High-quality spectrophotometry of the planetary nebula in the Fornax dSph ^*

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany e-mail: akniazev@saao.ac.za
² Current address: South African Astronomical Observatory, Observatory Road, Cape Town, South Africa
³ Special Astrophysical Observatory, Nizhnij Arkhyz, Karachai-Circassia 369167 Russia
⁴ Astronomical Institute of the University of Basel, Department of Physics and Astronomy, Venusstrasse 7, 4102 Binningen, Switzerland
⁵ Astronomisches Rechen-Institut, Zentrum für Astronomie Heidelberg, University of Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany

Received: 26 January 2006
Accepted: 16 March 2007

Abstract

We present results of NTT spectroscopy of the one known planetary nebula (PN) in the dwarf spheroidal galaxy Fornax, a gas-deficient Local Group galaxy that stopped its star formation activity a few hundred million years ago. We detected the [] λ4363 line with a signal-to-noise ratio of ~22. For the first time we detected the weak [] 6717,6731 lines (I(6717+6731) ≈ 0.01 I(Hβ)), determined the electron number density (N_e(SII) = 750 cm^-3), and calculated O, N, Ne, Ar, S, Cl, Fe, He and C abundances. The abundance analysis presented here is based on the direct calculation of the electron temperature T_e and yields an oxygen abundance of 12+log(O/H) = . The analysis of the O, Ne, Ar and S abundances shows that the original ISM oxygen abundance was  dex lower and that third-dredge-up self-pollution in oxygen took place. The blue spectrum shows weak Wolf-Rayet features, and the progenitor star is classified as a weak emission-line star. Four of the five PNe in dwarf spheroidal galaxies are now known to show WR wind features. Overall, the metallicity of the progenitor of the PN fits in well with stellar spectroscopic abundances derived in previous studies as well as with the stellar age-metallicity relation of Fornax.