Searching for shell stars in LAMOST DR4 by probing the Fe 42 multiplet lines

Abstract

Shell stars, in particular the cooler ones, often do not show conspicuous Balmer-line emission and may consequently be missed in surveys that specifically search for emission signatures in the Hα line. The present work is aimed at identifying stars with shell-signatures via a search for strong Fe ii multiplet 42 lines at λλ4924, 5018, 5169 Å in archival LAMOST spectra. Candidates were selected by probing the Fe ii (42) lines in the spectra of a sample of colour-preselected early-type stars using a modified version of the MKCLASS code and then categorised by visual inspection of their spectra. We identified 75 stars showing conspicuous shell features, 43 Am/CP1 stars, 12 Ap/CP2 stars, and three objects with composite spectra. Spectral types and equivalent width measurements of the Fe ii (42) lines are presented for the sample of shell stars. Except for three objects, all shell stars appear significantly removed from the ZAMS in the colour-magnitude diagram, which is likely due to extinction by circumstellar material. We find a correlation between the equivalent width of the λ5169 Å line and the distance to the locus of the main-sequence stars (the larger the IR-excess, the stronger the λ5169 Å line) and studied the variability of the shell star sample using TESS data, identifying a very high proportion of double stars. All but 14 shell stars are new discoveries, which highlights the efficiency of the here presented novel approach to identify stars with subtle shell features. This study may be used as a blueprint for discovering these objects in massive spectral databases.