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1. A grid of MARCS model atmospheres for S stars
ID:
ivo://CDS.VizieR/J/A+A/601/A10
Title:
A grid of MARCS model atmospheres for S stars
Short Name:
J/A+A/601/A10
Date:
21 Oct 2021
Publisher:
CDS
Description:
S-type stars are late-type giants whose atmospheres are enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing asymptotic giant branch. A grid of MARCS model atmospheres has been computed for S stars, covering the range 2700<=Teff(K)<=4000, 0.50<=C/O<0.99, 0<=logg<=5, [Fe/H]=0., -0.5dex, and [s/Fe]= 0, 1, and 2 dex (where the latter quantity refers to the global overabundance of s-process elements). The MARCS models make use of a new ZrO line list. Synthetic spectra computed from these models are used to derive photometric indices in the Johnson and Geneva systems, as well as TiO and ZrO band strengths. A method is proposed to select the model best matching any given S star, a non-trivial operation since the grid contains more than 3500 models covering a five-dimensional parameter space. The method is based on the comparison between observed and synthetic photometric indices and spectral band strengths, and has been applied on a vast subsample of the Henize sample of S stars. Our results confirm the old claim by Piccirillo (1980MNRAS.190..441P) that ZrO bands in warm S stars (Teff > 3200K) are not caused by the C/O ratio being close to unity, as traditionally believed, but rather by some Zr overabundance. The TiO and ZrO band strengths, combined with V-K and J-K photometric indices, are used to select Teff, C/O, [Fe/H] and [s/Fe]. The Geneva U-B_1 and B_2-V_1 indices (or any equivalent) are good at selecting the gravity. The defining spectral features of dwarf S stars are outlined, but none is found among the Henize S stars. More generally, it is found that, at Teff=3200K, a change of C/O from 0.5 to 0.99 has a strong impact on V-K (2mag). Conversely, a range of 2 mag in V-K corresponds to a 200K shift along the (Teff, V-K) relationship (for a fixed C/O value). Hence, the use of a (Teff, V-K) calibration established for M stars will yield large errors for S stars, so that a specific calibration must be used, as provided in the present paper. Using the atmospheric parameters derived by our method for the sample of Henize S stars, we show that the extrinsic-intrinsic dichotomy among S stars reveals itself very clearly as a bimodal distribution in the effective temperatures. Moreover, the increase of s-process element abundances with increasing C/O ratios and decreasing temperatures is apparent among intrinsic stars, confirming theoretical expectations.
2. A(Li) and 6Li/7Li 3D NLTE corrections
ID:
ivo://CDS.VizieR/J/A+A/618/A16
Title:
A(Li) and 6Li/7Li 3D NLTE corrections
Short Name:
J/A+A/618/A16
Date:
21 Oct 2021
Publisher:
CDS
Description:
The purpose of this work is to provide corrections for the lithium abundance, A(Li), and the ^6^Li/^7^Li ratio that can easily be applied to correct 1D LTE lithium abundances in G and F dwarf stars of approximately solar mass and metallicity for three-dimensional (3D) and non-LTE (NLTE) effects. The corrections for A(Li) and ^6^Li/^7^Li are computed using grids of 3D NLTE and 1D LTE synthetic lithium line profiles, generated from 3D hydro-dynamical CO5BOLD and 1D hydrostatic LHD model atmospheres, respectively. For comparative purposes, all calculations are performed for three different line lists representing the LiI 670.8nm spectral region. The 3D NLTE corrections are then approximated by analytical expressions as a function of the stellar parameters (Teff, logg, [Fe/H], vsini, A(Li), ^6^Li/^7^Li). The derived 3D NLTE corrections range between -0.01 and +0.11dex for A(Li), and between -4.9 and -0.4% for the ^6^Li/^7^Li ratio, depending on the adopted stellar parameters. Our findings show a general consistency with recent works on lithium abundance corrections. In the case of the ^6^Li/^7^Li ratio, our corrections are always negative, showing that 1D LTE analysis can significantly overestimate the presence of ^6^Li (up to 4.9 percentage points) in the atmospheres of solar-like dwarf stars. These results emphasize the importance of reliable 3D model atmospheres combined with NLTE line formation for deriving precise isotopic lithium abundances. Although 3D NLTE spectral synthesis implies an extensive computational effort, the results can be made accessible with parametric tools like the ones presented in this work.
3. ALMA maps of 6 sources of star forming regions
ID:
ivo://CDS.VizieR/J/A+A/636/A67
Title:
ALMA maps of 6 sources of star forming regions
Short Name:
J/A+A/636/A67
Date:
21 Oct 2021
Publisher:
CDS
Description:
As a building block for amino acids, formamide (NH_2_CHO) is an important molecule in astrobiology and astrochemistry, but its formation path in the interstellar medium is not understood well. We aim to find empirical evidence to support the chemical relationships of formamide to HNCO and H_2_CO. We examine high angular resolution (~0.2") Atacama Large Millimeter/submillimeter Array (ALMA) maps of six sources in three high-mass star-forming regions and compare the spatial extent, integrated emission peak position, and velocity structure of HNCO and H_2_CO line emission with that of NH_2_CHO by using moment maps. Through spectral modeling, we compare the abundances of these three species. In these sources, the emission peak separation and velocity dispersion of formamide emission is most often similar to HNCO emission, while the velocity structure is generally just as similar to H_2_CO and HNCO (within errors). From the spectral modeling, we see that the abundances between all three of our focus species are correlated, and the relationship between NH_2_CHO and HNCO reproduces the previously demonstrated abundance relationship. In this first interferometric study, which compares two potential parent species to NH_2_CHO, we find that all moment maps for HNCO are more similar to NH_2_CHO than H_2_CO in one of our six sources (G24 A1). For the other five sources, the relationship between NH_2_CHO, HNCO, and H_2_CO is unclear as the different moment maps for each source are not consistently more similar to one species as opposed to the other.
4. (Al2O3)n, n=1-10, clusters data
ID:
ivo://CDS.VizieR/J/A+A/658/A167
Title:
(Al2O3)n, n=1-10, clusters data
Short Name:
J/A+A/658/A167
Date:
22 Feb 2022
Publisher:
CDS
Description:
Aluminium oxide (alumina; Al_2_O_3_) is a promising candidate as a primary dust condensate in the atmospheres of oxygen-rich evolved stars. Therefore, alumina 'seed' particles might trigger the onset of stellar dust formation and of stellar mass loss in the wind. However, the formation of alumina dust grains is not well understood. Aims. We aim to shed light on the initial steps of cosmic dust formation (i.e. nucleation) in oxygen-rich environments via a quantum- chemical bottom-up approach. Starting with an elemental gas-phase composition, we construct a detailed chemical-kinetic network that describes the formation and destruction of aluminium-bearing molecules and dust- forming (Al_2_O_3_)_n_ clusters up to the size of dimers (n=2) coagulating to tetramers (n=4). Intermediary species include the prevalent gas- phase molecules AlO and AlOH as well as Al_x_O_y_ clusters with x=1-5, y=1-6. The resulting extensive network is applied to two model stars, which represent a semi-regular variable and a Mira type, and to different circumstellar gas trajectories, including a non-pulsating outflow and a pulsating model. The growth of larger-sized (Al_2_O_3_)_n_ clusters with n=4-10 is described by the temperature-dependent Gibbs free energies of the most favourable structures (i.e. the global minima clusters) as derived from global optimisation techniques and calculated via density functional theory. We provide energies, bond characteristics, electrostatic properties, and vibrational spectra of the clusters as a function of size, n, and compare these to corundum, which corresponds to the crystalline bulk limit (n to infinity). The circumstellar aluminium gas-phase chemistry in oxygen- rich giants is primarily controlled by AlOH and AlO, which are tightly coupled by the reactions AlO+H_2_, AlO+H_2_O, and their reverse. Models of semi-regular variables show comparatively higher AlO abundances, as well as a later onset and a lower efficiency of alumina cluster formation when compared to Mira-like models. The Mira-like models exhibit an efficient cluster production that accounts for more than 90% of the available aluminium content, which is in agreement with the most recent ALMA observations. Chemical equilibrium calculations fail to predict both the alumina cluster formation and the abundance trends of AlO and AlOH in the asymptotic giant branch dust formation zone. Furthermore, we report the discovery of hitherto unreported global minimum candidates and low-energy isomers for cluster sizes n=7, 9, and 10. A homogeneous nucleation scenario, where Al2O3 monomers are successively added, is energetically viable. However, the formation of the Al2O3 monomer itself represents an energetic bottleneck. Therefore, we provide a bottom-up interpolation of the cluster characteristics towards the bulk limit by excluding the monomer, approximately following an n^(-1/3)^ dependence.
5. Analysis of hot Jupiters in Kepler Q2
ID:
ivo://CDS.VizieR/J/AJ/143/39
Title:
Analysis of hot Jupiters in Kepler Q2
Short Name:
J/AJ/143/39
Date:
21 Oct 2021
Publisher:
CDS
Description:
In this paper, we present the results of searching the Kepler Q2 public data set for the secondary eclipses of 76 hot Jupiter planet candidates from the list of 1235 candidates published by Borucki et al., 2011, Cat. J/ApJ/736/19. This search has been performed by modeling both the Kepler pre-search data conditioned light curves and new light curves produced via our own photometric pipeline. We derive new stellar and planetary parameters for each system, while calculating robust errors for both. We find 16 systems with 1{sigma}-2{sigma}, 14 systems with 2{sigma}-3{sigma}, and 6 systems with >3{sigma} confidence level secondary eclipse detections in at least one light curve produced via the Kepler pre-search data conditioned light curve or our own pipeline; however, results can vary depending on the light curve modeled and whether eccentricity is allowed to vary or not. We estimate false alarm probabilities of 31%, 10%, and 6% for the 1{sigma}-2{sigma}, 2{sigma}-3{sigma}, and >3{sigma} confidence intervals, respectively.
6. APOGEE-Kepler Cool Dwarf star ages determination
ID:
ivo://CDS.VizieR/J/ApJ/888/43
Title:
APOGEE-Kepler Cool Dwarf star ages determination
Short Name:
J/ApJ/888/43
Date:
21 Oct 2021
Publisher:
CDS
Description:
We use models of stellar angular momentum evolution to determine ages for ~500 stars in the APOGEE-Kepler Cool Dwarfs sample. We focus on lower-main-sequence stars, where other age-dating tools become ineffective. Our age distributions are compared to those derived from asteroseismic and giant samples and solar analogs. We are able to recover gyrochronological ages for old, lower-main-sequence stars, a remarkable improvement over prior work in hotter stars. Under our model assumptions, our ages have a median relative uncertainty of 14%, comparable to the age precision inferred for more massive stars using traditional methods. We investigate trends of Galactic {alpha}-enhancement with age, finding evidence of a detection threshold between the age of the oldest {alpha}-poor stars and that of the bulk {alpha}-rich population. We argue that gyrochronology is an effective tool reaching ages of 10-12Gyr in K and early M dwarfs. Finally, we present the first effort to quantify the impact of detailed abundance patterns on rotational evolution. We estimate a ~15% bias in age for cool, {alpha}-enhanced (+0.4dex) stars when standard solar-abundance-pattern rotational models are used for age inference, rather than models that appropriately account for {alpha}-enrichment.
7. A search for multiplanet systems with TESS
ID:
ivo://CDS.VizieR/J/AJ/158/243
Title:
A search for multiplanet systems with TESS
Short Name:
J/AJ/158/243
Date:
21 Oct 2021
Publisher:
CDS
Description:
Transiting exoplanets in multiplanet systems exhibit non-Keplerian orbits as a result of the gravitational influence from companions, which can cause the times and durations of transits to vary. The amplitude and periodicity of the transit time variations are characteristic of the perturbing planet's mass and orbit. The objects of interest from the Transiting Exoplanet Survey Satellite (TESS) are analyzed in a uniform way to search for transit timing variations (TTVs) with sectors 1-3 of data. Due to the volume of targets in the TESS candidate list, artificial intelligence is used to expedite the search for planets by vetting nontransit signals prior to characterizing the light-curve time series. The residuals of fitting a linear orbit ephemeris are used to search for TTVs. The significance of a perturbing planet is assessed by comparing the Bayesian evidence between a linear and nonlinear ephemeris, which is based on an N-body simulation. Nested sampling is used to derive posterior distributions for the N-body ephemeris and in order to expedite convergence, custom priors are designed using machine learning. A dual-input, multi-output convolutional neural network is designed to predict the parameters of a perturbing body given the known parameters and measured perturbation (O-C). There is evidence for three new multiplanet candidates (WASP-18, WASP-126, TOI 193) with nontransiting companions using the two-minute cadence observations from TESS. This approach can be used to identify stars in need of longer radial velocity and photometric follow-up than those already performed.
8. Atmospheric parameters in metal-poor stars. II
ID:
ivo://CDS.VizieR/J/A+A/350/955
Title:
Atmospheric parameters in metal-poor stars. II
Short Name:
J/A+A/350/955
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present non-LTE corrections to abundances of Fe, O, Na, and Mg derived from LTE analyses of F-K stars over a broad range of gravities and metal abundances; they were obtained using statistical equilibrium calculations and new model atoms. Line opacity was considered by means of an empirical procedure where it was attributed to a veil of weak Fe I lines; in the case of solar-type dwarfs, results were compared with those obtained using (LTE) mean intensities computed from OSMARCS models. We think that the empirical procedure produces better results for metal-poor stars, while mean intensities should perhaps be preferred for the Sun (where departures from LTE are anyway not very large). Collisions with both electrons and H I atoms were considered. Since cross sections for this second mechanism are very poorly known, we calibrated them empirically by matching observations of RR Lyrae variables at minimum light (discussed in Clementini et al., 1995, Cat. <J/AJ/110/2319>). These stars were selected because non-LTE effects are expected to be larger in these stars than in those usually considered in the study of the chemical evolution of the Galaxy (cool main sequence and red giant branch stars). We found that different non-LTE mechanisms are important for the different species and transitions considered; on the whole, our calculations yielded moderate corrections to LTE abundances for high excitation O lines in warm dwarfs and giants, Na and Mg lines in giants and supergiants, and Fe I lines in F-supergiants (where corrections becomes very large for IR O lines). Non-LTE corrections were found to be negligible in the other cases studied.
9. Calculated chemical composition of galactic PN
ID:
ivo://CDS.VizieR/J/AZh/82/437
Title:
Calculated chemical composition of galactic PN
Short Name:
J/AZh/82/437
Date:
21 Oct 2021
Publisher:
CDS
Description:
The results of a study of the chemical compositions of Galactic planetary nebulae taking into account two types of inhomogeneity in the nebular gas density in their envelopes are reported. New analytical expressions for the ionization correction factors have been derived and are used to determine the chemical compositions of the nebular gas in Galactic planetary nebulae. The abundances of He, N, O, Ne, S, and Ar have been found for 193 objects. The Y-Z diagrams for various He abundances are analyzed for type II planetary nebulae separately and jointly with HII regions. The primordial helium abundance Yp and enrichment ratio dY/dZ are determined, and the resulting values are compared with the data of other authors. Radial abundance gradients in the Galactic disk are studied using type II planetary nebulae.
10. Calibration of Stromgren phot. for late-type stars
ID:
ivo://CDS.VizieR/J/A+A/498/527
Title:
Calibration of Stromgren phot. for late-type stars
Short Name:
J/A+A/498/527
Date:
21 Oct 2021
Publisher:
CDS
Description:
The use of model atmospheres for deriving stellar fundamental parameters, such as Teff, log(g), and [Fe/H], will increase as we find and explore extreme stellar populations where empirical calibrations are not yet available. Moreover, calibrations for upcoming large satellite missions of new spectrophotometric indices, similar to the uvby-Hbeta system, will be needed. We aim to test the power of theoretical calibrations based on a new generation of MARCS models by comparisons with observational photometric data. We calculated synthetic uvby-Hbeta colour indices from synthetic spectra. A sample of 367 field stars, as well as stars in globular clusters, is used for a direct comparison of the synthetic indices versus empirical data and for scrutizing the possibilities of theoretical calibrations for temperature, metallicity, and gravity. We show that the temperature sensitivity of the synthetic (b-y) colour is very close to its empirical counterpart, whereas the temperature scale based upon Hbeta shows a slight offset. The theoretical metallicity sensitivity of the m1 index (and for G-type stars its combination with c1) is somewhat higher than the empirical one, based upon spectroscopic determinations. The gravity sensitivity of the synthetic c1 index shows satisfactory behaviour when compared to observations of F stars. For stars cooler than the sun, a deviation is significant in the c1-(b-y) diagram. The theoretical calibrations of (b-y), (v-y), and c1 seem to work well for Pop II stars and lead to effective temperatures for globular cluster stars supporting recent claims that atomic diffusion occurs in stars near the turnoff point of NGC 6397. Synthetic colours of stellar atmospheres can indeed be used, in many cases, to derive reliable fundamental stellar parameters. The deviations seen when compared to observational data could be due to incomplete linelists but are possibly also due to the effects of assuming plane-parallel or spherical geometry and LTE.

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