Steps toward interstellar silicate mineralogy

Jäger, Cornelia; Dorschner, J.; Mutschke, H.; Posch, Th.; Henning, Th.

doi:10.1051/0004-6361:20030916

Cited by 283 publications

(313 citation statements)

References 41 publications

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“…For the pyroxene sample, the bands between 1100 and 800 cm −1 can be attributed to the asymmetric and symmetric stretching vibrations of the SiO 4 tetrahedron. The peak at 646 cm −1 corresponds to the bending vibration of Si−O bonds owing to the interaction with [15,36,37]. Compared with the pyroxene sample, the peak at 1076 cm −1 for enstatite exhibited no splitting, which is similar to the results reported by Jäger et al [37,38].…”

Section: Resultssupporting

confidence: 87%

“…The peak at 646 cm −1 corresponds to the bending vibration of Si−O bonds owing to the interaction with [15,36,37]. Compared with the pyroxene sample, the peak at 1076 cm −1 for enstatite exhibited no splitting, which is similar to the results reported by Jäger et al [37,38]. The band at 796 cm −1 can be ascribed to a frequency shift of the in-plane bending vibrations from amorphous SiO 2 [38].…”

Section: Resultssupporting

confidence: 85%

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Catalytic conversion of acetylene to polycyclic aromatic hydrocarbons over particles of pyroxene and alumina

Tian

Liu

Hammonds

et al. 2013

Phil. Trans. R. Soc. A.

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Polycyclic aromatic hydrocarbons (PAHs) are known to be present in many astrophysical objects and environments, but our understanding of their formation mechanism(s) is far from satisfactory. In this paper, we describe an investigation of the catalytic conversion reaction of acetylene gas to PAHs over pyroxene and alumina. Crystalline silicates such as pyroxenes (with general formula [Mg, Fe]SiO 3 ) and alumina (Al 2 O 3 ) are observed astrophysically through their infrared spectra and are likely to promote grain surface chemical reactions. In the experiments reported here, gas-phase PAHs were produced by the catalytic reaction of acetylene over crystalline silicates and alumina using a pulsed jet expansion technique and the gaseous products detected using time-of-flight mass spectrometry. In a separate experiment, the catalytic formation of PAHs from acetylene was further confirmed with acetylene gas at atmospheric pressure flowing continuously through a fixed-bed reactor. The gas effluent and carbonaceous compounds deposited on the catalysts were dissolved separately in dichloromethane and analysed using gas chromatography–mass spectrometry. Among the samples studied, alumina showed higher activity than the pyroxene-type grains for the acetylene reaction. It is proposed that formation of the PAHs relies on the Mg 2+ ions in the pyroxenes and Al 3+ ions in alumina, where these ions act as Lewis acid sites. X-ray diffraction, Fourier transform infrared and high-resolution transmission electron microscopy techniques were used to characterize the structure and physical properties of the pyroxene and alumina samples.

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Section: Resultssupporting

confidence: 87%

Section: Resultssupporting

confidence: 85%

Catalytic conversion of acetylene to polycyclic aromatic hydrocarbons over particles of pyroxene and alumina

Tian

Liu

Hammonds

et al. 2013

Phil. Trans. R. Soc. A.

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“…We adopted the halfopening angle of the density enhancement of 45 • and a density ratio of 4 as derived in Paper I, because the overall structure of the inner dust clouds seen in 2016 is similar to what was seen in 2015. For the dust opacities, we computed the absorption and scattering coefficients as well as the scattering matrix elements for spherical grains using the code of Bohren & Huffman (1983) from the complex refractive indices of Al 2 O 3 measured by Koike et al (1995, their "Alumina" sample), MgSiO 3 , and Mg 2 SiO 4 measured by Jäger et al (2003). The free parameters of our modeling are the optical depth of the dust shell in the radial direction, the outer radius of the dust shell, the exponent of the density distribution p, and the grain size.…”

Section: Monte-carlo Radiative Transfer Modelingmentioning

confidence: 99%

Clumpy dust clouds and extended atmosphere of the AGB star W Hydrae revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Ohnaka

Weigelt

Hofmann

2016

A&A

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Aims. Our recent visible polarimetric images of the well-studied AGB star W Hya taken at pre-maximum light (phase 0.92) with VLT/SPHERE-ZIMPOL have revealed clumpy dust clouds close to the star at ∼2 R . We present second-epoch SPHERE-ZIMPOL observations of W Hya at minimum light (phase 0.54) as well as high-spectral resolution long-baseline interferometric observations with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). Methods. We observed W Hya with VLT/SPHERE-ZIMPOL at three wavelengths in the continuum (645, 748, and 820 nm), in the Hα line at 656.3 nm, and in the TiO band at 717 nm. The VLTI/AMBER observations were carried out in the wavelength region of the CO first overtone lines near 2.3 µm with a spectral resolution of 12 000. Results. The high-spatial resolution polarimetric images obtained with SPHERE-ZIMPOL have allowed us to detect clear time variations in the clumpy dust clouds as close as 34-50 mas (1.4-2.0 R ) to the star. We detected the formation of a new dust cloud as well as the disappearance of one of the dust clouds detected at the first epoch. The Hα and TiO emission extends to ∼150 mas (∼6 R ), and the Hα images obtained at two epochs reveal time variations. The degree of linear polarization measured at minimum light, which ranges from 13 to 18%, is higher than that observed at pre-maximum light. The power-law-type limb-darkened disk fit to the AMBER data in the continuum results in a limb-darkened disk diameter of 49.1 ± 1.5 mas and a limb-darkening parameter of 1.16±0.49, indicating that the atmosphere is more extended with weaker limb-darkening compared to pre-maximum light. Our Monte Carlo radiative transfer modeling shows that the second-epoch SPHERE-ZIMPOL data can be explained by a shell of 0.1 µm grains of Al 2 O 3 , Mg 2 SiO 4 , and MgSiO 3 with a 550 nm optical depth of 0.6±0.2 and an inner and outer radii of 1.3 R and 10±2 R , respectively. Our modeling suggests the predominance of small (0.1 µm) grains at minimum light, in marked contrast to the predominance of large (0.5 µm) grains at pre-maximum light. Conclusions. The variability phase dependence of the characteristic grain size implies that small grains might just have started to form at minimum light in the wake of a shock, while the pre-maximum light phase might have corresponded to the phase of efficient grain growth.

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“…As these short wavelength data are important for determining the temperature of the grains, we used the optical constants given by Koike et al (1995) at λ < 7.7 μm. To model the silicate emission, we retrieved data for silicate dust species from the JENA database of optical constants from the works of Jäger et al (1994), Dorschner et al (1995), Mutschke et al (1998), andJäger et al (2003). The dust species that are considered are given in Table A.1.…”

Section: Mcmax and Model Assumptionsmentioning

confidence: 99%

Dusty wind of W Hydrae

Khouri

Waters

Koter

et al. 2015

A&A

107

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Context. Low-and intermediate-mass stars go through a period of intense mass-loss at the end of their lives, during the asymptotic giant branch (AGB) phase. While on the AGB a significant part, or even most, of their initial mass is expelled in a stellar wind. This process controls the final stages of the evolution of these stars and contributes to the chemical evolution of galaxies. However, the wind-driving mechanism of AGB stars is not yet well understood, especially so for oxygen-rich sources. Characterizing both the present-day mass-loss rate and wind structure and the evolution of the mass-loss rate of such stars is paramount to advancing our understanding of this processes. Aims. We study the dusty wind of the oxygen-rich AGB star W Hya to understand its composition and structure and shed light on the mass-loss mechanism. Methods. We modelled the dust envelope of W Hya using an advanced radiative transfer code. We analysed our dust model in the light of a previously calculated gas-phase wind model and compared it with measurements available in the literature, such as infrared spectra, infrared images, and optical scattered light fractions. Results. We find that the dust spectrum of W Hya can partly be explained by a gravitationally bound dust shell that probably is responsible for most of the amorphous Al 2 O 3 emission. The composition of the large (∼0.3 μm) grains needed to explain the scattered light cannot be constrained, but probably is dominated by silicates. Silicate emission in the thermal infrared was found to originate from beyond 40 AU from the star. In our model, the silicates need to have substantial near-infrared opacities to be visible at such large distances. The increase in near-infrared opacity of the dust at these distances roughly coincides with a sudden increase in expansion velocity as deduced from the gas-phase CO lines. The dust envelope of W Hya probably contains an important amount of calcium but we were not able to obtain a dust model that reproduces the observed emission while respecting the limit set by the gas mass-loss rate. Finally, the recent mass loss of W Hya is confirmed to be highly variable and we identify a strong peak in the mass-loss rate that occurred about 3500 years ago and lasted for a few hundred years.

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Steps toward interstellar silicate mineralogy

Cited by 283 publications

References 41 publications

Catalytic conversion of acetylene to polycyclic aromatic hydrocarbons over particles of pyroxene and alumina

Catalytic conversion of acetylene to polycyclic aromatic hydrocarbons over particles of pyroxene and alumina

Clumpy dust clouds and extended atmosphere of the AGB star W Hydrae revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Dusty wind of W Hydrae

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