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Register a new userPredictions of polarized dust emission from interstellar clouds: spatial variations in the efficiency of radiative torque alignment
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Veli-Matti Pelkonen
Publication date
2009
fields
Physics
and research's language is
English
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Polarization carries information about the magnetic fields in interstellar clouds. The observations of polarized dust emission are used to study the role of magnetic fields in the evolution of molecular clouds and the initial phases of star-formation. We study the grain alignment with realistic simulations, assuming the radiative torques to be the main mechanism that spins the grains up. The aim is to study the efficiency of the grain alignment as a function of cloud position and to study the observable consequences of these spatial variations. Our results are based on the analysis of model clouds derived from MHD simulations. The continuum radiative transfer problem is solved with Monte Carlo methods to estimate the 3D distribution of dust emission and the radiation field strength affecting the grain alignment. We also examine the effect of grain growth in cores. We are able to reproduce the results of Cho & Lazarian using their assumptions. However, the anisotropy factor even in the 1D case is lower than their assumption of $gamma = 0.7$, and thus we get less efficient radiative torques. Compared with our previous paper, the polarization degree vs. intensity relation is steeper because of less efficient grain alignment within dense cores. Without grain growth, the magnetic field of the cores is poorly recovered above a few $A_{rm V}$. If grain size is doubled in the cores, the polarization of dust emission can trace the magnetic field lines possibly up to $A_{rm V} sim 10$ magnitudes. However, many of the prestellar cores may be too young for grain coagulation to play a major role. The inclusion of direction dependent radiative torque efficiency weakens the alignment. Even with doubled grain size, we would not expect to probe the magnetic field past a few magnitudes in $A_{rm V}$.
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Interstellar dust is an essential component of the interstellar medium (ISM) and plays critical roles in astrophysics. Achieving an accurate model of interstellar dust is therefore of great importance. Interstellar dust models are usually built based on observational constraints such as starlight extinction and polarization, but dynamical constraints such as grain rotation are not considered. In this paper, we show that a newly discovered effect by Hoang et al., so-called RAdiative Torque Disruption (RATD), can act as an important dynamical constraint for dust models. Using this dynamical constraint, we derive the maximum size of grains that survive in the ISM for different dust models, including contact binary, composite, silicate-core, and amorphous carbon mantle, and compact grain model for the different radiation fields. We find that the different dust models have different maximum size due to their different tensile strengths, and the largest maximum size corresponds to compact grains with the highest tensile strength. We show that the composite grain model cannot be ruled out if constituent particles are very small with radius $a_{p}le$ 25 nm, but large composite grains would be destroyed if the particles are large with $a_{p}ge 50$ nm. We suggest that grain internal structures can be constrained with observations using the dynamical RATD constraint for strong radiation fields such as supernova, nova, or star-forming regions. Finally, our obtained results suggest that micron-sized grains perhaps have compact/core-mantle structures or have composite structures but located in regions with slightly higher gas density and weaker radiation intensity than the average ISM.
Observations of far-infrared (FIR) and submillimeter (SMM) polarized emission are used to study magnetic fields and dust grains in dense regions of the interstellar medium (ISM). These observations place constraints on models of molecular clouds, star-formation, grain alignment mechanisms, and grain size, shape, and composition. The FIR/SMM polarization is strongly dependent on wavelength. We have attributed this wavelength dependence to sampling different grain populations at different temperatures. To date, most observations of polarized emission have been in the densest regions of the ISM. Extending these observations to regions of the diffuse ISM, and to microwave frequencies, will provide additional tests of grain and alignment models. An understanding of polarized microwave emission from dust is key to an accurate measurement of the polarization of the cosmic microwave background. The microwave polarization spectrum will put limits on the contributions to polarized emission from spinning dust and vibrating magnetic dust.
Context. Planck observations demonstrated that the grain alignment efficiency is almost constant in the diffuse ISM. Aims. We test if the Radiative Torque (RAT) theory is compatible with observational constraints on grain alignment. Methods. We combine a numerical simulation with the radiative transfer code POLARIS that incorporates a physical dust model and the detailed grain alignment physics of RATs. A dust model is designed to reproduce the spectral dependence of extinction of the ISM. From a RAMSES simulation of interstellar turbulence, we extract a cube representative of the diffuse ISM. We post-process the cube with POLARIS to get the grain temperature and RATs to simulate synthetic dust polarization maps. Results. In our simulation the grain alignment efficiency is correlated with gas pressure, but not with the RAT intensity. Because of the low dust extinction, the magnitude of RATs varies little, decreasing only for high column densities $N_H$. Comparing our maps with a uniform alignment efficiency, we find no systematic difference. The dependence of polarization fraction $p$ with $N_H$ or polarization dispersion $S$ is similar. The drop of RATs in dense regions barely affects the polarization pattern, the signal being dominated by the LOS and magnetic field geometry. If a star is inserted, the polarization increases, with no specific pattern around the star. The angle-dependence of RATs is not observed in the maps, and is weak using a uniform magnetic field. Conclusions. RATs are compatible with Planck data for the diffuse ISM such that both uniform alignment and RAT alignment lead to similar observations. To further test the predictions of RATs where an important drop of grain alignment is expected, polarization observations of dense regions must be confronted to numerical simulations sampling high column densities through dense clouds, with enough statistics.
The quantization of energy levels in very nanoparticles suppresses dissipative processes that convert grain rotational kinetic energy into heat. For grains small enough to have GHz rotation rates, the suppression of dissipation can be extreme. As a result, alignment of such grains is suppressed. This applies both to alignment of the grain body with its angular momentum J, and to alignment of J with the local magnetic field B_0. If the anomalous microwave emission is rotational emission from spinning grains, it will be negligibly polarized at GHz frequencies, with P < 10^{-6} at frequencies above 10 GHz.
Interstellar grain alignment studies are currently experiencing a renaissance due to the development of a new quantitative theory based on Radiative Alignment Torques (RAT). One of the distinguishing predictions of this theory is a dependence of the grain alignment efficiency on the relative angle ($Psi$) between the magnetic field and the anisotropy direction of the radiation field. In an earlier study we found observational evidence for such an effect from observations of the polarization around the star HD 97300 in the Chamaeleon I cloud. However, due to the large uncertainties in the measured visual extinctions, the result was uncertain. By acquiring explicit spectral classification of the polarization targets, we have sought to perform a more precise reanalysis of the existing polarimetry data. We have obtained new spectral types for the stars in our for our polarization sample, which we combine with photometric data from the literature to derive accurate visual extinctions for our sample of background field stars. This allows a high accuracy test of the grain alignment efficiency as a function of $Psi$. We confirm and improve the measured accuracy of the variability of the grain alignment efficiency with $Psi$, seen in the earlier study. We note that the grain temperature (heating) also shows a dependence on $Psi$ which we interpret as a natural effect of the projection of the grain surface to the illuminating radiation source. This dependence also allows us to derive an estimate of the fraction of aligned grains in the cloud.
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