arXiv subjects

H. -G. Ludwig

Publications and source records attributed to H. -G. Ludwig.

At least 37 records · Page 2Linked to original sources

arXiv2019

Masses of the Hyades white dwarfs: A gravitational redshift measurement

Context. It is possible to accurately measure the masses of the white dwarfs (WDs) in the Hyades cluster using gravitational redshift, because the radial velocity of the stars can be obtained independently of spectroscopy from astrometry and the cluster has a low velocity dispersion. Aims. We aim to obtain an accurate measurement of the Hyades WD masses by determining the mass-to-radius ratio (M/R) from the observed gravitational redshift, and to compare them with masses derived from other methods. Methods. We analyse archive high-resolution UVES-VLT spectra of six WDs belonging to the Hyades to measure their Doppler shift, from which M/R is determined after subtracting the astrometric radial velocity. We estimate the radii using Gaia photometry as well as literature data. Results. The M/R error associated to the gravitational redshift measurement is about 5%. The radii estimates, evaluated with different methods, are in very good agreement, though they can differ by up to 4% depending on the quality of the data. The masses based on gravitational redshift are systematically smaller than those derived from other methods, by a minimum of $\sim 0.02$ up to 0.05 solar masses. While this difference is within our measurement uncertainty, the fact that it is systematic indicates a likely real discrepancy between the different methods. Conclusions. We show that the M/R derived from gravitational redshift measurements is a powerful tool to determine the masses of the Hyades WDs and could reveal interesting properties of their atmospheres. The technique can be improved by using dedicated spectrographs, and can be extended to other clusters, making it unique in its ability to accurately and empirically determine the masses of WDs in open clusters. At the same time we prove that gravitational redshift in WDs agrees with the predictions of stellar evolution models to within a few percent.

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arXiv2019

Two-dimensional non-LTE \ion{O}{I} 777\,nm line formation in radiation hydrodynamics simulations of Cepheid atmospheres

Oxygen abundance measurements are important for understanding stellar structure and evolution. Measured in Cepheids, they further provide clues on the metallicity gradient and chemo-dynamical evolution in the Galaxy. However, most of the abundance analyses of Cepheids to date have been based on one-dimensional (1D) hydrostatic model atmospheres. Here, we test the validity of this approach for the key oxygen abundance diagnostic, the \ion{O}{I} $777\,\mathrm{nm}$~triplet lines. We carry out 2D non-LTE radiative transfer clculations across two different 2D radiation hydrodynamics simulations of Cepheid atmospheres, having stellar parameters of $T_\mathrm{eff}= 5600$ K, solar chemical compositions, and $\log\,g= 1.5$ and $2.0$, corresponding to pulsation periods of 9 and 3 days, respectively. We find that the 2D non-LTE versus 1D LTE abundance differences range from $-1.0$~dex to $-0.25$~dex depending on pulsational phase. The 2D non-LTE versus 1D non-LTE abundance differences range from $-0.2$~dex to $0.8$~dex. The abundance differences are smallest when the Cepheid atmospheres are closest to hydrostatic equilibrium, corresponding to phases of around $0.3$ to $0.8$, and we recommend these phases for observers deriving the oxygen abundance from \ion{O}{I} $777\,\mathrm{nm}$ triplet with 1D hydrostatic models.

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arXiv2019

4MOST Consortium Survey 2: The Milky Way Halo High-Resolution Survey

We will study the formation history of the Milky Way, and the earliest phases of its chemical enrichment, with a sample of more than 1.5 million stars at high galactic latitude. Elemental abundances of up to 20 elements with a precision of better than 0.2 dex will be derived for these stars. The sample will include members of kinematically coherent substructures, which we will associate with their possible birthplaces by means of their abundance signatures and kinematics, allowing us to test models of galaxy formation. Our target catalogue is also expected to contain 30,000 stars at a metallicity of less than one hundredth that of the Sun. This sample will therefore be almost a factor of 100 larger than currently existing samples of metal-poor stars for which precise elemental abundances are available (determined from high-resolution spectroscopy), enabling us to study the early chemical evolution of the Milky Way in unprecedented detail.

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arXiv2018

Observations of the solar chromosphere with ALMA and comparison with theoretical models

In this work we use solar observations with the ALMA radio telescope at the wavelength of 1.21 mm. The aim of the analysis is to improve understanding of the solar chromosphere, a dynamic layer in the solar atmosphere between the photosphere and corona. The study has an observational and a modeling part. In the observational part full-disc solar images are analyzed. Based on a modified FAL atmospheric model, radiation models for various observed solar structures are developed. Finally, the observational and modeling results are compared and discussed.

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arXiv2018

Systematic investigation of chemical abundances derived using IR spectra obtained with GIANO

Detailed chemical abundances of Galactic stars are needed in order to improve our knowledge of the formation and evolution of our galaxy, the Milky Way. We took advantage of the GIANO archive spectra to select a sample of Galactic disc stars in order to derive their chemical inventory and to compare the abundances we derived from these infrared spectra to the chemical pattern derived from optical spectra. We analysed high-quality spectra of 40 stars observed with GIANO. We derived the stellar parameters from the photometry and the Gaia data-release 2 (DR2) parallax; the chemical abundances were derived with the code MyGIsFOS. For a subsample of stars we compared the chemical pattern derived from the GIANO spectra with the abundances derived from optical spectra. We derived P abundances for all 40 stars, increasing the number of Galactic stars for which phosphorus abundance is known. We could derive abundances of 14 elements, 8 of which are also derived from optical spectra. The comparison of the abundances derived from infrared and optical spectra is very good. The chemical pattern of these stars is the one expected for Galactic disc stars and is in agreement with the results from the literature. GIANO is providing the astronomical community with an extremely useful instrument, able to produce spectra with high resolution and a wide wavelength range in the infrared.

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arXiv2018

Spectroscopic and astrometric radial velocities: Hyades as a benchmark

We study the accuracy of spectroscopic RVs by comparing spectroscopic and astrometric RVs for stars of the Hyades open cluster. Rather accurate astrometric RVs are available for the Hyades' stars, based on Hipparcos and on the first Gaia data release. We obtained HARPS spectra for a large sample of Hyades stars, and homogeneously analysed them. After cleaning the sample from binaries, RV variables, and outliers, 71 stars remained. The distribution of the observed RV difference (spectroscopic -- astrometric) is skewed and depends on the star right ascension. This is consistent with the Hyades cluster rotating at 42.3 +/- 10 m/s/pc. The two Hyades giants in the sample show, as predicted by gravitational redshift (GR), a spectroscopic RV that is blue-shifted with respect to the dwarfs, and the empirical GR slope is of 626 +/- 131 m/s, in agreement with the theoretical prediction. The difference between spectroscopic and astrometric RVs is very close to zero, within the uncertainties. In particular, the mean difference is of -33 m/s and the median is of -16 m/s when considering the Gaia-based RVs (corrected for cluster rotation), with a sigma of 347 m/s, very close to the expected cluster velocity dispersion. We also determine a new value of the cluster centroid spectroscopic RV: 39.36 +/- 0.26 km/s. The spectroscopic RV measurements are expected, from simulations, to depend on stellar rotation, but our data do not confirm these predictions. We finally discuss other phenomena that can influence the RV difference, such as cluster expansion, stellar activity, general relativity, and Galactic potential. Clusters within the reach of current telescopes are expected to show differences of several hundreds m/s, depending on their position in the Galaxy.

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arXiv2018

Calibration of the mixing-length parameter $α$ for the MLT and FST models by matching with CO$^5$BOLD models

The CoRoT and Kepler missions provided a wealth of high-quality data for solar-like oscillations. To make the best of such data for seismic inferences, we need theoretical models with precise near-surface structure, which has significant influence on solar-like oscillation frequencies. The mixing-length parameter, $α$, is a key factor for the near-surface structure. In the convection formulations used in evolution codes, the $α$ is a free parameter that needs to be properly specified. We calibrated $α$ values by matching entropy profiles of 1D envelope models with those of 3D CO$^5$BOLD models. For such calibration, previous works concentrated on the classical mixing-length theory (MLT). Here we also analyzed the full spectrum turbulence (FST) models. For the atmosphere part in the 1D models, we use the Eddington grey $T(τ)$ relation and the one with the solar-calibrated Hopf-like function. For both the MLT and FST models with a mixing length $l=αH_p$, calibrated $α$ values increase with increasing $g$ or decreasing $T_{\rm eff}$. For the FST models, we also calibrated values of $α^*$ defined as $l=r_{\rm top}-r+α^*H_{p,{\rm top}}$. $α^*$ is found to increase with $T_{\rm eff}$ and $g$. As for the correspondence to the 3D models, the solar Hopf-like function gives a photospheric-minimum entropy closer to a 3D model than the Eddington $T(τ)$. The structure below the photosphere depends on the convection model. However, not a single convection model gives the best correspondence since the averaged 3D quantities are not necessarily related via an EOS. Although the FST models with $l=r_{\rm top}-r+α^*H_{p,{\rm top}}$ are found to give the frequencies closest to the solar observed ones, a more appropriate treatment of the top part of the 1D convective envelope is necessary.

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arXiv2018

TOPoS V: Abundance ratios in a sample of very metal-poor turn-off stars

Extremely metal-poor stars are keys to understand the early evolution of our Galaxy. The ESO large programme TOPoS has been tailored to analyse a new set of metal-poor turn-off stars, whereas most of the previously known extremely metal-poor stars are giant stars. Sixty five turn-off stars (preselected from SDSS spectra) have been observed with the X-Shooter spectrograph at the ESO VLT Unit Telescope 2, to derive accurate and detailed abundances of magnesium, silicon, calcium, iron, strontium and barium. We analysed medium-resolution spectra (R ~ 10 000) obtained with the ESO X-Shooter spectrograph and computed the abundances of several alpha and neutron-capture elements using standard one-dimensional local thermodynamic equilibrium (1D LTE) model atmospheres. Our results confirms the super-solar [Mg/Fe] and [Ca/Fe] ratios in metal-poor turn-off stars as observed in metal-poor giant stars. We found a significant spread of the [alpha/Fe] ratios with several stars showing sub-solar [Ca/Fe] ratios. We could measure the abundance of strontium in 12 stars of the sample, leading to abundance ratios [Sr/Fe] around the Solar value. We detected barium in two stars of the sample. One of the stars (SDSS J114424-004658) shows both very high [Ba/Fe] and [Sr/Fe] abundance ratios (>1 dex).

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arXiv2018

Influence of metallicity on the near-surface effect on oscillation frequencies

The CoRoT and Kepler missions have provided high-quality measurements of the frequency spectra of solar-like pulsators, enabling us to probe stellar interiors with a very high degree of accuracy by comparing the observed and modeled frequencies. However, the frequencies computed with 1D models suffer from systematic errors related to the poor modeling of the uppermost layers of stars. These biases are what is commonly named the near surface effect. The dominant effect is related to the turbulent pressure that modifies the hydrostatic equilibrium and thus the frequencies. This has already been investigated using grids of 3D RMHD simulations, which also were used to constrain the parameters of the empirical correction models. However, the effect of metallicity has not been considered so far. We study the impact of metallicity on the surface effect across the HR diagram, and provide a method for accounting for it when using the empirical correction models. We computed a grid of patched 1D stellar models with the stellar evolution code CESTAM in which poorly modeled surface layers have been replaced by averaged stratification computed with the 3D RMHD code CO5BOLD. We found that metallicity has a strong impact on the surface effect: keeping T_eff and log g constant, the frequency residuals can vary by up to a factor two. Therefore, the influence of metallicity cannot be neglected. We found that a correct way of accounting for it is to consider the surface Rosseland mean opacity. It allowed us to give a physically-grounded justification as well as a scaling relation for the frequency differences at nu_max as a function of T_eff, log g and kappa. Finally, we provide prescriptions for the fitting parameters of the correction functions. We show that the impact of metallicity through the Rosseland mean opacity must be taken into account when studying and correcting the surface effect.

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arXiv2018

Pure-helium 3D model atmospheres of white dwarfs

We present the first grid of 3D simulations for the pure-helium atmospheres of DB white dwarfs. The simulations were computed with the CO5BOLD radiation-hydrodynamics code and cover effective temperatures and surface gravities between 12000 K < Teff < 34000 K and 7.5 < log(g) (cgs units) < 9.0, respectively. In this introductory work, synthetic spectra calculated from the 3D simulations are compared to appropriate 1D model spectra under a differential approach. This results in the derivation of 3D corrections for the spectroscopically derived atmospheric parameters of DB stars with respect to the 1D ML2/alpha = 1.25 mixing-length parameterisation. No significant Teff corrections are found for the V777 Her instability strip region, and therefore no 3D revision is expected for the empirical blue and red edges of the strip. However, large log(g) corrections are found in the range 12000 K < Teff < 23000 K for all log(g) values covered by the 3D grid. These corrections indicate that 1D model atmospheres overpredict log(g), reminiscent of the results found from 3D simulations of pure-hydrogen white dwarfs. The next step will be to compute 3D simulations with mixed helium and hydrogen atmospheres to comprehend the full implications for the stellar parameters of DB and DBA white dwarfs.

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arXiv2018

Abundance of zinc in the red giants of Galactic globular cluster 47 Tucanae

We investigate possible relations between the abundances of zinc and the light elements sodium, magnesium, and potassium in the atmospheres ofred giant branch (RGB) stars of the Galactic globular cluster 47~Tuc and study connections between the chemical composition and dynamical properties of the cluster RGB stars. The abundance of zinc was determined in 27 RGB stars of 47~Tuc using 1D local thermal equilibrium (LTE) synthetic line profile fitting to the high-resolution 2dF spectra obtained with the Anglo-Australian Telescope (AAT). Synthetic spectra used in the fitting procedure were computed with the SYNTHE code and 1D ATLAS9 stellar model atmospheres. The average 1D~LTE zinc-to-iron abundance ratio and its RMS variations due to star-to-star abundance spread determined in the sample of 27 RGB stars is $\langle{\rm[Zn/Fe]}\rangle^{\rm 1D~LTE}=0.11\pm0.09$. We did not detect any statistically significant relations between the abundances of zinc and those of light elements. Neither did we find any significant correlation or anticorrelation between the zinc abundance in individual stars and their projected distance from the cluster center. Finally, no statistically significant relation between the absolute radial velocities of individual stars and the abundance of zinc in their atmospheres was detected. The determined average [Zn/Fe]$^{\rm~1DLTE}$ ratio agrees well with those determined in this cluster in earlier studies and nearly coincides with that of Galactic field stars at this metallicity. All these results suggest that nucleosynthesis of zinc and light elements proceeded in separate, unrelated pathways in 47~Tuc.

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arXiv2018

Abundances of Mg and K in the atmospheres of turn-off stars in Galactic globular cluster 47 Tucanae

We determined abundances of Mg and K in the atmospheres of 53 (Mg) and 75 (K) turn-off (TO) stars of the Galactic globular cluster 47 Tuc. The obtained abundances, together with those of Li, O, and Na, were used to search for possible relations between the abundances of K and other light elements, Li, O, Na, and Mg, as well as the connections between the chemical composition of TO stars and their kinematical properties. Abundances of Mg and K were determined using archival VLT FLAMES/GIRAFFE spectra, in combination with the 1D NLTE spectral synthesis methodology. Spectral line profiles were computed with the MULTI code, using ATLAS9 stellar model atmospheres. We also utilized 3D hydrodynamical CO5BOLD and 1D hydrostatic LHD model atmospheres for computing 3D-1D LTE abundance corrections for the spectral lines of Mg and K, in order to assess the influence of convection on their formation in the atmospheres of TO stars. The determined average abundance-to-iron ratios and their RMS variations due to star-to-star abundance spreads were $\langle{\rm[Mg/Fe]}\rangle^{\rm 1D NLTE}=0.47\pm0.12$, and $\langle{\rm [K/Fe]}\rangle^{\rm 1D NLTE}=0.39\pm0.09$. Although the data suggest the possible existence of a weak correlation in the [K/Fe]-[Na/Fe] plane, its statistical significance is low. No statistically significant relations between the abundance of K and other light elements were detected. Also, we did not find any significant correlations or anti-correlations between the [Mg/Fe] and [K/Fe] ratios and projected distance from the cluster center. Similarly, no relations between the absolute radial velocities of individual stars and abundances of Mg and K in their atmospheres were detected. The 3D-1D abundance corrections were found to be small ($\leq 0.1$\,dex) for the lines of Mg and K used in this study, thus indicating that the influence of convection on their formation is small.

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arXiv2018

Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours. I. Grids of 3D corrections in the UBVRI, 2MASS, Hipparcos, Gaia, and SDSS systems

The atmospheres of cool stars are temporally and spatially inhomogeneous due to the effects of convection.The influence of this inhomogeneity, referred to as granulation, on colours has never been investigated over a large range of effective temperatures and gravities. We aim to study, in a quantitative way, the impact of granulation on colours. We use the CIFIST grid of CO5BOLD hydrodynamical models to compute emerging fluxes. These in turn are used to compute theoretical colours in the UBVRI,2MASS,Hipparcos,Gaia and SDSS systems. Every CO5BOLD model has a corresponding one dimensional (1D) plane-parallel LHD model computed for the same atmospheric parameters, which we used to define a "3D correction" that can be applied to colours computed from fluxes computed from any 1D model atmosphere code. The 3D corrections on colours are generally small, of the order of a few hundredths of a magnitude, yet they are far from negligible. We find that ignoring granulation effects can lead to underestimation of Teff by up to 200K and overestimation of gravity by up to 0.5dex, when using colours as diagnostics. We have identified a major shortcoming in how scattering is treated in the current version of the CIFIST grid, which could lead to offsets of the order 0.01mag, especially for colours involving blue and UV bands.We have investigated the Gaia and Hipparcos photometric systems and found that the (G-H_p),(BP-RP) diagram is immune to the effects of granulation. In addition, we point to the potential of the RVS photometry as a metallicity diagnostic. Our investigation shows that the effects of granulation should not be neglected if one wants to use colours as diagnostics of the stellar parameters of F,G,K stars. A limitation is that scattering is treated as true absorption in our current computations, thus our 3D corrections are likely an upper limit to the true effect. (Abridged)

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arXiv2018

Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours. II. The role of convection accross the H-R diagram

We studied the influence of convection on the spectral energy distributions, photometric magnitudes, and colour indices of different types of stars across the H-R diagram. The 3D hydrodynamical CO5BOLD, averaged <3D>, and 1D hydrostatic LHD model atmospheres were used to compute spectral energy distributions of stars on the main sequence (MS), main sequence turn-off (TO), subgiant branch (SGB), and red giant branch (RGB), in each case at two different effective temperatures and two metallicities, [M/H]=0.0 and -2.0. Using the obtained spectral energy distributions, we calculated photometric magnitudes and colour indices in the broad-band Johnson-Cousins $UBVRI$ and 2MASS $JHK_{\rm s}$, and the medium-band Strömgren $uvby$ photometric systems. The 3D-1D differences in photometric magnitudes and colour indices are small in both photometric systems and typically do not exceed $\pm0.03$ mag. Only in the case of the coolest giants located on the upper RGB are the differences in the $U$ and $u$ bands able reach $\approx-0.2$ mag at [M/H]=0.0 and $\approx-0.1$ mag at [M/H]=-2.0. Generally, the 3D-1D differences are largest in the blue-UV part of the spectrum and decrease towards longer wavelengths. They are also sensitive to the effective temperature and are significantly smaller in hotter stars. Metallicity also plays a role and leads to slightly larger 3D-1D differences at [M/H]=0.0. All these patterns are caused by a complex interplay between the radiation field, opacities, and horizontal temperature fluctuations that occur due to convective motions in stellar atmospheres. Although small, the 3D-1D differences in the magnitudes and colour indices are nevertheless comparable to or larger than typical photometric uncertainties and may therefore cause non-negligible systematic differences in the estimated effective temperatures.

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arXiv2018

TOPoS: IV. Chemical abundances from high-resolution observations of seven EMP stars

Extremely metal-poor stars provide us with indirect information on the first generations of massive stars. The TOPoS survey has been designed to increase the census of these stars and to provide a chemical inventory that is as detailed as possible. Seven of the most iron-poor stars have been observed with the UVES spectrograph at the ESO VLT Kueyen 8.2m telescope to refine their chemical composition. We analysed the spectra based on 1D LTE model atmospheres, but also used 3D hydrodynamical simulations of stellar atmospheres. We measured carbon in six of the seven stars: all are carbon-enhanced and belong to the low-carbon band, defined in the TOPoS II paper. We measured lithium (A(Li)=1.9) in the most iron-poor star (SDSS J1035+0641, [Fe/H] < -5.2 ). We were also able to measure Li in three stars at [Fe/H]~ -4.0, two of which lie on the Spite plateau. We confirm that SDSS J1349+1407 is extremely rich in Mg, but not in Ca. It is also very rich in Na. Several of our stars are characterised by low alpha-to-iron ratios. The lack of high-carbon band stars at low metallicity can be understood in terms of evolutionary timescales of binary systems. The detection of Li in SDSS J1035+0641 places a strong constraint on theories that aim at solving the cosmological lithium problem. The Li abundance of the two warmer stars at [Fe/H]~ -4.0 places them on the Spite plateau, while the third, cooler star, lies below. We argue that this suggests that the temperature at which Li depletion begins increases with decreasing [Fe/H]. SDSS J1349+1407 may belong to a class of Mg-rich EMP stars. We cannot assess if there is a scatter in alpha-to-iron ratios among the EMP stars or if there are several discrete populations. However, the existence of stars with low alpha-to-iron ratios is supported by our observations.

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arXiv2017

Spectroscopic properties of a two-dimensional time-dependent Cepheid model II. Determination of stellar parameters and abundances

Standard spectroscopic analyses of variable stars are based on hydrostatic one-dimensional model atmospheres. This quasi-static approach has theoretically not been validated. We aim at investigating the validity of the quasi-static approximation for Cepheid variables. We focus on the spectroscopic determination of the effective temperature $T_\mathrm{eff}$, surface gravity $\log \,g$, microturbulent velocity $ξ_\mathrm{t}$, and a generic metal abundance $\log\,A$ -- here taken as iron. We calculate a grid of 1D hydrostatic plane-parallel models covering the ranges in effective temperature and gravity encountered during the evolution of a two-dimensional time-dependent envelope model of a Cepheid computed with the radiation-hydrodynamics code CO5BOLD. We perform 1D spectral syntheses for artificial iron lines in local thermodynamic equilibrium varying the microturbulent velocity and abundance. We fit the resulting equivalent widths to corresponding values obtained from our dynamical model. For the four-parametric case, the stellar parameters are typically underestimated exhibiting a bias in the iron abundance of $\approx-0.2\,\mbox{dex}$. To avoid biases of this kind it is favourable to restrict the spectroscopic analysis to photometric phases $ϕ_\mathrm{ph}\approx0.3\ldots 0.65$ using additional information to fix effective temperature and surface gravity. Hydrostatic 1D model atmospheres can provide unbiased estimates of stellar parameters and abundances of Cepheid variables for particular phases of their pulsations. We identified convective inhomogeneities as the main driver behind potential biases. For obtaining a complete view on the effects when determining stellar parameters with 1D models, multi-dimensional Cepheid atmosphere models are necessary for variables of longer period than investigated here.

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arXiv2017

Spectroscopic properties of a two-dimensional time-dependent Cepheid model I. Description and validation of the model

Standard spectroscopic analyses of Cepheid variables are based on hydrostatic one-dimensional model atmospheres, with convection treated using various formulations of mixing-length theory. This paper aims to carry out an investigation of the validity of the quasi-static approximation in the context of pulsating stars. We check the adequacy of a two-dimensional time-dependent model of a Cepheid-like variable with focus on its spectroscopic properties. With the radiation-hydrodynamics code CO5BOLD, we construct a two-dimensional time-dependent envelope model of a Cepheid with $T_\mathrm{eff}= 5600$ K, $\log g=2.0$, solar metallicity, and a 2.8-day pulsation period. Subsequently, we perform extensive spectral syntheses of a set of artificial iron lines in local thermodynamic equilibrium. The set of lines allows us to systematically study effects of line strength, ionization stage, and excitation potential. We evaluate the microturbulent velocity, line asymmetry, projection factor, and Doppler shifts. The mean Doppler shift is non-zero and negative, -1 km/s, after averaging over several full periods and lines. This residual line-of-sight velocity (related to the "K-term") is primarily caused by horizontal inhomogeneities, and consequently we interpret it as the familiar convective blueshift ubiquitously present in non-pulsating late-type stars. Limited statistics prevent firm conclusions on the line asymmetries. Our two-dimensional model provides a reasonably accurate representation of the spectroscopic properties of a short-period Cepheid-like variable star. Some properties are primarily controlled by convective inhomogeneities rather than by the Cepheid-defining pulsations.

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arXiv2017

Carbon-enhanced metal-poor 3D model atmospheres

We present our latest 3D model atmospheres for carbon-enhanced metal-poor (CEMP) stars computed with the CO5BOLD code. The stellar parameters are representative of hot turn-off objects (Teff ~ 6250 K, log g=4.0, [Fe/H]=-3.0). The main purpose of these models is to investigate the role of 3D effects on synthetic spectra of the CH G-band (4140-4400 A), the CN BX-band (3870-3890 A), and several UV OH transitions (3122-3128 A). By comparison with the synthetic spectra from standard 1D model atmospheres (assuming local thermodynamic equilibrium, LTE), we derive 3D abundance corrections for carbon and oxygen of up to -0.5 and -0.7 dex, respectively.

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