Gas phase ion mobility measurements have been used to examine the geometries of silicon-doped carbon cluster cations, CnSi+ (n = 3−69). The observed isomers resemble those found for pure carbon clusters:  linear chains, monocyclic, bicyclic, and polycyclic rings, graphite sheets, and fullerenes. Generally, the silicon atom substitutes for a carbon atom and has only a minor effect on the structure. The linear chain and monocyclic ring isomers coexist for CnSi+ clusters over a wider range than for pure carbon cluster cations. The results provide the first conclusive experimental evidence that C2n-1Si+ fullerenes with an even number of atoms are closed-cage structures with the silicon atom networked into the cage. For C2nSi+ fullerenes with an odd number of atoms the silicon atom appears to be exohedral.

Structures of Silicon-Doped Carbon Clusters

Observations towards IRC +10216 of CS, SiO, SiS, NaCl, KCl, AlCl, AlF, and NaCN have been carried out with the IRAM 30-m telescope in the 80-357.5 GHz frequency range. A large number of rotational transitions covering a wide range of energy levels, including highly excited vibrational states, are detected in emission and serve to trace different regions of the envelope. Radiative transfer calculations based on the LVG formalism have been performed to derive molecular abundances from the innermost out to the outer layers. The excitation calculations include infrared pumping to excited vibrational states and inelastic collisions, for which up-to-date rate coefficients for rotational and, in some cases, ro-vibrational transitions are used. We find that in the inner layers CS, SiO, and SiS have abundances relative to H$_2$ of 4e-6, 1.8e-7, and 3e-6, respectively, and that CS and SiS have significant lower abundances in the outer envelope, which implies that they actively contribute to the formation of dust. Moreover, in the inner layers, the amount of sulfur and silicon in gas phase molecules is only 27 % for S and 5.6 % for Si, implying that these elements have already condensed onto grains, most likely in the form of MgS and SiC. Metal-bearing molecules lock up a relatively small fraction of metals, although our results indicate that NaCl, KCl, AlCl, AlF, and NaCN, despite their refractory character, are not significantly depleted in the cold outer layers. In these regions a few percent of the metals Na, K, and Al survive in the gas phase, either in atomic or molecular form, and are therefore available to participate in the gas phase chemistry in the outer envelope.

/pdf/molecular-abundances-in-the-inner-layers-of-irc-10216-5dc9sfb8bc.pdf

Molecular abundances in the inner layers of IRC +10216

A spectral survey of IRC+10216 has been carried out in the range 11 to 14 � m with a spectral resolution of about 4 km s −1 . We have identified a forest of lines in six bands of C2H2 involving the vibrational states from the ground to 3ν5 and in two bands of HCN, involving the vibrational states from the ground up to 2ν2. Some of these transitions are observed also in H 13 CCH and H 13 CN. We have estimated the kinetic, vibrational, and rotational temperatures, and the abundances and column densities of C2H2 and HCN between 1 and 300 R∗ (≃ 1.5 × 10 16 cm) by fitting about 300 of these ro-vibrational lines. The envelope can be divided into three regions with approximate boundaries at 0 ′′ .019 (the stellar photosphere), 0 ′′ .1 (the inner dust formation zone), and 0 ′′ .4 (outer dust formation zone). Most of the lines might require a large microturbulence broadening. The derived abundances of C2H2 and HCN increase by factors of 10 and 4, respectively, from the innermost envelope outwards. The derived column densities for both C2H2 and HCN are ≃ 1.6 ×10 19 cm −2 . Vibrational states up to 3000 K above ground are populated, suggesting pumping by near-infrared radiation from the star and innermost envelope. Low rotational levels can be considered under LTE while those with J > 20 −30 are not thermalized. A few lines require special analysis to deal with effects like overlap with lines of other molecules. Subject headings: line: identification — line: profiles — surveys — stars: AGB and post-AGB — stars: carbon — stars: individual (IRC+10216)

/pdf/a-detailed-analysis-of-the-dust-formation-zone-of-irc-10216-3qmv8if05h.pdf

A Detailed Analysis of the Dust Formation Zone of IRC +10216 Derived from Mid-Infrared Bands of C2H2 and HCN

Following discovery of the free radical SiCN in the C-star envelope IRC+10216, we report the detection in the same source of its isomer SiNC. The microwave spectra of SiNC and SiCN were studied in the laboratory and their rotational transition frequencies are accurately known. The ground fine structure state of SiNC, 2 Π1/2, gives rise to a series of rotational transitions, spaced by 12.8 GHz, each with Λ-doubling. Five weak lines are detected with the IRAM 30-m telescope at the frequencies of the J = 6.5 → 5. 5( e), 7.5 → 6.5 (f), 8.5 → 7.5 (f) and 10.5 → 9.5 (e) and (f) rotational transitions. Other SiNC lines from these or adjacent rotational transitions are found to be blended with stronger lines from known molecules. The lines assigned to SiNC have a cusped shape, characteristic of species confined to a hollow shell in the outer circumstellar envelope. They are twice weaker than their SiCN counterparts, which have the same shape, and presumably arise in the same region of the envelope. SiNC and SiCN have about the same abundance in IRC+10216, ∼4 × 10 −9 with respect to H2 .T his contrasts with HCN, HC3 Na nd HC 5N, for which the cyanide to isocyanide abundance ratio is >100.

/pdf/detection-of-the-sinc-radical-in-irc-10216-3f8l2pkuls.pdf

Detection of the SiNC radical in IRC+10216

Aims. We model the chemistry of the inner wind of the carbon star IRC+10216 and consider the effects of periodic shocks induced by the stellar pulsation on the gas to follow the non-equilibrium chemistry in the shocked gas layers. We consider a very complete set of chemical families, including hydrocarbons and aromatics, hydrides, halogens, and phosphorous-bearing species. Our derived abundances are compared to those for the latest observational data from large surveys and the Herschel telescope.Methods. A semi-analytical formalism based on parameterised fluid equations is used to describe the gas density, velocity, and temperature from 1 R ⋆  to 5 R ⋆ . The chemistry is described using a chemical kinetic network of reactions and a set of stiff, ordinary, coupled differential equations is solved.Results. The shocks induce an active non-equilibrium chemistry in the dust formation zone of IRC+10216 where the collision destruction of CO in the post-shock gas triggers the formation of O-bearing species such as H2 O and SiO. Most of the modelled molecular abundances agree very well with the latest values derived from Herschel data on IRC+10216. The hydrides form a family of abundant species that are expelled into the intermediate envelope. In particular, HF traps all the atomic fluorine in the dust formation zone. The halogens are also abundant and their chemistry is independent of the C/O ratio of the star. Therefore, HCl and other Cl-bearing species should also be present in the inner wind of O-rich AGB or supergiant stars. We identify a specific region ranging from 2.5 R ⋆  to 4 R ⋆ , where polycyclic aromatic hydrocarbons form and grow. The estimated carbon dust-to-gas mass ratio derived from the mass of aromatics formed ranges from 1.2 × 10-3 to 5.8 × 10-3 and agrees well with existing values deduced from observations. This aromatic formation region is situated outside hot layers where SiC2 is produced as a bi-product of silicon carbide dust synthesis. The MgS grains can form from the gas phase but in lower quantities than those necessary to reproduce the strength of the 30 μ m emission band. Finally, we predict that some molecular lines will show a flux variation with pulsation phase and time (e.g., H2 O), while other species will not (e.g., CO). These variations merely reflect the non-equilibrium chemistry that destroys and reforms molecules over a pulsation period in the shocked gas of the dust formation zone.

https://www.aanda.org/articles/aa/pdf/2012/09/aa18542-11.pdf

The inner wind of IRC+10216 revisited: new exotic chemistry and diagnostic for dust condensation in carbon stars

We have obtained high angular resolution ~ 3”), and high sensitivity maps of IRC+10216. SiC2 is found both in a spherical shell and in the very central region, indicating it is formed both in the inner envelope close to the star, and in the outer shell. The molecules SiS and CS are mostly found in the inner parts of the envelope, but are still detectable in the outer region (r ~ 15”) where the products of photochemistry are found. The maps show that IRC+10216 has a very clumpy envelope, with strong departures from spherical symmetry; an axis oriented NS-SW (P.A. 20°) can be seen in all maps. The radial brightness distribution of CS has secondary maxima, at the radius where the SiC2 shell has its peak emission. A preliminary map shows CN in the same shell, but also in a still larger outer shell. Time variations in the mass loss rate, could be invoked to explain the multiple shell structure of this envelope.

C. Kahane

Papers

Plateau De Bure Observations of IRC+10216: High Sensitivity Maps of SiC2, SIS, and CS