Paper detail

On the nature of sn stars. I. A detailed abundance study

The sn stars present sharp Balmer lines, sharp metallic lines and broad coreless He I lines. Initially Abt & Levato proposed a shell-like nature to explain the sn stars, although this scenario was subsequently questioned. We aim to derive abundances for a sample of 9 stars, including sn and non-sn stars, to determine the possible relation between sn and CP stars. We analysed the photospheric chemical composition of sn stars and show that approximately 40% of them display chemical peculiarities (such as He-weak and HgMn stars) within a range of temperature of 10300 - 14500 K. We have studied the possible contribution of different processes to the broad He I lines present in the sn stars. Although NLTE effects could not be completely ruled out, it seems that NLTE is not directly related to the broad He I profiles observed in the sn stars. The broad-line He I 4026 is the clearest example of the sn characteristics in our sample. We succesfully fit this line in 4 out of 7 sn stars by using the appropriate Stark broadening tables, while small differences appear in the other 3 stars. Studying the plots of abundance vs depth for the He I lines resulted in some sn stars probably being stratified in He. However, a further study of variability in the He I lines would help for determining whether a possible non-uniform He superficial distribution could also play a role in these sn stars. We conclude that the broad He I lines that characterize the sn class could be modelled (at least in some of these stars) by the usual radiative transfer process with Stark broadening, without needing another broadening mechanism. The observed line broadening in sn stars seems to be related to the "normal" He line formation that originates in these atmospheres. (abridged)