On the Origin of Crystalline Silicate in Circumstellar Envelopesof Oxygen-rich Asymptotic Giant Branch Stars

Sogawa, Hiromitsu; Kozasa, Takashi

doi:10.1086/311992

Cited by 52 publications

(51 citation statements)

References 9 publications

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“…Unfortunately, condensation models are currently unable to say anything concrete about the physical structure of the solid materials they predict (Demyk et al 2000). Although Sogawa and Kozasa (1999) have shown crystalline grains can be produced in the outflows of high mass loss rate AGB stars by the annealing of heterogeneously condensed silicate mantles on alumina cores, that crystalline grains form by direct condensation is unlikely. Crystal growth only proceeds in relatively quiescent, slowly cooling vapors and is very slow, yielding only very small crystals.…”

Section: Formation Of Cometary Silicate Crystalsmentioning

confidence: 99%

Crystalline comet dust: Laboratory experiments on a simple silicate system

Thompson

Fonti

Verrienti

et al. 2003

Meteorit & Planetary Scien

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Abstract-Spectra for certain comets show the presence of crystalline silicate dust grains believed to have been incorporated during comet formation. While grain crystallization is widely assumed to result from the thermal annealing of precursor amorphous grains, the physical processes behind the silicate amorphous-to-crystalline transition are poorly understood. This makes it difficult to place constraints on the evolutionary histories of both grains and comets, and consequently, on the nebular conditions in which they formed. It has, therefore, become necessary to study this process in the laboratory using simulated grain materials. In this paper, we discuss recent results from laboratory investigations into a basic amorphous MgSiO 3 silicate annealed in the region of 1000 K. Our object is not to model the behavior of dust grains per se, but to study the underlying process of crystallization and separate the physics of the material from the astrophysics of dust grains. In our experiments, we bring together spectroscopic measurements made in the infrared with the high resolution structural probing capabilities of synchrotron X-ray powder diffraction. The combined use of these complementary techniques provides insights into the crystallization process that would not be easily obtained if each was used in isolation. In particular, we focus on the extent to which the identification of certain spectral features attributed to crystalline phases extends to the physical structure of the grain material itself. Specifically, we have identified several key features in the way amorphous MgSiO 3 behaves when annealed. Rather than crystallize directly to enstatite (MgSiO 3 ) structures, in crystallographic terms, amorphous MgSiO 3 can enter a mixed phase of crystalline forsterite (Mg 2 SiO 4 ) and SiO 2 -rich amorphous silicate where structural evolution appears to stall. Spectroscopically, the evolution of the 10 µm band does not appear to correlate directly with structural evolution, and therefore, may be a poor indicator of the degree of crystallinity. Indeed, certain features in this band may not be indicators of crystal type. However, the 20 µm band is found to be a good indicator of crystal structure. We suggest that forsterite forms from the ordering of pre-existing regions rich in SiO 4 and that this phase separation is aided by a dehydrogenation processes that results in the evolutionary stall. The implications of this work regarding future observations of comets are discussed.

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Section: Formation Of Cometary Silicate Crystalsmentioning

confidence: 99%

Crystalline comet dust: Laboratory experiments on a simple silicate system

Thompson

Fonti

Verrienti

et al. 2003

Meteorit & Planetary Scien

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“…However, detailed investigations about dust formation processes suggested that the dust formation temperature is higher for more dusty stars with stronger stellar winds (e.g. Kozasa et al 1984;Gail & Sedlmayr 1999;Sogawa & Kozasa 1999). If the dust formation temperature is much lower than 1000 K for low mass-loss rate O-rich AGB stars, the annealing would not be effective for those stars.…”

Section: Crystallization Of Dust Grains During a Pulsation Cyclementioning

confidence: 99%

Modeling IR spectra of OH/IR stars at different phases

Suh

Kim

2002

A&A

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Abstract. We investigate the spectral energy distributions (SEDs) of OH/IR stars (OH127.8+0.0 and OH26.5+0.6) having thick dust envelopes at different pulsation phases. Using new infrared observational data including the Infrared Space Observatory (ISO) data, we determine the new pulsation parameters. The deep silicate absorption features show significant variations depending on the pulsation phase. The variations are mainly due to changes in the properties of dust envelopes around the OH/IR stars. Comparing the results of detailed radiative model calculations with observations, we explore the changes of the relevant parameters of the envelopes and central stars depending on the pulsation phase. We find that when the central luminosity increases from the minimum to maximum phase, the inner radius of the dust shell increases with velocity faster than the outer shell expansion velocity and the dust shell optical depth decreases. During the phase change from the minimum to maximum, we find that dust formation ceases and about a half of the dust grains in the volume difference should have evaporated. During the phase change from the maximum to minimum, we find that the dust formation should be enhanced because the inner radius is decreasing. In the outer radii of the dust shell, the constant dust winds are easily maintained. We expect that the dust evaporation process driven by pulsation could be a mechanism for crystallizing the dust grains in inner regions of the dust shells around OH/IR stars.

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“…Dust condensation theory suggests that the density is critical in the formation of crystalline silicates (Tielens et al 1998;Gail & Sedlmayr 1999;Sogawa & Kozasa 1999), but this has still not been verified by observations. Secondly, how much crystalline silicate material is deposited by AGB stars to the diffuse ISM?…”

Section: Introductionmentioning

confidence: 96%

“…Theoretical studies predict that the formation of crystalline silicate dust is dependent on the gas density (Tielens et al 1998;Gail & Sedlmayr 1999;Sogawa & Kozasa 1999). Higher densities are more favorable for the formation of crystals than low densities.…”

Section: Introductionmentioning

confidence: 99%

Determining the forsterite abundance of the dust around asymptotic giant branch stars

Vries

Min

Waters

et al. 2010

A&A

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Aims. We present a diagnostic tool to determine the abundance of the crystalline silicate forsterite in AGB stars surrounded by a thick shell of silicate dust. Using six infrared spectra of high mass-loss oxygen rich AGB stars we obtain the forsterite abundance of their dust shells. Methods. We use a monte carlo radiative transfer code to calculate infrared spectra of dust enshrouded AGB stars. We vary the dust composition, mass-loss rate and outer radius. We focus on the strength of the 11.3 and the 33.6 μm forsterite bands, that probe the most recent (11.3 μm) and older (33.6 μm) mass-loss history of the star. Simple diagnostic diagrams are derived, allowing direct comparison to observed band strengths. Results. Our analysis shows that the 11.3 μm forsterite band is a robust indicator for the forsterite abundance of the current mass-loss period for AGB stars with an optically thick dust shell. The 33.6 μm band of forsterite is sensitive to changes in the density and the geometry of the emitting dust shell, and so a less robust indicator. Applying our method to six high mass-loss rate AGB stars shows that AGB stars can have forsterite abundances of 12% by mass and higher, which is more than the previously found maximum abundance of 5%.

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On the Origin of Crystalline Silicate in Circumstellar Envelopesof Oxygen-rich Asymptotic Giant Branch Stars

Cited by 52 publications

References 9 publications

Crystalline comet dust: Laboratory experiments on a simple silicate system

Crystalline comet dust: Laboratory experiments on a simple silicate system

Modeling IR spectra of OH/IR stars at different phases

Determining the forsterite abundance of the dust around asymptotic giant branch stars

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