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New ExoMol line lists AloHa for aluminium hydride (AlH and AlD) are presented improving the previous line lists WYLLoT. The revision is motivated by the recent experimental measurements and astrophysical findings involving the highly excited rotational states of AlH in its A 1Π − X 1Σ+ system. A new high-resolution emission spectrum of 10 bands from the A 1Π − X 1Σ+ system of AlD, in the region 17 300–32 000 cm−1 was recorded with a Fourier transform spectrometer, which probes the predissociative A 1Π v  = 2 state. The AlD new line positions are combined with all available experimental data on AlH and AlD to construct a comprehensive set of empirical rovibronic energies of AlH and AlD covering the X 1Σ+ and A 1Π electronic states using the MARVEL approach. We then refine the spectroscopic model WYLLoT to our experimentally derived energies using the nuclear-motion code Duo and use this fit to produce improved line lists for 27AlH, 27AlD, and 26AlH with a better coverage of the rotationally excited states of A 1Π in the predissociative energy region. The lifetimes of the predissociative states are estimated and are included in the line list using the new ExoMol data structure, alongside the temperature-dependent continuum contribution to the photoabsorption spectra of AlH. The new line lists are shown to reproduce the experimental spectra of both AlH and AlD well, and to describe the AlH absorption in the recently reported Proxima Cen spectrum, including the strong predissociative line broadening. The line lists are included into the ExoMol data base www.exomol.com. [ABSTRACT FROM AUTHOR]

The link between stellar host properties, be it chemical, physical, dynamical, or galactic in nature, with the presence of planetary companions, has been one that has been repeatedly tested in the literature. Several corroborated work has argued that the correlation between a stellar atmosphere's chemistry and the presence of gas giant companions is primordial in nature, implying that the chemical budget in a protoplanetary disc, and by proxy the eventual stellar host, increases the likelihood of gas giant formation. In this work, we aim to use the power of computer vision to build and test a machine learning classifier capable of discriminating between gas giant host stars and a comparison sample, using spectral data of the host stars in the visible regime. High-resolution spectra are used to preserve any inherent information which may contribute to the classification, and are fed into a stacked ensemble design incorporating several convolutional neural networks. The spectral range is binned such that each is assigned to a first-level voter, with the meta-learner aggregating their votes into a final classification. We contextualize and elaborate on the model design and results presented in a prior proceedings publication, and present an amended architecture incorporating semisupervized learning. Both models achieve relatively strong performance metrics and generalize over the holdout sets well, yet still present signs of overfitting. [ABSTRACT FROM AUTHOR]

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

Cool astrophysical objects, such as (exo)planets, brown dwarfs, or asymptotic giant branch stars, can be strongly affected by condensation. Condensation not only directly affects the chemical composition of the gas phase by removing elements but the condensed material also influences other chemical and physical processes in these objects. This includes, for example, the formation of clouds in planetary atmospheres and brown dwarfs or the dust-driven winds of evolved stars. In this study, we introduce fastchem cond , a new version of the fastchem equilibrium chemistry code that adds a treatment of equilibrium condensation. Determining the equilibrium composition under the impact of condensation is complicated by the fact that the number of condensates that can exist in equilibrium with the gas phase is limited by a phase rule. However, this phase rule does not directly provide information as to which condensates are stable. A major advantage of fastchem cond is its ability to automatically select the set stable condensates satisfying the phase rule. Besides the normal equilibrium condensation, fastchem cond can also be used with the rainout approximation that is commonly employed in atmospheres of brown dwarfs or (exo)planets. fastchem cond is available as open-source code, released under the GNU General Public Licence 3 (GPLv3). In addition to the c++  code, fastchem cond also offers a python interface. Together with the code update, we also add about 290 liquid and solid condensate species to fastchem. [ABSTRACT FROM AUTHOR]

A semi-empirical IR/Vis line list, SOLIS, for the sulphur monoxide molecule 32S16O is presented. SOLIS includes accurate empirical rovibrational energy levels, uncertainties, lifetimes, quantum number assignments, and transition probabilities in the form of Einstein A coefficients covering the |$X\, {}^{3}\Sigma ^{-}$| |$, a\, {}^{1}\Delta , b\, {}^{1}\Sigma ^{+}, A\, {}^{3}\Pi , B\, {}^{3}\Sigma ^{-}, A^{\prime \prime }\, {}^{3}\Sigma ^{+}, A^{\prime }\, {}^{3}\Delta$|⁠ , and |$e\, {}^{1}\Pi$| systems and wavenumber range up to 43 303.5 cm−1 (≥230.93 nm) with J ≤ 69. SOLIS has been computed by solving the rovibronic Schrödinger equation for diatomics using the general purpose variational code Duo and starting from a published ab initio spectroscopic model of SO (including potential energy curves, coupling curves, (transition) dipole moment curves) which is refined to experimental data. To this end, a database of 50 106 experimental transitions, 48 972 being non-redundant, has been compiled through the analysis of 29 experimental sources, and a self-consistent network of 8558 rovibronic energy levels for the X, a, b, A, B , and C electronic states has been generated with the marvel algorithm covering rotational and vibrational quantum numbers J ≤ 69 and v ≤ 30 and energies up to 52 350.40 cm−1. No observed transitions connect to the |$B\, {}^{3}\Sigma ^{-}$| (v  = 0) state which is required to model perturbations correctly, so we leave fitting the |$B\, {}^3\Sigma ^-$| and |$C\, {}^3\Pi$| state UV model to a future project. The SO line list is available at ExoMol from www.exomol.com. [ABSTRACT FROM AUTHOR]

We present a photometric variability survey of young planetary-mass objects using the New Technology Telescope in the JS and KS bands. Surface gravity plays an important role in the atmospheric structure of brown dwarfs, as young low-gravity L dwarfs have a higher variability rate than field L dwarfs. In this study, we extend variability studies to young T-type planetary-mass objects and investigate the effects of surface gravity on the variability of L and T dwarfs across a large sample. We conduct continuous monitoring for 18 objects with spectral types from L5 to T8 and detect four new variables and two variable candidates. Combining with previous variability surveys of field and young L and T objects, we find that young objects tend to be more variable than field objects within peak-to-peak variability amplitude ranges of 0.5–10 per cent and period ranges of 1.5–20 h. For the first time, we constrain the variability rate of young T dwarfs to be  |$56_{-18}^{+20}$|  per cent compared to  |$25_{-7}^{+8}$|  per cent for field T dwarfs. Both field and young samples have higher variability rates at the L/T transition than outside the L/T transition. The differences in the variability rates between field and young samples are about 1σ and therefore larger sample sizes are needed to confirm and refine the results. Besides the L/T transition, young L dwarfs with strong variability tend to assemble in a narrow spectral type range of L6–L7.5. This work supports the critical role of surface gravity on the atmospheric structure from L to T spectral types. [ABSTRACT FROM AUTHOR]

Recently, a novel idea has been proposed to use a spacecraft orbiting closely around the L4 or L5 Lagrangian points of the Earth–Moon system as a vantage location for astronomical observations under unique and advantageous conditions, e.g. to investigate the Solar corona or Kordylewski clouds. In this paper, we explore the scientific advantages of this configuration to observe lunar occultations. By using simulations based on actual orbital parameters and detector performance, we find that even a telescope as small as 10 cm in a L4 orbit would enable better light-curve sampling and higher angular resolution than from ground, and open up at the same time new spectral windows such as e.g. in the UV. We focus in particular on angular diameter measurements, which we find would be possible at the 100  |$\mu$| as level even at magnitudes normally considered faint in the context of ground-based LOs. Considerable benefits would also be afforded in the area of small separation binary objects, not just in terms of angular resolution but also by the capability of L4-based observations to repeat an LO scan of a source at multiple different position angles. Additional science cases would be made possible by the high time resolution intrinsic to the LO technique and the access to UV wavelengths. On-board data pipeline codes already exist, greatly simplifying the automatic pre-processing and the data transfer requirements. [ABSTRACT FROM AUTHOR]

We present a catalogue of 62 polluted white dwarfs observed by the 9th Low-Resolution Data Release of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST LRS DR9v1; R ≈1800) and the Early Data Release 3 (EDR3) of the Gaia Mission. Among these stellar remnants, 30 are new discoveries with previously unknown traces of calcium pollution. To generate our catalogue, we used a data base of 4324 unique Gaia EDR3 white dwarf candidates with LAMOST LRS DR9v1 observations, many of which have been spectroscopically confirmed by other telescopes. For these stars, we developed a quantitative method to detect calcium absorption in their spectra between 3900–4000 Å, which we then validated through visual inspection and multiple literature cross-checks. Our catalogue provides the astrometric and photometric properties of the white dwarf candidates, incorporates supplementary data (e.g. Montreal White Dwarf Database, MWDD; PanSTARRS; the Hubble Space Telescope), and indicates the possibility of calcium pollution in their atmospheres. For our final sample of polluted white dwarfs, we also determine the main atmospheric properties of 23 sources with effective temperatures T eff ≤ 25 000 K and no existing calcium abundances in the MWDD. Our analysis represents a first step towards measuring the full atmospheric composition of these stars and learning about the bulk properties of their accreted material. As we venture into the era of wide-field spectroscopic surveys, our work highlights the importance of combining large-scale data bases for identifying and characterizing new polluted white dwarfs. [ABSTRACT FROM AUTHOR]

Empirical line lists BRYTS for the open shell molecule 89Y16O (yttrium oxide) and its isotopologues are presented. The line lists cover the six lowest electronic states: |$X\, {}^{2}\Sigma ^{+}$|⁠ , |$A\, {}^{2}\Pi$|⁠ , |$A^{\prime }\, {}^{2}\Delta$|⁠ , |$B\, {}^{2}\Sigma ^{+}$|⁠ , |$C\, {}^{2}\Pi$|⁠ , and |$D\, {}^{2}\Sigma ^{+}$| up to 60 000 cm−1 (<0.167  |$\mu$| m) for rotational excitation up to J  = 400.5. An ab initio spectroscopic model consisting of potential energy curves (PECs), spin–orbit, and electronic angular momentum couplings is refined by fitting to experimentally determined energies of YO, derived from published YO experimental transition frequency data. The model is complemented by empirical spin-rotation and Λ-doubling curves and ab initio dipole moment and transition dipole moment curves computed using multireference configuration interaction (MRCI). The ab initio PECs computed using the complete basis set limit extrapolation and the coupled-cluster CCSD(T) method with its higher quality provide an excellent initial approximation for the refinement. Non-adiabatic coupling curves for two pairs of states of the same symmetry A / C and B / D are computed using a state-averaged complete active space self-consistant field theory (CASSCF) and used to build diabatic representations for the |$A\, {}^{2}\Pi$|⁠ , |$C\, {}^{2}\Pi$|⁠ , |$B\, {}^{2}\Sigma ^{+}$|⁠ , and |$D\, {}^{2}\Sigma ^{+}$| curves. The experimentally derived energies of 89Y16O utilized in the fit are used to replace the corresponding calculated energy values in the BRYTS line list. Simulated spectra of YO show excellent agreement with the experiment, where it is available. Calculated lifetimes of YO are tuned to agree well with the experiment, where available. The BRYTS YO line lists are available from the ExoMol database (www.exomol.com). [ABSTRACT FROM AUTHOR]

Aims. We explore different scenarios to explain the chemical difference found in the remarkable giant-giant binary system HD 138202 + CD−30 12303. For the first time, we suggest how to distinguish these scenarios by taking advantage of the extensive convective envelopes of giant stars. Methods. We carried out a high-precision determination of stellar parameters and abundances by applying a full line-by-line differential analysis on GHOST high-resolution spectra. We used the FUNDPAR program with ATLAS12 model atmospheres and specific opacities calculated for an arbitrary composition through a doubly iterated method. Physical parameters were estimated with the isochrones package and evolutionary tracks were calculated via MIST models. Results. We found a significant chemical difference between the two stars (Δ[Fe/H] ∼ 0.08 dex), which is largely unexpected considering the insensitivity of giant stars to planetary ingestion and diffusion effects. We tested the possibility of engulfment events by using several different combinations of stellar mass, ingested mass, metallicity of the engulfed object and different convective envelopes. However, the planetary ingestion scenario does not seem to explain the observed differences. For the first time, we distinguished the source of chemical differences using a giant-giant binary system. By ruling out other possible scenarios such as planet formation and evolutionary effects between the two stars, we suggest that primordial inhomogeneities might explain the observed differences. This remarkable result implies that the metallicity differences that were observed in at least some main-sequence binary systems might be related to primordial inhomogeneities rather than engulfment events. We also discuss the important implications of finding primordial inhomogeneities, which affect chemical tagging and other fields such as planet formation. We strongly encourage the use of giant-giant pairs. They are a relevant complement to main-sequence pairs for determining the origin of the observed chemical differences in multiple systems. [ABSTRACT FROM AUTHOR]

Context. Studies of the element abundances and kinematics of stars belonging to the Galactic halo have revealed the existence of two distinct populations: accreted stars with a low [α/Fe] ratio and in situ born stars with a higher ratio. Aims. Previous work on the abundances of C, O, Na, Mg, Si, Ca, Ti, Cr, Mn, Fe, Ni, Cu, and Zn in high-α and low-α halo stars is extended to include the abundances of Sc, V, and Co, enabling us to study the nucleosynthesis of all iron-peak elements along with the lighter elements. Methods. The Sc, V, and Co abundances were determined from a 1D MARCS model-atmosphere analysis of equivalent widths of atomic lines in high signal-to-noise, high resolution spectra assuming local thermodynamic equilibrium (LTE). In addition, new 3D and/or non-LTE calculations were used to correct the 1D LTE abundances for several elements including consistent 3D non-LTE calculations for Mg. Results. The two populations of accreted and in situ born stars are well separated in diagrams showing [Sc/Fe], [V/Fe], and [Co/Fe] as a function of [Fe/H]. The [X/Mg] versus [Mg/H] trends for high-α and low-α stars were used to determine the yields of core-collapse and Type Ia supernovae. The largest Type Ia contribution occurs for Cr, Mn, and Fe, whereas Cu is a pure core-collapse element. Sc, Ti, V, Co, Ni, and Zn represent intermediate cases. A comparison with yields calculated for supernova models shows poor agreement for the core-collapse yields. The Ia yields suggest that sub-Chandrasekhar-mass Type Ia supernovae provide a dominant contribution to the chemical evolution of the host galaxies of the low-α stars. A substructure in the abundances and kinematics of the low-α stars suggests that they arise from at least two different satellite accretion events, Gaia-Sausage-Enceladus and Thamnos. [ABSTRACT FROM AUTHOR]

We perform nonlocal thermodynamic equilibrium (NLTE) model atmosphere analyses of the three hottest hypervelocity stars (space velocities between ≈1500–2800 km s−1) known to date, which were recently discovered spectroscopically and identified as runaways from Type Ia supernovae. The hottest of the three (J0546+0836, effective temperature Teff = 95 000 ± 15 000 K, surface gravity log 푔 = 5.5 ± 0.5) has an oxygen-dominated atmosphere with a significant amount of carbon (C = 0.10 ± 0.05, O = 0.90 ± 0.05, mass fractions). Its mixed absorption+emission line spectrum exhibits photospheric absorption lines from O v and O vi as well as O iii and O iv emission lines that are formed in a radiation-driven wind with a mass-loss rate of the order of Ṁ= 10−8M⊙ yr−1. Spectroscopically, J0546+0836 is a [WC]–PG1159 transition-type pre-white dwarf. The second object (J0927–6335) is a PG1159-type white dwarf with a pure absorption-line spectrum dominated by C III/C IV and O III/O IV. We find Teff = 60 000 ± 5000 K, log 푔 = 7.0 ± 0.5, and a carbon-and oxygen-dominated atmosphere with C = 0.47 ± 0.25, O = 0.48 ± 0.25, and possibly a minute amount of helium (He = 0.05 ± 0.05). Comparison with post-AGB evolutionary tracks suggests a mass of M ≈ 0.5 M⊙ for both objects, if such tracks can safely be applied to these stars. We find the third object (J1332−3541) to be a relatively massive (M = 0.89 M⊙) hydrogen-rich (DAO) white dwarf with Teff = 65 657 ± 2390 K, log 푔 = 8.38 ± 0.08, and abundances H = 0.65 ± 0.04 and He = 0.35 ± 0.04. We discuss our results in the context of the "dynamically driven double-degenerate double-detonation" (D6) scenario proposed for the origin of these stars. [ABSTRACT FROM AUTHOR]

Context. Large spectroscopic surveys devoted to the study of the Milky Way, including Gaia, use automated pipelines to determine the atmospheric parameters of millions of stars. The Gaia FGK benchmark stars are reference stars with Teff and log 푔 derived through fundamental relations, independently of spectroscopy, to be used as anchors for the parameter scale. The first and second versions of the sample have been extensively used for that purpose, and more generally to help constrain stellar models. Aims. We provide the third version of the Gaia FGK benchmark stars, an extended set intended to improve the calibration of spectroscopic surveys, and their interconnection. Methods. We have compiled about 200 candidates that have precise measurements of angular diameters and parallaxes. We determined their bolometric fluxes by fitting their spectral energy distribution. Masses were determined using two sets of stellar evolution models. In a companion paper, we describe the determination of metallicities and detailed abundances. Results. We provide a new set of 192 Gaia FGK benchmark stars with their fundamental Teff and log 푔, and with uncertainties lower than 2% for most stars. Compared to the previous versions, the homogeneity and accuracy of the fundamental parameters are significantly improved thanks to the high quality of the Gaia photometric and astrometric data. [ABSTRACT FROM AUTHOR]

Utilizing a simplified quantum model approach, the low-energy inelastic collision processes between yttrium atoms (ions) and hydrogen atoms have been studied. Rate coefficients corresponding to the mutual neutralization, ion-pair formation, excitation, and de-excitation processes for the above collision systems have been provided in the temperature range of 1000–10 000 K. Three ionic states and 73 covalent states are considered in calculations for the collisions of yttrium atoms with hydrogen atoms, which include six molecular symmetries and 4074 partial inelastic reaction processes. For the collisions of yttrium ions with hydrogen atoms, one ionic state and 116 covalent states are included, which are related to three molecular symmetries and 13 572 partial inelastic collision processes. It is found that the rate coefficients for the mutual neutralization process have a maximum at T = 6000 K, which is an order of magnitude higher than those of other processes. Notably, the positions of optimal windows for the collisions of yttrium atoms and ions with hydrogen atoms are found near electronic binding energy −2 eV (Y) and −4.4 eV (Y+), respectively. The scattering channels located in or near these optimal windows have intermediate-to-large rate coefficients (greater than 10−12 cm3 s−1). The reported data should be useful in the study of non-local thermodynamic equilibrium modelling. [ABSTRACT FROM AUTHOR]

New molecular line lists for lithium hydroxide (6Li16O1H and 7Li16O1H isotopologues) covering wavelengths λ > 1  |$\mu$| m (0–10 000 cm−1 range) are presented. Each line list contains around 300 million transitions between rotation–vibration energy levels with total angular momentum up to J  = 95, applicable for temperatures up to T ≈ 3500 K. Line list calculations were based on a previously published, high-level ab initio potential energy surface and a newly computed dipole moment surface of the ground |$\tilde{X}\, ^1\Sigma ^+$| electronic state. Lithium-containing molecules are important in a variety of stellar objects and there is potential for LiOH to be observed in the atmospheres of exoplanets. Transit spectra are simulated using the rocky super-Earth 55 Cancri e as an example with two different atmospheric scenarios including LiOH. This work provides the first, comprehensive line lists of LiOH to facilitate its future molecular detection. The OYT7 line lists along with the associated temperature- and pressure-dependent molecular opacities can be downloaded from the ExoMol database at www.exomol.com and the CDS astronomical database. [ABSTRACT FROM AUTHOR]

Mass-loss is a key parameter throughout the evolution of massive stars. In this work we probe the radial clumping stratification of OB stars in the intermediate and outer wind regions (r ≳ 2 R *; r , radial distance to photosphere), derive upper limits for mass-loss rates, Ṁ max, and compare them to current theoretical mass-loss recipes implemented in evolutionary models. A key conclusion of our analysis regards the derived upper-limit mass-loss rates of B supergiants, independently of clumping, which calls for an urgent revision of the role recombination of iron-like elements plays in determining the mass-loss rates of objects that cross the bi-stability region, and a careful analysis of corresponding effects for stellar evolution models. [ABSTRACT FROM AUTHOR]

The VLT/FLAMES Tarantula Survey (Evans et al. 2011) identified a group of slowly-rotating nitrogen-rich O-type stars that cannot be explained by current evolutionary models. Here we present high-quality VLT/UVES observations of four of these stars that allow a detailed quantitative spectroscopic analysis. We present the analysis of the spectra with a genetic algorithm, and discuss the future steps to be taken to further investigate the cause of the nitrogen enrichment. [ABSTRACT FROM AUTHOR]

Spectroscopy can decode the radiation from stars in an appropriate way and derive many properties of different stellar objects. In this work we seek to derive simultaneously stellar and wind parameters of massive stars. To model the data we use the radiative transport code Fastwind with the hydrodynamic solutions derived using our stationary code Hydwind as input, instead of the β-law. Then, ISOSCELES, our grid of stellar atmosphere and hydrodynamic models of massive stars, is used to derive the physical properties of the observed spectra through spectral line fittings. This quantitative spectroscopic analysis provide an estimation about the line–force parameters, whose theoretical calculations are complex. In addition, we expect to confirm that the hydrodynamic δ-slow solutions, describe quite reliable the radiation line-driven winds of A and late B supergiant stars and, at the same time, explain disagreements between observational data and theoretical models for the Wind–Momentum Luminosity Relationship (WLR). [ABSTRACT FROM AUTHOR]

B-type supergiants show enormous potential as resourceful tools to address a wide range of astrophysical questions concerning stellar atmospheres, stellar and galactic evolution and the cosmic distance scale. For the purposes of a comprehensive analysis of these objects we test a hybrid non-LTE approach – line-blanketed model atmospheres computed under the assumptions of local thermodynamic equilibrium (LTE) in combination with non-LTE line-formation calculations. An observational sample of 14 Galactic B-type supergiants with masses below about 30 M ȯ is investigated on the basis of high-resolution Echelle spectra. The results of this analysis – atmospheric and fundamental stellar parameters, the characterisation of the interstellar sightlines to the objects, as well as derived spectroscopic distances and multi-species abundances – are subjected to multiple tests of consistency. [ABSTRACT FROM AUTHOR]

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

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

We present chemical composition and fundamental parameters (the effective temperature, surface gravity, and radius) for four sharp-lined A-type stars γ Gem (HD 41705), o Peg (HD 214994), θ Vir (HD 114330), and ν Cap (HD 193432). Our analysis is based on a self-consistent model fitting of high-resolution spectra and spectrophotometric observations over a wide wavelength range. We refined the fundamental parameters of the stars with the sme  (Spectroscopy Made Easy) package and verified their accuracy by comparing with the spectral energy distribution and hydrogen line profiles. We found T eff/log  g  = 9190 ± 130 K/3.56 ± 0.08, 9600 ± 50 K/3.81 ± 0.04, 9600 ± 140 K/3.61 ± 0.12, and 10200 ± 220 K/3.88 ± 0.08 for γ Gem, o Peg, θ Vir, and ν Cap, respectively. Our detailed abundance analysis employs a hybrid technique for spectrum synthesis based on classical model atmospheres calculated in local thermodynamic equilibrium (LTE) assumption together with the non-LTE (NLTE) line formation for 18 of 26 investigated species. Comparison of the abundance patterns observed in A stars of different types (normal A, Am, and Ap) with similar fundamental parameters reveals significant abundance diversity that cannot be explained by the current mechanisms of abundance peculiarity formation in stellar atmospheres. We found a rise of the heavy element (Zn, Sr, Y, Zr, and Ba) abundance excess up to  +1 dex with T eff increasing from 7200 to 10 000 K, with a further decrease down to solar value at T eff = 13 000 K, indicating that stars with solar element abundances can be found among late B-type stars. [ABSTRACT FROM AUTHOR]

We conduct a line-by-line differential analysis of a sample of 125 co-moving pairs of stars (dwarfs and subgiants near solar metallicity). We obtain high-precision stellar parameters with average uncertainties in effective temperature, surface gravity, and metallicity of 16.5 K, 0.033 and 0.014 dex, respectively. We classify the co-moving pairs of stars into two groups, chemically homogeneous (conatal; |Δ[Fe/H]| ≤ 0.04 dex) and inhomogeneous (non-conatal), and examine the fraction of chemically homogeneous pairs as a function of separation and effective temperature. The four main conclusions from this study are (1) A spatial separation of Δ s  = 106 au is an approximate boundary between homogeneous and inhomogeneous pairs of stars, and we restrict our conclusions to only consider the 91 pairs with Δ s  ≤ 106 au; (2) There is no trend between velocity separation and the fraction of chemically homogeneous pairs in the range Δ v  ≤ 4 km s−1; (3) We confirm that the fraction of chemically inhomogeneous pairs increases with increasing T eff and the trend matches a toy model of that expected from planet ingestion; (4) Atomic diffusion is not the main cause of the chemical inhomogeneity. A major outcome from this study is a sample of 56 bright co-moving pairs of stars with chemical abundance differences ≤ 0.02 dex (5 per cent) which is a level of chemical homogeneity comparable to that of the Hyades open cluster. These important objects can be used, in conjunction with star clusters and the Gaia 'benchmark' stars, to calibrate stellar abundances from large-scale spectroscopic surveys. [ABSTRACT FROM AUTHOR]

In previous works, we computed abundances for the red giant in nearly four dozen S-type symbiotic systems (SySt). The abundances provide information about metallicity, evolutionary status, and possible memberships in Galactic stellar populations. Here, we extend our studies with a northern hemisphere sample of SySt. This northern sample is dominated by Galactic disc/halo objects, whereas our previous southern sample is heavily biased toward the bulge population. Spectrum synthesis of high-resolution (R ∼ 50 000), near- IR spectra using standard LTE analysis and atmospheric models have been used to measure abundances of CNO and elements around the iron peak (Fe, Ti, Ni, and Sc) in the atmospheres of the red giant component. The SySt sample shows generally slightly sub-solar metallicity, as expected for an older disc population, with a median at [Fe/H]  ∼ −0.2 dex. Enhanced 14N, depleted 12C, and decreased 12C/13C, indicate that all these giants have experienced the first dredge-up. Comparison with theoretical predictions indicates that additional mixing processes had to occur to explain the observed C and N abundances. Relative O and Fe abundances agree with those represented by Galactic disc and bulge giant populations in the APOGEE data, with a few cases that can be attributed to membership in the extended thick-disc/halo. As an interesting byproduct of this study, we observed a blue-shifted additional component on the wings of absorption lines in the spectra of AG Peg, which could be connected with accretion on to the hot component. [ABSTRACT FROM AUTHOR]

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

Accurately quantifying the rates d M /d t at which massive stars lose mass is essential to any understanding of their evolution. All d M /d t estimates to date assume wind clumping factors; not allowing for clumping leads to overestimates of d M /d t and underestimates of lifetimes and masses when these stars explode as supernovae. Mid-infrared spectroscopy suggested that the wind of the nearest Wolf–Rayet (WR) star, γ2 Vel, is resolved with a full width at 10 per cent intensity of 0.5 arcsec, or 171 au at the 342 pc distance of the star. As the Zorro speckle imager on Gemini South is capable of ∼0.02 arcsec resolution, we have used it to image γ2 Vel at two orbital phases (0.30 and 0.44) with two narrow-band and two intermediate-band filters in an attempt to resolve its wind. Our observations demonstrate that γ2 Vel's wind may be resolved as a ∼0.07 arcsec westward elongation through an 832 nm filter at orbital phase 0.3. If confirmed, this is the smallest scale (∼24 au) at which a WR star wind asymmetry has been directly imaged. Similar imaging at multiple phases is needed to determine if the asymmetry is due to stochastic wind clumping, co-rotating interaction regions or colliding-wind, cone-shaped shocks. [ABSTRACT FROM AUTHOR]

We present a detailed abundance analysis of nine early B-type stars in the λ Orionis group. High- and medium-resolution spectra of the target stars were observed at the Ankara University Kreiken Observatory or acquired from the European Southern Observatory and Observatoire de Haute-Provence archives. The atmospheric parameters of each star were estimated from the hydrogen Balmer, ionized silicone, and helium lines. Non-LTE model atmosphere calculations determined the precise abundances of helium, carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron from the spectra of each target star. While the abundances of the light elements He, C, N, O, Ne, Si, and also Fe are close to solar, Mg, Al, and S are deficient, and Ar is overabundant compared to the Sun. The phosphorus absorption lines were only accessible on the optical spectra of HD 43112, HD 34989, 37 Ori, and HD 36895. The phosphorus mean abundance values of these four stars are underabundant compared to the Sun. Argon was only detected on the spectra of HD 43112 and HD 36262 and showed an overabundance of 0.33 dex. The sulfur element deficiency was detected in all stars except for 35 Orionis. Sulfur may be a good signature to distinguish the λ Orionis group members from the Orion star-forming complex. Overall, chemical homogeneity was detected in all B-type star members of the λ Orionis group. Furthermore, the mass and age of each member were calculated and plotted on the H–R diagram. [ABSTRACT FROM AUTHOR]

There has been a concerted effort in recent years to identify the astrophysical sites of the r -process that can operate early in the galaxy. The discovery of many r -process-enhanced (RPE) stars (especially by the R -process Alliance collaboration) has significantly accelerated this effort. However, only limited data exist on the detailed elemental abundances covering the primary neutron-capture peaks. Subtle differences in the structure of the r -process pattern, such as the relative abundances of elements in the third peak, in particular, are expected to constrain the r -process sites further. Here, we present a detailed elemental-abundance analysis of four bright RPE stars selected from the HESP–GOMPA survey. Observations were carried out with the 10-m class telescope Gran Telescopio Canarias (GTC), Spain. The high spectral signal-to-noise ratios obtained allow us to derive abundances for 20 neutron-capture elements, including the third r -process peak element osmium (Os). We detect thorium (Th) in two stars, which we use to estimate their ages. We discuss the metallicity evolution of Mg, Sr, Ba, Eu, Os, and Th in r -II and r -I stars, based on a compilation of RPE stars from the literature. The strontium (Sr) abundance trend with respect to europium (Eu) suggests the need for an additional production site for Sr (similar to several earlier studies); this requirement could be milder for yttrium (Y) and zirconium (Zr). We also show that there could be some time delay between r -II and r -I star formation, based on the Mg/Th abundance ratios. [ABSTRACT FROM AUTHOR]

As a cool star evolves, it loses mass and angular momentum due to magnetized stellar winds that affect its rotational evolution. This change has consequences that range from the alteration of its activity to influences over the atmosphere of any orbiting planet. Despite their importance, observations constraining the properties of stellar winds in cool stars are extremely limited. Therefore, numerical simulations provide a valuable way to understand the structure and properties of these winds. In this work, we simulate the magnetized winds of 21 cool main-sequence stars (F-type to M-dwarfs) using a state-of-the-art 3D MHD code driven by observed large-scale magnetic field distributions. We perform a qualitative and quantitative characterization of our solutions, analysing the dependencies between the driving conditions (e.g. spectral type, rotation, and magnetic field strength) and the resulting stellar wind parameters (e.g. Alfvén surface size, mass-loss rate, angular momentum loss rate, and stellar wind speeds). We compare our models with the current observational knowledge on stellar winds in cool stars and explore the behaviour of the mass-loss rate as a function of the Rossby number. Furthermore, our 3D models encompass the entire classical Habitable Zones (HZ) of all the stars in our sample. This allows us to provide the stellar wind dynamic pressure at both edges of the HZ and analyse the variations of this parameter across spectral type and orbital inclination. The results here presented could serve to inform future studies of stellar wind-magnetosphere interactions and stellar wind erosion of planetary atmospheres via ion escape processes. [ABSTRACT FROM AUTHOR]

The convective dredge-up of carbon from the interiors of hydrogen-deficient white dwarfs has long been invoked to explain the presence of carbon absorption features in the spectra of cool DQ stars (⁠|$T_{\rm eff} \lt 10\,000\,$| K). It has been hypothesized that this transport process is not limited to DQ white dwarfs and also operates, albeit less efficiently, in non-DQ hydrogen-deficient white dwarfs within the same temperature range. This non-DQ population is predominantly composed of DC white dwarfs, which exhibit featureless optical spectra. However, no direct observational evidence of ubiquitous carbon pollution in DC stars has thus far been uncovered. In this Letter, we analyse data from the Galaxy Evolution Explorer to reveal the photometric signature of ultraviolet carbon lines in most DC white dwarfs in the |$8500\, {\rm K} \le T_{\rm eff} \le 10\,500\,$| K temperature range. Our results show that the vast majority of hydrogen-deficient white dwarfs experience carbon dredge-up at some point in their evolution. [ABSTRACT FROM AUTHOR]

Standard stellar evolution theory poorly predicts the surface abundances of chemical species in low-mass, red giant branch (RGB) stars. Observations show an enhancement of p–p chain and CNO cycle products in red giant envelopes, which suggests the existence of non-canonical mixing that brings interior burning products to the surface of these stars. The 12C/13C ratio is a highly sensitive abundance metric used to probe this mixing. We investigate extra RGB mixing by examining: (1) how 12C/13C is altered along the RGB, and (2) how 12C/13C changes for stars of varying age and mass. Our sample consists of 43 red giants, spread over 15 open clusters from the Sloan Digital Sky Survey's APOGEE DR17, that have reliable 12C/13C ratios derived from their APOGEE spectra. We vetted these 12C/13C ratios and compared them as a function of evolution and age/mass to the standard mixing model of stellar evolution, and to a model that includes prescriptions for RGB thermohaline mixing and stellar rotation. We find that the observations deviate from standard mixing models, implying the need for extra mixing. Additionally, some of the abundance patterns depart from the thermohaline model, and it is unclear whether these differences are due to incomplete observations, issues inherent to the model, our assumption of the cause of extra mixing, or any combination of these factors. Nevertheless, the surface abundances across our age/mass range clearly deviate from the standard model, agreeing with the notion of a universal mechanism for RGB extra mixing in low-mass stars. [ABSTRACT FROM AUTHOR]

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

We revisit the abundances of neutron-capture elements in the metal-poor ([Fe/H] = −2.9) r-process-rich halo star CS 31082−001. Partly motivated by the development of the new near-ultraviolet Cassegrain U-band Efficient Spectrograph for the Very Large Telescope, we compiled an expanded line list for heavy elements over the range 3000–4000 Å, including hyperfine structure for several elements. Combining archival near-ultraviolet spectra of CS 31082−001 from the Hubble Space Telescope and the Very Large Telescope, we investigate the abundances and nucleosynthesis of 35 heavy elements (Ge, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sn, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Os, Ir, Pt, Pb, Bi, Th, and U). Our analysis includes the first abundance estimates for tin, holmium, and ytterbium from these data, and the first for lutetium from ground-based data, enabling a more complete view of the abundance pattern of this important reference star. In general, the r-process-dominated elements are as enhanced as those in the Sun, particularly for elements with Z  ≥ 56 (Ba and heavier). However, the abundances for the lighter elements in our sample, from Ge to Sn (31 ≤  Z  ≤ 50), do not scale with the solar abundance pattern. Moreover, the Ge abundance is deficient relative to solar, indicating that it is dominantly an iron-peak rather than neutron-capture element. Our results (or upper limits) on Sn, Pt, Au, Pb, and Bi all pose further questions, prompting further study on the origin and evolution of the known r-rich and actinide-rich, metal-poor stars. [ABSTRACT FROM AUTHOR]

Magnetic features on the surface of stars, such as spots and faculae, cause stellar spectral variability on time-scales of days and longer. For stars other than the Sun, the spectral signatures of faculae are poorly understood, limiting our ability to account for stellar pollution in exoplanet transit observations. Here we present the first facular contrasts derived from magnetoconvection simulations for K0, M0, and M2 main-sequence stars and compare them to previous calculations for G2 main-sequence stars. We simulate photospheres and immediate subsurface layers of main-sequence spectral types between K0 and M2, with different injected vertical magnetic fields (0 G, 100 G, 300 G, and 500 G) using MURaM, a 3D radiation-magnetohydrodynamics code. We show synthetic spectra and contrasts from the UV (300 nm) to the IR (10 000 nm) calculated using the ATLAS9 radiative transfer code. The calculations are performed for nine viewing angles to characterize the facular radiation across the disc. The brightness contrasts of magnetic regions are found to change significantly across spectral type, wavelength, and magnetic field strength, leading to the conclusion that accurate contrasts cannot be found by scaling solar values. This is due to features of different size, apparent structure and spectral brightness emerging in the presence of a given magnetic field for different spectral types. [ABSTRACT FROM AUTHOR]

Aims. The evolutionary status of the blue supergiant Sher 25 and its membership to the massive cluster NGC 3603 are investigated. Methods. A hybrid non-local thermodynamic equilibrium (non-LTE) spectrum synthesis approach is employed to analyse a high-resolution optical spectrum of Sher 25 and five similar early B-type comparison stars in order to derive atmospheric parameters and elemental abundances. Fundamental stellar parameters are determined by considering stellar evolution tracks, Gaia Data Release 3 (DR3) data, and complementary distance information. Interstellar reddening and the reddening law along the sight line towards Sher 25 are constrained employing UV photometry for the first time in addition to optical and infrared data. The distance to NGC 3603 is reevaluated based on Gaia DR3 data of the innermost cluster O-stars. Results. The spectroscopic distance derived from the quantitative analysis implies that Sher 25 lies in the foreground of NGC 3603, which is found to have a distance of dNGC3603 = 6250 ± 150 pc. A cluster membership is also excluded as the hourglass nebula is unaffected by the vigorous stellar winds of the cluster stars and from the different excitation signatures of the hourglass nebula and the nebula around NGC 3603. Sher 25 turns out to have a luminosity of log L/L⊙ = 5.48 ± 0.14, equivalent to that of a ~27 M⊙ supergiant in a single-star scenario, which is about half of the mass assumed so far, bringing it much closer in its characteristics to Sk−69º202, the progenitor of SN 1987A. Sher 25 is significantly older than NGC 3603. Further arguments for a binary (merger) evolutionary scenario of Sher 25 are discussed. [ABSTRACT FROM AUTHOR]

We report on the spectroscopic confirmation of 68 new bright (G = 13.5–17.2 mag) and blue (pre-)white dwarfs (WDs). This finding has allowed us to almost double the number of the hottest (Teff ≥ 60 kK) known WDs brighter than G = 16 mag. We increased the number of known ultra-high excitation (UHE) WDs by 20%, found one unambiguous close binary system consisting of one DA WD with an irradiated low-mass companion, one DAO, and one DOA WD that are likely in their transformation phase of becoming pure DA WDs, one rare, naked O(H) star, two DA and two DAO WDs with Teff possibly in excess of 100 kK, three new DOZ WDs, and three of our targets are central stars of (possible) planetary nebulae. Using non-local thermodynamic equilibrium models, we derived the atmospheric parameters of these stars and by fitting their spectral energy distribution we derived their radii, luminosities, and gravity masses. In addition, we derived their masses in the Kiel and Hertzsprung-Russell diagram (HRD). We find that Kiel, HRD, and gravity mass agree only in half of the cases. This is not unexpected and we attribute this to the neglect of metal opacities, possibly stratified atmospheres, as well as possible uncertainties of the parallax zero point determination. Furthermore, we carried out a search for photometric variability in our targets using archival data, finding that 26% of our targets are variable. This includes 15 new variable stars, with only one of them being clearly an irradiation effect system. Strikingly, the majority of the variable stars exhibit non-sinusoidal light-curve shapes, which are unlikely explained in terms of close binary systems. We propose that a significant fraction of all (not just UHE) WDs develop spots when entering the WD cooling phase. We suggest that this could be related to the on-set of weak magnetic fields and possibly diffusion. [ABSTRACT FROM AUTHOR]

Context. B supergiants (BSGs) are evolved stars with effective temperatures between ~10 to ~30 kK. Knowing the properties of these objects is important to understand massive star evolution. Located on the cool end of the line-driven wind regime, the study of their atmospheres can help us to understand the physics of their winds and phenomena such as the bi-stability jump. Aims. Despite being well-studied stars, key UV features of their spectra have so far not been reproduced by atmosphere models for spectral types later than B1. In this study, we aim to remedy this situation by performing quantitative spectral analyzes that account for the effects of X-rays and clumping in the wind. In addition, we also briefly investigate the evolutionary status of our sample stars based on the stellar parameters we obtained. Methods. We determined photospheric and wind parameters via quantitative spectroscopy using atmosphere models computed with CMFGEN and PoWR. These models were compared to high-resolution UV and optical spectra of four BSGs: HD206165, HD198478, HD53138, and HD164353. We further employed GENEC and MESA tracks to study the evolutionary status of our sample. Results. When including both clumping and X-rays, we obtained a good agreement between synthetic and observed spectra for our sample stars. For the first time, we reproduced key wind lines in the UV, where previous studies were unsuccessful. To model the UV spectra, we require a moderately clumped wind (fV∞ ≳ 0.5). We also infer a relative X-ray luminosity of about 10−7.5 to 10−8, which is lower than the typical ratio of 10−7. Moreover, we find a possible mismatch between evolutionary mass predictions and the derived spectroscopic masses, which deserves deeper investigation as this might relate to the mass-discrepancy problem present in other types of OB stars. Conclusions. Our results provide direct spectroscopic evidence that both X-rays and clumping need to be taken into account to describe the winds of cool BSGs. However, their winds seem to be much less structured than in earlier OB-type stars. Our findings are in line with observational X-rays and clumping constraints as well as recent hydrodynamical simulations. The evolutionary status of BSGs seems to be diverse with some objects potentially being post-red supergiants or merger products. The obtained wind parameters provide evidence for a moderate increase of the mass-loss rate around the bi-stability jump. [ABSTRACT FROM AUTHOR]

We present the homogeneous abundance analysis for a combined sample of 185 giants in the bulge globular cluster (GC) NGC 6388. Our results are used to describe the multiple stellar populations and differences or analogies with bulge field stars. Proton-capture elements indicate that a single class of first-generation polluters is sufficient to reproduce both the extreme and intermediate parts of the anti-correlations among light elements O, Na, Mg, and Al, which is at odds with our previous results based on a much smaller sample. The abundance pattern of other species in NGC 6388 closely tracks the trends observed in bulge field stars. In particular, the α-elements, including Si, rule out an accreted origin for NGC 6388, confirming our previous results based on iron-peak elements, chemo-dynamical analysis, and the age-metallicity relation. The neutron-capture elements are generally uniform, although the [Zr/Fe] ratio shows an intrinsic scatter, correlated to Na and Al abundances. Instead, we do not find enhancement in neutron-capture elements for stars whose photometric properties would classify NGC 6388 as a type II GC. Together with the homogeneity in [Fe/H] we found in a previous paper, this indicates we need to better understand the criteria to separate classes of GCs, coupling photometry, and spectroscopy. These results are based on abundances of 22 species (O, Na, Mg, Al, Si, Ca, Ti, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Ba, La, Ce, Nd, and Eu) from UVES spectra sampling proton-, α-, neutron-capture elements, and Fe-peak elements. For 12 species, we also obtain abundances in a large number of giants (up to 150) from GIRAFFE spectra. [ABSTRACT FROM AUTHOR]

Stellar winds of cool and pulsating asymptotic giant branch (AGB) stars enrich the interstellar medium with large amounts of processed elements and various types of dust. We present the first study on the influence of gas-to-dust drift on ab initio simulations of stellar winds of M-type stars driven by radiation pressure on forsterite particles. Our study is based on our radiation hydrodynamic model code T-800 that includes frequency-dependent radiative transfer, dust extinction based on Mie scattering, grain growth and ablation, gas-to-dust drift using one mean grain size, a piston that simulates stellar pulsations, and an accurate high spatial resolution numerical scheme. To enable this study, we calculated new gas opacities based on the EXOMOL database, and we extended the model code to handle the formation of minerals that may form in M-type stars. We determine the effects of drift by comparing drift models to our new and extant non-drift models. Three out of four new drift models show high drift velocities, 87–310 km s−1. Our new drift model mass-loss rates are 1.7–13 per cent of the corresponding values of our non-drift models, but compared to the results of two extant non-drift models that use the same stellar parameters, these same values are 0.33–1.5 per cent. Meanwhile, a comparison of other properties such as the expansion velocity and grain size show similar values. Our results, which are based on single-component forsterite particles, show that the inclusion of gas-to-drift is of fundamental importance in stellar wind models driven by such transparent grains. Assuming that the drift velocity is insignificant, properties such as the mass-loss rate may be off from more realistic values by a factor of 50 or more. [ABSTRACT FROM AUTHOR]

Context. A new, self-consistent, scale-free theory of stellar convection was recently developed (SFCT) in which velocities, dimensions, and energy fluxes carried by the convective elements are defined in a rest frame co-moving with the convective element itself. As the dynamics of the problem is formulated in a different framework with respect to the mixing length theory (MLT), the SFCT equations are sufficient to determine all the properties of stellar convection in accordance with the physics of the environment alone, with no need for the mixing length parameter (MLP). Subsequently, the SFCT was improved by introducing suitable boundary conditions at the surface of the external convective zones of the stars, and the first stellar models and evolutionary tracks on the Hertzsprung–Russell diagram were calculated. Aims. The SFCT received alternatively positive and negative attention that spurred us to reconsider the whole problem. In this work, we aim to re-examine the physical foundations and results of the SFCT, elucidate some misconceptions on its physical foundations, reply to reported criticisms, and present some recent improvements to the SFCT. Methods. The analysis was done using the same formalism of the previous studies, but novel arguments and demonstrations are added to better justify the controversial points, in particular the relaxation of instantaneous hydrostatic equilibrium between a convective element and the surrounding medium. Results. The main results include (i) a novel detailed discussion of the boundary conditions to ensure that the temperature gradients in the outermost regions of a star are adequate for analyses of stability or instability in asteroseismology; (ii) a quantitative comparison with the MLT; and, finally, (iii) the recovery of the MLT as a particular case of the SFCT, but also in this case with no need for the MLP. Conclusions. In conclusion, the SFCT is a step forward with respect to the classical MLT. [ABSTRACT FROM AUTHOR]

Analysis of all archival 5–14 micron spectra of field ultracool dwarfs from the Infrared Spectrograph on the Spitzer Space Telescope has shown that absorption by silicates in the 8–11 micron region is seen in most L-type (1300 to 2200 K) dwarfs. The absorption is caused by silicate-rich clouds in the atmospheres of L dwarfs and is strongest at L4–L6 spectral types. Herein we compare averages of the mid-infrared silicate absorption signatures of L3–L7 dwarfs that have low (≲104.5 cm s−2) versus high (≳105 cm s−2) surface gravity. We find that the silicate absorption feature is sensitive to surface gravity, with young atmospheres having a broader, redder, and more asymmetric absorption profile. This indicates a difference in grain size and composition between dust condensates in young and old mid-L dwarfs. The mean silicate absorption profile of low-gravity mid-L dwarfs matches expectations for ∼1 micron-sized amorphous iron- and magnesium-bearing pyroxene (Mg x Fe1 − x SiO3) grains. High-gravity mid-L dwarfs have silicate absorption better represented by smaller (≲0.1 μ m) and more volatile amorphous enstatite (MgSiO3) or SiO grains. This is the first direct spectroscopic evidence for gravity-dependent sedimentation of dust condensates in ultracool atmospheres. It confirms theoretical expectations for lower sedimentation efficiencies in low-gravity atmospheres and independently confirms their increased dustiness. [ABSTRACT FROM AUTHOR]

We present a study of high-resolution spectra of RS Canum Venaticorum (RS CVn), a prototype of active binary systems. Our data were obtained from 1998 to 2017 using different telescopes. We analyse the chromospheric activity indicators |$\rm{Ca~{\small II}}$| IRT, Hα, |$\rm{Na~{\small I}}$| D1, D2 doublet, |$\rm{He~{\small I}}$| D3, and Hβ using a spectral subtraction technique. The chromospheric emission stems mainly from the K2 IV primary star, while the F5 V secondary star only shows weak emission features in a few of our spectra. We find excess absorption features in the subtracted Hα lines and other activity indicators from spectra taken near primary eclipse, which we ascribe to prominence-like material associated with the primary star. We estimate size limits of these tentative prominences based on the geometry of the binary system, and investigate the physical properties of the strongest prominence. An optical flare, characterized by |$\rm{He~{\small I}}$| D3 line emission, together with stronger emission in other activity lines, was detected. The flare energy is roughly comparable to strong flares observed on other RS CVn-type stars. The chromospherically active longitudes of RS CVn most frequently appear near the two quadratures of the system and display changes between observing runs, which indicates an ongoing evolution of its active regions. [ABSTRACT FROM AUTHOR]

We present a detailed model atmosphere analysis of 14001 DA white dwarfs from the Montreal White Dwarf Database with ultraviolet photometry from the GALEX mission. We use the 100 pc sample, where the extinction is negligible, to demonstrate that there are no major systematic differences between the best-fitting parameters derived from optical only data and the optical + UV photometry. GALEX FUV and NUV data improve the statistical errors in the model fits, especially for the hotter white dwarfs with spectral energy distributions that peak in the UV. Fitting the UV to optical spectral energy distributions also reveals UV-excess or UV-deficit objects. We use two different methods to identify outliers in our model fits. Known outliers include objects with unusual atmospheric compositions, strongly magnetic white dwarfs, and binary white dwarfs, including double degenerates and white dwarf + main-sequence systems. We present a list of 89 newly identified outliers based on GALEX UV data; follow-up observations of these objects will be required to constrain their nature. Several current and upcoming large-scale spectroscopic surveys are targeting >105 white dwarfs. In addition, the ULTRASAT mission is planning an all-sky survey in the NUV band. A combination of the UV data from GALEX and ULTRASAT and optical data on these large samples of spectroscopically confirmed DA white dwarfs will provide an excellent opportunity to identify unusual white dwarfs in the solar neighbourhood. [ABSTRACT FROM AUTHOR]

We present a new library of semi-empirical stellar population models that are based on the empirical MILES and semi-empirical sMILES stellar libraries. The models span a large range of age and metallicity, in addition to an [α/Fe] coverage from −0.2 to +0.6 dex, at MILES resolution (FWHM  =  |$2.5\,$|  Å) and wavelength coverage (⁠|$3540.5-7409.6\,$|  Å). These models are aimed at exploring abundance ratios in the integrated light from stellar populations in star clusters and galaxies. Our approach is to build SSPs from semi-empirical stars at particular [α/Fe] values, thus producing new SSPs at a range of [α/Fe] values from sub-solar to super-solar. We compare these new SSPs with previously published and well-used models and find similar abundance pattern predictions, but with some differences in age indicators. We illustrate a potential application of our new SSPs, by fitting them to the high signal-to-noise data of stacked SDSS galaxy spectra. Age, metallicity, and [α/Fe] trends were measured for galaxy stacks with different stellar velocity dispersions and show systematic changes, in agreement with previous analyses of subsets of those data. These new SSPs are made publicly available. [ABSTRACT FROM AUTHOR]

The Gaia colour–magnitude diagram reveals a striking separation between hydrogen-atmosphere white dwarfs and their helium-atmosphere counterparts throughout a significant portion of the white dwarf cooling track. However, pure-helium atmospheres have Gaia magnitudes that are too close to the pure-hydrogen case to explain this bifurcation. To reproduce the observed split in the cooling sequence, it has been shown that trace amounts of hydrogen and/or metals must be present in the helium-dominated atmospheres of hydrogen-deficient white dwarfs. Yet, a complete explanation of the Gaia bifurcation that takes into account known constraints on the spectral evolution of white dwarfs has thus far not been proposed. In this work, we attempt to provide such a holistic explanation by performing population synthesis simulations coupled with state-of-the-art model atmospheres and evolutionary calculations that account for element transport in the envelopes of white dwarfs. By relying on empirically grounded assumptions, these simulations successfully reproduce the bifurcation. We show that the convective dredge-up of optically undetectable traces of carbon from the deep interior is crucial to account for the observations. Neither the convective dilution/mixing of residual hydrogen nor the accretion of hydrogen or metals can be the dominant drivers of the bifurcation. Finally, we emphasize the importance of improving theoretical models for the average ionization level of carbon in warm dense helium, which governs the shape of the diffusive tail of carbon and in turn the predicted amount of dredged-up carbon. [ABSTRACT FROM AUTHOR]

Well-studied very metal-poor (VMP, [Fe/H] <−2) stars in the inner Galaxy are few in number, and they are of special interest because they are expected to be among the oldest stars in the Milky Way. We present high-resolution spectroscopic follow-up of the carbon-enhanced metal-poor (CEMP) star Pristine_184237.56-260624.5 (hereafter Pr184237) identified in the Pristine Inner Galaxy Survey. This star has an apocentre of ∼2.6 kpc. Its atmospheric parameters (T eff = 5100 K, log  g = 2.0, and [Fe/H] = −2.60) were derived based on the non-local thermodynamic equilibrium (NLTE) line formation. We determined abundances for 32 elements, including 15 heavy elements beyond the iron group. The NLTE abundances were calculated for 13 elements from Na to Pb. Pr184237 is strongly enhanced in C, N, and O, and both s- and r-process elements from Ba to Pb; it reveals a low carbon isotope ratio of 12C/13C = 7. The element abundance pattern in the Na–Zn range is typical of halo stars. With [Ba/Eu] = 0.32, Pr184237 is the first star of the CEMP-r/s subclass identified in the inner Galaxy. Variations in radial velocity suggest binarity. We tested whether a pollution by the s- or i-process material produced in the more massive and evolved companion can form the observed abundance pattern and find that an i-process in the asymptotic giant branch star with a progenitor mass of 1.0–2.0 |$\, {\rm M}_{\odot }$| can be the solution. [ABSTRACT FROM AUTHOR]

A blue depression is found in the spectra of M dwarfs from 4000 to 4500 Å. This depression shows an increase towards lower temperatures, though it is particularly sensitive to gravity and metallicity. It is the single strongest and most sensitive feature in the optical spectra of M dwarfs. The depression appears as centred on the neutral calcium resonance line at 4227 Å and leads to nearby features being weaker by about two orders of magnitude than predicted. We consider a variety of possible causes for the depression, including temperature, gravity, metallicity, dust, damping constants, and atmospheric stratification. We also consider relevant molecular opacities that might be the cause identifying AlH, SiH, and NaH in the spectral region. However, none of these solutions are satisfactory. In the absence of a more accurate determination of the broadening of the calcium line perturbed by molecular hydrogen, we find a promising empirical fit using a modified Lorentzian line profile for the calcium resonance line. Such fits provide a simplistic line-broadening description for this calcium resonance line and potentially other unmodelled resonance lines in cool high-pressure atmospheres. Thus, we claim that the most plausible cause of the blue depression in the optical spectra of M dwarfs is a lack of appropriate treatment of line broadening for atomic calcium. The broad wings of the calcium resonance line develop at temperatures below about 4000 K and are analogous to the neutral sodium and potassium features, which dominate the red optical spectra of L dwarfs. [ABSTRACT FROM AUTHOR]

We present the surface magnetic field conditions of the brightest pulsating RV Tauri star, R Sct. Our investigation is based on the longest spectropolarimetric survey ever performed on this variable star. The analysis of high resolution spectra and circular polarization data give sharp information on the dynamics of the atmosphere and the surface magnetism, respectively. Our analysis shows that surface magnetic field can be detected at different phases along a pulsating cycle, and that it may be related to the presence of a radiative shock wave periodically emerging out of the photosphere and propagating throughout the stellar atmosphere. [ABSTRACT FROM AUTHOR]

We investigated the chemical composition of the planetary host halo star HD47536 via high-resolution spectral observations recorded using a 1.5 meter Cerro Tololo Inter-American Observatory (CTIO) telescope (Chile). Furthermore, we determined the abundances of 38 chemical elements. Both light and heavy elements were overabundant compared to the iron group elements. The abundance pattern of HD47536 was similar to that of halo-type stars, with an enrichment of heavy elements. We analyzed the relationships between the relative abundances of chemical elements and their second ionization potentials and condensation temperatures. We demonstrated that the interplay of charge-exchange reactions owing to the accretion of interstellar matter and the gas-dust separation mechanism can influence the initial abundances and can be used to qualitatively explain the abundance patterns in the atmosphere of HD47536.

We provide in this paper new Stark broadening parameters for the four ions Ga IV, Ga V, Ge IV, and Ge V. The calculations have been performed using our quantum mechanical method. To the best of our knowledge, the only available Stark broadening results are those of the Ge IV ion, where the semiclassical perturbation and the modified semi-empirical methods have been used. For the three other ions, no data have been found. In 2020 and for the first time, about five hundred lines of trans-iron elements including gallium (Ga) and germanium (Ge) have been identified in the ultraviolet (UV) spectrum of a DAO-type white dwarf BD-22°3467. Other precedent observations showed the existence of Ga IV–V lines in two white dwarfs (G191-B2B and RE 0503-289), and those of Ge IV and Ge V in the UV spectrum of RE 0503-289. Theoretical evaluations and measurements of atomic and Stark broadening data are a pre-requisite for stellar-atmosphere modelling, which is a tool for the determination of the photospheric Ga and Ge abundance in white dwarfs. Since there is no Stark broadening data for the three ions Ga IV, Ga V, and Ge V, our calculations come to fill this gap, and the obtained results can be used for the abundance determination of elements, the calculation of stellar opacity, the interpretation and modelling of stellar spectra, and the estimation of the relative transfer through stellar plasmas. They will be also implemented into the data base of Stark broadening parameters, STARK-B. [ABSTRACT FROM AUTHOR]