arXiv subjects

M. Min

Publications and source records attributed to M. Min.

123 records · Page 7Linked to original sources

arXiv2008

The infrared emission spectra of compositionally inhomogeneous aggregates composed of irregularly shaped constituents

In order to deduce properties of dust in astrophysical environments where dust growth through aggregation is important, knowledge of the way aggregated particles interact with radiation, and what information is encoded in the thermal radiation they emit, is needed. The emission characteristics are determined by the size and structure of the aggregate and the composition and shape of the constituents. We thus aim at performing computations of compositionally inhomogeneous aggregates composed of irregularly shaped constituents. In addition we aim at developing an empirical recipe to compute the optical properties of such aggregates in a fast and accurate manner. We performed CDA computations for aggregates of irregularly shaped particles with various compositions. The constituents of the aggregate are assumed to be in the Rayleigh regime, and in addition we assume that the dominant interaction of the aggregate constituents is through dipole-dipole interactions. We computed the spectral structure of the emission efficiency in the 10 micron region for aggregates with 30% amorphous carbon and 70% silicates by volume with various fractions of crystalline and amorphous components. We find that the spectral appearance of the various components of the aggregate are very different and depend on their abundances. Most notably, materials that have a very low abundance appear spectroscopically as if they were in very small grains, while more abundant materials appear, spectroscopically to reside in larger grains. We construct a fast empirical approximate method, based on the idea of an effective medium approximation, to construct the spectra for these aggregates which almost perfectly reproduces the more exact computations. This new method is fast enough to be easily implemented in fitting procedures.

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arXiv2007

Dust-grain processing in circumbinary discs around evolved binaries. The RV Tauri spectral twins RU Cen and AC Her

Context: We study the structure and evolution of circumstellar discs around evolved binaries and their impact on the evolution of the central system. Aims: To study in detail the binary nature of RUCen and ACHer, as well as the structure and mineralogy of the circumstellar environment. Methods: We combine multi-wavelength observations with a 2D radiative transfer study. Our radial velocity program studies the central stars, while our Spitzer spectra and broad-band SEDs are used to constrain mineralogy, grain sizes and physical structure of the circumstellar environment. Results: We determine the orbital elements of RUCen showing that the orbit is highly eccentric with a rather long period of 1500 days. The infrared spectra of both objects are very similar and the spectral dust features are dominated by Mg-rich crystalline silicates. The small peak-to-continuum ratios are interpreted as being due to large grains. Our model contains two components with a cold midplain dominated by large grains, and the near- and mid-IR which is dominated by the emission of smaller silicates. The infrared excess is well modelled assuming a hydrostatic passive irradiated disc. The profile-fitting of the dust resonances shows that the grains must be very irregular. Conclusions: These two prototypical RVTauri pulsators with circumstellar dust are binaries where the dust is trapped in a stable disc. The mineralogy and grain sizes show that the dust is highly processed, both in crystallinity and grain size. The cool crystals show that either radial mixing is very efficient and/or that the thermal history at grain formation has been very different from that in outflows. The physical processes governing the structure of these discs are similar to those observed in protoplanetary discs around young stellar objects.

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arXiv2007

The circumbinary disc around the J-type C-star IRAS 18006-3213

In the generally accepted, but poorly documented model, silicate J-type C-stars are binary objects for which the silicate emission originates from a circumbinary or a circumcompanion disc. We aim at testing this hypothesis by a thorough spectral and spatial observational study of one object: IRAS18006-3213. We obtained, analysed and modeled high spatial resolution interferometric VLTI/MIDI observations on multiple baselines ranging from 45 m to 100 m. All observations resolved the object and show the very compact nature of the N-band emission (~30 mas). In addition, the highest spatial resolution data show a significant differential phase jump around 8.3 micron. This demonstrates the asymmetric nature of the N-band emission. Moreover, the single telescope N-band spectrum shows the signature of highly processed silicate grains. These data are used to confirm the model on silicate J-type C-stars for IRAS18006-3213. We show that the most favourable model of the dust geometry is a stable circumbinary disc around the system, seen under an intermediate inclination. The data presented on the silicate J-type C-star IRAS18006-3213 provide evidence that the oxygen rich dust is trapped in a circumbinary disc. The formation of this disc is probably linked to the binary nature of the central star.

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arXiv2006

The shape and composition of interstellar silicate grains

We investigate the composition and shape distribution of silicate dust grains in the interstellar medium. The effect of the amount of magnesium in the silicate lattice is studied. We fit the spectral shape of the interstellar 10 mu extinction feature as observed towards the galactic center. We use very irregularly shaped coated and non-coated porous Gaussian Random Field particles as well as a statistical approach to model shape effects. For the dust materials we use amorphous and crystalline silicates with various composition and SiC. The results of our analysis of the 10 mu feature are used to compute the shape of the 20 mu silicate feature and to compare this with observations. By using realistic particle shapes we are, for the first time, able to derive the magnesium fraction in interstellar silicates. We find that the interstellar silicates are highly magnesium rich (Mg/(Fe+Mg)>0.9) and that the stoichiometry lies between pyroxene and olivine type silicates. This composition is not consistent with that of the glassy material found in GEMS in interplanetary dust particles indicating that these are, in general, not unprocessed remnants from the interstellar medium. Also, we find a significant fraction of SiC (~3%). We discuss the implications of our results for the formation and evolutionary history of cometary and circumstellar dust. We argue that the fact that crystalline silicates in cometary and circumstellar grains are almost purely magnesium silicates is a natural consequence of our findings that the amorphous silicates from which they were formed were already magnesium rich.

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arXiv2006

Resolving the compact dusty discs around binary post-AGB stars using N-band interferometry

We present the first mid-IR long baseline interferometric observations of the circumstellar matter around binary post-AGB stars. Two objects, SX Cen and HD 52961, were observed using the VLTI/MIDI instrument during Science Demonstration Time. Both objects are known binaries for which a stable circumbinary disc is proposed to explain the SED characteristics. This is corroborated by our N-band spectrum showing a crystallinity fraction of more than 50 % for both objects, pointing to a stable environment where dust processing can occur. Surprisingly, the dust surrounding SX Cen is not resolved in the interferometric observations providing an upper limit of 11 mas (or 18 AU at the distance of this object) on the diameter of the dust emission. This confirms the very compact nature of its circumstellar environment. The dust emission around HD 52961 originates from a very small but resolved region, estimated to be ~ 35 mas at 8 micron and ~ 55 mas at 13 micron. These results confirm the disc interpretation of the SED of both stars. In HD 52961, the dust is not homogeneous in its chemical composition: the crystallinity is clearly concentrated in the hotter inner region. Whether this is a result of the formation process of the disc, or due to annealing during the long storage time in the disc is not clear.

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arXiv2005

The 10 micron amorphous silicate feature of fractal aggregates and compact particles with complex shapes

We model the 10 micron absorption spectra of nonspherical particles composed of amorphous silicate. We consider two classes of particles, compact ones and fractal aggregates composed of homogeneous spheres. For the compact particles we consider Gaussian random spheres with various degrees of non-sphericity. For the fractal aggregates we compute the absorption spectra for various fractal dimensions. The 10 micron spectra are computed for ensembles of these particles in random orientation using the well-known Discrete Dipole Approximation. We compare our results to spectra obtained when using volume equivalent homogeneous spheres and to those computed using a porous sphere approximation. We conclude that, in general, nonspherical particles show a spectral signature that is similar to that of homogeneous spheres with a smaller material volume. This effect is overestimated when approximating the particles by porous spheres with the same volume filling fraction. For aggregates with fractal dimensions typically predicted for cosmic dust, we show that the spectral signature characteristic of very small homogeneous spheres (with a volume equivalent radius r_V<0.5 micron) can be detected even in very large particles. We conclude that particle sizes are underestimated when using homogeneous spheres to model the emission spectra of astronomical sources. In contrast, the particle sizes are severely overestimated when using equivalent porous spheres to fit observations of 10 micron silicate emission.

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arXiv2005

Eta Car through the eyes of interferometers

The core of the nebula surrounding Eta Carinae has recently been observed with VLT/NACO, VLTI/VINCI, VLTI/MIDI and VLTI/AMBER in order to spatially and spectrally constrain the warm dusty environment and the central object. Narrow-band images at 3.74 and 4.05 micron reveal the structured butterfly-shaped dusty environment close to the central star with an unprecedented spatial resolution of about 60 mas. VINCI has resolved the present-day stellar wind of Eta Carinae on a scale of several stellar radii owing to the spatial resolution of the order of 5 mas (11 AU). The VINCI observations show that the object is elongated with a de-projected axis ratio of approximately 1.5. Moreover the major axis is aligned with that of the large bipolar nebula that was ejected in the 19th century. Fringes have also been obtained in the Mid-IR with MIDI using baselines of 75m. A peak of correlated flux of 100 Jy is detected 0.3" south-east from the photocenter of the nebula at 8.7 micron is detected. This correlated flux is partly attributed to the central object but it is worth noting that at these wavelengths, virtually all the 0.5" x 0.5" central area can generate detectable fringes witnessing the large clumping of the dusty ejecta. These observations provide an upper limit for the SED of the central source from 3.8 to 13.5 micron and constrain some parameters of the stellar wind which can be compared to Hillier's model. Lastly, we present the great potential of the AMBER instrument to study the numerous near-IR emissive lines from the star and its close vicinity. In particular, we discuss its ability to detect and follow the faint companion.

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arXiv2005

The composition and size distribution of the dust in the coma of comet Hale-Bopp

We discuss the composition and size distribution of the dust in the coma of comet Hale-Bopp. We do this by fitting simultaneously the infrared emission spectrum measured by the infrared space observatory (ISO) and the measured degree of linear polarization of scattered light at various phase angles and 12 different wavelengths. The effects of particle shape on the modeled optical properties of the dust grains are taken into account. We constrain our fit by forcing the abundances of the major rock forming chemical elements to be solar. The infrared spectrum at long wavelengths reveals that large grains are needed in order to fit the spectral slope. The size and shape distribution we employ allows us to estimate the sizes of the crystalline silicates. The ratios of the strength of various forsterite features show that the crystalline silicate grains in Hale-Bopp must be submicron sized. We exclude the presence of large crystalline silicate grains in the coma. Because of this lack of large crystalline grains combined with the fact that we do need large amorphous grains to fit the emission spectrum at long wavelengths, we need only approximately 4% of crystalline silicates by mass. After correcting for possible hidden crystalline material included in large amorphous grains, our best estimate of the total mass fraction of crystalline material is approximately 7.5%, significantly lower than deduced in previous studies in which the typical derived crystallinity is 20-30%. The implications of this on the possible origin and evolution of the comet are discussed. The crystallinity we observe in Hale-Bopp is consistent with the production of crystalline silicates in the inner solar system by thermal annealing and subsequent radial mixing to the comet forming region.

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arXiv2005

A 10 micron spectroscopic survey of Herbig Ae star disks: grain growth and crystallization

We present spectroscopic observations of a large sample of Herbig Ae stars in the 10 micrometer spectral region. We perform compositional fits of the spectra based on properties of homogeneous as well as inhomogeneous spherical particles, and derive the mineralogy and typical grain sizes of the dust responsible for the 10 $μ$m emission. Several trends are reported that can constrain theoretical models of dust processing in these systems: i) none of the sources consists of fully pristine dust comparable to that found in the interstellar medium, ii) all sources with a high fraction of crystalline silicates are dominated by large grains, iii) the disks around more massive stars (M >~ 2.5 M_sun, L >~ 60 L_sun) have a higher fraction of crystalline silicates than those around lower mass stars, iv) in the subset of lower mass stars (M <~ 2.5 M_sun) there is no correlation between stellar parameters and the derived crystallinity of the dust. The correlation between the shape and strength of the 10 micron silicate feature reported by van Boekel et al. (2003) is reconfirmed with this larger sample. The evidence presented in this paper is combined with that of other studies to present a likely scenario of dust processing in Herbig Ae systems. We conclude that the present data favour a scenario in which the crystalline silicates are produced in the innermost regions of the disk, close to the star, and transported outward to the regions where they can be detected by means of 10 micron spectroscopy. Additionally, we conclude that the final crystallinity of these disks is reached very soon after active accretion has stopped.

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arXiv2005

Absorption and scattering properties of arbitrarily shaped particles in the Rayleigh domain: A rapid computational method and a theoretical foundation for the statistical approach

We provide a theoretical foundation for the statistical approach for computing the absorption properties of particles in the Rayleigh domain. We present a general method based on the Discrete Dipole Approximation (DDA) to compute the absorption and scattering properties of particles in the Rayleigh domain. The method allows to separate the geometrical aspects of a particle from its material properties. Doing the computation of the optical properties of a particle once, provides them for any set of refractive indices, wavelengths and orientations. This allows for fast computations of e.g. absorption spectra of arbitrarily shaped particles. Other practical applications of the method are in the interpretation of atmospheric and radar measurements as well as computations of the scattering matrix of small particles as a function of the scattering angle. In the statistical approach, the optical properties of irregularly shaped particles are represented by the average properties of an ensemble of particles with simple shapes. We show that the absorption cross section of an ensemble of arbitrarily shaped particles with arbitrary orientations can always be uniquely represented by the average absorption cross section of an ensemble of spheroidal particles with the same composition and fixed orientation. This proves for the first time that the statistical approach is generally viable in the Rayleigh domain.

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arXiv2005

Modeling optical properties of cosmic dust grains using a distribution of hollow spheres

In this paper we study the combined effects of size and shape of small solid state particles on the absorption, emission and scattering characteristics. We use the statistical approach to calculate these optical properties. In this approach the average optical properties of an ensemble of particles in random orientation are represented by the average optical properties of an ensemble of simple shapes. The validity of this approach is studied in detail for a uniform distribution of hollow spheres where the fractional volume of the central inclusion is varied. We apply the results to two different areas of interest, i) infrared spectroscopy and ii) polarization of scattered light. The effects of particle size and shape on the optical characteristics are discussed. We compare the results using the distribution of hollow spheres with those obtained by using randomly oriented spheroids. Also we compare the results with observations and laboratory measurements. The distribution of hollow spheres is very successful in reproducing laboratory measurements of the scattering angle distribution of the degree of linear polarization for incident unpolarized light of randomly oriented irregular quartz particles. Furthermore, we show that we are able to derive the size distribution of dust grains by fitting the measured degree of linear polarization using computational result for hollow spheres. It is shown that the distribution of hollow spheres is a powerful tool for studying light scattering, absorption and emission by cosmic dust grains and in particular when large numbers of particle parameters need to be considered since the computational demand of the distribution of hollow spheres is small.

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arXiv2005

The sub-arcsecond dusty environment of Eta Carinae

The core of the nebula surrounding Eta Carinae has been observed with the VLT Adaptive Optics system NACO and with the interferometer VLTI/MIDI to constrain spatially and spectrally the warm dusty environment and the central object. In particular, narrow-band images at 3.74 and 4.05 micron reveal the butterfly shaped dusty environment close to the central star with unprecedented spatial resolution. A void whose radius corresponds to the expected sublimation radius has been discovered around the central source. Fringes have been obtained in the Mid-IR which reveal a correlated flux of about 100Jy situated 0.3" south-east of the photocenter of the nebula at 8.7 micron, which corresponds with the location of the star as seen in other wavelengths. This correlated flux is partly attributed to the central object, and these observations provide an upper limit for the SED of the central source from 2.2 to 13.5 micron. Moreover, we have been able to spectrally disperse the signal from the nebula itself at PA=318 degree, i.e. in the direction of the bipolar nebula 310 degree) within the MIDI field of view of 3". A large amount of corundum (Al2O3) is discovered, peaking at 0.6-1.2" south-east from the star, whereas the dust content of the Weigelt blobs is dominated b silicates. We discuss the mechanisms of dust formation which are closely related to the geometry of this Butterfly nebulae.

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arXiv2003

Spectroscopic diagnostic for the mineralogy of large dust grains

We examine the thermal infrared spectra of large dust grains of different chemical composition and mineralogy. Strong resonances in the optical properties result in detectable spectral structure even when the grain is much larger than the wavelength at which it radiates. We apply this to the thermal infrared spectra of compact amorphous and crystalline silicates. The weak resonances of amorphous silicates at 9.7 and 18 micron virtually disappear for grains larger than about 10 micron. In contrast, the strong resonances of crystalline silicates produce emission dips in the infrared spectra of large grains; these emission dips are shifted in wavelength compared to the emission peaks commonly seen in small crystalline silicate grains. We discuss the effect of a fluffy or compact grain structure on the infrared emission spectra of large grains, and apply our theory to the dust shell surrounding Vega.

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arXiv2003

The mineralogy, geometry and mass-loss history of IRAS 16342-3814

We present the 2-200 um Infrared Space Observatory (ISO) spectrum and 3.8-20 um ISAAC and TIMMI2 images of the extreme OH/IR star IRAS 16342-3814. Amorphous silicate absorption features are seen, together with crystalline silicate absorption features up to almost 45 um. No other OH/IR star is known to have crystalline silicate features in absorption up to these wavelengths. This suggests that IRAS 16342-3814 must have, or recently had, an extremely high mass-loss rate. Preliminary radiative transfer calculations suggest that the mass-loss rate may be as large as 10^{-3} Msun/yr. The 3.8 um ISAAC image shows a bipolar reflection nebula with a dark equatorial waist or torus, similar to that seen in optical Hubble Space Telescope (HST) images. The position angle of the nebula decreases significantly with increasing wavelength, suggesting that the dominant source of emission changes from scattering to thermal emission. Still, even up to 20 um the nebula is oriented approximately along the major axis of the nebula seen in the HST and ISAAC images, suggesting that the torus must be very cold, in agreement with the very red ISO spectrum. The 20 um image shows a roughly spherically symmetric extended halo, approximately 6'' in diameter, which is probably due to a previous phase of mass-loss on the AGB, suggesting a transition from a (more) spherically symmetric to a (more) axial symmetric form of mass-loss at the end of the AGB. We estimate the maximum dust particle sizes in the torus and in the reflection nebula to be 1.3 and 0.09 um respectively. The size of the particles in the torus is large compared to typical ISM values, but in agreement with high mass-loss rate objects like AFGL 4106 and HD161796. We discuss the possible reason for the difference in particle size between the torus and the reflection nebula.

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arXiv2003

Water ice growth around evolved stars

We present a model of the growth of water ice on silicate grains in the circumstellar envelopes of Asymptotic Giant Branch (AGB) stars and Red Super Giants. We consider the growth of ice by gas grain collisions, the thermal evaporation of ice from a grain, and sputtering. Our model contains several improvements compared to earlier models, including a detailed treatment of the effects of sputtering, a detailed calculation of the radiation pressure on the grain, and the treatment of subsonic drift velocities. In terms of drift velocity between the grains and gas in the envelope, we find that the ice formation process can be divided into three regimes: (i) a sputtering dominated regime where ice growth is heavily suppressed, (ii) an intermediate regime with moderately efficient condensation and (iii) a thermally dominated regime where ice formation is highly efficient. Sputtering is the critical factor which determines if ice formation can occur at all. We find that in Red Supergiants, ice formation is suppressed, while the winds of OH/IR stars allow for efficient condensation and can convert significant fractions of the available water vapor (tens of percent) into ice mantles on grains. Population II AGB stars hardly form ice due to their low dust to gas ratios. We also modify an analytical equation describing condensation and depletion (Jura and Morris, 1985) in order to give reasonable results for high and low drift velocities. Initially, ice will condense in crystalline form, but continuing condensation at low temperatures, and damage caused by interstellar UV photons favor the production of amorphous ice as well. We predict that a significant fraction of the ice formed will be amorphous.

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