A Molecular Line Survey of the Highly Evolved Carbon Star Cit 6

Abstract

We present a spectral line survey of the C-rich envelope CIT 6 in the λ 2 mm and 1.3 mm bands carried out with the Arizona Radio Observatory (ARO) 12 m telescope and the Heinrich Hertz Submillimeter Telescope (SMT). The observations cover the frequency ranges of 131-160 GHz, 219-244 GHz, and 252-268 GHz with typical sensitivity limit of T R < 10 mK. A total of 74 individual emission features are detected, of which 69 are identified to arise from 21 molecular species and isotopologues, with 5 faint lines remaining unidentified. Two new molecules (C 4 H and CH 3 CN) and seven new isotopologues (C 17 O, 29 SiC 2 , 29 SiO, 30 SiO, 13 CS, C 33 S, and C 34 S) are detected in this object for the first time. The column densities, excitation temperatures, and fractional abundances of the detected molecules are determined using rotation diagram analysis. Comparison of the spectra of CIT 6 to that of IRC+10216 suggests that the spectral properties of CIT 6 are generally consistent with those of IRC+10216. For most of the molecular species, the intensity ratios of the lines detected in the two objects are in good agreement with each other. Nevertheless, there is evidence suggesting enhanced emission from CN and HC 3 N and depleted emission from HCN, SiS, and C 4 H in CIT 6. Based on their far-IR spectra, we find that CIT 6 probably has a lower dust-to-molecular gas ratio than IRC+10216. To investigate the chemical evolution of evolved stars, we compare the molecular abundances in the AGB envelopes CIT 6 and IRC+10216 and those in the bright proto-planetary nebula CRL 618. The implication on the circumstellar chemistry is discussed.

A Comprehensive Survey of Hydrogen Chloride in the Galaxy

“…As shown by the red curve in the appropriate panel in Figure 2, this approach adequately fits the line profile, yielding a solution that indicates that the HCl emission originated from a circumstellar shell with an expansion velocity of 4.8 km s −1 , compared to an observed terminal velocity of 14.5 km s −1 (see, for example, Schöier & Olofsson 2001). The [H 35 Cl]/[H 37 Cl] isotopic ratio in the shell is likely in excess of 4 from the optical depth shown in Table 2. CIT6 is widely regarded as similar to IRC+10216 with comparable chemical and physical characteristics, but a lower dust-to-molecular mass ratio (Zhang et al 2009). HCl is no exception in that regard.…”

“…It may be interesting to note that we derive an upper limit of 8 × 10 −8 for the HCL abundance in CIT6 by treating the low signal-to-noise detection as a 0.2 K line with 20 km s −1 in line width. Similar to the procedure used in Sgr A * and G1.6−0.025, the resulting HCl column density is 5 × 10 12 cm −2 , with an adopted gas temperature of 40 K and an H 2 column density of 6.3 × 10 19 cm −2 (Zhang et al 2009). …”

New Identifications of the CCH Radical in Planetary Nebulae: A Connection to C60?

“…One species of interest is CCH, the ethynyl radical. CCH is a common constituent of molecular clouds and C-rich circumstellar envelopes (e.g., Zhang et al 2009;Nishimura et al 2016), and has also been identified in a few protoplanetary nebulae and toward diffuse clouds (e.g., Lucas & Liszt 2000;Pardo et al 2007;Park et al 2008;Gerin et al 2011). It has been detected in three PNe to date: NGC 7027, NGC 6537, and the Helix (e.g., Zhang et al 2008;Tenenbaum et al 2009;Edwards & Ziurys 2013).…”

A Molecular Line Survey of CRL 2688 at 1 mm and 3 mm Wavelengths

“…The integration time for each frequency setting is 1-2 hours, achieving a typical sensitivity of T R < 10 mK. Using the same observation settings, we also observed other C-rich CSEs including IRC+10216, CIT 6, CRL 3068, and NGC 7027 (He et al 2008;Zhang, et al 2008Zhang, et al , 2009a, which cover an evolutionary sequence from AGB to PN stages. The detected molecules include CO, CN, CS, SiO, SiS, C 2 H, HCN, HNC, SiC 2 , C 3 H, C 3 N, C 4 H, C 5 H, HCO + , H 2 CO, H 2 CS, HC 3 N, HC 5 N, CH 3 CN, and their isotopomers.…”