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Register a new user3D radiative transfer of intrinsically polarized dust emission based on aligned aspherical grains
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Gesa H.-M. Bertrang
Publication date
2017
fields
Physics
and research's language is
English
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(Sub-)Millimeter observations of the polarized emission of aligned aspherical dust grains enable us to study the magnetic fields within protoplanetary disk. However, the interpretation of these observations is complex. One must consider the various effects that alter the measured polarized signal, such as the shape of dust grains, the efficiency of grain alignment, the magnetic field properties, and the projection of the signal along the line of sight. We aim at analyzing observations of the polarized dust emission by disentangling the effects on the polarization signal in the context of 3D radiative transfer simulations. For this purpose, we developed a code capable of simulating dust grain alignment of aspherical grains and intrinsical polarization of thermal dust emission. We find that the influence of thermal polarization and dust grain alignment on the polarized emission displayed as spatially resolved polarization map or as spectral energy distribution trace disk properties which are not traced in total (unpolarized) emission such as the magnetic field topology. The radiative transfer simulations presented in this work enable the 3D analysis of intrinsically polarized dust emission - observed with, e.g., ALMA - which is essential to constrain magnetic field properties.
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Polarized continuum emission from aligned grains in disks around young stellar objects can be used to probe the magnetic field, radiation anisotropy, or drift between dust and gas, depending on whether the non-spherical grains are aligned magnetically, radiatively or mechanically. We show that it can also be used to probe another key disk property -- the temperature gradient -- along sight lines that are optically thick, independent of the grain alignment mechanism. We first illustrate the technique analytically using a simple 1D slab model, which yields an approximate formula that relates the polarization fraction to the temperature gradient with respect to the optical depth tau at the tau=1 surface. The formula is then validated using models of stellar irradiated disks with and without accretion heating. The promises and challenges of the technique are illustrated with a number of Class 0 and I disks with ALMA dust polarization data, including NGC 1333 IRAS4A1, IRAS 16293B, BHB 07-11, L1527, HH 212 and HH 111. We find, in particular, that the sight lines passing through the near-side of a highly inclined disk trace different temperature gradient directions than those through the far-side, which can lead to a polarization orientation on the near-side that is orthogonal to that on the far-side, and that the HH 111 disk may be such a case. Our technique for probing the disk temperature gradient through dust polarization can complement other methods, particularly those using molecular lines.
Context. The mechanisms governing the opening of cavities in transition disks are not fully understood. Several processes have been proposed but their occurrence rate is still unknown. Aims. We present spatially resolved observations of two transition disks and aim at constraining their vertical and radial structure using multiwavelength observations. Methods. We have obtained near-IR scattered light observations with VLT/SPHERE of the transition disks J1608 and J1852. We complement our datasets with ALMA observations and with unresolved photometric observations covering a wide range of wavelengths. We performed radiative transfer modeling to analyze the morphology of the disks and compare the results with a sample of 20 other transition disks observed with both SPHERE and ALMA. Results. The scattered light image of J1608 reveals a very inclined disk, with two bright lobes and a large cavity. J1852 shows an inner ring extending beyond the coronagraphic radius up to 15au, a gap and a second ring at 42au. Our radiative transfer model of J1608 indicates that the millimeter-sized grains are less extended vertically and radially than the micron-sized grains, indicating advanced settling and radial drift. We find good agreement with the observations of J1852 with a similar model, but due to the low inclination of the system, the model remains partly degenerate. The analysis of 22 transition disks shows that, in general, the cavities observed in scattered light are smaller than the ones detected at millimeter wavelengths. Conclusions. The analysis of a sample of transition disks indicates that the small grains can flow inward of the region where millimeter grains are trapped. While 15 out of the 22 cavities in our sample could be explained by a planet of less than 13 Jupiter masses, the others either require the presence of a more massive companion or of several low-mass planets.
The reconstruction of the solar spectral irradiance (SSI) on various time scales is essential for the understanding of the Earths climate response to the SSI variability. The driver of the SSI variability is understood to be the intensity contrast of magnetic features present on the Sun with respect to the largely non-magnetic quiet Sun. However, different spectral synthesis codes lead to diverging projections of SSI variability. In this study we compare three different radiative transfer codes and carry out a detailed analysis of their performance. We perform the spectral synthesis at the continuum wavelength of 665 nm with the Code for Solar Irradiance (COSI), and the Rybicki-Hummer (RH), and Max Planck University of Chicago Radiative MHD (MURaM) codes for three 3D MHD simulations snapshots, a non-magnetic case, and MHD simulations with 100 G, and 200 G magnetic field strength. We determine the intensity distributions, the intensity differences and ratios for the spectral synthesis codes. We identify that the largest discrepancies originate in the intergranular lanes where the most field concentration occurs. Overall, the applied radiative transfer codes give consistent intensity distributions. Also, the intensity variation as a function of magnetic field strength for the particular 100 G and 200 G snapshots agree within the 2-3% range.
We compare maps of scattering polarization signals obtained from three-dimensional (3D) radiation transfer calculations in a magneto-convection model of the solar atmosphere using formal solvers based on the short characteristics (SC) and the long characteristics (LC) methods. The SC method requires less computational work, but it is known to introduce spatial blurring in the emergent radiation for inclined lines of sight. For polarized radiation this effect is generally more severe due to it being a signed quantity and to the sensitivity of the scattering polarization to the models inhomogeneities. We study the differences in the polarization signals of the emergent spectral line radiation calculated with such formal solvers. We take as a case study already published results of the scattering polarization in the Sr I $4607~unicode{xC5}$ line obtained with the SC method, demonstrating that in high-resolution grids it is accurate enough for that type of study. In general, the LC method is the preferred one for accurate calculations of the emergent radiation, reason why it is now one of the options in the public version of the 3D radiative transfer code PORTA.
HYPERION is a new three-dimensional dust continuum Monte-Carlo radiative transfer code that is designed to be as generic as possible, allowing radiative transfer to be computed through a variety of three-dimensional grids. The main part of the code is problem-independent, and only requires an arbitrary three-dimensional density structure, dust properties, the position and properties of the illuminating sources, and parameters controlling the running and output of the code. HYPERION is parallelized, and is shown to scale well to thousands of processes. Two common benchmark models for protoplanetary disks were computed, and the results are found to be in excellent agreement with those from other codes. Finally, to demonstrate the capabilities of the code, dust temperatures, SEDs, and synthetic multi-wavelength images were computed for a dynamical simulation of a low-mass star formation region. HYPERION is being actively developed to include new features, and is publicly available (http://www.hyperion-rt.org).
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