Stellar Atmospheric Parameters Research Articles

SPECTROSCOPIC STUDY OF BARIUM STARS

We present the abundance results from the high-resolution spectral analysis of fifteen barium stars. The spectral resolution is R ~ 86000 with a wave length coverage of 3900 – 9000˚A. Stellar atmospheric parameters the effective temperature, surface gravity, metallic ties are determined from the local thermodynamic equilibrium analys is using model atmospheres. The effective temperature of the programme stars ranges from 3535 – 7656 K. The surface gravity covers a range from 1.17 – 4.07 cgs and metallicity ranges from -1.41 to - 0.13.We have also obtained temperature from photometric calculations and log g from parallax method. These values are found to match closely with the spectroscopic values within errors. The elemental abundances are determined from the spectrum synthesis calculation of clean and unblended lines due to neutral and ionized elements using iSpec. The abundances of elements such as O, Na, Mg, Ca, Sc, Ti, Mn, Ni, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Pr, Sm and Eu are determined. The star HD 22589 shows enhancement in Ba with [Ba/Fe] > 1.33. HD 40430 is found to be Ba normal with [Ba/Fe]0.10.The mass and the age of the programme stars are also determined from their locations in the H-R diagram. The estimated mass of the programmestars ranges from 1.20-4.50M☉ and age from 0.53to3.36Gyr

The Value-added Catalog for LAMOST DR8 Low-resolution Spectra

Abstract We present a value-added catalog containing stellar parameters estimated from 7.10 million low-resolution spectra for 5.16 million unique stars with spectral signal-to-noise ratios (S/N) higher than 10 obtained by the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) Galactic spectroscopic surveys. The catalog presents values of stellar atmospheric parameters (effective temperature T eff, surface gravity log g, metallicity [Fe/H]/[M/H]), α-element to metal abundance ratio [α/M], carbon and nitrogen to iron abundance ratios [C/Fe] and [N/Fe], and 14 bands’ absolute magnitudes deduced from LAMOST spectra using the neural network method. The spectrophotometric distances of those stars are also provided based on the distance modulus. For stars with a spectral S/N larger than 50, precisions of T eff, log g, [Fe/H], [M/H], [C/Fe], [N/Fe], and [α/M] are 85 K, 0.098 dex, 0.05 dex, 0.05 dex, 0.052 dex, 0.082 dex, and 0.027 dex, respectively. The errors of 14 band’s absolute magnitudes are 0.16–0.22 mag for stars with a spectral S/N larger than 50. The spectrophotometric distance is accurate to 8.5% for stars with a spectral S/N larger than 50 and is more accurate than the geometrical distance for stars with a distance larger than 2.0 kpc. Our estimates of [Fe/H] are reliable down to [Fe/H] ∼−3.5 dex, significantly better than previous results. The catalog provides 26,868 unique very metal-poor star candidates ([Fe/H] ≤−2.0). The catalog would be a valuable dataset to study the structure and evolution of the galaxy, especially the solar neighborhood and the outer disk.

Chemical Composition of Bright Stars in the Northern Hemisphere: Star–Planet Connection

Abstract In fulfilling the aims of the planetary and asteroseismic research missions, such as that of the NASA Transiting Exoplanet Survey Satellite (TESS) space telescope, accurate stellar atmospheric parameters and a detailed chemical composition are required as inputs. We have observed high-resolution spectra for all 848 bright (V < 8 mag) stars that are cooler than F5 spectral class in the area up to 12 deg surrounding the northern TESS continuous viewing zone and uniformly determined the main atmospheric parameters, ages, orbital parameters, velocity components, and precise abundances of up to 24 chemical species (C(C2), N(CN), [O i], Na i, Mg i, Al i, Si i, Si ii, Ca i, Ca ii, Sc i, Sc ii, Ti i, Ti ii, V i, Cr i, Cr ii, Mn i, Fe i, Fe ii, Co i, Ni i, Cu i, and Zn i) for 740 slowly rotating stars. The analysis of 25 planet-hosting stars in our sample drove us to the following conclusions: the dwarf stars hosting high-mass planets are more metal rich than those with low-mass planets. We find slightly negative C/O and Mg/Si slopes toward the stars with high-mass planets. All the low-mass planet hosts in our sample show positive Δ[El/Fe] versus condensation temperature slopes, in particular, the star with the largest number of various planets. The high-mass planet hosts have a diversity of slopes, but in more metal-rich, older, and cooler stars, the positive elemental abundance slopes are more common.

Estimation of Stellar Atmospheric Parameters with Light Gradient Boosting Machine Algorithm and Principal Component Analysis

Abstract In this paper, we propose a new method to estimate stellar atmospheric parameters with photometric data, which is based on principal component analysis (PCA) and light gradient boosting machine (LightGBM) algorithms. We first use PCA to transform all band photometric data (u, v, g, r, i, and z) and then utilize LightGBM to estimate stellar atmospheric parameters. The experimental results show that the root mean square errors of the method for estimating the effective temperature, surface gravity, and metallicity are 90 K, 0.40 dex, and 0.20 dex, respectively. We then compare PCA + LightGBM with the original photometry data (OPD) + LightGBM and the color index data (CID) + LightGBM. The experimental results show that the performance of PCA + LightGBM is better than that of CID + LightGBM and OPD + LightGBM, and PCA + LightGBM can solve the problems of model instability and inaccurate estimation results caused by direct use of OPD or CID as input for LightGBM. We believe the new features obtained by PCA can be used on photometric data collected by the Chinese Space Station Telescope.

J-PLUS: Stellar parameters, C, N, Mg, Ca, and [α/Fe] abundances for two million stars from DR1

Context. The Javalambre Photometric Local Universe Survey (J-PLUS) has obtained precise photometry in 12 specially designed filters for large numbers of Galactic stars. Deriving their precise stellar atmospheric parameters and individual elemental abundances is crucial for studies of Galactic structure and the assembly history and chemical evolution of our Galaxy. Aims. Our goal is to estimate not only stellar parameters (effective temperature, Teff, surface gravity, log g, and metallicity, [Fe/H]), but also [α/Fe] and four elemental abundances ([C/Fe], [N/Fe], [Mg/Fe], and [Ca/Fe]) using data from the first data release (DR1) of J-PLUS. Methods. By combining recalibrated photometric data from J-PLUS DR1, Gaia DR2, and spectroscopic labels from the Large sky Area Multi-Object fiber Spectroscopic Telescope, we designed and trained a set of cost-sensitive neural networks, the CSNet, to learn the nonlinear mapping from stellar colours to their labels. Special attention was paid to the poorly populated regions of the label space by giving different weights according to their density distribution. Results. We achieved precisions of δ Teff ∼ 55 K, δlog g ∼ 0.15 dex, and δ [Fe/H] ∼ 0.07 dex, respectively, over a wide range of temperatures, surface gravities, and metallicities. The uncertainties of the abundance estimates for [α/Fe] and the four individual elements are in the 0.04–0.08 dex range. We compare our parameter and abundance estimates with those from other spectroscopic catalogs such as the Apache Point Observatory for Galactic Evolution Experiment and the Galactic Archaeology with High Efficiency and Resolution Multi-Element Spectrograph and find an overall good agreement. Conclusions. Our results demonstrate the potential of well-designed, high-quality photometric data for determinations of stellar parameters as well as individual elemental abundances. Applying the method to J-PLUS DR1, we obtained the aforementioned parameters for about two million stars, providing an outstanding dataset for chemo-dynamic analyses of the Milky Way. The catalog of the estimated parameters is publicly accessible.

Survey for Distant Solar Twins (SDST) – I. epic method for stellar parameter measurement

ABSTRACT Solar twins are stars of key importance to the field of astronomy and offer a multitude of science applications. Only a small number (≲200) of solar twins are known today, all of which are relatively close to our Sun (${\lesssim}{800}\, {\rm pc}$). The goal of our Survey for Distant Solar Twins (SDST) is to identify many more solar twin and solar analogue stars out to much larger distances (${\sim}{4}\, {\rm kpc}$). In this paper, we present a new method to identify solar twins using relatively low S/N, medium resolving power ($R\sim 28\, 000$) spectra that will be typical of such distant targets observed with HERMES on the ${3.9}\, {\rm m}$ Anglo-Australian Telescope (AAT). We developed a novel approach, namely epic, to measure stellar parameters (SPs) which we use to identify stars similar to our Sun. epic determines the stellar atmospheric parameters (effective temperature Teff, surface gravity log g, and metallicity [Fe/H]) using differential equivalent width (EW) measurements of selected spectroscopic absorption features and a simple model, trained on previously analysed spectra, that connects these EWs to the SPs. The reference for the EW measurements is a high S/N solar spectrum which is used to minimize several systematic effects. epic is fast, optimized for Sun-like stars and yields SP measurements with small enough uncertainties to enable spectroscopic identification of solar twin and analogue stars up to ${\sim}{4}\, {\rm kpc}$ away using AAT/HERMES, i.e. $\sigma \left(T_{\mathrm{eff}}, \log g, \textrm {[Fe/H]}\right) = \left({50}\, {\rm K}, {0.08}\, {\rm dex}, {0.03}\, {\rm dex}\right)$ on average at S/N = 25.

The SAPP pipeline for the determination of stellar abundances and atmospheric parameters of stars in the core program of the PLATO mission

We introduce the SAPP (Stellar Abundances and atmospheric Parameters Pipeline), the prototype of the code that will be used to determine parameters of stars observed within the core program of the PLATO space mission. The pipeline is based on the Bayesian inference and provides effective temperature, surface gravity, metallicity, chemical abundances, and luminosity. The code in its more general version has a much wider range of potential applications. It can also provide masses, ages, and radii of stars and can be used with stellar types not targeted by the PLATO core program, such as red giants. We validate the code on a set of 27 benchmark stars that includes 19 FGK-type dwarfs, 6 GK-type subgiants, and 2 red giants. Our results suggest that combining various observables is the optimal approach, as this allows the degeneracies between different parameters to be broken and yields more accurate values of stellar parameters and more realistic uncertainties. For the PLATO core sample, we obtain a typical uncertainty of 27 (syst.) ± 37 (stat.) K for Teff, 0.00 ± 0.01 dex for log g, 0.02 ± 0.02 dex for metallicity [Fe/H], −0.01 ± 0.03 R⊙ for radii, −0.01 ± 0.05 M⊙ for stellar masses, and −0.14 ± 0.63 Gyr for ages. We also show that the best results are obtained by combining the νmax scaling relation with stellar spectra. This resolves the notorious problem of degeneracies, which is particularly important for F-type stars.

Dynamical parallax, physical parameters, and evolutionary status of the components of the bright eclipsing binary α Draconis

Aims. Both components of the bright eclipsing binary α Dra have been resolved using long baseline interferometry and the secondary component has been shown to contribute approximately 15% of the total flux; however, a spectroscopic detection of the companion star has so far been unsuccessful. We aim for a firm spectroscopic detection of the secondary component of α Dra using state-of-the-art spectroscopic analysis methods for very high-quality spectroscopic observations. This will allow the determination of fundamental and atmospheric properties of the components in the system with high precision and accuracy. Methods. To achieve our goals, we use a combined data set from interferometry with the Navy Precision Optical Interferometer (NPOI), photometry with the TESS space observatory, and high-resolution spectroscopy with the HERMES fibre-fed spectrograph at the La Palma observatory. We use the method of spectral disentangling to search for the contribution of a companion star in the observed composite HERMES spectra, to separate the spectral contributions of both components, and to determine orbital elements of the α Dra system. TESS light curves are analysed in an iterative fashion with spectroscopic inference of stellar atmospheric parameters to determine fundamental stellar properties and their uncertainties. Finally, NPOI interferometric measurements are used for determination of the orbital parameters of the system and angular diameters of both binary components. Results. We report the first firm spectroscopic detection of the secondary component in α Dra and deliver disentangled spectra of both binary components. The components’ masses and radii are inferred with high precision and accuracy, and are MA = 3.186 ± 0.044 M⊙, RA = 4.932 ± 0.036 R⊙, and MB = 2.431 ± 0.019 M⊙, RB = 2.326 ± 0.052 R⊙ for the primary and secondary components, respectively. Combined astrometric and spectroscopic analysis yields the semi-major axis of the system, which is ultimately used to derive the dynamical parallax of π = 11.48 ± 0.13 mas, and the distance d = 87.07 ± 1.03 pc to the α Dra system. Evolutionary analysis of both binary components with MESA stellar structure and evolution models suggests the primary is an evolved post-TAMS A-type star, while the companion is a main-sequence A-type star with a convective core mass of Mcc = 0.337 ± 0.011 M⊙. Positions of both binary components in the Kiel- and HR-diagrams suggest a value of the convective core overshooting parameter fov well below 0.010 Hp, and we infer the age of the system to be 310 ± 25 Myr. Conclusions. The inferred near-core mixing properties of both components do not support a dependence of the convective core overshooting on the stellar mass. At the same time, the α Dra system provides extra support to hypothesise that the mass discrepancy in eclipsing spectroscopic double-lined binaries is associated with inferior atmospheric modelling of intermediate- and high-mass stars, and less so with the predictive powerof stellar structure and evolution models as to the amount of near-core mixing and mass of the convective core.

Beyond Spectroscopy. I. Metallicities, Distances, and Age Estimates for Over 20 Million Stars from SMSS DR2 and Gaia EDR3

Abstract Accurate determinations of stellar parameters and distances for large complete samples of stars are keys for conducting detailed studies of the formation and evolution of our Galaxy. Here we present stellar atmospheric parameters (effective temperature, luminosity classifications, and metallicity) estimates for some 24 million stars determined from the stellar colors of SMSS DR2 and Gaia EDR3, based on training data sets with available spectroscopic measurements from previous high/medium/low-resolution spectroscopic surveys. The number of stars with photometric-metallicity estimates is 4–5 times larger than that collected by the current largest spectroscopic survey to date—LAMOST—over the course of the past decade. External checks indicate that the precision of the photometric-metallicity estimates are quite high, comparable to or slightly better than that derived from spectroscopy, with typical values around 0.05–0.15 dex for both dwarf and giant stars with [Fe/H] > −2.01.0, 0.10–0.20 dex for giant stars with −2.0 < [Fe/H] ≤ −1.0, and 0.20–0.25 dex for giant stars with [Fe/H] ≤ −2.0, and include estimates for stars as metal-poor as [Fe/H] ∼ −3.5, substantially lower than previous photometric techniques. Photometric-metallicity estimates are obtained for an unprecedented number of metal-poor stars, including a total of over three million metal-poor (MP; [Fe/H] ≤ −1.0) stars, over half a million very metal-poor (VMP; [Fe/H] ≤ −2.0) stars, and over 25,000 extremely metal-poor (EMP; [Fe/H] ≤ −3.0) stars. Moreover, distances are determined for over 20 million stars in our sample. For the over 18 million sample stars with accurate Gaia parallaxes, stellar ages are estimated by comparing with theoretical isochrones. Astrometric information is provided for the stars in our catalog, along with radial velocities for ∼10% of our sample stars, taken from completed/ongoing large-scale spectroscopic surveys.

STEPARSYN: A Bayesian code to infer stellar atmospheric parameters using spectral synthesis

Context. STEPARSYN is an automatic code written in Python 3.X designed to infer the stellar atmospheric parameters Teff, log g, and [Fe/H] of FGKM-type stars following the spectral synthesis method. Aims. We present a description of the STEPARSYN code and test its performance against a sample of late-type stars that were observed with the HERMES spectrograph mounted at the 1.2-m Mercator Telescope. This sample contains 35 late-type targets with well-known stellar parameters determined independently from spectroscopy. The code is available to the astronomical community in a GitHub repository. Methods. STEPARSYN uses a Markov chain Monte Carlo sampler to explore the parameter space by comparing synthetic model spectra generated on the fly to the observations. The synthetic spectra are generated with an spectral emulator. Results. We computed Teff, log g, and [Fe/H] for our sample stars and discussed the performance of the code. We calculated an internal scatter for these targets of −12 ± 117 K in Teff, 0.04 ± 0.14 dex in log g, and 0.05 ± 0.09 dex in [Fe/H]. In addition, we find that the log g values obtained with STEPARSYN are consistent with the trigonometric surface gravities to the 0.1 dex level. Finally, STEPARSYN can compute stellar parameters that are accurate down to 50 K, 0.1 dex, and 0.05 dex for Teff, log g, and [Fe/H] for stars with v sin i ≤ 30 km s−1.

J-PLUS: Searching for very metal-poor star candidates using the SPEEM pipeline

Context. We explore the stellar content of the Javalambre Photometric Local Universe Survey (J-PLUS) Data Release 2 and show its potential for identifying low-metallicity stars using the Stellar Parameters Estimation based on Ensemble Methods (SPEEM) pipeline. Aims. SPEEM is a tool used to provide determinations of atmospheric parameters for stars and separate stellar sources from quasars based on the unique J-PLUS photometric system. The adoption of adequate selection criteria allows for the identification of metal-poor star candidates that are suitable for spectroscopic follow-up investigations. Methods. SPEEM consists of a series of machine-learning models that use a training sample observed by both J-PLUS and the SEGUE spectroscopic survey. The training sample has temperatures, Teff, between 4800 K and 9000 K, values of log g between 1.0 and 4.5, as well as −3.1 < [Fe/H] < +0.5. The performance of the pipeline was tested with a sample of stars observed by the LAMOST survey within the same parameter range. Results. The average differences between the parameters of a sample of stars observed with SEGUE and J-PLUS, obtained with the SEGUE Stellar Parameter Pipeline and SPEEM, respectively, are ΔTeff ~ 41 K, Δlog g ~ 0.11 dex, and Δ[Fe/H] ~ 0.09 dex. We define a sample of 177 stars that have been identified as new candidates with [Fe/H] < −2.5, with 11 of them having been observed with the ISIS spectrograph at the William Herschel Telescope. The spectroscopic analysis confirms that 64% of stars have [Fe/H] < −2.5, including one new star with [Fe/H] < −3.0. Conclusions. Using SPEEM in combination with the J-PLUS filter system has demonstrated their potential in estimating the stellar atmospheric parameters (Teff, log g, and [Fe/H]). The spectroscopic validation of the candidates shows that SPEEM yields a success rate of 64% on the identification of very metal-poor star candidates with [Fe/H] < −2.5.

The CARMENES search for exoplanets around M dwarfs

We determined effective temperatures, surface gravities, and metallicities for a sample of 343 M dwarfs observed with CARMENES, the double-channel, high-resolution spectrograph installed at the 3.5 m telescope at Calar Alto Observatory. We employed STEPARSYN, a Bayesian spectral synthesis implementation particularly designed to infer the stellar atmospheric parameters of late-type stars following a Markov chain Monte Carlo approach. We made use of the BT-Settl model atmospheres and the radiative transfer code turbospectrum to compute a grid of synthetic spectra around 75 magnetically insensitive Fe I and Ti I lines plus the TiO γ and ϵ bands. To avoid any potential degeneracy in the parameter space, we imposed Bayesian priors on Teff and logg based on the comprehensive, multi-band photometric data available for the sample. We find that this methodology is suitable down to M7.0 V, where refractory metals such as Ti are expected to condense in the stellar photospheres. The derived Teff, logg, and [Fe/H] range from 3000 to 4200 K, 4.5 to 5.3 dex, and −0.7 to 0.2 dex, respectively. Although our Teff scale is in good agreement with the literature, we report large discrepancies in the [Fe/H] scales, which might arise from the different methodologies and sets of lines considered. However, our [Fe/H] is in agreement with the metallicity distribution of FGK-type stars in the solar neighbourhood and correlates well with the kinematic membership of the targets in the Galactic populations. Lastly, excellent agreement in Teff is found for M dwarfs with interferometric angular diameter measurements, as well as in the [Fe/H] between the components in the wide physical FGK+M and M+M systems included in our sample.

Orbital parameters of the hierarchical triplet system CN Lyn

ABSTRACT CN Lyn is an intriguing triplet system that contains three equal-mass components. However, the orbital parameters of this rare triple system are still unknown. By monitoring the system since 2016 October 19, 13 eclipse times have been obtained. We discover that the observed minus calculated (O − C) diagram of CN Lyn shows a cyclical variation with a period of 15.80 (± 0.40) years and a projected semi-major axis of 0.0074 (± 0.0008) days. The orbital properties of the third body are investigated by analysing the light travel time effect. It is found that the third body is orbiting the central binary in an eccentric orbit (e3 ∼ 0.38 (± 0.18)). The orbital inclination is determined to be i3 = 25°, which reveals that the third body is non-coplanar to the orbit of the central eclipsing binary. This is the reason why investigators did not detect a clear periodicity in the radial velocities of the third body. By analysing one low-dispersion spectrum obtained with the 2.16-m telescope in Xinglong station, stellar atmospheric parameters of CN Lyn were derived. It is shown that the system has a low metallicity that is only about 21.5 per cent of the solar metallicity. The rare composition, eccentric orbit, low metallicity, and low orbital inclination of the triplets all indicate that the CN Lyn system can provide valuable information on the formation of multiple star systems, a matter of active research and debate.

GSP-spec line list for the parametrisation of Gaia-RVS stellar spectra

Context. The Gaia mission is a magnitude-limited whole-sky survey that collects an impressive quantity of astrometric, spectro-photometric and spectroscopic data. Among all the on-board instruments, the Radial Velocity Spectrometer (RVS) produces millions of spectra up to a magnitude of GRVS ~ 16. For the brightest RVS targets, stellar atmospheric parameters and individual chemical abundances are automatically estimated by the Generalized Stellar Parametriser – spectroscopy group (GSP-Spec). These data will be published with the third Gaia Data Release. Aims. Some major ingredients of the determination of these stellar parameters include the atomic and molecular line lists that are adopted to compute reference synthetic spectra, on which the parametrisation methods rely. We aim to build such a specific line list optimised for the analysis of RVS late-type star spectra. Methods. Starting from the Gaia-ESO line lists, we first compared the observed and synthetic spectra of six well-known reference late-type stars in the wavelength range covered by the RVS instrument. We then improved the quality of the atomic data for the transitions presenting the largest mismatches. Results. The new line list is found to produce very high-quality synthetic spectra for the tested reference stars and has thus been adopted within GSP-Spec. We note, however, that a couple of atomic line profiles, in particular the calcium infrared triplet lines, still show some deviations compared to the reference spectra, probably because of the adopted line-transfer assumptions (local thermodynamical equilibrium, hydrostatic, and no chromosphere). Future works should focus on such lines and should extend the present work towards OBA and M-type stellar spectra.

Improve the Search of Very Metal-poor Stars Using the Deep Learning Method

Abstract Very metal-poor (VMP) stars have [Fe/H] < −2.0 dex. They are among the oldest stars in the universe, and their unique metallicity can help explore the enrichment mechanism and evolutionary history of the chemical elements of stars in the early universe. However, most current stellar parameter estimation methods do not perform well in determining the stellar parameters of VMP stars, which limits our ability to discover and exploit the properties of VMP stars. In this study, we propose a new model based on a convolutional neural network to determine the stellar atmospheric parameters of VMP stars. We tested our model on the LAMOST spectra; our model can determine the effective temperature (T eff), surface gravity (log g), metallicity ([Fe/H]), and carbon abundance ([C/Fe]) of the LAMOST spectra with precisions σ(T eff) = 134.82 K, σ ( log g ) = 0.33 dex, σ ([Fe/H]) = 0.20 dex, and σ([C/Fe]) = 0.35 dex. Furthermore, our model can distinguish VMP stars from normal stars with an accuracy of 88.65%. We also compared this model with other widely used methods, and found that this method performs better than other methods. It can be applied to the stellar parameter pipelines of upcoming large surveys such as 4MOST, WEAVES, and MOONS to search for VMP stars and identify carbon-enhanced metal-poor stars.

Detailed elemental abundances of binary stars: searching for signatures of planet formation and atomic diffusion

ABSTRACT Binary star systems are assumed to be co-natal and coeval, and thus to have identical chemical composition. In this work, we aim to test the hypothesis that there is a connection between observed element abundance patterns and the formation of planets using binary stars. Moreover, we also want to test how atomic diffusion might influence the observed abundance patterns. We conduct a strictly line-by-line differential chemical abundance analysis of seven binary systems. Stellar atmospheric parameters and elemental abundances are obtained with extremely high precision (<3.5 per cent) using the high-quality spectra from Very Large Telescope/ultraviolet–visual Echelle spectrograph and Keck/high-resolution Echelle spectrometer. We find that four of seven binary systems show subtle abundance differences (0.01–0.03 dex) without clear correlations with the condensation temperature, including two planet-hosting pairs. The other three binary systems exhibit similar degree of abundance differences correlating with the condensation temperature. We do not find any clear relation between the abundance differences and the occurrence of known planets in our systems. Instead, the overall abundance offsets observed in the binary systems (four of seven) could be due to the effects of atomic diffusion. Although giant planet formation does not necessarily imprint chemical signatures on to the host star, the differences in the observed abundance trends with condensation temperature, on the other hand, are likely associated with diverse histories of planet formation (e.g. formation location). Furthermore, we find a weak correlation between abundance differences and binary separation, which may provide a new constraint on the formation of binary systems.

TITANS metal-poor reference stars

Context. Several large stellar spectroscopic surveys are producing enormous amounts of data that can be used to determine stellar atmospheric parameters and chemical abundances. Nonetheless, the accuracy achieved in the derived astrophysical parameters is still insufficient, mainly because of the paucity of adequate calibrators, particularly in the metal-poor regime ([Fe/H] ≤−1.0). Aims. Our aim is to increase the number of metal-poor stellar calibrators that have accurate parameters. Here, we introduce the TITANS metal-poor reference stars: a sample of 41 dwarf and subgiant stars with accurate, but model-dependent, parameters. Methods. Effective temperatures (Teff) were derived by fitting observed Hα profiles with synthetic lines computed using three-dimensional (3D) hydrodynamic model atmospheres that take local thermodynamic equilibrium departures into account (non-LTE effects). Surface gravities (log g) were computed using evolutionary tracks and parallaxes from Gaia early-data release 3. Results. The same methods recover the Teff values of the Gaia benchmark stars, which are mostly based on interferometric measurements, with a 1σ dispersion of ±50 K. We assume this to be the accuracy of the Hα profiles computed from 3D non-LTE models for metal-poor dwarfs and subgiants, although this is likely an upper-bound estimate dominated by the uncertainty of the standard Teff values. We achieved an internal precision typically between 30 and 40 K; these errors are dominated by instrumental effects. The final total uncertainty for the Teff values of the TITANS are thus estimated to be of the order of 1%. The typical error for log g is ≤0.04 dex. In addition, we identified a few members of Gaia-Enceladus, of Sequoia, and of the Helmi stream in our sample. These stars can pave the way for the accurate chemical characterisation of these Galactic substructures. Conclusions. Using the TITANS as a reference, large stellar surveys will be able to improve the internal calibration of their astrophysical parameters. Ultimately, this sample will help users of data from Gaia and large surveys to reach their goal of redefining our understanding of stars, stellar systems, and the Milky Way.

Spectroscopic study of CEMP-(s & r/s) stars

Context. The origin of the enhanced abundances of both s- and r-process elements observed in a subclass of carbon-enhanced metal-poor (CEMP) stars, denoted CEMP-r/s stars, still remains poorly understood. The i-process nucleosynthesis has been suggested as one of the most promising mechanisms for the origin of these stars. Aims. Our aim is to better understand the chemical signatures and formation mechanism(s) of five previously claimed potential CH star candidates HE 0017+0055, HE 2144−1832, HE 2339−0837, HD 145777, and CD−27 14351 through a detailed systematic follow-up spectroscopic study based on high-resolution spectra. Methods. The stellar atmospheric parameters, the effective temperature Teff, the microturbulent velocity ζ, the surface gravity log g, and the metallicity [Fe/H] are derived from local thermodynamic equilibrium analyses using model atmospheres. Elemental abundances of C, N, α-elements, iron-peak elements, and several neutron-capture elements are estimated using the equivalent width measurement technique as well as spectrum synthesis calculations in some cases. In the context of the double enhancement observed in four of the programme stars, we have critically examined whether the literature i-process model yields ([X/Fe]) of heavy elements can explain the observed abundance distribution. Results. The estimated metallicity [Fe/H] of the programme stars ranges from −1.63 to −2.74. All five stars show enhanced abundance for Ba, and four of them exhibit enhanced abundance for Eu. Based on our analysis, HE 0017+0055, HE 2144−1832, and HE 2339−0837 are found to be CEMP-r/s stars, whereas HD 145777 and CD−27 14351 show characteristic properties of CEMP-s stars. From a detailed analysis of different classifiers of CEMP stars, we have identified the one which best describes the CEMP-s and CEMP-r/s stars. We found that for both CEMP-s and CEMP-r/s stars, [Ba/Eu] and [La/Eu] exhibit positive values and [Ba/Fe] ≥ 1.0. However, CEMP-r/s stars satisfy [Eu/Fe] ≥ 1.0, 0.0 ≤ [Ba/Eu] ≤ 1.0, and/or 0.0 ≤ [La/Eu] ≤ 0.7. CEMP-s stars normally show [Eu/Fe] < 1.0 with [Ba/Eu] > 0.0 and/or [La/Eu] > 0.5. If [Eu/Fe] ≥ 1.0, then the condition on [Ba/Eu] and/or [La/Eu] for a star to be a CEMP-s star is [Ba/Eu] > 1.0 and/or [La/Eu] > 0.7. Using a large sample of similar stars from the literature we have examined whether the ratio of heavy-s to light-s process elements [hs/ls] alone can be used as a classifier, and if there are any limiting values for [hs/ls] that can be used to distinguish between CEMP-s and CEMP-r/s stars. Even though they peak at different values of [hs/ls], CEMP-s and CEMP-r/s stars show an overlap in the range 0.0 < [hs/ls] < 1.5, and hence this ratio cannot be used to distinguish between CEMP-s and CEMP-r/s stars. We have noticed a similar overlap in the case of [Sr/Ba] as well, in the range −1.6 < [Sr/Ba] < −0.5, and hence this ratio also cannot be used to separate the two subclasses.

Targeting Bright Metal-poor Stars in the Disk and Halo Systems of the Galaxy

Abstract We present the results of spectroscopic follow-up for 1897 low-metallicity star candidates, selected from the Best & Brightest (B&B) Survey, carried out with the GMOS-N/S (Gemini North/South telescopes) and Goodman (SOAR Telescope) spectrographs. From these low-resolution (R ∼ 2000) spectra, we estimate stellar atmospheric parameters, as well as carbon and magnesium abundance ratios. We confirm that 56% of our program stars are metal-poor ([Fe/H] < − 1.0), 30% are very metal-poor (VMP; [Fe/H] < − 2.0), and 2% are extremely metal-poor (EMP; [Fe/H] < − 3.0). There are 191 carbon-enhanced metal-poor (CEMP) stars, resulting in CEMP fractions of 19% and 43% for the VMP and EMP regimes, respectively. A total of 94 confirmed CEMP stars belong to Group I (A(C) ≳ 7.25) and 97 to Group II (A(C) ≲ 7.25) in the Yoon–Beers A(C)−[Fe/H] diagram. Moreover, we combine these data with Gaia EDR3 astrometric information to delineate new target-selection criteria, which have been applied to the Goodman/SOAR candidates, to more than double the efficiency for identification of bona fide VMP and EMP stars in comparison to random draws from the B&B catalog. We demonstrate that this target-selection approach can achieve success rates of 96%, 76%, 28%, and 4% for [Fe/H] ≤ − 1.5, ≤ − 2.0, ≤ − 2.5 and ≤ − 3.0, respectively. Finally, we investigate the presence of dynamically interesting stars in our sample. We find that several VMP/EMP ([Fe/H] ≤ − 2.5) stars can be associated with either the disk system or halo substructures like Gaia-Sausage/Enceladus and Sequoia.

CSS J213934.3-050020: A New Double-mode High-amplitude δ Scuti-type Pulsating Star

CSS J213934.3-050020 (hereafter J213934) was originally found as an EW-type eclipsing binary star with a period of 0.225456 days by Catalina Real-time Transient Survey. With the stellar atmospheric parameters from the LAMOST, it is found that the temperature is much higher than those of normal EW-type binaries and does not follow the period–temperature relation of EW-type eclipsing binary stars. To study the physical properties of the variable star, new B and V light curves were obtained by using the 70 cm telescope at the Lijiang station of Yunnan Observatories. It is detected that this variable star is a high-amplitude δ Scuti-type pulsating star rather than an EW-type eclipsing binary. On the diagram, J213934 lies inside the classical Cepheid instability strip where most δ Scuti stars are located. The results of pulsating frequency analysis show the fundamental frequency f 0 = 6.8195 day−1, the first overtone f 1 = 8.8709 day−1 and 4 harmonics or linear combinations. The mass of J213934 is estimated as . The period is revised as 0.11272960 days by using 16 new determined times of light maximum. Meanwhile, it is discovered that its O − C curve shows an upward parabolic variation and a sinusoidal variation. The upward parabolic variation may be a part of a cyclic change or a long-time pulsation period increase caused by its evolution. The sinusoidal variation may be caused by the light-travel-time effect via the presence of a companion star.