The abundance of SiS in circumstellar envelopes around AGB stars

Schoeier, F. L.; Bast, J.; Olofsson, H.; Lindqvist, Michael

doi:10.1051/0004-6361:20077887

Cited by 69 publications

(107 citation statements)

References 41 publications

(82 reference statements)

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“…When taking the abundance uncertainty into account, this agrees with the results of Schöier et al (2007), who find a value of 2 × 10 −6 from lowexcitation SiS lines. From observations of the 13.5 μm fundamental band of SiS, Boyle et al (1994) obtained a gradient in the abundance of SiS, going from 4.3 × 10 −6 at a distance of 12 R and rising to 4.3×10 −5 close to the stellar surface.…”

Section: Sissupporting

confidence: 81%

Silicon in the dust formation zone of IRC +10216

Decin

Cernicharo

Barlow

et al. 2010

A&A

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The interstellar medium is enriched primarily by matter ejected from evolved low and intermediate mass stars. The outflows from these stars create a circumstellar envelope in which a rich gas-phase and dust-nucleation chemistry takes place. We observed the nearest carbon-rich evolved star, IRC +10216, using the PACS (55−210 μm) and SPIRE (194−672 μm) spectrometers on board Herschel. We find several tens of lines from SiS and SiO, including lines from the v = 1 vibrational level. For SiS these transitions range up to J = 124−123, corresponding to energies around 6700 K, while the highest detectable transition is J = 90−89 for SiO, which corresponds to an energy around 8400 K. Both species trace the dust formation zone of IRC +10216, and the broad energy ranges involved in their detected transitions permit us to derive the physical properties of the gas and the particular zone in which each species has been formed. This allows us to check the accuracy of chemical thermodynamical equilibrium models and the suggested depletion of SiS and SiO due to accretion onto dust grains.

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Section: Sissupporting

confidence: 81%

Silicon in the dust formation zone of IRC +10216

Decin

Cernicharo

Barlow

et al. 2010

A&A

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“…More generally, Cherchneff (2006) studied the inner wind composition as a function of the carbon-to-oxygen (C/O) ratio of the stellar photosphere and confirmed the formation of O-bearing species in carbon stars and that of C-bearing molecules in O-rich AGB stars as a result of non-equilibrium chemistry induced by periodic shocks. Several observations of high energy rotational transitions of HCN, SiS, CS, and SiO in AGBs and supergiants at mm and submm wavelengths corroborated these results (e.g., Schöier et al 2006Schöier et al , 2007Ziurys et al 2007Ziurys et al , 2009Decin et al 2008).…”

Section: Introductionsupporting

confidence: 67%

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

Cherchneff

2012

A&A

122

133

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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 H 2 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 SiC 2 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 μm emission band. Finally, we predict that some molecular lines will show a flux variation with pulsation phase and time (e.g., H 2 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.

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“…As concerns previous observational studies, there is a general consensus on the presence of a gradient in the abundance of SiS, with reported values in the range 10 −6 −10 −5 in the inner layers (>6.5 × 10 −7 by Turner 1987; 7.5 × 10 −6 by Bieging & Nguyen-Q-Rieu 1989; 4.3 × 10 −5 close to the stellar surface decreasing down to 4.3 × 10 −6 at 12 R * by Boyle et al 1994) and reported values in the range 10 −7 −10 −6 in the outer envelope (2.4 × 10 −7 by Sahai et al 1984; 5 × 10 −7 by Nguyen-Q-Rieu et al 1984; 5 × 10 −7 by Henkel et al 1983; 1.5−4 × 10 −7 by Henkel et al 1985;6.5 × 10 −7 by Bieging & Nguyen-Q-Rieu 1989; 1.4 × 10 −6 by Schöier et al 2007). The more recent study of Decin et al (2010a), based on observations with SPIRE and PACS that are particularly sensitive to the warm inner envelope, reports a SiS abundance of 4 × 10 −6 , in good agreement with our finding for the inner layers.…”

Section: Molecular Abundancesmentioning

confidence: 99%

Molecular abundances in the inner layers of IRC +10216

Agúndez

Fonfría

Cernicharo

et al. 2012

A&A

136

259

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Context. The inner layers of circumstellar envelopes around asymptotic giant branch stars are sites where a variety of processes such as thermochemical equilibrium, shocks induced by the stellar pulsation, and condensation of dust grains determine the chemical composition of the material that is expelled into the outer envelope layers and, ultimately, into interstellar space. Aims. We aim at studying the abundances, throughout the whole circumstellar envelope of the carbon star IRC +10216, of several molecules formed in the inner layers in order to constrain the different processes at work in such regions. Methods. 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. Results. We find that in the inner layers CS, SiO, and SiS have abundances relative to H 2 of 4 × 10 −6 , 1.8 × 10 −7 , and 3 × 10 −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.

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The abundance of SiS in circumstellar envelopes around AGB stars

Cited by 69 publications

References 41 publications

Silicon in the dust formation zone of IRC +10216

Silicon in the dust formation zone of IRC +10216

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

Molecular abundances in the inner layers of IRC +10216

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