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Register a new userEvolution of Gas and Dust in Circumstellar Disks
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David W. Koerner
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A clear understanding of the chemical processing of matter, as it is transferred from a molecular cloud to a planetary system, depends heavily on knowledge of the physical conditions endured by gas and dust as these accrete onto a disk and are incorporated into planetary bodies. Reviewed here are astrophysical observations of circumstellar disks which trace their evolving properties. Accretion disks that are massive enough to produce a solar system like our own are typically larger than 100 AU. This suggests that the chemistry of a large fraction of the infalling material is not radically altered upon contact with a vigorous accretion shock. The mechanisms of accretion onto the star and eventual dispersal are not yet well understood, but timescales for the removal of gas and optically thick dust appear to be a few times 10$^6$ yrs. At later times, tenuous ``debris disks of dust remain around stars as old as a few times 10$^8$ yrs. Features in the morphology of the latter, such as inner holes, warps, and azimuthal asymmetries, are likely to be the result of the dynamical influence of large planetary bodies. Future observations will enlighten our understanding of chemical evolution and will focus on the search for disks in transition from a viscous accretion stage to one represented by a gas-free assemblage of colliding planetesimals. In the near future, comparative analysis of circumstellar dust and gas properties within a statistically significant sample of young stars at various ages will be possible with instrumentation such as SIRTF and SOFIA. Well-designed surveys will help place solar system analogs in a general context of a diversity of possible pathways for circumstellar evolution, one which encompasses the formation of stellar and brown-dwarf companions as well as planetary systems.
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We present a compositional analysis of 8-13um spectra of 32 young stellar objects (YSOs). Our sample consists of 5 intermediate-mass stars and 27 low-mass stars. While the spectra and first scientific results have already been published by Przygodda et al. (2003) and Kessler-Silacci et al. (2004) we perform a more detailed analysis of the 10um silicate feature. In our analysis we assume that this emission feature can be represented by a linear superposition of the wavelength-dependent opacity $kappa_{rm abs}(lambda)$ describing the optical properties of silicate grains with different chemical composition, structure, and grain size. The determination of an adequate fitting equation is another goal of this study. Using a restricted number of fitting parameters we investigate which silicate species are necessary for the compositional fitting. Particles with radii of 0.1um- and 1.5um consisting of amorphous olivine and pyroxene, forsterite, enstatite, and quartz have been considered. Only compact, homogeneous dust grains have been used in the presented fitting procedures. In this context we show that acceptable fitting results can also be achieved if emission properties of porous silicate grains are considered instead. Although some previous studies give reasons for the similarity between the dust in circumstellar disks of TTauri stars and Herbig Ae/Be stars, a quantitative comparison has been missing, so far. Therefore, we conclude with a discussion of the results of a 10um spectroscopic survey of van Boekel et al. (2005) who focus on Herbig Ae/Be stars, the higher mass counterparts of T Tauri stars and draw comparisons to this and other studies. We find that the results of our study of T Tauri systems partly agree with previous studies of Herbig Ae/Be stars.
The long-term evolution of a circumstellar disk starting from its formation and ending in the T Tauri phase was simulated numerically with the purpose of studying the evolution of dust in the disk with distinct values of viscous alpha-parameter and dust fragmentation velocity v_frag. We solved numerical hydrodynamics equations in the thin-disk limit, which are modified to include a dust component consisting of two parts: sub-micron-sized dust and grown dust with a maximum radius a_r. The former is strictly coupled to the gas, while the latter interacts with the gas via friction. The conversion of small to grown dust, dust growth, and dust self-gravity are also considered. We found that the process of dust growth known for the older protoplanetary phase also holds for the embedded phase of disk evolution. The dust growth efficiency depends on the radial distance from the star - a_r is largest in the inner disk and gradually declines with radial distance. In the inner disk, a_r is limited by the dust fragmentation barrier. The process of small-to-grown dust conversion is very fast once the disk is formed. The total mass of grown dust in the disk (beyond 1 AU) reaches tens or even hundreds of Earth masses already in the embedded phase of star formation and even a greater amount of grown dust drifts in the inner, unresolved 1 AU of the disk. Dust does not usually grow to radii greater than a few cm. A notable exception are models with alpha <= 10^{-3}, in which case a zone with reduced mass transport develops in the inner disk and dust can grow to meter-sized boulders in the inner 10 AU. Grown dust drifts inward and accumulates in the inner disk regions. This effect is most pronounced in the alpha <= 10^{-3} models where several hundreds of Earth masses can be accumulated in a narrow region of several AU from the star by the end of embedded phase. (abridged).
Accurate temperature calculations for circumstellar disks are particularly important for their chemical evolution. Their temperature distribution is determined by the optical properties of the dust grains, which, among other parameters, depend on their radius. However, in most disk studies, only average optical properties and thus an average temperature is assumed to account for an ensemble of grains with different radii. We investigate the impact of subdividing the grain radius distribution into multiple sub-intervals on the resulting dust temperature distribution and spectral energy distribution (SED). These quantities were computed for two different scenarios: (1) Radius distribution represented by 16 logarithmically distributed radius intervals, and (2) radius distribution represented by a single grain species with averaged optical properties (reference). Within the considered parameter range, i.e., of grain radii between 5 nm and 1 mm and an optically thin and thick disk with a parameterized density distribution, we obtain the following results: In optically thin disk regions, the temperature spread can be as large as ~63% and the relative grain surface below a certain temperature is lower than in the reference disk. With increasing optical depth, the difference in the midplane temperature and the relative grain surface below a certain temperature decreases. Furthermore, below ~20K, this fraction is higher for the reference disk than for the case of multiple grain radii, while it shows the opposite behavior for temperatures above this threshold. The thermal emission in the case of multiple grain radii at short wavelengths is stronger than for the reference disk. The freeze-out radius is a function of grain radius, spanning a radial range between the coldest and warmest grain species of ~30AU.
We investigate the behaviour of dust in protoplanetary disks under the action of gas drag using our 3D, two-fluid (gas+dust) SPH code. We present the evolution of the dust spatial distribution in global simulations of planetless disks as well as of disks containing an already formed planet. The resulting dust structures vary strongly with particle size and planetary gaps are much sharper than in the gas phase, making them easier to detect with ALMA than anticipated. We also find that there is a range of masses where a planet can open a gap in the dust layer whereas it doesnt in the gas disk. Our dust distributions are fed to the radiative transfer code MCFOST to compute synthetic images, in order to derive constraints on the settling and growth of dust grains in observed disks.
Four Ophiuchus binaries, two Class I systems and two Class II systems, with separations of ~450-1100 AU, were observed with the Owens Valley Radio Observatory (OVRO) millimeter interferometer. In each system, the 3 mm continuum maps show dust emission at the location of the primary star, but no emission at the position of the secondary. This result is different from observations of less evolved Class 0 binaries, in which dust emission is detected from both sources. The nondetection of secondary disks is, however, similar to the dust distribution seen in wide Class II Taurus binaries. The combined OVRO results from the Ophiuchus and Taurus binaries suggest that secondary disk masses are significantly lower than primary disk masses by the Class II stage, with initial evidence that massive secondary disks are reduced by the Class I stage. Although some of the secondaries retain hot inner disk material, the early dissipation of massive outer disks may negatively impact planet formation around secondary stars. Masses for the circumprimary disks are within the range of masses measured for disks around single T Tauri stars and, in some cases, larger than the minimum mass solar nebula. More massive primary disks are predicted by several formation models and are broadly consistent with the observations. Combining the 3 mm data with previous 1.3 mm observations, the dust opacity power-law index for each primary disk is estimated. The opacity index values are all less than the scaling for interstellar dust, possibly indicating grain growth within the circumprimary disks.
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