A Study of Emission Lines in Carbon Stars

Abstract Carbon stars are chemically peculiar stars with high carbon abundance, showing strong carbon molecule bands and rare emission lines in their spectra. This paper explores the stellar activity of carbon stars by identifying emission line features in their spectra. An outlier detection method based on morphological feature extraction and interval representation is used to identify 88 targets presenting emission lines from 3546 carbon star spectra. Of these, 55 targets present Balmer series emissions, 35 show forbidden lines ([O i ] λ 6301 Å, [O iii ], [N ii ], [S ii ], with only 2 of the 35 spectra that present forbidden lines not showing [O i ]) and only 2 objects presenting two types of emission lines, and 51 spectra showing Balmer series emissions are C-N stars. This indicates the existence of a continuous stellar wind forming shock waves around the star, suggesting the presence of mass loss or tidal effects and exchange of materials from the companion star. In addition, 23 spectra presenting [O i ] emissions are from C-H carbon stars. These are mainly concentrated in a high-Galactic-latitude region with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="∣" open="∣"> <mml:mrow> <mml:mi>b</mml:mi> </mml:mrow> </mml:mfenced> <mml:mo>&gt;</mml:mo> <mml:mn>3</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>∘</mml:mo> </mml:mrow> </mml:msup> </mml:math> , where the influence of halo clusters could lead to relatively strong magnetic activity. Other objects in the Galactic disk may also show stronger magnetic activity due to the influence of a dense medium or magnetic field, rotation, and convection within stars. In addition, one spectrum shows both [O i ] and Balmer series emissions, probably caused by magnetic activity and wave shock. Another spectrum shows both Balmer series and [O iii ] emissions, presumably contaminated by an unknown nebula.