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Astron. Astrophys. 317, 178-184 (1997)

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1. Introduction

This work is closely connected to the two papers by Lemke (1989, 1990) dealing with the chemical composition of 16 bright, sharp-lined normal main-sequence A stars. Lemke`s work reveals a wide spread in abundance patterns for these stars that is contrary to their `normal' classification. The Ti/Fe and Si/Fe ratios are solar while the Ba/Fe and Sr/Fe ratios vary from star to star. Both elements are typically enhanced by 1.0 dex relative to the sun. Calcium and carbon on the other hand show a scatter of one order of magnitude.

Subsequent work on the same stars by Holweger (1992) and Holweger & Stürenburg (1993) reveals a tight anticorrelation between carbon and silicon which was interpreted as a signature of gas-dust separation in the protostellar or circumstellar environment. Volatile elements like C, N, O remain in the gas phase, hence stars with a large C/Si ratio have preferentially accreted gas. Accretion of dust on the other hand leads to a small C/Si ratio, because refractory elements like silicon and iron have condensed onto dust grains. In addition diffusion processes in the atmosphere partly contaminate the chemical composition obtained by differential accretion of gas or dust.

Recently this picture has been elaborated further (Holweger et al. 1995): while for the metal-poor [FORMULA] Bootis stars the accretion model still holds true, it fails for the `normal' A stars. The abundance patterns of the latter reveal clear signatures of diffusion. According to theoretical calculations of radiative acceleration (Michaud et al. 1976, Gonzalez et al. 1995), in A stars carbon and nitrogen should settle, because at the bottom of the convection zone their radiative acceleration outwards is smaller than the gravity acceleration inwards.

The aim of this paper is to investigate a possible correlation between carbon and nitrogen abundances in the `normal' A stars in order to support or reject the above models of chemical separation in these stars. Newly CCD spectra of these stars are therefore analyzed to obtain reliable non-LTE abundances of nitrogen. Lines of sulphur and iron in the same spectral range are analyzed as well. The derived abundances are discussed in the context of earlier work.

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© European Southern Observatory (ESO) 1997