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Register a new userNew constraints on dust grain size and distribution in CQ Tau
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Grain growth in circumstellar disks is expected to be the first step towards the formation of planetary systems. There is now evidence for grain growth in several disks around young stars. Radially resolved images of grain growth in circumstellar disks are believed to be a powerful tool to constrain the dust evolution models and the initial stage for the formation of planets. In this paper we attempt to provide these constraints for the disk surrounding the young star CQ Tau. This system was already suggested from previous studies to host a population of grains grown to large sizes. We present new high angular resolution (0.3-0.9 arcsec) observations at wavelengths from 850um to 3.6cm obtained at the SMA, IRAM-PdBI and NRAO-VLA interferometers. We perform a combined analysis of the spectral energy distribution and of the high-resolution images at different wavelengths using a model to describe the dust thermal emission from the circumstellar disk. We include a prescription for the gas emission from the inner regions of the system. We detect the presence of evolved dust by constraining the disk averaged dust opacity coefficient beta (computed between 1.3 and 7mm) to be 0.6+/-0.1. This confirms the earlier suggestions that the disk contains dust grains grown to significant sizes and puts this on firmer grounds by tightly constraining the gas contamination to the observed fluxes at mm-cm wavelengths. We report some evidence of radial variations in dust properties, but current resolution and sensitivity are still too low for definitive results.
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It has recently been shown that turbulence in the interstellar medium (ISM) can significantly accelerate the growth of dust grains by accretion of molecules, but the turbulent gas-density distribution also plays a crucial role in shaping the grain-size distribution. The growth velocity, i.e., the rate of change of the mean grain radius, is proportional to the local gas density if the growth species (molecules) are well-mixed in the gas. As a consequence, grain growth happens at vastly different rates in different locations, since the gas-density distribution of the ISM shows a considerable variance. Here, it is shown that grain-size distribution (GSD) rapidly becomes a reflection of the gas-density distribution, irrespective of the shape of the initial GSD. This result is obtained by modelling ISM turbulence as a Markov process, which in the special case of an Ornstein-Uhlenbeck process leads to a lognormal gas-density distribution, consistent with numerical simulations of isothermal compressible turbulence. This yields an approximately lognormal GSD; the sizes of dust grains in cold ISM clouds may thus not follow the commonly adopted power-law GSD with index -3.5, but corroborates the use of a log-nomral GSD for large grains, suggested by several studies. It is also concluded that the very wide range of gas densities obtained in the high Mach-number turbulence of molecular clouds must allow formation of a tail of very large grains reaching radii of several microns.
We investigate the abundance and properties (especially, grain size) of dust in galaxy halos using available observational data in the literature. There are two major sets of data. One is (i) the reddening curves at redshifts $zsim 1$ and 2 derived for Mg II absorbers, which are assumed to trace the medium in galaxy halos. The other is (ii) the cosmic extinction up to $zsim 2$ mainly traced by distant background quasars. For (i), the observed reddening curves favor a grain radius of $asim 0.03~mu$m for silicate, while graphite is not supported because of its strong 2175 AA bump. Using amorphous carbon improves the fit to the reddening curves compared with graphite if the grain radius is $alesssim 0.03~mu$m. For (ii), the cosmic extinction requires $etagtrsim 10^{-2}$ ($eta$ is the ratio of the halo dust mass to the stellar mass; the observationally suggested value is $etasim 10^{-3}$) for silicate if $asim 0.03~mu$m as suggested by the reddening curve constraint. Thus, for silicate, we do not find any grain radius that satisfies both (i) and (ii) unless the halo dust abundance is much larger than suggested by the observations. For amorphous carbon, in contrast, a wide range of grain radius ($asim 0.01$--0.3~$mu$m) is accepted by the cosmic extinction; thus, we find that a grain radius range of $asim 0.01$--0.03 $mu$m is supported by combining (i) and (ii). We also discuss the origin of dust in galaxy halos, focusing on the importance of grain size in the physical mechanism of dust supply to galaxy halos.
Based on a one-zone evolution model of grain size distribution in a galaxy, we calculate the evolution of infrared spectral energy distribution (SED), considering silicate, carbonaceous dust, and polycyclic aromatic hydrocarbons (PAHs). The dense gas fraction ($eta_mathrm{dense}$) of the interstellar medium (ISM), the star formation time-scale ($tau_mathrm{SF}$), and the interstellar radiation field intensity normalized to the Milky Way value ($U$) are the main parameters. We find that the SED shape generally has weak mid-infrared (MIR) emission in the early phase of galaxy evolution because the dust abundance is dominated by large grains. At an intermediate stage ($tsim 1$ Gyr for $tau_mathrm{SF}=5$ Gyr), the MIR emission grows rapidly because the abundance of small grains increases drastically by the accretion of gas-phase metals. We also compare our results with observational data of nearby and high-redshift ($zsim 2$) galaxies taken by textit{Spitzer}. We broadly reproduce the flux ratios in various bands as a function of metallicity. We find that small $eta_mathrm{dense}$ (i.e. the ISM dominated by the diffuse phase) is favoured to reproduce the 8 $mu$m intensity dominated by PAHs both for the nearby and the $zsim 2$ samples. A long $tau_mathrm{SF}$ raises the 8 $mu$m emission to a level consistent with the nearby low-metallicity galaxies. The broad match between the theoretical calculations and the observations supports our understanding of the grain size distribution, but the importance of the diffuse ISM for the PAH emission implies the necessity of spatially resolved treatment for the ISM.
Dust is formed out of stellar material and is constantly affected by different mechanisms occurring in the ISM. Dust grains behave differently under these mechanisms depending on their sizes, and therefore the dust grain size distribution also evolves as part of the dust evolution itself. Following how the grain size distribution evolves is a difficult computing task that is just recently being overtaking. Smoothed particle hydrodynamic (SPH) simulations of a single galaxy as well as cosmological simulations are producing the first predictions of the evolution of the dust grain size distribution. We compare for the first time the evolution of the dust grain size distribution predicted by the SPH simulations with the results provided by the observations. We analyse how the radial distribution of the small to large grain mass ratio (D(S)/D(L)) changes over the whole discs in three galaxies: M 101, NGC 628 and M 33. We find good agreement between the observed radial distribution of D(S)/D(L) and what is obtained from the SPH simulations of a single galaxy. The central parts of NGC 628, at high metallicity and with a high molecular gas fraction, are mainly affected not only by accretion but also by coagulation of dust grains. The centre of M 33, having lower metallicity and lower molecular gas fraction, presents an increase of D(S)/D(L), showing that shattering is very effective in creating a large fraction of small grains. Observational results provided by our galaxies confirm the general relations predicted by the cosmological simulations based on the two grain size approximation. However, we present evidence that the simulations could be overestimating the amount of large grains in high massive galaxies.
We revisit the evolution model of grain size distribution in a galaxy for the ultimate purpose of implementing it in hydrodynamical simulations. We simplify the previous model in such a way that some model-dependent assumptions are replaced with simpler functional forms. For the first test of the developed framework, we apply it to a one-zone chemical evolution model of a galaxy, confirming that our new model satisfactorily reproduces the previous results and that efficient coagulation of small grains produced by shattering and accretion is essential in reproducing the so-called MRN grain size distribution. For the next step, in order to test if our model can be treated together with the hydrodynamical evolution of the interstellar medium (ISM), we post-process a hydrodynamical simulation of an isolated disc galaxy using the new grain evolution model. We sample hydrodynamical particles representing each of the dense and diffuse ISM phases. By this post-processing, we find that the processes occurring in the dense gas (grain growth by accretion and coagulation) are important in reproducing the grain size distribution consistent with the Milky Way extinction curve. In our model, the grain size distributions are similar between the dense and diffuse ISM, although we observe a larger dispersion in the dense ISM. Moreover, we also show that even if we degrade the grain radius resolution (with 16 grid points), the overall shape of grain size distribution (and of resulting extinction curve) can be captured.
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