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 H emission, we derive a nebular inclination of i ¼ 24 AE 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 À1 , 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 À1 , 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 Â 10 3 10 4 cm À3 , 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 Â 10 À4 (>7 Â 10 À5 ) and $6 Â 10 À5 ($4 Â 10 À5 ) 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 d2 yr, which is substantially lower than the kinematical age of the lobes (d180 yr). This result suggests that a fast wind is currently active in CRL 618 and keeps shocking the circumstellar material.

Echelle Long‐Slit Optical Spectroscopy of Evolved Stars

“…Our interpretation is also consistent with the fact that most PPNs exhibit intrinsic polarization of the continuum as well as some emission lines, which are most likely due to scattering off dust grains (Trammell et al 1994). In particular, Balmer lines seem to come from an H ii region close to the star, which is blocked from direct view by, e.g.,the thick equatorial dust waists commonly present in these objects, and are seen in reflection off the nebular dust (see also, e.g., Sánchez Contreras et al 2002).…”

The chemistry in circumstellar envelopes of evolved stars: Following the origin of the elements to the origin of life

“…For example, CRL 2688, the Egg Nebula, exhibits both low (Ͻ20 km⅐s Ϫ1 ) and high velocity (Ͼ100 km⅐s Ϫ1 ) outflows (24,25). A more evolved object is CRL 618; here, the star, which has lost its dusty shroud, is producing sufficient UV radiation to create a small, nearby HII region (26,27).…”

Magnetic field in IRC+10216 and other C-rich evolved stars

“…AFGL618, also named the Westbrook Nebula, is a young proto-PN (∼200 years Kwok & Bignell 1984) exhibiting a B0 central star, a central compact HII region (Sánchez Contreras et al 2002), and two pairs of rapidly expanding well collimated lobes (Balick et al 2013). SMA continuum and CO-line-polarisation observations of Sabin et al (2014) have revealed a magnetic field well aligned and organised along the polar direction, suggesting a magnetic-outflowlaunching mechanism.…”