A Model of Silicate Grain Nucleation and Growth in Circumstellar Outflows

Paquette, John; Ferguson, Frank T.; Nuth, Joseph A.

doi:10.1088/0004-637x/732/2/62

Cited by 24 publications

(24 citation statements)

References 33 publications

(40 reference statements)

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“…Donn & Nuth [12] were initially rather critical of CNT for these reasons, although very recently they have shown that the lack of chemical equilibrium is perhaps not such a serious problem [13]. These workers have also introduced a scaled version of CNT [11]. Cherchneff & Dwek [14] have also recently discussed the shortcomings of CNT in some detail.…”

Section: Introductionmentioning

confidence: 99%

On the nucleation of dust in oxygen-rich stellar outflows

Plane

2013

Phil. Trans. R. Soc. A.

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Understanding the nature of dust condensation in the outflow from oxygen-rich asymptotic giant branch stars is a continuing problem. A kinetic model has been developed to describe the formation of gas-phase precursors from Ca, Mg, Fe, SiO and TiO in an outflow cooling from 1500 to 1000 K. Electronic structure calculations are used to identify efficient reaction pathways that lead to the formation of metal titanates and silicates. The molecular properties of the stationary points on the relevant potential energy surfaces are then used in a multi-well master equation solver to calculate pertinent rate coefficients. The outflow model couples an explicit treatment of gas-phase chemistry to a volume-conserving particle growth model. CaTiO 3 is shown to be the overwhelming contributor to the formation of condensation nuclei (CN), with less than 0.01 per cent provided by CaSiO 3 , (TiO 2 ) 2 and FeTiO 3 . Magnesium species make a negligible contribution. Defining CN as particles with radii greater than 2 nm, the model shows that for stellar mass loss rates above 3×10 −5 M ⊙ yr −1 , more than 10 −13 CN per H nucleus will be produced when the outflow temperature is still well above 1000 K. This is sufficient to explain the observed number density of grains in circumstellar dust shells.

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Section: Introductionmentioning

confidence: 99%

On the nucleation of dust in oxygen-rich stellar outflows

Plane

2013

Phil. Trans. R. Soc. A.

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“…This suggestion is based on the theoretical calculations done by Dominik et al (2016) and the laboratory studies of Krause & Blum (2004). The nucleation models by Paquette et al (2011) and the dynamical models by Fleischer (1994) suggest a narrow grain size distribution with a strong peak at small grain sizes in the inner wind region 14 . Further out in the wind, in the region beyond 400 mas, the relative motions of the dust particles will be governed by thermal (Brownian) motions caused by the random collisions with the gas molecules and turbulence caused by the relative difference in stopping time (captured in the Stokes number) for various dust particles.…”

Section: Scenario 2: Enforced Dynamics In An Isotropic Windmentioning

confidence: 95%

“…This equation bears the implicit assumption of no further grain growth when the dust particles travel through the wind. However, an interesting finding by Paquette et al (2011) is that silicate grain nucleation occurs over a wide range in stellar radius -from approximately sevento more than 20 stellar radii in their models. This additional nucleation would provide more grains to capture photons and accelerate dust (and hence gas) farther from the star.…”

Section: Scenario 2: Enforced Dynamics In An Isotropic Windmentioning

confidence: 99%

ALMA spectral line and imaging survey of a low and a high mass-loss rate AGB star between 335 and 362 GHz

Decin

Richards

Danilovich

et al. 2019

A&A

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Context. Low and intermediate mass stars are known to power strong stellar winds when evolving through the asymptotic giant branch (AGB) phase. Initial mass, luminosity, temperature, and composition determine the pulsation characteristics of the star and the dust species formed in the pulsating photospheric layers. Radiation pressure on these grains triggers the onset of a stellar wind. However, as of today, we still cannot predict the wind mass-loss rates and wind velocities from first principles neither do we know which species are the first to condense in the upper atmospheric regions. Aims. We aim to characterise the dominant physical, dynamical, and chemical processes in the inner wind region of two archetypical oxygen-rich (C/O<1) AGB stars, that is, the low mass-loss rate AGB star R Dor (Ṁ ∼1×10 −7 M /yr) and the high mass-loss rate AGB star IK Tau (Ṁ ∼5×10 −6 M /yr). The purpose of this study is to observe the key molecular species contributing to the formation of dust grains and to cross-link the observed line brightnesses of several species to the global and local properties of the star and its wind. Methods. A spectral line and imaging survey of IK Tau and R Dor was made with ALMA between 335 and 362 GHz (band 7) at a spatial resolution of ∼150 mas, which corresponds to the locus of the main dust formation region of both targets. Results. Some two hundred spectral features from 15 molecules (and their isotopologues) were observed, including rotational lines in both the ground and vibrationally excited states (up to v=5 for SiO). Detected species include the gaseous precursors of dust grains such as SiO, AlO, AlOH, TiO, and TiO2. We present a spectral atlas for both stars and the parameters of all detected spectral features. A clear dichotomy for the sulphur chemistry is seen: while CS, SiS, SO, and SO2 are abundantly present in IK Tau, only SO and SO2 are detected in R Dor. Also other species such as NaCl, NS, AlO, and AlOH display a completely different behaviour. From some selected species, the minor isotopologues can be used to assess the isotopic ratios. The channel maps of many species prove that both large and small-scale inhomogeneities persist in the inner wind of both stars in the form of blobs, arcs, and/or a disk. The high sensitivity of ALMA allows us to spot the impact of these correlated density structures in the spectral line profiles. The spectral lines often display a half width at zero intensity much larger than expected from the terminal velocity, v∞, previously derived for both objects (36 km/s versus v∞ ∼17.7 km/s for IK Tau and 23 km/s versus v∞ ∼5.5 km/s for R Dor). Both a more complex 3D morphology and a more forceful wind acceleration of the (underlying) isotropic wind can explain this trend. The formation of fractal grains in the region beyond ∼400 mas can potentially account for the latter scenario. From the continuum map, we deduce a dust mass of ∼ 3.7 × 10 −7 M and ∼ 2 × 10 −8 M for IK Tau and R Dor, respectively. Conclusions. The observations presented here provide importa...

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“…Among all the gas-phase species of refractory elements that could be involved in this process, SiO is one of the most abundant. For this reason, it has been often discussed whether cluster formation by this species is the starter reaction for the sequence of processes that ultimately leads to the condensation of silicate dust particles (Donn et al 1981;Nuth & Donn 1982, 1983Gail & Sedlmayr 1986, 1998aAli & Castleman 2005;Nuth & Ferguson 2006;Reber et al 2006;Paquette et al 2011;, although other possibilities have also been discussed recently (Goumans & Bromley 2012;Plane 2013;Bromley et al 2014;Gobrecht et al 2016). …”

Section: Introductionmentioning

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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A Model of Silicate Grain Nucleation and Growth in Circumstellar Outflows

Cited by 24 publications

References 33 publications

On the nucleation of dust in oxygen-rich stellar outflows

On the nucleation of dust in oxygen-rich stellar outflows

ALMA spectral line and imaging survey of a low and a high mass-loss rate AGB star between 335 and 362 GHz

Silicate condensation in Mira variables

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