CCD images and long-slit spectroscopy of the ring nebula around θ Mus

We present the first digital CCD images and long-slit spectroscopy of the optical ring nebula around the Wolf–Rayet star θ Mus. The CCD images obtained through narrow-band filters centred at O  iii ] and Hα show that the nebula has a filamentary structure, similar to supernova remnants, mainly seen in O  iii ]. A spatial detachment between O  iii ] and Hα images suggests excitation stratification, or multiple rings. An analysis of the physical conditions in the nebula was performed by means of long-slit CCD spectra. The spectral images show that the nebula is of low density and medium excitation. By means of quotients of recombination and collisional spectral line fluxes we determine that the principal excitation mechanism is photoionization. We have determined the electronic temperature and density, and chemical abundances for the oxygen at different sites within the nebula. Nebular chemical abundances are found to be similar to the Galactic ISM, indicating that the nebula is mainly composed of swept up material.