Silicon in the dust formation zone of IRC +10216

Decin, L.; Cernicharo, J.; Barlow, M. J.; Royer, P.; Vandenbussche, B.; Wesson, R.; Polehampton, E. T.; Beck, E. De; Agúndez, M.; Blommaert, J. A. D. L.; Cohen, Martin; Daniel, F.; Meester, W. De; Exter, Katrina; Feuchtgruber, H.; Fonfría, J. P.; Gear, Walter Kieran; Goicoechea, J. R.; Gomez, Haley Louise; Groenewegen, M. A. T.; Hargrave, Peter Charles; Huygen, R.; Imhof, P.; Ivison, R. J.; Jean, C.; Kerschbaum, F.; Leeks, S. J.; Lim, T.; Matsuura, M.; Olofsson, G.; Posch, Th.; Regibo, S.; Savini, G.; Sibthorpe, B.; Swinyard, Bruce; Tercero, B.; Waelkens, C.; Witherick, D. K.; Yates, J. A.

doi:10.1051/0004-6361/201014562

Cited by 33 publications

(56 citation statements)

References 30 publications

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“…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 Abundancessupporting

confidence: 92%

“…ISO observed a full infrared scan from 2.4 to 197 μm, albeit with a low spatial and spectral resolution, that probed the inner layers of IRC +10216 through the lines of abundant molecules such as CO, C 2 H 2 , and HCN (Cernicharo et al 1996(Cernicharo et al , 1999. More recently, spectral surveys carried out with Herschel with a high angular resolution have detected many high energy rotational transitions of molecules such as CO, HCN, CS, SiS, SiO, and SiC 2 , at low spectral resolution using SPIRE and PACS in the 55−672 μm range (Decin et al 2010a), and at high spectral resolution in the 488−1901 GHz range covered by HIFI .…”

Section: Introductionmentioning

confidence: 99%

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Molecular abundances in the inner layers of IRC +10216

Agúndez

Fonfría

Cernicharo

et al. 2012

A&A

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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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Section: Molecular Abundancessupporting

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Molecular abundances in the inner layers of IRC +10216

Agúndez

Fonfría

Cernicharo

et al. 2012

A&A

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134

259

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“…The dust nucleation zone starts around 5 R (or 100-150 mas, Decin et al 2010b). This is similar to the resolution of these images, but we cannot distinguish faint details this close to the strong stellar peak.…”

Section: Stellar and Dust Propertiessupporting

confidence: 76%

ALMA data suggest the presence of spiral structure in the inner wind of CW Leonis

Decin

Richards

Neufeld

et al. 2015

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Context. Evolved low-mass stars lose a significant fraction of their mass through stellar winds. While the overall morphology of the stellar wind structure during the asymptotic giant branch (AGB) phase is thought to be roughly spherically symmetric, the morphology changes dramatically during the post-AGB and planetary nebula phase, during which bipolar and multi-polar structures are often observed. Aims. We aim to study the inner wind structure of the closest well-known AGB star CW Leo. Different diagnostics probing different geometrical scales have implied a non-homogeneous mass-loss process for this star: dust clumps are observed at milli-arcsec scale, a bipolar structure is seen at arcsecond-scale, and multi-concentric shells are detected beyond 1 . Methods. We present the first ALMA Cycle 0 band 9 data around 650 GHz (450 μm) tracing the inner wind of CW Leo. The fullresolution data have a spatial resolution of 0. 42 × 0. 24, allowing us to study the morpho-kinematical structure of CW Leo within ∼6 . Results. We have detected 25 molecular emission lines in four spectral windows. The emission of all but one line is spatially resolved. The dust and molecular lines are centered around the continuum peak position, which is assumed to be dominated by stellar emission. The dust emission has an asymmetric distribution with a central peak flux density of ∼2 Jy. The molecular emission lines trace different regions in the wind acceleration region and imply that the wind velocity increases rapidly from about 5 R , almost reaching the terminal velocity at ∼11 R . The images prove that vibrational lines are excited close to the stellar surface and that SiO is a parent molecule. The channel maps for the brighter lines show a complex structure; specifically, for the 13 CO J = 6-5 line, different arcs are detected within the first few arcseconds. The curved structure in the position-velocity (PV) map of the 13 CO J = 6-5 line can be explained by a spiral structure in the inner wind of CW Leo, probably induced by a binary companion. From modelling the ALMA data, we deduce that the potential orbital axis for the binary system lies at a position angle of ∼10-20• to the north-east and that the spiral structure is seen almost edge-on. We infer an orbital period of 55 yr and a binary separation of 25 au (or ∼8.2 R ). We tentatively estimate that the companion is an unevolved low-mass main-sequence star. Conclusions. A scenario of a binary-induced spiral shell can explain the correlated structure seen in the ALMA PV images of CW Leo. Moreover, this scenario can also explain many other observational signatures seen at different spatial scales and in different wavelength regions, such as the bipolar structure and the almost concentric shells. ALMA data hence for the first time provide the crucial kinematical link between the dust clumps seen at milli-arcsecond scale and the almost concentric arcs seen at arcsecond scale.

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“…To test our results, we assess the mass of AC dust that can grow from the C 6 H 6 and C 2 H 2 masses in the radius range of interest, compare the derived AC mass to the gas mass, and derive dust-to-gas mass ratios. It has been found that IRC+10216 is heavily dust-rich and has a dust-to-gas mass ratio comprised between 1.4 × 10 −3 and 4 × 10 −3 based on various studies (e.g., Ivezić & Elitzur 1996;Groenewegen 1998;Decin et al 2010a). In Table 5, we list the abundances and total masses of C 2 H 2 and C 6 H 6 formed as a function of radius.…”

Section: Carbon Dust Precursors: Hydrocarbons and Aromaticsmentioning

confidence: 99%

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

Cherchneff

2012

A&A

119

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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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Silicon in the dust formation zone of IRC +10216

Cited by 33 publications

References 30 publications

Molecular abundances in the inner layers of IRC +10216

Molecular abundances in the inner layers of IRC +10216

ALMA data suggest the presence of spiral structure in the inner wind of CW Leonis

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

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