Vapor Pressure and Evaporation Coefficient Measurements at Elevated Temperatures with a Knudsen Cell and a Quartz Crystal Microbalance: New Data for SiO

Wetzel, Steffen; Pucci, Annemarie; Gail, H. P.

doi:10.1021/je300199a

Cited by 12 publications

(14 citation statements)

References 18 publications

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“…Our results (Wetzel et al 2012b) extend the range of measured vapour pressures down to the pressure range relevant to circumstellar dust shells. Only moderate extrapolation of vapour pressures measured at higher temperatures in the laboratory to the lower temperatures of circumstellar dust condensation is required.…”

Section: Why Solid Silicon Monoxide?supporting

confidence: 71%

“…Calculations with T c = 800 K result in model spectra that are not significantly different. The old results of Gail & Sedlmayr (1986) seem to indicate a temperature for onset of SiO condensation as low as 600 K, but this conclusion can no longer be maintained since that calculation is based on older vapour-pressure measurements of solid SiO for which it is now known that they have to be revised downwards substantially (Ferguson & Nuth 2008;Wetzel et al 2012b;Ferguson & Nuth 2012), which increases the condensation temperature.…”

Section: Model Calculationsmentioning

confidence: 99%

“…New determinations of the vapour pressure of solid SiO (Ferguson & Nuth 2008Wetzel et al 2012b) have now shown that the older measurements were seriously in error and had strongly overestimated the vapour pressure. Our own results (Wetzel et al 2012b, Gail et al, in prep.…”

Section: Why Solid Silicon Monoxide?mentioning

confidence: 99%

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Laboratory measurement of optical constants of solid SiO and application to circumstellar dust

Wetzel

Klevenz

Gail

et al. 2013

A&A

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Context. Silicate minerals belong to the most abundant solids that form in cosmic environments. Their formation requires that a sufficient number of oxygen atoms per silicon atom are freely available. For the standard cosmic element mixture this can usually be taken for granted, but it becomes a problem at the transition from the oxygen-rich chemistry of M-stars to the carbon-rich chemistry of C-stars. In the intermediate type S-stars, most of the oxygen and carbon is consumed by formation of CO and SiO molecules, and left-over oxygen to build SiO 4 -tetrahedrons in solids becomes scarce. Under such conditions SiO molecules from the gas phase may condense into solid SiO. The infrared absorption spectrum of solid SiO differs from that of normal silicates by the absence of Si-O-Si bending modes around 18 μm whereas the absorption band due to Si-O bond stretching modes at about 10 μm is present. Observations show that exactly this particular characteristic can be found in some S-star spectra. Aims. We demonstrate that this observation may be explained by the formation of solid SiO as a major dust component at C/O abundance ratios close to unity. Methods. The infrared absorption properties of solid SiO are determined by laboratory transmission measurements of thin films of SiO produced by vapour deposition on a Si(111) wafer in the range between 100 cm −1 and 5000 cm −1 (2 μm and 100 μm). From the measured spectra the dielectric function of SiO is derived by using a Brendel-oscillator model, particularly suited to the representation of optical properties of amorphous materials. The results are used in model calculations of radiative transfer in circumstellar dust shells with solid SiO dust in order to determine the spectral features due to SiO dust. Results. Comparison of synthetic and observed spectra shows that reasonable agreement is obtained between the main spectral characteristics of emission bands due to solid silicon monoxide and an emission band centred on 10 μm, but without the accompanying 18μm band, observed in some S-stars. We propose that solid SiO is the carrier material of this 10 μm spectral feature.

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Section: Why Solid Silicon Monoxide?supporting

confidence: 71%

Section: Model Calculationsmentioning

confidence: 99%

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Laboratory measurement of optical constants of solid SiO and application to circumstellar dust

Wetzel

Klevenz

Gail

et al. 2013

A&A

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“…Fundamentally, the mass-loss rate and vapour pressure are measured by a quartz crystal micro balance and the Knudsen cell is heated by electron bombardment with an electron beam evaporator. All experimental results, as well as detailed information regarding the experimental conditions and the samples, are defined in Wetzel et al (2012).…”

Section: Vapour Pressure Of Siomentioning

confidence: 99%

Seed particle formation for silicate dust condensation by SiO nucleation

Gail¹,

Wetzel²,

Pucci³

et al. 2013

A&A

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Context. Dust formation in stellar outflows is initiated by the formation of some seed particles that form the growth centres for macroscopic dust grains. The nature of the seed particles for silicate dust in stellar outflows with an oxygen-rich element mixture is still an open question. Clustering of the abundant SiO molecules has been discussed several times as a possible mechanism and investigated both theoretically and by laboratory experiments. The initial results seemed to indicate, however, that condensation temperatures obtained by model calculations based on this mechanism are significant lower than what is really observed, which renders SiO nucleation unlikely. Aims. This negative result strongly rests on experimental data on the vapour pressure of SiO. The case for SiO nucleation may be not as bad as it previously seemed and needs to be discussed again because new determinations of the vapour pressure of SiO molecules over solid SiO have shown the older data on SiO vapour pressure to be seriously in error. Here we aim to check again the possibility that SiO nucleation triggers the cosmic silicate dust formation in light of improved new data. Methods. First we present results of our measurements of vapour pressure of solid SiO. Second, we use the improved vapour pressure data to recalibrate existing experimental data on SiO nucleation from the literature. Third, we use the recalibrated data on SiO nucleation in a simple model program for dust-driven winds to determine the condensation temperature of silicate in stellar outflows from AGB stars. Results. Our measurements extend the temperature range of measurements for the vapour pressure to lower temperatures and pressures than ever before. This improves the reliability of the required extrapolation from the temperature range where laboratory data can be obtained to the temperature range where circumstellar dust condensation is observed. We determine an analytical fit for the nucleation rate of SiO from recalibrated literature data and show that the onset of nucleation under circumstellar conditions commences at a higher temperature than was previously found. This brings calculated condensation temperatures of silicate dust much closer to the observed condensation temperatures derived from analysis of infrared spectra from dust-enshrouded M stars. Calculated condensation temperatures are still by about 100 K lower than observed ones, but this may be due to the greenhouse effect of silicate dust temperatures, which is not considered in our model calculation. Conclusions. The assumption that the onset of dust formation in late-type stars with oxygen-rich element mixtures is triggered by the cluster formation of SiO is compatible with dust condensation temperatures derived from infrared observations.

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“…(in units cm −3 s −1 ) following the suggestions of Nuth & Donn (1982) and using their experimental data, but now based on the redetermination of the vapour pressure of solid SiO by Wetzel et al (2012) and ; see also Nuth & Ferguson (2006) and Ferguson & Nuth (2008). Here S = p vap /p g,SiO is the supersaturation ratio, determined by the vapour pressure…”

Section: Nucleation By Siomentioning

confidence: 99%

Silicate condensation in Mira variables

Gail

Scholz

Pucci

2016

A&A

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Context. The formation of dust in winds of cool and highly evolved stars and the rate of injection of dust into the interstellar medium is not yet completely understood, despite the importance of the process for the evolution of stars and galaxies. This holds in particular for oxygen-rich stars, where it is still not known which process is responsible for the formation of the necessary seed particles of their silicate dust. Aims. We study whether the condensation of silicate dust in Mira envelopes could be caused by cluster formation by the abundant SiO molecules. Methods. We solve the dust nucleation and growth equations in the co-moving frame of a fixed mass element for a simplified model of the pulsational motions of matter in the outer layers of a Mira variable, which is guided by a numerical model for Mira pulsations. It is assumed that seed particles form through the clustering of SiO. The calculation of the nucleation rate is based on published experimental data. The quantity of dust formed is calculated via a moment method and the calculation of radiation pressure on dusty gas is based on a dirty silicate model. Results. Dust nucleation occurs in the model at the upper culmination of the trajectory of a gas parcel where it stays for a considerable time at low temperatures. Subsequent dust growth occurs during the descending part of the motion and continues after the next shock reversed motion. It is found that sufficient dust forms that radiation pressure exceeds the gravitational pull of the stars such that the mass element is finally driven out of the star. Conclusions. Nucleation of dust particles by clustering of the abundant SiO molecules could be the mechanism that triggers silicate dust formation in Miras.

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Vapor Pressure and Evaporation Coefficient Measurements at Elevated Temperatures with a Knudsen Cell and a Quartz Crystal Microbalance: New Data for SiO

Cited by 12 publications

References 18 publications

Laboratory measurement of optical constants of solid SiO and application to circumstellar dust

Laboratory measurement of optical constants of solid SiO and application to circumstellar dust

Seed particle formation for silicate dust condensation by SiO nucleation

Silicate condensation in Mira variables

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