A Survey for C II Emission-Line Stars in the Large Magellanic Cloud

Abstract

We present a narrow-band imaging survey of the Large Magellanic Cloud, designed to isolate the C ii λλ7231, 7236 emission lines in objects as faint as m λ7400 ∼ 18. The work is motivated by the recent serendipitous discovery in the LMC of the first confirmed extragalactic [WC11] star, whose spectrum is dominated by C ii emission, and the realization that the number of such objects is currently largely unconstrained. The survey, which imaged ∼50 deg 2 using on-band and off-band filters, will significantly increase the total census of these rare stars. In addition, each new LMC [WC] star has a known luminosity, a quantity quite uncertain in the Galactic sample. Multiple known C ii emitters were easily recovered, validating the survey design. We find 38 new C ii emission candidates; spectroscopy of the complete sample will be needed to ascertain their nature. In a preliminary spectroscopic reconnaissance, we observed three candidates, finding C ii emission in each. One is a new [WC11]. Another shows both the narrow C ii emission lines characteristic of a [WC11], but also broad emission of C iv, O v, and He ii characteristic of a much hotter [WC4] star; we speculate that this is a binary [WC]. The third object shows weak C ii emission, but the spectrum is dominated by a dense thicket of strong absorption lines, including numerous O ii transitions. We conclude it is likely an unusual hot, hydrogen-poor post-AGB star, possibly in transition from [WC] to white dwarf. Even lacking a complete spectroscopic program, we can infer that late [WC] stars do not dominate the central stars