STIS optical spectroscopy of the lobes of CRL 618

Abstract

Context. Many proto-planetary nebulae show collimated structures sometimes showing multiple outflows.Aims. We present the results of new optical spectroscopic observations (both medium (with a dispersion of 0.56 A pixel-1 ) and low (2.73 and 4.92 A pixel-1 for the G430L and G750L gratings) spectral resolution) of the lobes of the proto-planetary nebula CRL 618 obtained with the Space Telescope Imaging Spectrograph on board of the Hubble Space Telescope. Methods. We analyse the density structure and the excitation conditions of the lobes of CRL 618. The spectra obtained at medium spectral resolution (~50 km s-1 ) allow us to quote the fraction of unscattered (intrinsic) Hα emission. We have also obtained de-reddened emission line ratios of several features from the low spectral resolution spectra.Results. We find that most of the analysed line ratios are reproduced by planar shocks moving through a dense medium (with pre-shock densities ~104 cm-3 ) with shock velocities from 30 to 40 km s-1 (except the [O III]/Hβ line ratios which require shock velocities of 80 to 90 km s-1 ). We find that the [S II]-weighted ionization fraction ranges from 0.015 to 0.06. The total densities derived from the electron density and the ionization fraction are ~105 to 106 cm-3 . Conclusions. We conclude that the spectra of the lobes of CRL 618 can be margially reproduced by steady plane–parallel shock models for shock velocities which are significantly lower than the velocities at which the jet moves outwards (~200 km s-1 ). These results are consistent with the predictions of a jet with a variable ejection velocity. The mirror-symmetry, the luminosity asymmetry between both lobes and the ejection velocity variability suggest that its central source may host a binary system.