Physical structure of the proto-planetary nebula CRL 618. I. Optical long-slit spectroscopy and imaging

Abstract

In this paper we present optical long-slit spectroscopy and imaging of the proto-planetary nebula CRL 618. The optical lobes of CRL 618 consist of shock-excited gas, which emits many recombination and forbidden lines, and dust, which scatters light from the innermost regions. From the analysis of the scattered Hα emission, we derive a nebular inclination of i = 24° ± 6°. The spectrum of the innermost part of the east lobe (visible as a bright, compact nebulosity close to the star in the Hα Hubble Space Telescope image) is remarkably different from that of the shocked lobes but similar to that of the inner H II region, suggesting that this region represents the outermost parts of the latter. We find a nonlinear radial variation of the gas velocity along the lobes. The largest projected LSR velocities (with respect to the systemic velocity), ~80 km s^-1, are measured at the tips of the lobes, where the direct images show the presence of compact bow-shaped structures. The velocity of the shocks in CRL 618 is in the range ~75-200 km s^-1, as derived from diagnostic line ratios and line profiles. We report a brightening (weakening) of [O III] λ5007 ([O I] λ6300) over the last ~10 years that may indicate a recent increase in the speed of the exciting shocks. From the analysis of the spatial variation of the nebular extinction, we find a large density contrast between the material inside the lobes and beyond them: the optical lobes seem to be "cavities" excavated in the asymptotic giant branch (AGB) envelope by interaction with a more tenuous post-AGB wind. The electron density, with a mean value n_e ~ 5 × 10^3-10^4 cm^-3, shows significant fluctuations but no systematic decrease along the lobes, in agreement with most line emission arising in a thin shell of shocked material (the lobe walls) rather than in the post-AGB wind filling the interior of the lobes. The masses of atomic and ionized gas, respectively, in the east (west) lobe are >1.3 × 10^-4 (>7 × 10^-5) and ~6 × 10^-5 (~4 × 10^-5) M_☉. The shocks in CRL 618 are in a radiative regime and may lead in the future to the evolution of the optically emitting lobes into a fast, bipolar molecular outflow. The time required by the dense, shocked gas to cool down significantly is ≲2 yr, which is substantially lower than the kinematical age of the lobes (≲180 yr). This result suggests that a fast wind is currently active in CRL 618 and keeps shocking the circumstellar material.