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Resumen:
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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 H? 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 H? 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] ?5007 ([O I] ?6300) 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.
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