Physical Conditions in the Proto–Planetary Nebula CRL 618 Derived from Observations of Vibrationally Excited HC₃N

Abstract

We used the Effelsberg 100 m and IRAM 30 m telescopes to observe vibrationally excited cyanoacetylene (HC 3 N) in several rotational transitions toward the proto-planetary nebula CRL 618. Lines from 9 different vibrationally excited states with energies ranging up to 1600 K above ground were detected. The lines show P Cygni profiles indicating that the HC 3 N emission originates from an expanding and accelerating molecular envelope. The HC 3 N rotational temperature varies with velocity, peaks at 520 K, 3 km s −1 blue-shifted from the systemic velocity and decreases with higher blueshift of the gas. The column density of the absorbing HC 3 N is 3-6×10 17 cm −2 . We modeled spectra based on spherical models of the expanding envelope which provide an excellent fit to the observations, and discuss the implications of the models. Additionally, lines from 13 C substituted cyanoacetylene were observed. They can be used to constrain the 12 C/ 13 C ratio in this source to 10 ± 2.

First Detection of Vibrationally Excited HNC in Space

“…For average source sizes of 0Љ .5, guided by the results of Wyrowski et al (2003) and Thorwirth et al (2003), the intensities of the vibrationally excited lines of both HCN and HNC are consistent with optically thick emission at 200-300 K, in agreement with the temperatures expected at these radii. The fact that the HNC lines are of comparable strength to the HCN lines can mean either that the HNC lines come from a similar radius, because for optically thick lines…”

“…INTRODUCTION The observation of rotational transitions of vibrationally excited molecules is a unique probe of very hot gas around stars and young stellar objects (see, e.g., Schilke et al 1992bSchilke et al , 2000Walmsley 1999 andreferences therein). This is because vibrational transitions are generally pumped by IR radiation, and thus the excitation analysis offers a probe of the IR radiation field close to the exciting object, often at wavelengths at which direct observations in the IR are not possible, because the dust in the object is optically thick there.…”

Millimeter-wave spectroscopy of HC5N in vibrationally excited states below 500 cm$\mathsf{^{-1}}$

“…More recently, mm-wave lines of vibrationally excited HC 5 N were observed toward the proto-planetary nebula CRL 618 (Cernicharo et al 2001;Thorwirth 2001;Wyrowski et al 2003), whose thick and dusty molecular envelope is particularly rich in small hydrocarbons, which produce a rather congested mm-wave spectrum. These recent observations stimulated the present laboratory work, whose objective is a detailed analysis of the mm-wave spectra for the low-lying Table 1.…”

Planckintermediate results. XVIII. The millimetre and sub-millimetre emission from planetary nebulae

“…Among them is the free-free emission from the central H ii region, whose variability has been reported by several authors (Kwok & Feldman 1981;Sánchez Contreras & Sahai 2004;Wyrowski et al 2003). This somewhat controversial behaviour, implyies that CRL 618 exhibits the activity of a post-AGB wind, which is poorly understood (García-Segura et al 2005).…”