a set of experimentally derived energy levels of |$^{15}$| NH |$_3$| is produced using the marvel (Measured Active Rotation Vibration Energy Level) procedure. To this end, 37 experimental sources of the line positions of |$^{15}$| NH |$_3$| available in the literature are collected, combined, and systematized to produce a self-consistent spectroscopic network of 21 095 |$^{15}$| NH |$_3$| transitions covering 40 vibrational bands ranging up to 6818 cm |$^{-1}$| and resulting in 2777 energy term values. These marvel energies are then used to replace the theoretical values in the CoYuTe-15 line list and also complemented by pseudo- marvel energies obtained by an isotopologue extrapolation using the previously reported marvel energies of the |$^{14}$| NH |$_3$| parent isotopologue of ammonia. A list of 53 856 high-resolution transitions between marvel states and theoretical intensities is provided in the HITRAN format. Comparison with the recent experimental spectra of |$^{15}$| NH |$_3$| illustrate the potential of the line list for detections and as an efficient assistant in spectroscopic assignments. The line list is available from www.exomol.com. [ABSTRACT FROM AUTHOR]

An analysis of the high-resolution ( R ∼ 48000 ) optical spectrum of hot (B1Ibe) post-AGB star LS 4331 (IRAS 17381−1616) is presented. The detailed identification of the observed absorption and emission features in the wavelength range 3700–9200 Å is carried out for the first time. The atmospheric parameters and chemical composition of the star are derived from the non-LTE analysis of absorption lines. We estimated T eff = 20900 ± 500 K, log g = 2.57 ± 0.08 , V r = - 51.7 ± 0.8 km s - 1 , ξ t = 24 ± 4 km s - 1 and v sin i = 30 ± 5 km s - 1 . An abundance analysis for C, N, O, Mg, Al, S, and Si reveals that the N and O abundance is close to solar while metal underabundances relative to the solar value (i.e., [ Mg / H ] = - 1.04 dex, [ Al / H ] = - 1.20 dex, [ Si / H ] = - 0.46 dex) are found. LS 4331 is a high galactic latitude metal-poor and carbon-deficient hot post-AGB star. The underabundance of carbon ( [ C / H ] = - 0.64 dex) is similar to that found in other hot post-AGB stars and indicates that the star's AGB phase of evolution was terminated before the third dredge-up. Plasma diagnostics are derived from the nebular emission lines. The presence of nebular emission lines in the spectrum of LS 4331 indicates that the photoionization of the circumstellar envelope has already started. The nebular parameters and expansion velocity of the nebula are derived. Using the Gaia DR3 distance, the absolute luminosity of the star is derived, and the star's position on the post-AGB evolutionary tracks suggests that its initial main sequence mass is about 1.2 M ⊙ . It is also reported that fast irregular brightness variations with an amplitude of up to 0.3 mag in the V band have been found in the star, typical of hot post-AGB objects. [ABSTRACT FROM AUTHOR]

During Big Bang nucleosynthesis (BBN) in the first 15 minutes of the Universe, some 7Li was created along with isotopes of H and He. The determination of that primordial value of Li can help constrain the conditions at that time. The oldest stars with known ages can be found in globular clusters which have well-determined ages through stellar evolution models. High-resolution spectra of Li have been obtained with the Keck I Telescope and HIRES in several unevolved stars in the clusters M13 and M71 with V magnitudes of 17.6–17.9. Abundances of Li have been determined with spectrum synthesis techniques and show a range of a factor of 4. We attribute that spread to differences in initial angular momentum resulting in different amounts of spin-down, related mixing, and destruction of Li. Our results are compared with similar results for main-sequence and turnoff stars in other globular clusters. The range in age of these clusters is 11.2–14.2 Gyr for an age span of 3 Gyr. These clusters range in [Fe/H] from −0.75 to −2.24 corresponding to a factor of 30 in metallicity. The maximum in the Li abundance for these unevolved stars in all eight clusters is the same corresponding to Li/H = 3.16 × 10−10, while the predicted Li abundance, based on the deuterium abundance and the BBN predictions, is 5.24 × 10−10. [ABSTRACT FROM AUTHOR]

We continue to study the ultra-luminous X-ray source NGC 7793 P13 in the optical range. In this work, we are testing the model of a spherically symmetric wind atmosphere of the donor star, previously identified as a B9 Ia supergiant. The model spectrum has shown good agreement with the observed one at a relatively high mass loss rate of ; other parameters turned out to be close to those expected for late B-supergiants. The increased mass loss rate can be explained by the high rotation velocity of the star. In addition, we have qualitatively demonstrated the effect of X-ray irradiation on the observed spectrum and discuss the fundamental possibility of wind acceleration under conditions of powerful irradiation. [ABSTRACT FROM AUTHOR]

The TESS light curve of the silicon Ap star MX TrA was modeled using the observational surface distribution of silicon, iron, helium, and chromium obtained previously with the Doppler Imaging technique. The theoretical light curve was calculated using a grid of synthetic fluxes from line-by-line stellar atmosphere models with individual chemical abundances. The observational TESS light curve was fitted by a synthetic one with an accuracy better than 0.001 mag. The influence of Si and Fe abundance stratification on the amplitude of variability was estimated. Also, the wavelength dependence of the photometric amplitude and phase of the maximum light was modeled showing the typical Ap Si star behavior with increased amplitude and anti-phase variability in far ultraviolet caused by the flux redistribution. [ABSTRACT FROM AUTHOR]

The physical properties of star cluster populations offer valuable insights into their birth, evolution, and disruption. However, individual stars in clusters beyond the nearest neighbours of the Milky Way are unresolved, forcing analyses of star cluster demographics to rely on integrated light, a process fraught with uncertainty. Here, we infer the demographics of the cluster population in the benchmark galaxy NGC 628 using data from the Legacy Extra-galactic UV Survey (LEGUS) coupled to a novel Bayesian forward-modelling technique. Our method analyses all 1178 clusters in the LEGUS catalogue, |$\sim 4$| times more than prior studies severely affected by completeness cuts. Our results indicate that the cluster mass function is either significantly steeper than the commonly observed slope of |$-2$| or is truncated at |$\approx 10^{4.5}$| M |$_\odot$|⁠ ; the latter possibility is consistent with proposed relations between truncation mass and star formation surface density. We find that cluster disruption is relatively mild for the first |$\approx 200$|   Myr of cluster evolution; no evidence for mass-dependent disruption is found. We find suggestive but not incontrovertible evidence that inner galaxy clusters may be more prone to disruption and outer galaxy clusters have a more truncated mass function, but confirming or refuting these findings will require larger samples from future observations of outer galaxy clusters. Finally, we find that current stellar track and atmosphere models, along with common forms for cluster mass and age distributions, cannot fully capture all features in the multidimensional photometric distribution of star clusters. While our forward-modelling approach outperforms earlier backward-modelling approaches, some systematic differences persist between observed and modelled photometric distributions. [ABSTRACT FROM AUTHOR]

We present an atmospheric retrieval analysis of the Y0 brown dwarf WISE J035934.06−540154.6 using the low-resolution 0.96–12 μ m James Webb Space Telescope (JWST) spectrum presented in Beiler et al. We obtain volume number mixing ratios of the major gas-phase absorbers (H2O, CH4, CO, CO2, PH3, and H2S) that are three to five times more precise than previous work that used Hubble Space Telescope (HST) spectra. We also find an order-of-magnitude improvement in the precision of the retrieved thermal profile, a direct result of the broad wavelength coverage of the JWST data. We used the retrieved thermal profile and surface gravity to generate a grid of chemical forward models with varying metallicity, (C/O)atm, and strengths of vertical mixing as encapsulated by the eddy diffusion coefficient K zz. Comparison of the retrieved abundances with this grid of models suggests that the deep atmosphere of WISE 0359−54 shows signs of vigorous vertical mixing with K zz = 109 [cm2 s−1]. To test the sensitivity of these results to our five-knot spline thermal profile model, we performed a second retrieval using the Madhusudhan & Seager thermal profile model. While the results of the two retrievals generally agree well, we do find differences between the retrieved values of mass and volume number mixing ratio of H2S with fractional differences of the median values of −0.64 and −0.10, respectively. In addition, the five-knot thermal profile is consistently warmer at pressure between 1 and 70 bar. Nevertheless, our results underscore the power that the broad-wavelength infrared spectra obtainable with the JWST have to characterize the atmospheres of cool brown dwarfs. [ABSTRACT FROM AUTHOR]

Ultra-hot Jupiter Kelt9b impels to reconsider existing models of the upper atmospheres of hot exoplanets, which were previously developed using examples of G or M star systems such as HD209458b and GJ436b. The unique conditions of interaction between the radiation of an A-class star and the atmosphere necessitate kinetic modeling of excited levels of elements, primarily the hydrogen atom. Kelt9b shows the absorption for several Balmer lines and lines of a number of heavy elements, the quantitative interpretation of which is an urgent problem. In this study, for the first time, 3D modeling of the atmosphere of a planet with a close location of the Roche lobe is implemented with allowance for the aeronomy and kinetics of excited hydrogen. [ABSTRACT FROM AUTHOR]

We report results from 8 h of JWST /MIRI low resolution spectroscopic (LRS) monitoring directly followed by 7 h of JWST /NIRSpec prism spectroscopic monitoring of the benchmark binary brown dwarf WISE 1049AB, the closest, brightest brown dwarfs known. We find water, methane, and CO absorption features in both components, including the 3.3  μ m methane absorption feature and a tentative detection of small grain (⁠|$\lt $| 1 μ m) silicate absorption at |$\gt $| 8.5 μ m in WISE 1049A. Both components vary significantly (⁠|$\gt 1~{{\rm per\ cent}}$|⁠), with WISE 1049B displaying larger variations than WISE 1049A. Using K -means clustering, we find three main transition points in wavelength for both components of the binary: (1) change in behaviour at |$\sim$| 2.3 μ m coincident with a CO absorption bandhead, (2) change in behaviour at 4.2 μ m, close to the CO fundamental band at |$\lambda \gt $|  4.4 µm, and (3) change in behaviour at 8.3–8.5 µm, potentially corresponding to silicate absorption. We interpret the light curves observed with both NIRSpec and MIRI as likely stemming from (1) a deep pressure level driving the double-peaked variability seen in WISE 1049B at wavelengths |$\lt $| 2.3 and |$\gt $| 8.5 µm, (2) an intermediate pressure level shaping the light-curve morphology between 2.3 and 4.2 µm, and (3) a higher altitude pressure level producing single-peaked and plateaued light-curve behaviour between 4.2 and 8.5 µm. [ABSTRACT FROM AUTHOR]

We present ionization structures of IC 2003, a planetary nebula with [WR] central star, using a 1-D dusty photo-ionization code: "CLOUDY 17.03". The photo-ionization model is constrained by archival UV emission line fluxes, medium-resolution optical spectroscopy, IRAS 25 μ m flux, absolute H β flux, and the mean angular size of the nearly spherical optical nebula. To constrain the carbon abundance and the effect of photo-electric heating in the ionized gas, we used UV emission lines. We considered an amorphous carbon dust grain with MRN and KMH size distributions to address the importance of photo-electric heating in the ionized nebula. We show that KMH grain size distribution with quantum dust heating reproduces the observations quite well. We construct the ionization structures of different elements at their different ionization stages in the nebula. We derive the physical properties of the planetary nebula and its chemical composition, as well as the parameters of its central star. The estimated nebular dust-to-gas mass ratio is 2.37 × 10 - 3 , and the enhanced photo-electric heating yielded by small dust grains is 9.4 % of the total heating. We considered the H-poor model atmosphere for the central star; the effective temperature of the central star is 177 kK, the specific gravity log(g) is 6, and its luminosity ( L ∗ ) is 6425 L ⊙ . The derived central star parameters plotted on stellar evolutionary tracks correspond to a central star mass of 0.636 M ⊙ and to a progenitor mass of 3.26 M ⊙ . [ABSTRACT FROM AUTHOR]

Excess chromospheric emissions within deep photospheric lines are effective proxies of stellar magnetism for FGK stars. This emission decays with stellar age and is a potential determinant of this important stellar quantity. We report absolutely calibrated H  |$\alpha$| chromospheric fluxes for 511 solar-type stars in a wide interval of precisely determined masses, [Fe/H], ages, and evolution states from high S/N, moderately high |$-$| resolution spectra. The comparison of H  |$\alpha$| and H + K chromospheric fluxes reveals a metallicity bias (absent from H  |$\alpha$|⁠) affecting Ca  ii H + K fluxes thereby metal-rich stars with deep line profiles mimic low chromospheric flux levels, and vice versa for metal-poor stars. This bias blurs the age–activity relation, precluding age determinations for old, inactive stars unless mass and [Fe/H] are calibrated into the relation. The H + K lines being the most widely studied tool to quantify magnetic activity in FGK stars, care should be exercised in its use whenever wide ranges of mass and [Fe/H] are involved. The H  |$\alpha$| age–activity–mass–metallicity calibration appears to be in line with the theoretical expectation that (other parameters being equal) more massive stars possess narrower convective zones and are less active than less massive stars, while more metal-rich stars have deeper convective zones and appear more active than metal-poorer stars. If regarded statistically in tandem with other age diagnostics, H  |$\alpha$| chromospheric fluxes may be suitable to constrain ages for FGK stars with acceptable precision. [ABSTRACT FROM AUTHOR]

We describe the Stagger code for simulations of magnetohydrodynamic (MHD) systems. This is a modular code with a variety of physics modules that will let the user run simulations of deep stellar atmospheres, sunspot formation, stellar chromospheres and coronae, proto-stellar disks, star formation from giant molecular clouds, and even galaxy formation. The Stagger code is efficiently and highly parallelizable, enabling such simulations with large ranges of both spatial and temporal scales. We describe the methodology of the code and present the most important of the physics modules, as well as its input and output variables. We show results of a number of standard MHD tests to enable comparison with other, similar codes. In addition, we provide an overview of tests that have been carried out against solar observations, ranging from spectral line shapes, spectral flux distribution, limb darkening, intensity and velocity distributions of granulation, to seismic power spectra and the excitation of p -modes. The Stagger code has proven to be a high-fidelity code with a large range of uses. [ABSTRACT FROM AUTHOR]

The precise measurement of stellar abundances plays a pivotal role in providing constraints on the chemical evolution of the Galaxy. However, before spectral lines can be employed as reliable abundance indicators, particularly for challenging elements such as helium, they must undergo thorough scrutiny. Galactic open clusters, representing well-defined single stellar populations, offer an ideal setting for unfolding the information stored in the helium spectral line feature. In this study, we characterise the profile and strength of the helium transition at around 10 830 Å (He 10 830) in nine giant stars in the Galactic open cluster Stock 2. To remove the influence of weak blending lines near the helium feature, we calibrated their oscillator strengths (log 푔f) by employing corresponding abundances obtained from simultaneously observed optical spectra. Our observations reveal that the He 10 830 in all the targets is observed in absorption, with line strengths categorised into two groups. Three stars exhibit strong absorption, including a discernible secondary component, while the remaining stars exhibit weaker absorption. The lines are in symmetry and align with or near their rest wavelengths, suggesting a stable upper chromosphere without a significant systematic mass motion. We find a correlation between the He 10 830 strength and the Ca II log R′HK index, with a slope similar to that reported in previous studies on dwarf stars. This correlation underscores the necessity of accounting for the stellar chromosphere structure when employing He 10 830 as a probe for the stellar helium abundance. The procedure of measuring the He 10 830 we developed in this study is applicable not only to other Galactic open clusters but also to field stars, and we plan to use it to map the helium abundance across various types of stars in the future. [ABSTRACT FROM AUTHOR]

Most Galactic globular clusters (GCs) harbour multiple populations of stars (MPs) that are composed of at least two generations: the first generation is characterised by a standard α-enhanced metal mixture, as observed in field halo stars of the Milky Way, and the second generation displays an anti-correlated CN–ONa chemical abundance pattern in combination with an enhanced helium fraction. Adequate collections of stellar spectra are needed to characterize the effect of these changes in the stellar abundance on the integrated light of GCs. We present a grid of synthetic stellar spectra to cover the atmospheric parameters relevant to old stellar populations at four subsolar metallicities and two abundance patterns that are representative of the first and second generations of stars in GCs. The integrated spectra of the populations were computed using our stellar grid and empirical stellar populations, namely, colour-magnitude diagrams from the literature for Galactic GCs. The spectra range from 290 to 1000 nm, where we measured the effect on several spectrophotometric indices due to the surface abundance variations attributed to MPs. We find non-negligible effects of the MPs on the spectroscopic indices that are sensitive to C, N, Ca, or Na, and on the Balmer indices; we also describe how MPs modify specific regions in the near-UV and near-IR that can be measured with narrow or medium photometric passbands. The effects vary with metallicity. A number of these changes remain detectable even when we account for the stochastic fluctuations due to the finite nature of the stellar population cluster. [ABSTRACT FROM AUTHOR]

Context. The chemically peculiar (CP) stars of the upper main sequence are defined by spectral peculiarities that indicate unusual elemental abundance patterns in the presence of diffusion in the calm, stellar atmospheres. Some of them have a stable local magnetic field of up to several kiloGauss. The pre-main-sequence evolution of these objects is still a mystery and contains many open questions. Aims. We identify CP stars on the pre-main sequence to determine possible mechanisms that lead to the occurrence of chemical peculiarities in the (very) early stages of stellar evolution. Methods. We identified likely pre-main-sequence stars by fitting the spectral energy distributions. The subsequent analysis using stellar spectra and photometric time series helped us to distinguish between CP and non-CP stars. Additionally, we compared our results to the literature to provide the best possible quality assessment. Results. Out of 45 candidates, about 70% seem to be true CP stars or CP candidates. Furthermore, 9 sources appear to be CP stars on the pre-main sequence, and all are magnetic. We finally report a possible CP2 star that is also a pre-main-sequence star and was not previously in the literature. Conclusions. The evolution of the peculiarities seems to be related to the (strong) magnetic fields in these CP2 stars. [ABSTRACT FROM AUTHOR]

Context. Blue supergiants (BSGs) are key objects for understanding the evolution of massive stars, which play a crucial role in the evolution of galaxies. However, discrepancies between theoretical predictions and empirical observations have opened up important questions yet to be answered. Studying statistically significant and unbiased samples of these objects can help to improve the situation. Aims. We perform a homogeneous and comprehensive quantitative spectroscopic analysis of a large sample of Galactic luminous blue stars (a majority of which are BSGs) from the IACOB spectroscopic database, providing crucial parameters to refine and improve theoretical evolutionary models. Methods. We derived the projected rotational velocity (υ sin i) and macroturbulent broadening (υmac) using IACOB-BROAD, which combines Fourier transform and line-profile fitting techniques. We compared high-quality optical spectra with state-of-the-art simulations of massive star atmospheres computed with the FASTWIND code. This comparison allowed us to derive effective temperatures (Teff), surface gravities (log 푔), microturbulences (ξ), surface abundances of silicon and helium, and to assess the relevance of stellar winds through a wind-strength parameter (log Q). Results. We provide estimates and associated uncertainties of the above-mentioned quantities for the largest sample of Galactic luminous O9 to B5 stars spectroscopically analyzed to date, comprising 527 targets. We find a clear drop in the relative number of stars at Teff ≈ 21 kK, coinciding with a scarcity of fast rotating stars below that temperature. We speculate that this feature (roughly corresponding to B2 spectral type) might be roughly delineating the location of the empirical terminal-age main sequence in the mass range between 15 and 85 M⊙. By investigating the main characteristics of the υ sin i distribution of O stars and BSGs as a function of Teff, we propose that an efficient mechanism transporting angular momentum from the stellar core to the surface might be operating along the main sequence in the high-mass domain. We find correlations between ξ,υmac and the spectroscopic luminosity 퓛 (defined as Teff4 / g). We also find that no more than 20% of the stars in our sample have atmospheres clearly enriched in helium, and suggest that the origin of this specific subsample might be in binary evolution. We do not find clear empirical evidence of an increase in the wind strength over the wind bi-stability region toward lower Teff. [ABSTRACT FROM AUTHOR]

Context. The manifestation of surface spots on magnetic chemically peculiar (mCP) stars is most commonly explained by the atomic diffusion theory, which requires a calm stellar atmosphere and only moderate rotation. While very successful and well described, this theory still needs to be revised and fine-tuned to the observations. Aims. Our study aims to enlarge the sample of known photometrically variable mCP stars (ACV variables) to pave the way for more robust and significant statistical studies. We derive accurate physical parameters for these objects and discuss our results in the framework of the atomic diffusion theory. Methods. We studied 1314 candidate ACV variables that were selected from the Zwicky Transient Factory catalogue of periodic variables based on light curve characteristics. We investigated these objects using photometric criteria, a colour-magnitude diagram, and spectroscopic data from the LAMOST and Gaia missions to confirm their status as ACV variables. Results. We present a sample of 1232 new ACV variables, including information on distance from the Sun, mass, fractional age on the main sequence, fraction of the radius between the zero-age and terminal-age main sequence, and the equatorial velocity and its ratio to the critical velocity. Conclusions. Our results confirm that the employed selection process is highly efficient for detecting ACV variables. We have identified 38 stars with vequ in excess of 150 km s−1 (with extreme values up to 260 km s−1). This challenges current theories that cannot explain the occurrence of such fast-rotating mCP stars. [ABSTRACT FROM AUTHOR]

We present a quantitative spectral analysis of the extreme nitrogen-enhanced supergiant HD 93840 (BN1 Ib) at an intermediate galactic latitude. Based on an optical high-resolution spectrum and complementary ultraviolet and infrared (spectro-)photometry, in addition to Gaia data, we carried out a full characterisation of the star's properties. We used both hydrostatic and unified (photosphere+wind) model atmospheres that account for deviations from local thermodynamic equilibrium. A highly unusual surface CNO-mixing signature and a marked stellar overluminosity compared to the mass imply a binary channel for the star's past evolution. The kinematics shows that it has reached its current position above the Galactic plane as a runaway star, likely ejected by the supernova explosion of its former companion star. Its current bulk composition, with a notably increased mean molecular weight due to core He- and progressed shell H-burning, suggests an advanced evolutionary stage. It is poised to yield a rare core-collapse supernova of a blue supergiant about ten OB star population scale heights above the Galactic disk relatively soon, contributing to the metal enrichment of the circumgalactic medium. [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]

The discrepancy between the observed and the predicted radii in low-mass stars is a well-known and yet to be resolved problem. Although various theoretical approaches have been developed since the first appearance of the problem, there is still no satisfactory mechanism or model which could successfully reproduce observed radii. The stellar magnetic field is among proposed resolutions but has not been investigated comprehensively for detached eclipsing binary stars. In this study, we present analysis of seven low-mass Kepler eclipsing binaries, KIC 4484356, KIC 4678171, KIC 5300878, KIC 6147573, KIC 8543278, KIC 9762519, and KIC 9821078. We investigate the average amplitude of the rotational modulation signal observed at out-of-eclipse phases, which could be used as the proxy for the strength of the stellar magnetic field, and possible discrepancies between observed and theoretically calculated radii of the components of the target systems. Our findings indicate radius discrepancy for both components of KIC 4678171 and KIC 9821078. Moreover, secondary components of KIC 9762519 and KIC 8543278 appear to have quite inflated radii. However, results from this limited sample do not indicate any correlation between average amplitude of the rotation modulation signal and observed radius discrepancies. [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]

Spectroscopic observations have shown for decades that the Wolf–Rayet (WR) phenomenon is ubiquitous among stars with different initial masses. Although much effort to understand the winds from massive WR stars has been presented in the literature, not much has been done for such type of stars in the low-mass range. Here we present an attempt to understand the winds from [WR]-type stars using results from spectral analyses with the full non-local thermodynamic equilibrium stellar atmosphere code PoWR. These results are put into context with the properties of massive WR stars. We found that WC + [WC] stars and WO + [WO] stars create independent sequences in the mass-loss rate (⁠|$\dot{M}$|⁠) and modified wind momentum (D mom) versus luminosity (L) diagrams. Our analysis indicates that even when the winds of WR and [WR] stars become optically thin, there is no breakdown of the general mass-loss trend, contrary to the observed 'weak wind phenomenon' in OB stars. We report that all WR-type stars studied here broadly define single sequences in the wind efficiency (η) versus transformed mass-loss rate (⁠|$\dot{M}_\mathrm{t}$|⁠), the |$\dot{M}_\mathrm{t}$| – T eff diagram, and the |$(L, T_\mathrm{eff}, \dot{M})$| space, which suggest these to be fundamental properties of the WR phenomenon (regardless of the mass range), at least for WR-type stars of the O and C sequences. Our analytical estimations could drive computations of future stellar evolution models for WR-type stars. [ABSTRACT FROM AUTHOR]

Stellar rotation at early ages plays a crucial role in the survival of primordial atmospheres around Earth-mass exoplanets. Earth-like planets orbiting fast-rotating stars may undergo complete photoevaporation within the first few hundred Myr driven by the enhanced stellar XUV [X-rays and extreme ultraviolet (EUV)] radiation, while planets orbiting slow-rotating stars are expected to experience difficulty in losing their primordial envelopes. Besides the action of stellar radiation, stellar winds induce additional erosion on these primordial atmospheres, altering their morphology, extent, and causing supplementary atmospheric losses. In this paper, we study the impact of activity-dependent stellar winds on primordial atmospheres to evaluate the extent to which the action of these winds can be significant in the whole planetary evolution at early evolutionary stages. We performed 3D magnetohydrodynamical (MHD) simulations of the interaction of photoevaporating atmospheres around unmagnetized Earth-mass planets in the time span between 50 and 500 Myr, analysing the joint evolution of stellar winds and atmospheres for both fast- and slow-rotating stars. Our results reveal substantial changes in the evolution of primordial atmospheres when influenced by fast-rotating stars, with a significant reduction in extent at early ages. In contrast, atmospheres embedded in the stellar winds from slow-rotating stars remain largely unaltered. The interaction of the magnetized stellar winds with the ionized upper atmospheres of these planets allows us to evaluate the formation and evolution of different MHD structures, such as double bow shocks and induced magnetospheres. This work will shed light on the first evolutionary stages of Earth-like exoplanets, which are of crucial relevance in terms of planet habitability. [ABSTRACT FROM AUTHOR]

Auroral emission lines result from the interaction between magnetic field and stellar wind, offering valuable insights into physical properties and processes occurring within magnetospheres of celestial bodies. While extensively studied in planetary and exoplanetary atmospheres, in ultracool dwarfs, and as radio emission from early-type stars, the presence of specific auroral emission lines in hot star spectra remains unexplored. In this study, we utilized tlusty code to simulate the auroral lines, while modelling the effect of the interaction between stellar wind and magnetosphere through X-ray irradiation. Utilizing high-resolution synthetic spectra generated from model atmospheres, we identified potential candidate lines indicative of auroral emission, which were absent in non-irradiated spectra. Emission lines in synthetic spectra were present primarily in the infrared domain. The most prominent line generated by irradiation was He  ii 69458 Å, which appeared in all our model atmospheres with effective temperatures ranging from 15 kK to 30 kK. We also calculated the minimum irradiation required to detect emission in this most prominent line. The presence of emission lines was interpreted by considering changes in the population of different excited states of given atoms. Besides the appearance of infrared emission lines, high-energy irradiation causes infrared excess. To complement our simulations, we also searched for auroral lines in Far Ultraviolet Spectroscopic Explorer (FUSE) observations, which are deposited in the Multimission Archive at Space Telescope catalogue. The comparison of observed spectra with synthetic spectra did not identify any possible candidate emission lines in FUSE spectra. [ABSTRACT FROM AUTHOR]

A new N I model atom has been constructed using the energy levels known from laboratory measurements and predicted in N I atomic structure calculations and up-to-date atomic data for calculating the radiative and collisional transition rates. The solar abundance has been determined from N I lines by the synthetic spectrum method with a classical one-dimensional (1D, MARCS) solar model atmosphere and by taking into account the departures from local thermodynamic equilibrium (non-LTE effects). By applying the 3D corrections of Amarsi et al. (2020), we have obtained for the Sun. Based on high-resolution spectra, we have derived the non-LTE nitrogen abundances for 11 unevolved A–F-type stars with reliably determined atmospheric parameters. Non-LTE leads to a strengthening of N I lines, and the non-LTE effects grow with increasing effective temperature. For each of the stars the departures from LTE lead to a decrease in the root-mean-square (rms) abundance error compared to the LTE case. For superficially normal A stars non-LTE removes the enhancements relative to the solar nitrogen abundance obtained in an LTE analysis. The Boo-type star HD 172167 (Vega) also has a nearly solar nitrogen abundance. Four Am stars exhibit a scatter, from a nitrogen underabundance with to a nitrogen overabundance with . The nitrogen abundances for the Sun and superficially normal A stars are consistent within the error limits with the nitrogen abundance in the interstellar gas and early B-type stars. [ABSTRACT FROM AUTHOR]

A homogeneous set of calcium and scandium abundances has been obtained for 54 A-type stars with strong metal lines (Am stars) by taking into account the departures from local thermodynamic equilibrium. A correlation of the Ca and Sc abundances with the effective temperature has been revealed, with the Ca and Sc abundances in stars with a surface gravity growing with increasing faster than in stars with . No correlation of the Ca and Sc abundances with the iron abundance and the stellar rotation velocity has been found. Am stars exhibit, on average, a higher value of [Ca/H] than that of [Sc/H] and . However, for K there is a hint at a systematic difference between stars with and . The iron overabundance is, on average, the same in the range . It is shown that when atomic diffusion is taken into account, evolution computations with the MESA code for stellar masses of 1.5–2.2 give surface abundances that are consistent with the Ca and Fe abundances in Am stars in three open clusters with an age greater than 600 Myr. Additional mechanisms for the separation of chemical elements are required to explain the Am phenomenon in young stars in the Pleiades cluster. Published diffusion models have been tested. The turbulent models of Richer et al. (2000) and Hui-Bon-Hoa et al. (2022) are consistent with the observations of Am stars in open clusters at large values of the free parameter : 1000 for Ca and Fe and 500 for Sc. None of the diffusion models corresponding to the mass and age of the Am star Sirius reproduces the abundances of the elements from He to Ni observed in it. The results obtained are important for a better understanding of the chemical peculiarity mechanisms in Am stars. [ABSTRACT FROM AUTHOR]