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 dereddened 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 preshock densities ∼10 4 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 ∼10 5 to 10 6 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.

New light on the multiple jets of CRL 618

“…Haro-Corzo et al (2009) analyzed the influence of the orbital motion on the optical emission of these objects, by means of 3D hydrodynamical simulations. Their predicted [N ii] emission line maps can produce different luminosities for the two lobes, as observed in CRL 618 (Sánchez Contreras et al 2002;Riera et al 2011). …”

“…Unfortunately, the slit was displaced A&A 561, A145 (2014) from the leading bow-shocks. Comparing the observations with plane-parallel shock models, Riera et al (2011) found that the shock velocities lie in a narrow range between 30 and 40 km s −1 , except for the [O iii]/Hβ line ratios which require higher shock velocities (80-90 km s −1 ). The predicted shock velocities are significantly lower than the velocities with which the jets move outward.…”

“…Among the four narrowband filters used, the F673N and F656N are in common in both sets. Note that the Hα (F656N) images have two components: the emission formed in the lobes and the scattered component (Hα emission scattered by the dust outflow) (Carsenty & Solf 1982;Riera et al 2011). We measured the proper motion of the bow-shaped and knotty features from the two [S ii] images, which proved to be the most suitable because the observed emission arises from the jets.…”

“…From early ground-based observations, the optical emission from the lobes has been known to be shock-excited (Trammell et al 1993;Sánchez Contreras et al 2002;Riera et al 2006). More recently, Riera et al (2011) have analyzed the HST STIS spectra of several features of the jets of CRL 618 with unprecedented spatial resolution. Unfortunately, the slit was displaced A&A 561, A145 (2014) from the leading bow-shocks.…”

An Asymmetric Jet-Launching Model for the Protoplanetary Nebula CRL 618

“…Figure 6 displays position-velocity (PV) diagrams (bottom panels) for model M1 (xz projection) considering both orientations with respect to the plane of the sky. As in Figure 3 of Riera et al (2011), the PV diagrams display several ramps in radial velocity. This is clearly shown in the enlargement (upper panel) of the white rectangular region in the left panel.…”