BD +30°549: a young helium-weak silicon star in the NGC 1333 star-forming region

Abstract

ABSTRACT We present results of the spectroscopic study of the chemically peculiar star BD +30°549, which is a bona fide member of the young NGC 1333 star-forming region. We found that the star possesses negligible rotation and a helium-weak spectroscopic pattern with strongly enhanced Si ii and Si iii lines. The fundamental parameters of the star, Teff = 13 100 K and log (L/L⊙) = 2.1, indicate that its age is about 2.7 Myr and that its position on the Hertzsprung–Russell diagram is at the end of the pre-main-sequence evolutionary track, close to the zero-age main sequence. Abundance analysis reveals the modest deficit of almost all elements, with the exception of Si , Fe , Ca  and P,  which are overabundant. We performed non-local thermodynamic equilibrium (non-LTE) calculations for Si ii/Si iii, Mg ii and Ca ii in order to check the influence of departures from LTE on line formation. Non-LTE calculations lead to much better reproduction of individual silicon line profiles, but do not completely remove the abundance discrepancy between Si ii and Si iii lines. We also investigate the effects of possible chemical stratification in BD +30°549. We suspect that the ‘Si ii/Si iii anomaly’ observed in the BD +30°549 spectrum arises under the combined action of the vertical and horizontal abundance gradients and non-LTE effects. We suppose that the evolutionary status and phenomena observed in BD +30°549 indicate that conditions favourable for the selective diffusion and formation of the surface chemical composition peculiarities (slow rotation and stabilization of the atmosphere) could already be built up at the pre-main-sequence phase.