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 $\lambda \lambda$7231, 7236 emission lines in objects as faint as $m_{\lambda 7400}\sim 18$. 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 $\sim$50 deg${}^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 of LMC planetary nebulae, and that the detected C II emitters are largely of an old population.

Accepted for publication in the Astrophysical Journal, 2020

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Cited by in corpus (4)

• Line Identification and Excitation of Autoionizing States in a Late-Type, Low Mass Wolf-Rayet Star
• Rapid evolution of [WC] stars in the Magellanic Clouds
• Testing Evolutionary Models with Red Supergiant and Wolf-Rayet Populations
• 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