scholarly journals Non-stoichiometric amorphous magnesium-iron silicates in circumstellar dust shells

2020 ◽  
Vol 644 ◽  
pp. A139
Author(s):  
Hans-Peter Gail ◽  
Akemi Tamanai ◽  
Annemarie Pucci ◽  
Ralf Dohmen
Keyword(s):  
Infrared Emission ◽  
Dust Particles ◽  
Circumstellar Dust ◽  
Expansion Velocity ◽  
Condensation Temperature ◽  
Dust Formation ◽  
Dust Grains ◽  
Dust Shell ◽  
Dust Shells ◽  
Stellar Outflows

Aims. We study the growth of dust in oxygen-rich stellar outflows in order to find out to which extent dust growth models can quantitatively reconcile with the quantities and nature of dust as derived from observations of the infrared emission from circumstellar dust shells. Methods. We use a set of nine well-observed massive supergiants with optically thin dust shells as testbeds because of the relatively simple properties of the outflows from massive supergiants, contrary to the case of AGB stars. Models of the infrared emission from their circumstellar dust shells are compared to their observed infrared spectra to derive the essential parameters that rule dust formation in the extended envelope of these stars. The results are compared with a model for silicate dust condensation. Results. For all objects, the infrared emission in the studied wavelength range, between 6 and 25 μm, can be reproduced rather well by a mixture of non-stoichiometric iron-bearing silicates, alumina, and metallic iron dust particles. For three objects (μ Cep, RW Cyg, and RS Per), the observed spectra can be sufficiently well reproduced by a stationary and (essentially) spherically symmetric outflow in the instantaneous condensation approximation. For these objects, the temperature at the onset of massive silicate dust growth is of the order of 920 K and the corresponding outflow velocity of the order of the sound velocity. This condensation temperature is only somewhat below the vapourisation temperature of the silicate dust and suggests that the silicate dust grows on the corundum dust grains that formed well inside of the silicate dust shell at a much higher temperature. The low expansion velocity at the inner edge of the silicate dust shell further suggests that, for these supergiants, the region inside the silicate dust shell has an only subsonic average expansion velocity, though a high degree of supersonic turbulence is indicated by the widths of spectral lines. Conclusions. Our results suggest that for the two major problems of dust formation in stellar outflows, that is (i) formation of seed nuclei and (ii) their growth to macroscopic dust grains, we are gradually coming close to a quantitative understanding of the second item.

scholarly journals Recurrent Dust Formation by Wolf-Rayet Stars

1991 ◽  
Vol 143 ◽  
pp. 417-420
Author(s):  
P.M. Williams ◽  
K.A. Van Der Hucht ◽  
P.S. Thé ◽  
P. Bouchet ◽  
G. Roberts
Keyword(s):  
Dust Emission ◽  
Infrared Emission ◽  
Circumstellar Dust ◽  
Stellar Winds ◽  
Dust Formation ◽  
Dust Grains

A number of Wolf-Rayet stars show variations of up to a factor of ten in their infrared emission on timescales of months to years while their photospheric luminosities remain unchanged. This can be interpreted in terms of variation in the rates at which dust grains form in their stellar winds. Our data show variable circumstellar dust emission from WR 70 (HD 137603), with an episode of enhanced dust formation in early 1989, and fading of emission by dust formed around WR 48a (in 1979) and WR 19 some time before our first observation in 1988. We consider their relation to WR 140 (HD 193793), which forms dust in its wind for a few months at intervals of 7.94 years.

scholarly journals Long Baseline Interferometric Observations of Circumstellar Dust Shells at 11 Microns

1994 ◽  
Vol 158 ◽  
pp. 383-386
Author(s):  
W.C. Danchi ◽  
L. Greenhill ◽  
M. Bester ◽  
C.G. Degiacomi ◽  
C.H. Townes ◽  
...  
Keyword(s):  
Spatial Distribution ◽  
Radiative Transfer ◽  
Circumstellar Dust ◽  
Dust Formation ◽  
Late Type ◽  
Dust Shells ◽  
Long Baseline ◽  
Higher Temperature ◽  
Single Baseline ◽  
Radiative Transfer Modeling

The spatial distribution of dust around a sample of well-known late-type stars has been studied with the Infrared Spatial Interferometer (ISI) located at Mt. Wilson. Currently operating with a single baseline as a heterodyne interferometer at 11.15 μm, the ISI has obtained visibility curves of these stars. Radiative transfer modeling of the visibility curves has yielded estimates of the inner radii of the dust shells, the optical depth at 11 μm, and the temperature of the dust at the inner radii. For stars in which the dust is resolved, estimates of the stellar diameter and temperature can also be made. Broadly speaking two classes of stars have been found. One class has inner radii of their dust shells very close to the photospheres of the stars themselves (3–5 stellar radii) and at a higher temperature (~ 1200 K) than previously measured. This class includes VY CMa, NML Tau, IRC +10216, and o Ceti. For the latter two the visibility curves change with the luminosity phase of the star and new dust appears to form at still smaller radii during minimum luminosity. The second class of stars has dust shells with substantially larger inner radii and very little dust close to the stars, and includes α Ori, α Sco, α Her, R Leo, and χ Cyg. This indicates sporadic production of dust and no dust formation within the last several decades.

scholarly journals Modelling the carbon AGB star R Sculptoris

2018 ◽  
Vol 614 ◽  
pp. A17 ◽  
Author(s):  
M. Brunner ◽  
M. Maercker ◽  
M. Mecina ◽  
T. Khouri ◽  
F. Kerschbaum
Keyword(s):  
Mass Loss ◽  
Spectral Energy Distribution ◽  
Expansion Velocity ◽  
Agb Stars ◽  
Dust Grains ◽  
Dust Shell ◽  
Shell Mass ◽  
Dust Mass ◽  
Dust Grain ◽  
Grain Properties

Context. On the asymptotic giant branch (AGB), Sun-like stars lose a large portion of their mass in an intensive wind and enrich the surrounding interstellar medium with nuclear processed stellar material in the form of molecular gas and dust. For a number of carbon-rich AGB stars, thin detached shells of gas and dust have been observed. These shells are formed during brief periods of increased mass loss and expansion velocity during a thermal pulse, and open up the possibility to study the mass-loss history of thermally pulsing AGB stars. Aims. We study the properties of dust grains in the detached shell around the carbon AGB star R Scl and aim to quantify the influence of the dust grain properties on the shape of the spectral energy distribution (SED) and the derived dust shell mass. Methods. We modelled the SED of the circumstellar dust emission and compared the models to observations, including new observations of Herschel/PACS and SPIRE (infrared) and APEX/LABOCA (sub-millimeter). We derived present-day mass-loss rates and detached shell masses for a variation of dust grain properties (opacities, chemical composition, grain size, and grain geometry) to quantify the influence of changing dust properties to the derived shell mass. Results. The best-fitting mass-loss parameters are a present-day dust mass-loss rate of 2 × 10−10 M⊙ yr−1 and a detached shell dust mass of (2.9 ± 0.3) × 10−5 M⊙. Compared to similar studies, the uncertainty on the dust mass is reduced by a factor of 4. We find that the size of the grains dominates the shape of the SED, while the estimated dust shell mass is most strongly affected by the geometry of the dust grains. Additionally, we find a significant sub-millimeter excess that cannot be reproduced by any of the models, but is most likely not of thermal origin.

scholarly journals Dust formation and evolution around carbon stars

2018 ◽  
Author(s):  
◽  
Hannah Groom
Keyword(s):  
Mass Loss ◽  
Spectral Feature ◽  
Model Parameters ◽  
Condensation Temperature ◽  
Dust Formation ◽  
Carbon Stars ◽  
Dust Shells ◽  
Low Mass ◽  
Dust Condensation ◽  
Loss Rates

Evolved intermediate-mass stars with carbon-to-oxygen ratios (C/O) above unity are known as carbon stars. Carbon stars are surrounded by dust shells dominated by carbon (C) and silicon carbide (SiC) grains. These SiC grains have a diagnostic spectral feature at [about]11 [mu]m. We have selected a sample of 9 carbon stars with low mass-loss rates such that their dust shells are sufficiently optically thin to allow abundance analysis of the stars' photospheres. This allows the study of how atomic abundances affect dust formation around carbons stars. We present the result of radiative transfer modeling for these stars, and compare the resulting dust shell parameters to published abundance measurements. To constrain model parameters, we use published mass-loss rates, expansion velocities, and theoretical dust condensation models to estimate the dust condensation temperature, and spectral types to constrain stellar effective temperatures. We found significant correlations for the single-shell modeling with graphite/iron grains and amorphous carbon (AmC)/iron grains: (1) [subscript]0.55[mu]m and gas-to-dust ratio, (2) iron grains and graphite or AmC grains, (3) graphite or AmC grains and Fe/H, and (4) iron grains and Fe/H. For the collated data the significant correlations we found were: (1) for dust formation temperature and the change of temperature from the formation of graphite grains to the formation of SiC grains (2) C/O and the change of temperature from the formation of graphite grains to the formation of SiC grains. Lastly, between the abundance of SiC grains when compared to the abundances of SiC grains in graphite, AmC, graphite and iron and AmC and iron grains models. This shows that there is no sensitivity in the continuum when choosing the type of carbon to model with.

scholarly journals Circumstellar Dust Shells

1975 ◽  
Vol 67 ◽  
pp. 165-171
Author(s):  
L. M. Shulman
Keyword(s):  
Optical Properties ◽  
Optical Thickness ◽  
Thin Shell ◽  
Graphite Particle ◽  
Circumstellar Dust ◽  
Dust Shell ◽  
Dust Shells ◽  
Consequent Increase

The optical properties of circumstellar graphite particle shells are considered. In the case of the optically thin shell (e.g. the shell of a RCB star after minimum), the temperature of the grains is calculated. It is shown that the dust shell always has a sharp inner border. The RCB phenomenon may be considered as a sudden moving of this border toward the star and the consequent increase in its optical thickness.

scholarly journals Interstellar Matter in Meteorites and Interplanetary Dust

2000 ◽  
Vol 197 ◽  
pp. 527-536
Author(s):  
S. Messenger
Keyword(s):  
Molecular Cloud ◽  
Interplanetary Dust ◽  
Chemical Processes ◽  
Dust Particles ◽  
Circumstellar Dust ◽  
Interstellar Matter ◽  
Dust Grains ◽  
Interplanetary Dust Particles ◽  
Stellar Nucleosynthesis ◽  
Isotopic Anomalies

Meteorites and interplanetary dust particles (IDPs) are primitive solar system materials which contain preserved nebular condensates, circumstellar dust grains and partially preserved molecular cloud matter. The circumstellar dust grains found in meteorites are direct samples of a variety of stars, and provide detailed constraints on models of stellar nucleosynthesis. Hydrogen and nitrogen isotopic anomalies in organic matter in meteorites and IDPs are thought to originate from chemical processes in a presolar molecular cloud. This material is better preserved, but less well characterized among IDPs.

Dynamical Models of Circumstellar Dust Shells around Long-Period Variables

2000 ◽  
Vol 177 ◽  
pp. 377-384
Author(s):  
A. J. Fleischer ◽  
J. M. Winters ◽  
E. Sedlmayr
Keyword(s):  
Complex Interaction ◽  
Time Dependent ◽  
Circumstellar Dust ◽  
Dust Formation ◽  
Dynamical Models ◽  
Dust Shells ◽  
Long Period ◽  
Detailed Treatment ◽  
Long Period Variables ◽  
Dust Complex

We present dynamical models of circumstellar dust shells around long-period variables which include time-dependent hydrodynamics and a detailed treatment of dust formation, growth and evaporation. Important effects due to the complex interaction between the dynamics of the pulsating atmosphere and the dust complex are demonstrated.

scholarly journals Temperature distributions in circumstellar dust shells

1987 ◽  
Vol 122 ◽  
pp. 557-558
Author(s):  
Th. Henning ◽  
J. Gürtler
Keyword(s):  
Temperature Distribution ◽  
Ir Spectrum ◽  
Circumstellar Dust ◽  
Dust Formation ◽  
Physical Processes ◽  
Dust Shells ◽  
Temperature Distributions

The knowledge of the temperature distribution in circumstellar dust shells (CDS) is needed for both the calculation of the emergent IR spectrum and the modelling of physical processes, e.g. dust formation.

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