Stellar Atmosphere Models Research Articles

Study of UV bright sources in globular cluster NGC 4590 using Ultraviolet Imaging Telescope (UVIT) observations

ABSTRACT We have studied ultraviolet (UV) bright sources in the Galactic globular cluster (GGC) NGC 4590 using Ultraviolet Imaging Telescope onboard the AstroSat satellite. Using UV-optical colour–magnitude diagrams (CMDs), we have identified and characterized the sources of different evolutionary stages i.e. blue horizontal branch stars (BHBs), extremely blue horizontal branch stars (EHBs), blue straggler stars (BSs), variable stars, etc. We estimated effective temperature (Teff), gravity (log (g)), luminosity (Lbol), and hence the radius (R) of these hot stars by fitting spectral energy distribution with the help of stellar atmosphere models. Two new far-UV bright cluster member stars situated near the core of the cluster have been detected; one of them is an EHB star and the other one is either in its post-blue hook evolutionary phase or in white dwarf phase. The evolutionary status of all the hot stars, identified in the cluster, has been investigated by using various evolutionary models. We find the massive and younger BSs are concentrated at the centre of the cluster whereas the older and less massive BSs are distributed throughout the cluster. The BSs normalized radial distribution seems to be bi-modal with a minimum located at rmin = 4.3 rc. We calculated A+ parameter of the cluster which is obtained using cumulative normalized radial distribution of horizontal branch stars (HBs) and BSs. We measured this value up to half-mass radius of the cluster to be +0.13, which indicates that NGC 4590 is one of the youngest clusters among dynamically intermediate age GGCs with a dynamical age of 0.423 ± 0.096 Gyr.

A white dwarf accreting planetary material determined from X-ray observations.

The atmospheres of a large proportion of white dwarf stars are polluted by heavy elements1 that are expected to sink out of visible layers on short timescales2,3. This has been interpreted as a signature of ongoing accretion of debris from asteroids4, comets5 and giant planets6. This scenario is supported by the detection of debris discs7 and transits of planetary fragments8 around some white dwarfs. However, photospheric metals are only indirect evidence for ongoing accretion, and the inferred accretion rates and parent body compositions heavily depend on models of diffusion and mixing processes within the white dwarf atmosphere9-11. Here we report a 4.4σ detection of X-rays from a polluted white dwarf, G29-38. From the measured X-ray luminosity, we derive an instantaneous accretion rate of [Formula: see text], which is independent of stellar atmosphere models. This rate is higher than estimates from past studies of the photospheric abundances of G29-38, suggesting that convective overshoot may be needed to model the spectra of debris-accreting white dwarfs. We measure a low plasma temperature ofkBT = 0.5 ± 0.2 keV, corroborating the predicted bombardment solution for white dwarfs accreting at low accretion rates12,13.

Relativistic corrections for measuring Hubble’s constant to 1% using stellar standard candles

We have estimated relativistic corrections for cosmic distance estimates based on stellar standard candles such as classical Cepheids and stars near the tip of the red giant branch (TRGB stars) with the goal of enabling a future unbiased 1% measurement of Hubble’s constant, H0. We considered four effects: K corrections, time dilation, the apparent change of host dust extinction due to non-comoving reference frames, and the change of observed color due to redshift. Using stellar model atmospheres, we computed extinction-dependent K corrections for a wide range of effective temperatures, between 3500 and 6000 K, iron abundances between [Fe/H] = −2.0 and 0.5, surface gravity between log g = 2.0 and 0.0, and host reddening (up to E(B − V)host = 0.5) for a range of redshifts corresponding to distances of ∼20−120 Mpc (z between 0.005 and 0.03) in several HST, JWST, and 2MASS filters. The optical-near-infrared (NIR) Wesenheit function applied by the Cepheid distance ladder is particularly useful for limiting the magnitude of K corrections and for mitigating complications arising from host dust extinction. Missing host extinction corrections related to the circumgalactic medium and circumstellar environments arising from stellar mass loss are discussed as potential systematics of TRGB distance measurements. However, their effect is estimated to be insufficient to explain differences in H0 values based on Cepheids or TRGB supernova calibrations. All stellar standard candle observations require relativistic corrections in order to achieve an unbiased 1% H0 measurement in the future. Applying the K correction, the redshift-Leavitt bias correction, and a correction for the Wesenheit slope redshift dependence, the Cepheid-based H0 measurement increases by 0.45 ± 0.05 km s−1 Mpc−1 to H0SH0ES = 73.65 ± 1.30 km s−1 Mpc−1, raising the tension with the early-Universe value reported by the Planck Collaboration from 4.2σ to 4.4σ. For TRGB-based H0 measurements, we estimate a ∼0.5% upward correction for the methodology employed by Freedman et al. (H0CCHP = 70.2 ± 1.7 km s−1 Mpc−1) and an even smaller −0.15% downward correction for the methodology employed by Anand et al. (H0EDD = 71.4 ± 1.8 km s−1 Mpc−1). The opposite sign of these corrections is related to different reddening systematics and reduces the difference between the studies by ∼0.46 km s−1 Mpc−1. The optical-NIR Wesenheit function is particularly attractive for accurate distance measurements because it advantageously combines measurements in filters where K corrections have opposite signs. The JWST/NIRCAM F277W filter is of particular interest for TRGB stars thanks to its insensitivity to (weak) host reddening and K corrections below the level of 1% at Coma cluster distances.

Inelastic Processes in Nickel–Hydrogen Collisions

Abstract The cross sections and rate coefficients for inelastic processes in low-energy collisions of nickel atoms and positive ions with hydrogen atoms and negative ions are calculated for the collisional energy range 10−4–100 eV and for the temperature range 1000–10,000 K. 74 covalent and three ionic states correlated to 11 molecular symmetries are considered. 3380 partial inelastic processes are treated in total. The study of nickel–hydrogen collisions is performed by the quantum model methods within the Born–Oppenheimer formalism. The electronic structure of the collisional quasimolecule is calculated by the semiempirical asymptotic method for each considered molecular symmetry. For nuclear dynamic calculations the simplified method in combination with the Landau–Zener model is used. Nuclear dynamics within each considered symmetry is treated separately, and the total rate coefficients for each inelastic process have been summed over all symmetries. The largest values of the rate coefficients (exceeding 10−8 cm3 s−1) correspond to the mutual neutralization processes in collisions Ni+(3d 9 2 D) + H−(1s 2 1 S) (the ground ionic state being the initial state), as well as in Ni+(3d 84s 4,2 F) + H−(1s 2 1 S) (the first excited and the second excited ionic states being the initial states) collisions. At the temperature of 6000 K, the rate coefficients with large magnitudes have the values from the ranges (1.35−5.87) × 10−8 cm3 s−1 and (1.02−6.77) × 10−8 cm3 s−1, respectively. The calculated rate coefficients with large and moderate values are important for non–local thermodynamic equilibrium stellar atmosphere modeling.

The Spectroscopic Binaries from the LAMOST Medium-resolution Survey. I. Searching for Double-lined Spectroscopic Binaries with a Convolutional Neural Network

Abstract We developed a convolutional neural network model to distinguish the double-lined spectroscopic binaries (SB2s) from others based on single-exposure medium-resolution spectra (R ∼ 7500). The training set consists of a large set of mock spectra of single stars and binaries synthesized based on the MIST stellar evolutionary model and ATLAS9 atmospheric model. Our model reaches a novel theoretic false-positive rate by adding a proper penalty on the negative sample (e.g., 0.12% and 0.16% for the blue/red arm when the penalty parameter Λ = 16). Tests show that the performance is as expected and favors FGK-type main-sequence (MS) binaries with high mass ratio (q ≥ 0.7) and large radial velocity separation (Δv ≥ 50 km s−1). Although the real false-positive rate cannot be estimated reliably, validating on eclipsing binaries identified from Kepler light curves indicates that our model predicts low binary probabilities at eclipsing phases (0, 0.5, and 1.0) as expected. The color–magnitude diagram also helps illustrate its feasibility and capability of identifying FGK MS binaries from spectra. We conclude that this model is reasonably reliable and can provide an automatic approach to identify SB2s with period ≲10 days. This work yields a catalog of binary probabilities for over 5 million spectra of 1 million sources from the LAMOST medium-resolution survey (MRS) and a catalog of 2198 SB2 candidates whose physical properties will be analyzed in a follow-up paper. Data products are made publicly available online, as well as our Github website.

Modeling the Hα Emission Surrounding Spica Using the Lyman Continuum from a Gravity-darkened Central Star

Abstract The large, faint Hα emission surrounding the early B-star binary Spica has been used to constrain the total hydrogen recombination rate of the nebula and indirectly probe the Lyman continuum luminosity of the primary star. Early analysis suggested that a stellar atmosphere model, consistent with Spica A’s spectral type, has a Lyman continuum luminosity about two times lower than required to account for the measured Hα surface brightness within the nebula. To more consistently model both the stellar and nebular emission, we have used a model atmosphere for Spica A that includes the effects of gravity darkening as input to photoionization models to produce synthetic Hα surface brightness distributions for comparison to data from the Southern Hα Sky Survey Atlas. This paper presents a method for the computation of projected surface brightness profiles from 1D volume emissivity models and constrains both stellar and nebular parameters. A mean effective temperature for Spica A of ≃24,800 K is sufficient to match both the observed absolute spectrophotometry, from the far-UV to the near-IR, and radial Hα surface brightness distributions. Model hydrogen densities increase with the distance from the star, more steeply and linearly toward the southeast. The northwest matter-bounded portion of the nebula is predicted to leak ∼17% of Lyman continuum photons. Model H ii region column densities are consistent with archival observations along the line of sight.

Pre-explosion Properties of Helium Star Donors to Thermonuclear Supernovae

Abstract Helium star–carbon-oxygen white dwarf (CO WD) binaries are potential single-degenerate progenitor systems of thermonuclear supernovae. Revisiting a set of binary evolution calculations using the stellar evolution code MESA, we refine our previous predictions about which systems can lead to a thermonuclear supernova and then characterize the properties of the helium star donor at the time of explosion. We convert these model properties to near-UV/optical magnitudes assuming a blackbody spectrum and support this approach using a matched stellar atmosphere model. These models will be valuable to compare with pre-explosion imaging for future supernovae, though we emphasize the observational difficulty of detecting extremely blue companions. The pre-explosion source detected in association with SN 2012Z has been interpreted as a helium star binary containing an initially ultra-massive WD in a multiday orbit. However, extending our binary models to initial CO WD masses of up to 1.2 M ⊙, we find that these systems undergo off-center carbon ignitions and thus are not expected to produce thermonuclear supernovae. This tension suggests that, if SN 2012Z is associated with a helium star–WD binary, then the pre-explosion optical light from the system must be significantly modified by the binary environment and/or the WD does not have a carbon-rich interior composition.

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

Context. Next to H and He, carbon is, together with oxygen, the most abundant element in the Universe and widely used when modelling the formation and evolution of galaxies and their stellar populations. For the Milky Way bulge, there are currently essentially no measurements of carbon in un-evolved stars, hampering our abilities to properly compare Galactic chemical evolution models to observational data for this still enigmatic stellar population. Aims. We aim to determine carbon abundances for our sample of 91 microlensed dwarf and subgiant stars in the Galactic bulge. Together with new determinations for oxygen this forms the first statistically significant sample of bulge stars that have C and O abundances measured, and for which the C abundances have not been altered by the nuclear burning processes internal to the stars. Methods. Our analysis is based on high-resolution spectra for a sample of 91 dwarf and subgiant stars that were obtained during microlensing events when the brightnesses of the stars were highly magnified. Carbon abundances were determined through spectral line synthesis of six C I lines around 9100 Å, and oxygen abundances using the three O I lines at about 7770 Å. One-dimensional (1D) MARCS model stellar atmospheres calculated under the assumption of local thermodynamic equilibrium (LTE) were used, and non-LTE corrections were applied when calculating the synthetic spectra for both C and O. Results. Carbon abundances was possible to determine for 70 of the 91 stars in the sample and oxygen abundances for 88 of the 91 stars in the sample. The [C/Fe] ratio evolves essentially in lockstep with [Fe/H], centred around solar values at all [Fe/H]. The [O/Fe]–[Fe/H] trend has an appearance very similar to that observed for other α-elements in the bulge, with the exception of a continued decrease in [O/Fe] at super-solar [Fe/H], where other α-elements tend to level out. When dividing the bulge sample into two sub-groups, one younger than 8 Gyr and one older than 8 Gyr, the stars in the two groups follow exactly the elemental abundance trends defined by the solar neighbourhood thin and thick disks, respectively. Comparisons with recent models of Galactic chemical evolution in the [C/O]–[O/H] plane show that the models that best match the data are the ones that have been calculated with the Galactic thin and thick disks in mind. Conclusions. We conclude that carbon, oxygen, and the combination of the two support the idea that the majority of the stars in the Galactic bulge have a secular origin; that is, they are formed from disk material. We cannot exclude that a fraction of stars in the bulge could be classified as a classical bulge population, but it would have to be small. More dedicated and advanced models of the inner region of the Milky Way are needed to make more detailed comparisons to the observations.

Refined fundamental parameters of Canopus from combined near-IR interferometry and spectral energy distribution

Context. Canopus, the brightest and closest yellow supergiant to our Solar System, offers a unique laboratory for understanding the physics of evolved massive stars. Aims. We aim at quantitatively exploring a large space of fundamental parameters of Canopus based on the combined analysis of its spectral energy distribution (SED) and optical-IR long baseline interferometry. Methods. We use the most recent high resolution near-IR data from the VLTI focal beam combiners PIONIER (H and K bands) and AMBER (K band), together with precise spectrophotometric measures that cover the SED of Canopus, from the UV to the IR, taken from ground and space observatories. Results. The accurate and precise PIONIER data allowed us to simultaneously measure the angular diameter and the limb darkening (LD) profile using different analytical laws. We found that the power-law LD, being also in agreement with predictions from stellar atmosphere models, reproduces the interferometric data well. For this model we measured an angular diameter of 7.184 ± 0.0017 ± 0.029 mas and an LD coefficient of 0.1438 ± 0.0015, which are respectively ≳5 and ~15−25 more precise than in our previous A&A paper on Canopus from 2008. From a dedicated analysis of the interferometric data, we also provide new constraints on the putative presence of weak surface inhomogeneities. Additionally, we analyzed the SED in a innovative way by simultaneously fitting the reddening-related parameters and the stellar effective temperature and gravity. We find that a model based on two effective temperatures is much better at reproducing the whole SED, from which we derived several parameters, including a new bolometric flux estimate: fbol = (59.22 ± 2.45) × 10−6 erg cm−2 s−1. We were also able to estimate the stellar mass from these measurements, and it is shown to be in agreement with additional predictions from evolutionary models, from which we inferred the age of Canopus as well. Conclusions. The Canopus angular diameter and LD measured in this work with PIONIER are the most precise to date, with a direct impact on several related fundamental parameters. Moreover, thanks to our joint analysis, we were able to determine a set of fundamental parameters that simultaneously reproduces both high-precision interferometric data and a good quality SED and, at the same time, agrees with stellar evolution models. This refined set of fundamental parameters constitutes a careful balance between the different methodologies used, providing invaluable observationally based constraints to models of stellar structure and evolution, which still present difficulties in simulating stars such as Canopus in detail.

Laboratory Observation of C and O Emission Lines of the White Dwarf H1504+65-like Atmosphere Model

Abstract White dwarfs play important roles in stellar evolution and help us gauge the age of our galaxy. The white dwarf H1504+65, the hottest known post-asymptotic giant branch star, is peculiar due to its C- and O-rich but He- and H- deficient atmosphere whose composition cannot be well predicted by current stellar evolution models. The analysis of the elemental abundance and the benchmark of stellar atmospheric models depends heavily on spectral data under cosmic conditions, which are currently extremely scarce. We created a well-defined, uniform, relatively large-scale ∼millimeter plasma sample in the laboratory with a temperature and a C/O ratio similar to those of H1504+65’s atmosphere. The emission spectra with high precision in the range of 10–80 nm were obtained and identified according to databases such as NIST and Kelly. A detailed comparison between our emission lines and the Chandra-observed white dwarf H1504+65 atmosphere’s absorption lines was performed. The stongly isolated O VI lines in the range of 10–13 nm are observed in both cases. We observed a wealth of O V lines in the range of 13–14 nm that cannot be well identified or predicted by models due to the weak flux and also probably due to the blending effect of Fe group elements in the Chandra spectrum. Long-wavelength lines ranging from 14 to 80 nm, which are not observed in the Chandra spectrum because of the high interstellar neutral hydrogen column density, show abundant O IV-V, C IV lines, and strong O VI lines. Moreover, the intensities of the lines at 62.973 and 17.216 nm are analyzed to characterize the plasma temperature.

MPS-ATLAS: A fast all-in-one code for synthesising stellar spectra

Context. Stellar spectral synthesis is essential for various applications, ranging from determining stellar parameters to comprehensive stellar variability calculations. New observational resources as well as advanced stellar atmosphere modelling, taking three dimensional effects from radiative magnetohydrodynamics calculations into account, require a more efficient radiative transfer. Aims. For accurate, fast and flexible calculations of opacity distribution functions (ODFs), stellar atmospheres, and stellar spectra, we developed an efficient code building on the well-established ATLAS9 code. The new code also paves the way for easy and fast access to different elemental compositions in stellar calculations. Methods. For the generation of ODF tables, we further developed the well-established DFSYNTHE code by implementing additional functionality and a speed-up by employing a parallel computation scheme. In addition, the line lists used can be changed from Kurucz’s recent lists. In particular, we implemented the VALD3 line list. Results. A new code, the Merged Parallelised Simplified ATLAS, is presented. It combines the efficient generation of ODF, atmosphere modelling, and spectral synthesis in local thermodynamic equilibrium, therefore being an all-in-one code. This all-in-one code provides more numerical functionality and is substantially faster compared to other available codes. The fully portable MPS-ATLAS code is validated against previous ATLAS9 calculations, the PHOENIX code calculations, and high-quality observations.

ISOSCELES: Grid of stellar atmosphere and hydrodynamic models of massive stars. The first results

AbstractIn this work we seek to derive simultaneously the stellar and wind parameters of massive stars, mainly A and B type supergiant stars. Our stellar properties encompass the effective temperature, the surface gravity, the micro-turbulence velocity and, silicon abundance. For wind properties we consider the line–force parameters (α, k and δ) obtained from the standard line-driven wind theory. To model the data we use the radiative transport code Fastwind considering the hydrodynamic solutions derived with the stationary code Hydwind. Then, ISOSCELES, a grid of stellar atmosphere and hydrodynamic models of massive stars is created. Together with the observed spectra and a semi-automatic tool the physical properties from these stars are determined through spectral line fittings. This quantitative spectroscopic analysis provide an estimation about the line–force parameters. In addition, we confirm that the hydrodynamic solutions, called δ-slow solutions, describe quite reliable the radiation line-driven winds of B supergiant stars.

RGB photometric calibration of 15 million Gaia stars

ABSTRACT Although a catalogue of synthetic RGB magnitudes, providing photometric data for a sample of 1346 bright stars, has been recently published, its usefulness is still limited due to the small number of reference stars available, considering that they are distributed throughout the whole celestial sphere, and the fact that they are restricted to Johnson V < 6.6 mag. This work presents synthetic RGB magnitudes for ∼15 million stars brighter than Gaia G = 18 mag, making use of a calibration between the RGB magnitudes of the reference bright star sample and the corresponding high-quality photometric G, GBP, and GRP magnitudes provided by the Gaia EDR3. The calibration has been restricted to stars exhibiting −0.5 < GBP − GRP < 2.0 mag, and aims to predict RGB magnitudes within an error interval of ±0.1 mag. Since the reference bright star sample is dominated by nearby stars with slightly undersolar metallicity, systematic variations in the predictions are expected, as modelled with the help of stellar atmosphere models. These deviations are constrained to the ±0.1-mag interval when applying the calibration only to stars scarcely affected by interstellar extinction and with metallicity compatible with the median value for the bright star sample. The large number of Gaia sources available in each region of the sky should guarantee high-quality RGB photometric calibrations.

Extended envelopes around Galactic Cepheids

Aims. We aim to investigate the infrared excess of 45 Milky Way (MW) Cepheids combining different observables in order to constrain the presence of circumstellar envelopes (CSEs). Methods. We used the SpectroPhoto-Interferometry of Pulsating Stars (SPIPS) algorithm, a robust implementation of the parallax-of-pulsation method that combines photometry, angular diameter, stellar effective temperature, and radial velocity measurements in a global modelling of the pulsation of the Cepheid. We obtained new photometric measurements at mid-infrared (mid-IR) with the VISIR instrument at the Very Large Telescope complemented with data gathered from the literature. We then compared the mean magnitude of the Cepheids from 0.5 μm to 70 μm with stellar atmosphere models to infer the IR excess, which we attribute to the presence of a circumstellar envelope. Results. We report that at least 29% of the Cepheids of our sample have a detected IR excess (> 3σ). We estimated a mean excess of 0.08 ± 0.04 mag at 2.2 μm and 0.13 ± 0.06 mag at 10 μm. Other Cepheids possibly also have IR excess, but they were rejected due to their low detection level compared to a single-star model. We do not see any correlation between the IR excess and the pulsation period as previously suspected for MW Cepheids, but a rather constant trend at a given wavelength. We also do not find any correlation between the CO absorption and the presence of a CSE, but rather with the stellar effective temperature, which confirms that the CO features previously reported are mostly photospheric. No bias caused by the presence of the circumstellar material is detected on the average distance estimates from a SPIPS analysis with a fitted colour excess. We also do not find correlation between the presence of IR excess and the evolution stage of the Cepheids. Conclusions. We report a fraction of 29% of Cepheids with an IR excess likely produced by the circumstellar envelope surrounding the stars. Longer period Cepheids do not exhibit greater excess than short periods as previously suspected from observations and theoretical dusty-wind models. Other mechanisms such as free-free emission, among others, may be at the origin of the formation of the CSEs. We also show that not fitting the colour excess leads to a bias on the distance estimates in our Galaxy.

Radiative Transfer with Opacity Distribution Functions: Application to Narrowband Filters

Abstract Modeling of stellar radiative intensities in various spectral passbands plays an important role in stellar physics. At the same time, direct calculation of the high-resolution spectrum and then integration of it over the given spectral passband is computationally demanding due to the vast number of atomic and molecular lines. This is particularly so when employing three-dimensional (3D) models of stellar atmospheres. To accelerate the calculations, one can employ approximate methods, e.g., the use of opacity distribution functions (ODFs). Generally, ODFs provide a good approximation of traditional spectral synthesis, i.e., computation of intensities through filters with strictly rectangular transmission functions. However, their performance strongly deteriorates when the filter transmission noticeably changes within its passband, which is the case for almost all filters routinely used in stellar physics. In this context, the aims of this paper are (a) to generalize the ODFs method for calculating intensities through filters with arbitrary transmission functions, and (b) to study the performance of the standard and generalized ODFs methods for calculating intensities emergent from 3D models of stellar atmospheres. For this purpose we use the newly developed MPS-ATLAS radiative transfer code to compute intensities emergent from 3D cubes simulated with the radiative magnetohydrodynamics code MURaM. The calculations are performed in the 1.5D regime, i.e., along many parallel rays passing through the simulated cube. We demonstrate that the generalized ODFs method allows accurate and fast syntheses of spectral intensities and their center-to-limb variations.

An Empirical Bayesian Approach to Limb Darkening in Modeling WASP-121b Transit Light Curves

Abstract We present a novel, iterative method using an empirical Bayesian approach for modeling the limb-darkened WASP-121b transit from the TESS light curve. Our method is motivated by the need to improve R p /R * estimates for exoplanet atmosphere modeling and is particularly effective with the limb-darkening (LD) quadratic law requiring no prior central value from stellar atmospheric models. With the nonlinear LD law, the method has all the advantages of not needing atmospheric models but does not converge. The iterative method gives a different R p /R * for WASP-121b at a significance level of 1σ when compared with existing noniterative methods. To assess the origins and implications of this difference, we generate and analyze light curves with known values of the LD coefficients (LDCs). We find that noniterative modeling with LDC priors from stellar atmospheric models results in an inconsistent R p /R * at a 1.5σ level when the known LDC values are the same as those previously found when modeling real data by the iterative method. In contrast, the LDC values from the iterative modeling yield the correct value of R p /R * to within 0.25σ. For more general cases with different known inputs, Monte Carlo simulations show that the iterative method obtains unbiased LDCs and correct R p /R * to within a significance level of 0.3σ. Biased LDC priors can cause biased LDC posteriors and lead to bias in the R p /R * of up to 0.82%, 2.5σ for the quadratic law and 0.32%, 1.0σ for the nonlinear law. Our improvement in R p /R * estimation is important when analyzing exoplanet atmospheres.

Abundances of neutron-capture elements in thin- and thick-disc stars in the solar neighbourhood

Aims. The aim of this work is to determine abundances of neutron-capture elements for thin- and thick-disc F, G, and K stars in several selected sky fields near the north ecliptic pole and to compare the results with the Galactic chemical evolution models, to explore elemental gradients according to stellar ages, mean galactocentric distances, and maximum heights above the Galactic plane. Methods. The observational data were obtained with the 1.65 m telescope at the Molėtai Astronomical Observatory and a fibre-fed high-resolution spectrograph covering a full visible wavelength range (4000−8500 Å). Elemental abundances were determined using a differential line-by-line spectrum synthesis using the TURBOSPECTRUM code with the MARCS stellar model atmospheres and accounting for the hyperfine-structure effects. Results. We determined abundances of Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, and Eu for 424 thin- and 82 thick-disc stars. The sample of thick-disc stars shows a clearly visible decrease in [Eu/Mg] with increasing metallicity compared to the thin-disc stars, bringing more evidence of a different chemical evolution in these two Galactic components. Abundance correlation with age slopes for the investigated thin-disc stars are slightly negative for the majority of s-process dominated elements, while r-process dominated elements have positive correlations. Our sample of thin-disc stars with ages spanning from 0.1 to 9 Gyr gives the [Y/Mg] = 0.022 (±0.015)−0.027 (±0.003)⋅age [Gyr] relation. However, for the thick-disc stars, when we also took data from other studies into account, we found that [Y/Mg] cannot serve as an age indicator. The radial abundance-to-iron gradients in the thin disc are negligible for the s-process dominated elements and become positive for the r-process dominated elements. The vertical gradients are negative for the light s-process dominated elements and become positive for the r-process dominated elements. In the thick disc, the radial abundance-to-iron slopes are negligible, and the vertical slopes are predominantly negative.

The First Radio Spectrum of a Rapidly Rotating A-type Star

Abstract The radio spectra of main-sequence stars remain largely unconstrained due to the lack of observational data to inform stellar atmosphere models. As such, the dominant emission mechanisms at long wavelengths, how they vary with spectral type, and how much they contribute to the expected brightness at a given radio wavelength are still relatively unknown for most spectral types. We present radio continuum observations of Altair, a rapidly rotating A-type star. We observed Altair with NOEMA in 2018 and 2019 at 1.34, 2.09, and 3.22 mm and with the Very Large Array in 2019 at 6.7 and 9.1 mm. In the radio spectra, we see a brightness temperature minimum at millimeter wavelengths followed by a steep rise to temperatures larger than the optical photosphere, behavior that is unexpected for A-type stars. We use these data to produce the first submillimeter to centimeter spectrum of a rapidly rotating A-type star informed by observations. We generated both PHOENIX and KINICH-PAKAL model atmospheres and determine the KINICH-PAKAL model better reproduces Altair’s radio spectrum. The synthetic spectrum shows a millimeter brightness temperature minimum followed by significant emission over that of the photosphere at centimeter wavelengths. Together, these data and models show how the radio spectrum of an A-type star can reveal the presence of a chromosphere, likely induced by rapid rotation, and that a Rayleigh Jean’s extrapolation of the stellar photosphere is not an adequate representation of a star’s radio spectrum.

Forward modelling of Kepler-band variability due to faculae and spots

ABSTRACT Variability observed in photometric light curves of late-type stars (on time-scales longer than a day) is a dominant noise source in exoplanet surveys and results predominantly from surface manifestations of stellar magnetic activity, namely faculae and spots. The implementation of faculae in light-curve models is an open problem, with scaling typically based on spectra equivalent to hot stellar atmospheres or assuming a solar-derived facular contrast. We modelled rotational (single period) light curves of active G2, K0, M0, and M2 stars, with Sun-like surface distributions and realistic limb-dependent contrasts for faculae and spots. The sensitivity of light-curve variability to changes in model parameters such as stellar inclination, feature area coverage, spot temperature, facular region magnetic flux density, and active band latitudes is explored. For our light-curve modelling approach we used actress, a geometrically accurate model for stellar variability. actress generates two-sphere maps representing stellar surfaces and populates them with user-prescribed spot and facular region distributions. From this, light curves can be calculated at any inclination. Quiet star limb darkening and limb-dependent facular contrasts were derived from MURaM 3D magnetoconvection simulations using ATLAS9. 1D stellar atmosphere models were used for the spot contrasts. We applied actress in Monte Carlo simulations, calculating light-curve variability amplitudes in the Kepler band. We found that, for a given spectral type and stellar inclination, spot temperature and spot area coverage have the largest effect on variability of all simulation parameters. For a spot coverage of $1{{\ \rm per\ cent}}$, the typical variability of a solar-type star is around 2 parts per thousand. The presence of faculae clearly affects the mean brightness and light-curve shape, but has relatively little influence on the variability.

Stark broadening of spectral lines for zirconium ions Zr iv−vi in hot white dwarf atmospheres

ABSTRACT Stark widths for 31 spectral lines of the zirconium ions Zr iv, Zr v and Zr vi are calculated using a quantum mechanical method, and for five other lines they are deduced from a linear fit. Among these lines, 23 are provided for the first time. The motivation for this work was the recent identification of 25 lines of zirconium ions in the UV spectrum of the hot white dwarf RE 0503−289, which have never been detected before in hot white dwarfs. The results are presented for the plasma conditions found in hot white dwarf atmospheres. To the best of our knowledge, Stark broadening calculations have only been carried out for the Zr iv ion. Therefore, in addition to the quantum calculations, we perform semiclassical perturbation calculations for Zr iv lines and compare them with previous results, taking into account the effects of strong and elastic collisions on Stark broadening. Furthermore, we compare the Stark broadening with the Doppler broadening in order to ascertain their relative importance in the atmospheric conditions of DO white dwarfs. These results, together with atomic radiative data, are useful for non-local thermodynamic equilibrium stellar-atmosphere modelling.