BD+30$^\circ$549: young helium-weak silicon star in NGC 1333 star-forming region

TL;DR

This study analyzes the chemically peculiar, helium-weak star BD+30°549 in the young NGC 1333 region, revealing its evolutionary status, chemical peculiarities, and the effects of non-LTE processes on spectral line formation.

Contribution

It provides the first detailed spectroscopic analysis of a young helium-weak silicon star in a star-forming region, highlighting the role of non-LTE effects and chemical stratification.

Findings

Star is young (~2.7 Myr) and near the Zero Age Main Sequence.

Silicon lines show non-LTE effects and abundance anomalies.

Chemical peculiarities may originate during the Pre-Main Sequence phase.

Abstract

We present results of the spectroscopic study of the chemically peculiar star BD+30$^\circ$549 which is bona-fide member of young NGC 1333 star forming region. We found that the star possesses negligible rotation and helium-weak spectroscopic pattern with strongly enhanced Si II and Si III lines. The fundamental parameters of the star $T_{eff}$=13100~K and $\log (L/L_{\odot})$=2.1 indicate its age of about 2.7 Myr and position on the Hertzsprung-Russell diagram 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 exception of Si, Fe, Ca and P which are overabundant. We performed the 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 does 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$^\circ$549. We suspect that the "Si II/Si III anomaly", observed in BD+30$^\circ$549 spectrum arises under the combined action of the vertical and horizontal abundance gradients and non-LTE effects. We suppose that evolutionary status and phenomena observed in BD+30$^\circ$549 indicate that conditions favorable for the selective diffusion and formation of the surface chemical composition peculiarities (slow rotation and stabilization of the atmosphere) can be built up already at the Pre-Main Sequence phase.