Hunting white dwarfs at the center of planetary nebulae

Abstract

We present Gemini-South observations of nine faint and extended planetary nebulae. Using direct images taken with the spectrograph GMOS, we built the $(u' - g')$ vs. $(g' - r')$ diagrams of the stars in the observed areas which allowed us, also considering their geometrical positions, to identify the probable central stars of the nebulae. Our stellar spectra of seven stars, also taken with GMOS, indicate that four (and probably two more) objects are white dwarfs of the DAO subtype. Moreover, the white dwarf status of the four stars is confirmed by the parameters $ T_{\mathrm{eff}}$ and $ \log g$ derived with the help of theoretical stellar spectra. Given this evidence, we propose that these hot stars are the central ionizing sources of the nebulae. With this work we hope to help improve the current scarce statistics on central white dwarfs in planetary nebulae.