A Survey for C ii Emission-line Stars in the Large Magellanic Cloud. II. Final Results and the Origin of C ii Emission in [WC] Spectra

We present the final results of an imaging and spectroscopic search for stars in the Large Magellanic Cloud (LMC) with C ii lambda lambda 7231, 7236 emission lines. The goal is to identify and study [WC11] stars, the coolest of the low-mass Wolf-Rayet sequence, and a subset of central stars of planetary nebulae where the C ii lines are known to be especially prominent. A recent serendipitous discovery of an LMC [WC11] raised the possibility that these objects, although difficult to identify, might in fact be more common than previously believed. Several new members of this rare class have been found in this survey. It now seems clear, however, that a significant number of these stars are not hiding among the general [WC] population. We point out that the C ii doublet intensity ratio observed in our spectra proves to neatly divide the objects into two distinct groups, with the C ii emission likely originating from either the stellar wind or a surrounding nebula. The physics of the C ii emission mechanism correctly explains this bifurcation. Spectral subtypes are suggested for most of the objects. The numerous spectroscopic clues now available for these objects should facilitate future detailed modeling.

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This paper presents the final results of a search for stars in the Large Magellanic Cloud (LMC) using imaging and spectroscopy. The goal of the study is to identify and study [WC11] stars, the coolest of the low-mass Wolf-Rayet sequence, as well as a subset of central stars of planetary nebulae. The authors found several new members of this rare class and propose that the intensity ratio of the C ii doublet observed in their spectra can divide the objects into two distinct groups.