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 Å pixel^-1) and low (2.73 and 4.92 Å 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.