L. Fossati

The effect of rotation on the abundances of the chemical elements of the A-type stars in the Praesepe cluster ?

Aims. We study how chemical abundances of late B‐, A‐ and early F‐type stars evolve with time, and we search for correlations between the abundance of chemical elements and other stellar parameters, such as e ective temperature and sini. Methods. We have observed a large number of B‐, A‐ and F‐type stars belonging to open clusters of di erent ages. In this paper we concentrate on the Praesepe cluster (logt = 8.85), for which we have obtained high resolution, high signal‐to‐noise ratio spectra of sixteen normal A‐ and F‐type stars and one Am star, using the SOPHIE spectrograph of the Observatoire de Haute‐Provence. For all the observed stars, we have derived fundamental parameters and chemical abundances. In addition, we discuss another eight Am stars belonging to the same cluster, for which the abundance analysis had been presented in a previous paper. Results. We find a strong correlation between peculiarity of Am stars and sini. The abundance of the elements underabundant in Am stars increases with sini, while it decreases for the overabundant elements. Chemical abundances of various elements appear correlated with the iron abundance.

Late stages of the evolution of A-type stars on the main sequence: comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster

Aims. We aim to provide observational constraints on diffusion models that predict peculiar chemical abundances in the atmospheres of Am stars. We also intend to check if chemical peculiarities and slow rotation can be explained by the presence of a weak magnetic field. Methods. We have obtained high resolution, high signal-to-noise ratio spectra of eight previously-classified Am stars, two normal A-type stars and one Blue Straggler, considered to be members of the Praesepe cluster. For all of these stars we have determined fundamental parameters and photospheric abundances for a large number of chemical elements, with a higher precision than was ever obtained before for this cluster. For seven of these stars we also obtained spectra in circular polarization and applied the LSD technique to constrain the longitudinal magnetic field. Results. No magnetic field was detected in any of the analysed stars. HD 73666, a Blue Straggler previously considered as an Ap (Si) star, turns out to have the abundances of a normal A-type star. Am classification is not confirmed for HD 72942. For HD 73709 we have also calculated synthetic Aa photometry that is in good agreement with the observations. There is a generally good agreement between abundance predictions of diffusion models and values that we have obtained for the remaining Am stars. However, the observed Na and S abundances deviate from the predictions by 0.6 dex and ≥0.25 dex respectively. Li appears to be overabundant in three stars of our sample.

The chemical abundance analysis of normal early A- and late B-type stars

Context. Modern spectroscopy of early-type stars often aims at studying complex physical phenomena such as stellar pulsation, the peculiarity of the composition of the photosphere, chemical stratification, the presence of a magnetic field, and its interplay with the stellar atmosphere and the circumstellar environment. Comparatively less attention is paid to identifying and studying the ”normal” A- and B-type stars and testing how the basic atomic parameters and standard spectral analysis allow one to fit the observations. By contrast, this kind of study is paramount eventually for allowing one to correctly quantify the impact of the various physical processes that occur inside the atmospheres of A- and B-type stars. Aims. We wish to establish whether the chemical composition of the solar photosphere can be regarded as a reference for early Aand late B-type stars. Methods. We have obtained optical high-resolution, high signal-to-noise ratio spectra of three slowly rotating early-type stars (HD 145788, 21 Peg and Cet) that show no obvious sign of chemical peculiarity, and performed a very accurate LTE abundance analysis of up to 38 ions of 26 elements (for 21 Peg), using a vast amount of spectral lines visible in the spectral region covered by our spectra. Results. We provide an exhaustive description of the abundance characteristics of the three analysed stars with a critical review of the line parameters used to derive the abundances. We compiled a table of atomic data for more than 1100 measured lines that may be used in the future as a reference. The abundances we obtained for He, C, Al, S, V, Cr, Mn, Fe, Ni, Sr, Y, and Zr are compatible with the solar ones derived with recent 3D radiative-hydrodynamical simulations of the solar photosphere. The abundances of the remaining studied elements show some degree of discrepancy compared to the solar photosphere. Those of N, Na, Mg, Si, Ca, Ti, and Nd may well be ascribed to non-LTE e ects; for P, Cl, Sc and Co, non-LTE e ects are totally unknown; O, Ne, Ar, and Ba show discrepancies that cannot be ascribed to non-LTE e ects. The discrepancies obtained for O (in two stars) and Ne agree with very recent non-LTE abundance analysis of early B-type stars in the solar neighbourhood.

The nature of p-modes and granulation in HD 49933 observed by CoRoT

Context. Recent observations of HD 49933 by the space-photometric mission CoRoT provide photometric evidence of solar type oscillations in a star other than our Sun. The first published reduction, analysis, and interpretation of the CoRoT data yielded a spectrum of p-modes with l = 0, 1, and 2. Aims. We present our own analysis of the CoRoT data in an attempt to compare the detected pulsation modes with eigenfrequencies of models that are consistent with the observed luminosity and surface temperature. Methods. We used the Gruberbauer et al. frequency set derived based on a more conservative Bayesian analysis with ignorance priors and fit models from a dense grid of model spectra. We also introduce a Bayesian approach to searching and quantifying the best model fits to the observed oscillation spectra. Results. We identify 26 frequencies as radial and dipolar modes. Our best fitting model has solar composition and coincides within the error box with the spectroscopically determined position of HD 49933 in the H-R diagram. We also show that lower-than-solar Z models have a lower probability of matching the observations than the solar metallicity models. To quantify the effect of the deficiencies in modeling the stellar surface layers in our analysis, we compare adiabatic and nonadiabatic model fits and find that the latter reproduces the observed frequencies better.

VALD — an atomic and molecular database for astrophysics

The VALD database of atomic and molecular data aims to ensure a robust and consistent analysis of astrophysical spectra. We offer a convenient e-mail and web-based user interface to a vast collection of spectral line parameters for all chemical elements and in the future also for molecules. An international team is working on the following tasks: collecting line parameters from relevant theoretical and experimental publications, computing line parameters, evaluating the data quality by comparison of similar data from different sources and by comparison with astrophysical observations, and incorporating the data into VALD. A unique feature of VALD is its capability to provide the most comprehensive spectral line lists for specific astrophysical plasma conditions defined by the user.

Improved fundamental parameters and LTE abundances of the CoRoT solar-type pulsator HD 49933

Aims. The knowledge of accurate stellar parameters is a key stone in several fields of stellar astrophysics, such as asteroseismology and stellar evolution. Although the parameters can be derived both from spectroscopy and multicolour photometry, the results obtained are sometimes affected by systematic uncertainties. In this paper, we present a self-consistent spectral analysis of the solar-type star HD 49933, which is a primary target for the CoRoT satellite. Methods. We used high-resolution and high signal-to-noise ratio spectra to carry out a consistent parameter estimation abundance analysis of HD 49933. The llmodels code was employed for model atmosphere calculations, while synth 3a ndwidth9 codes were used for line profile calculation and LTE abundance analysis. Results. We provide a detailed description of the methodology adopted to derive the fundamental parameters and the abundances. Although the parameters obtained differ from the ones previously derived by other authors, we show that only the set obtained in this work is able to fit the observed spectrum accurately. In particular, the surface gravity was adjusted to fit pressure-sensitive spectral features. Conclusions. We confirm the importance of a consistent analysis of relevant spectroscopic features, application of advanced model atmospheres, and the use of up-to-date atomic line data for the determination of stellar parameters. These results are crucial for further studies, e.g., detailed theoretical modelling of the observed pulsation frequencies.

A comparative analysis of the laboratory and theoretical transition probabilities of the Fe-peak elements for a new release of VALD

We carried out a comparative analysis of the recent atomic data for iron-peak elements, mainly Ti, Cr and Fe, for a new release of the Vienna Atomic Line Database (VALD3). New data were compared with those available in VALD2 and were checked using high-resolution, high signal-to-noise spectra of sharp-lined chemically normal stars including the Sun, and the zero-rotation extremely Cr- and Fe-rich chemically peculiar star HD 133792. The observed spectrum of the latter star allowed for comparison with transition probability calculations based on the orthogonal operator technique with the Cowan code for Cr II and Fe II lines for lower level energies between 2 eV and 11 eV in the wavelength region 3100 to 9000 A. In general, the agreement between the new experimental transition probabilities and those currently available in VALD2 is fairly good, which helps to validate the stellar abundance data derived with the VALD2 atomic parameters. We also found that, for a few important Ti II and Fe II lines in the visible spectral region, new transition probabilities are not consistent within their quoted accuracy. In a series of recent works on experimental f-values for Fe II it was shown that calculations based on the orthogonal operator technique agree better with the experimental data than the Cowan code calculations and, hence, should have preference for stellar spectroscopy. Our analysis of the Ap star HD 133792 spectrum clearly demonstrates that there are quite a number of high-excitation Cr II and Fe II lines which are fitted reasonably well when using the transition probabilities calculated with the Cowan code. As a rule these lines have their upper energy levels classified differently in both methods of calculations.

Abundance analysis of seven δ Scuti stars

Context. The current knowledge of the abundance pattern in δ Scuti stars is based on analysis of just a few field stars. Aims. We aim to determine the general chemical properties of the atmospheres of δ Scuti stars based on a statistically relevant sample of stars and investigate whether the abundance pattern is close to solar, an assumption generally made for pulsation models. Methods. We analysed high-resolution, high signal-to-noise ratio spectra of seven field δ Scuti stars. We derived the fundamental parameters and the photospheric abundances and than compared them to a similar sample of cluster δ Scuti stars. Results. With the use of a t-test we demonstrate that there is no difference between the two samples, which allows us to merge them, resulting in a sample of fifteen δ Scuti stars. We do not find any substantial difference between the abundance pattern of our sample of δ Scuti stars and a sample of normal early A- and late F-type stars. One field star in our sample, HD 124953, is most likely a pulsating Am star.

Abundance analysis of seven

Context. The current knowledge of the abundance pattern in δ Scuti stars is based on analysis of just a few field stars. Aims. We aim to determine the general chemical properties of the atmospheres of δ Scuti stars based on a statistically relevant sample of stars and investigate whether the abundance pattern is close to solar, an assumption generally made for pulsation models. Methods. We analysed high-resolution, high signal-to-noise ratio spectra of seven field δ Scuti stars. We derived the fundamental parameters and the photospheric abundances and than compared them to a similar sample of cluster δ Scuti stars. Results. With the use of a t-test we demonstrate that there is no difference between the two samples, which allows us to merge them, resulting in a sample of fifteen δ Scuti stars. We do not find any substantial difference between the abundance pattern of our sample of δ Scuti stars and a sample of normal early A- and late F-type stars. One field star in our sample, HD 124953, is most likely a pulsating Am star.