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



Downloads per month over past year

Contreras, C. S. and Sahai, R. and Gil de Paz, Armando (2002) Physical structure of the proto-planetary nebula CRL 618. I. Optical long-slit spectroscopy and imaging. Astrophysical journal, 578 (1). pp. 269-289. ISSN 0004-637X

[thumbnail of gildepaz140libre.pdf]

Official URL:


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.

Item Type:Article
Additional Information:

© 2002. The American Astronomical Society. All rights reserved. The authors are grateful to Noam Soker for reading and commenting on this paper, to A. Castro Carrizo for helping during the long-slit observations, and to C.-F. Lee for fruitful conversations during the writing of this paper. This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration, and has been partially supported by a NASA Long Term Space Astrophysics grant to R. S. This research has made use of the USNOFS Image and Catalog Archive operated by the United States Naval Observatory, Flagstaff Station (http://www.nofs. The authors also acknowledge the use of NASA’s Astrophysical Data System Abstract Service (ADS).

Uncontrolled Keywords:Mass-loss rates; Molecular outflow; Bipolar outflow; Evolved stars; Protoplanetary nebulae; Radio-continuum; AFGL-618; CRL-618; Jets; Emission
Subjects:Sciences > Physics > Astrophysics
Sciences > Physics > Astronomy
ID Code:36057
Deposited On:01 Mar 2016 15:08
Last Modified:10 Dec 2018 15:05

Origin of downloads

Repository Staff Only: item control page