Modeling of C stars with core/mantle grains: Amorphous carbon + SiC

Lorenz-Martins, S.; Araújo, F. X. de; Landaberry, S. J. C.; Almeida, W.G. de; Nader, Rundsthen Vasques de

doi:10.1051/0004-6361:20000411

Cited by 16 publications

(23 citation statements)

References 33 publications

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“…The inner radius of the dust shell is close to 5 R (see Table 5) as determined by Lorenz-Martins et al (2001). The optical constants of AmC from Rouleau & Martin (1991) and SiC from Pégourié (1988), together with the fractional abundance of SiC/AmC of 0.08 found by Lorenz-Martins et al (2001), are also in good agreement with the values found in this work. These last two characteristics allow us to determine the opacities of SiC used to interpret the spectro-interferometric data of R Scl with the help of the dynamic model (see Sect.…”

Section: Determination Of the Sic Dust Parameterssupporting

confidence: 88%

“…The AmC dust grain optical constants from Rouleau & Martin (1991) are the same as adopted for the hydrodynamic calculation (Andersen et al 2003;Höfner et al 2003). Good agreement is also found with the works of Dehaes et al (2007) and Lorenz-Martins et al (2001), who determined a visual optical depth of 0.5 +0.2 −0.3 and a 1.0 μm optical depth of 0.1, respectively. The dust shell inner boundary temperature of 1000 +300 −200 K found by Dehaes et al (2007) agrees well with the value determined in this work.…”

Section: Determination Of the Sic Dust Parameterssupporting

confidence: 78%

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Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

Sacuto

Aringer

Hron

et al. 2010

A&A

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Context. We study the circumstellar environment of the carbon-rich star R Sculptoris using the near-and mid-infrared high spatial resolution observations from the ESO-VLTI focal instruments VINCI and MIDI, respectively. Aims. These observations aim at increasing our knowledge of the dynamic processes in play within the very close circumstellar environment where the mass loss of AGB stars is initiated. Methods. We first compare the spectro-interferometric measurements of the star at different epochs to detect the dynamic signatures of the circumstellar structures at different spatial and spectral scales. We then interpret these data using a self-consistent dynamic model atmosphere to discuss the dynamic picture deduced from the observations. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort is first applied to determining these parameters. Results. Interferometric observations do not show any significant variability effect at the 16 m baseline between phases 0.17 and 0.23 in the K band, and for both the 15 m baseline between phases 0.66 and 0.97 and the 31 m baseline between phases 0.90 and 0.97 in the N band. We find fairly good agreement between the dynamic model and the spectrophotometric data from 0.4 to 25 μm. The model agrees well with the time-dependent flux data at 8.5 μm, whereas it is too faint at 11.3 and 12.5 μm. The VINCI visibility measurements are reproduced well, meaning that the extension of the model is suitable in the K-band. In the mid-infrared, the model has the proper extension to reveal molecular structures of C 2 H 2 and HCN located above the stellar photosphere. However, the windless model used is not able to reproduce the more extended and dense dusty environment. Conclusions. Among the different explanations for the discrepancy between the model and the measurements, the strong nonequilibrium process of dust formation is one of the most probable. The transition from windless atmospheres to models with considerable mass-loss rates occurs in a very narrow range of stellar parameters, especially for the effective temperature, the C/O ratio, and the pulsation amplitude. A denser sampling of such critical regions of the parameter space with additional models might lead to a better representation of the extended structures of low mass-loss carbon stars like R Sculptoris. The complete dynamic coupling of gas and dust and the approximation of grain opacities with the small-particle limit in the dynamic calculation could also contribute to the difference between the model and the data.

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Section: Determination Of the Sic Dust Parameterssupporting

confidence: 88%

Section: Determination Of the Sic Dust Parameterssupporting

confidence: 78%

Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

Sacuto

Aringer

Hron

et al. 2010

A&A

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“…Sopka et al 1985;Groenewegen 1997;Blanco et al 1998;Lorenz-Martins et al 2001;Gautschy-Loidl et al 2004), our models follow a steeper density distribution ( ∝r −2.3 for AFGL 5379 and CW Leo, ∝r −2.5 for AFGL 3068) (see the density power index "n" in Table 3). …”

Section: Density Distributionmentioning

confidence: 52%

870 μm observations of evolved stars with LABOCA

Ladjal

Justtanont

Groenewegen

et al. 2010

A&A

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Context. During their evolution, asymptotic giant branch (AGB) stars experience a high mass loss which leads to the formation of a circumstellar envelope (CSE) of dust and gas. The mass loss process is the most important phenomenon during this evolutionary stage. In order to understand it, it is important to study the physical parameters of the CSE. The emission of the CSE in the (sub)millimetre range is dominated by the dust continuum. This means that (sub)millimetre observations are a key tool in tracing the dust and improving our knowledge of the mass loss process. Aims. The aim of this study is to use new submillimetre observations of a sample of evolved stars to constrain the CSE physical parameters. Methods. We used aperture photometry to determine the fluxes at 870 μm and to investigate the extended emission observed with the new APEX bolometer LABoCa. We computed the spectral energy distribution (SEDs) with the 1D radiative transfer code DUSTY, which we compared to literature data. Grain properties were calculated with both the spherical grains distribution and the continuous distribution of ellipsoids (CDE), and a comparison between the two is drawn. Synthetic surface brightness maps were derived from the modelling and were compared to the LABoCa brightness maps. Results. A sample of nine evolved stars with different chemistry was observed with LABoCa. We detected extended emission around four stars. Physical parameters of the circumstellar envelope were derived from SED modelling, like the dust chemical composition, the dust condensation temperature and the total mass of the envelope. It proved to be difficult to fit the SED and the intensity profile simultaneously however. The use of the CDE leads to "broad" SEDs when compared to spherical grains, and this results in steep density distributions (∝r −2.2 typically).

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“…The main dust species detected in carbon-rich stars are amorphous carbon, silicon carbide and magnesium sulfide (Treffers & Cohen 1974;Goebel & Moseley 1985;Martin & Rogers 1987;Andersen et al 1999;Chan & Kwok 1990;Hony et al 2002;Zijlstra et al 2006;Lagadec et al 2007;Leisenring et al 2008). One of the key questions in the carbon-species formation is the importance of mantle chemistry, for example the nucleation of amorphous carbon and magnesium sulfides as mantles around silicon carbide grains have been proposed as important dust formation mechanisms (Lorenz-Martins et al 2001;Zhukovska & Gail 2008).…”

Section: Dust Formation In Agb Starsmentioning

confidence: 99%

TheSpitzerspectroscopic survey of S-type stars

Smolders

Neyskens

Blommaert

et al. 2012

A&A

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Context. S-type AGB stars are thought to be in the transitional phase between M-type and C-type AGB stars. Because the composition of the circumstellar environment reflects the photospheric abundances, one may expect a strong influence of the stellar C/O ratio on the molecular chemistry and the mineralogy of the circumstellar dust. Aims. In this paper, we present a large sample of 87 intrinsic galactic S-type AGB stars, observed at infrared wavelengths with the Spitzer Space Telescope, and supplemented with ground-based optical data. Methods. On the one hand, we derive the stellar parameters from the optical spectroscopy and photometry, using a grid of model atmospheres. On the other, we decompose the infrared spectra to quantify the flux-contributions from the different dust species. Finally, we compare the independently determined stellar parameters and dust properties. Results. For the stars without significant dust emission features, we detect a strict relation between the presence of SiS absorption in the Spitzer spectra and the C/O ratio of the stellar atmosphere. These absorption bands can thus be used as an additional diagnostic for the C/O ratio. For stars with significant dust emission, we define three distinct groups, based on the relative contribution of certain dust species to the infrared flux. We find a strong link between group-membership and C/O ratio. Furthermore, we show that these groups can be explained by assuming that the dust-condensation can be cut short before silicates are produced, while the remaining free atoms and molecules can then be used to form the observed magnesium sulfides or the carriers of the unidentified 13 µm and 20 µm features. Finally, we present the detection of emission features attributed to molecules and dust characteristic to C-type stars, such as molecular SiS, hydrocarbons and magnesium sulfide grains. We show that we often detect magnesium sulfides together with molecular SiS and we propose that it is formed by a reaction of SiS molecules with Mg.

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Modeling of C stars with core/mantle grains: Amorphous carbon + SiC

Cited by 16 publications

References 33 publications

Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

870 μm observations of evolved stars with LABOCA

TheSpitzerspectroscopic survey of S-type stars

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