R. Monier

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.

Chemical composition of A and F dwarfs members of the Hyades open cluster

Aims. Abundances of 15 chemical elements have been derived for 28 F and 16 A stars members of the Hyades open cluster in order to set constraints on self-consistent evolutionary models that include radiative and turbulent diffusion. Methods. A spectral synthesis, iterative procedure was applied to derive the abundances from selected high-quality lines in highresolution, high-signal-to-noise spectra obtained with SOPHIE and AURELIE at the Observatoire de Haute Provence. Results. The abundance patterns found for A and F stars in the Hyades resemble those observed in Coma Berenices and Pleiades clusters. In graphs representing the abundances versus the effective temperature, A stars often display much more scattered abundances around their mean values than the coolest F stars do. Large star-to-star variations are detected in the Hyades A dwarfs in their abundances of C, Na, Sc, Fe, Ni, Sr, Y, and Zr, which we interpret as evidence of transport processes competing with radiative diffusion. In A and Am stars, the abundances of Cr, Ni, Sr, Y, and Zr are found to be correlated with that of Fe as in the Pleiades and in Coma Berenices. The ratios [C/Fe] and [O/Fe] are found to be anticorrelated with [Fe/H] as in Coma Berenices. All Am stars in the Hyades are deficient in C and O and overabundant in elements heavier than Fe but not all are deficient in Ca and/or Sc. The F stars have solar abundances for almost all elements except for Si. The overall shape of the abundance pattern of the slow rotator HD 30210 cannot be entirely reproduced by models including radiative diffusion and different amounts of turbulent diffusion. Conclusions. While part of the discrepancies between derived and predicted abundances could come from non-LTE effects, including competing processes such as rotational mixing and/or mass loss seems necessary in order to improve the agreement between the observed and predicted abundance patterns.

Normal A0−A1 stars with low rotational velocities - I. Abundance determination and classification

Context. The study of rotational velocity distributions for normal stars requires an accurate spectral characterization of the objects in order to avoid polluting the results with undetected binary or peculiar stars. This piece of information is a key issue in the understanding of the link between rotation and the presence of chemical peculiarities. Aims. A sample of 47 low v sini A0‐A1 stars (v sini < 65km s 1 ), initially selected as main-sequence normal stars, are investigated with high-resolution and high signal-to-noise spectroscopic data. The aim is to detect spectroscopic binaries and chemically peculiar stars, and eventually establish a list of confirmed normal stars. Methods. A detailed abundance analysis and spectral synthesis is performed to derive abundances for 14 chemical species. A hierarchical classification, taking measurement errors into account, is applied to the abundance space and splits the sample into two di erent groups, identified as the chemically peculiar stars and the normal stars. Results. We show that about one third of the sample is actually composed of spectroscopic binaries (12 double-lined and five singlelined spectroscopic binaries). The hierarchical classification breaks down the remaining sample into 13 chemically peculiar stars (or uncertain) and 17 normal stars.

Four new HgMn stars: HD 18104, HD 30085, HD 32867, and HD 53588

We have detected four new HgMn stars, while monitoring a sample of apparently slowly rotating superficially normal bright late B and early A stars in the northern hemisphere. Important classification lines of Hg II and Mn II are found as conspicuous features in the high resolution SOPHIE spectra of these stars (R = 75000). Several lines of Hg II, Mn II and Fe II have been synthesized using model atmospheres and the spectrum synthesis code SYNSPEC48 including hyperfine structure of various isotopes when relevant. These synthetic spectra have been compared to high resolution high signal-to-noise observations of these stars in order to derive abundances of these key elements. The four stars are found to have distinct enhancements of Hg and Mn which show that these stars are not superficially normal B and A stars, but actually are new HgMn stars and should reclassified as such.