ON MAGNESIUM SULFIDE AS THE CARRIER OF THE 30 μm EMISSION FEATURE IN EVOLVED STARS

Zhang, Ke; Jiang, Biwei; Li, Aigen

doi:10.1088/0004-637x/702/1/680

Cited by 45 publications

(48 citation statements)

References 23 publications

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“…Another explanation is that MgS is not the carrier of the 30 μm band. A critical assessment of all previous MgS studies was made by Zhang et al (2009) who pointed out that the mass of MgS derived from the emission at 30 μm violated the available abundances of Mg and S in the stellar atmospheres, owing to the use of improper optical constants for MgS in the optical and UV wavelength domains. Alternative solids have been proposed (e.g., hydrogenated amorphous carbon, HAC, Grishko et al 2001).…”

Section: Summary and Discussionmentioning

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

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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Section: Summary and Discussionmentioning

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

133

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“…Neither S nor Zn shows much affinity for dust, although MgS has been recently proposed as a possible grain constituent by Zhang et al (2009). Thus, depletion onto dust in DLAs probably cannot explain this difference.…”

Section: [S/mentioning

confidence: 94%

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Spite

Caffau

Andrievsky

et al. 2011

A&A

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Context. Precise S abundances are important in the study of the early chemical evolution of the Galaxy. In particular the site of the formation remains uncertain because, at low metallicity, the trend of this α-element versus [Fe/H] remains unclear. Moreover, although sulfur is not bound significantly in dust grains in the ISM, it seems to behave differently in DLAs and old metal-poor stars. Aims. We attempt a precise measurement of the S abundance in a sample of extremely metal-poor stars observed with the ESO VLT equipped with UVES, taking into account NLTE and 3D effects. Methods. The NLTE profiles of the lines of multiplet 1 of S I were computed with a version of the program MULTI, including opacity sources from ATLAS9 and based on a new model atom for S. These profiles were fitted to the observed spectra. ratio in EMP stars is solar, as also found in DLAs. We derive an upper limit to the sulfur abundance [S/Fe] < +0.5 for the ultra metal-poor star CS 22949-037. This, along with a previously reported measurement of zinc, argues against the conjecture that the light-element abundance pattern of this star (and by analogy, the hyper iron-poor stars HE 0107-5240 and HE 1327-2326) would be due to dust depletion.

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“…We show that MgS is a viable candidate to explain the 30 μm feature independent of the absorbing efficiencies of these grains at λ < 10 μm. Moreover, we show that if one assumes thermal contact between the dust species in the outflow, the mass problem in HD 56126 reported by Zhang et al (2009) does not occur in LL Peg. Finally, we discuss the formation of composite grains in AGB outflows.…”

Section: Introductionmentioning

confidence: 71%

“…In an analysis of this data set, Hony et al (2002) concluded that the shape of the 30 μm feature can be best reproduced when the grains are not perfect spheres. Zhang et al (2009) modeled the 30 μm feature in the proto-planetary nebula HD 56126 using pure MgS grains. Assuming the grains to be irradiated by unattenuated stellar light, their analysis needed the optical properties of MgS at wavelengths λ < 10 μm.…”

Section: Introductionmentioning

confidence: 99%

Observational evidence for composite grains in an AGB outflow

Lombaert

Vries

Koter

et al. 2012

A&A

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The broad 30 μm feature in carbon stars is commonly attributed to MgS dust particles. However, reproducing the 30 μm feature with homogeneous MgS grains would require much more sulfur relative to the solar abundance. Direct gas-phase condensation of MgS occurs at a low efficiency. Precipitation of MgS on SiC precursor grains provides a more efficient formation mechanism, such that the assumption of homogeneous MgS grains may not be correct. Using a Monte Carlo-based radiative transfer code, we aim to model the 30 μm feature of the extreme carbon star LL Peg with MgS dust particles. We find that for LL Peg this modeling is insensitive to the unknown MgS optical properties at λ < 10 μm. When MgS is allowed to be in thermal contact with amorphous carbon and SiC, the amount of MgS required to reproduce the strength of 30 μm feature agrees with the solar abundance of sulfur, thereby resolving the reported MgS mass problem. We conclude that MgS is a valid candidate to be the carrier of the 30 μm feature when it is part of a composite grain population that has optical properties representative of an ensemble of particle shapes.

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ON MAGNESIUM SULFIDE AS THE CARRIER OF THE 30 μm EMISSION FEATURE IN EVOLVED STARS

Cited by 45 publications

References 23 publications

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

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

First stars

Observational evidence for composite grains in an AGB outflow

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