HYPERION: An open-source parallelized three-dimensional dust continuum radiative transfer code

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).Comment: Accepted for publication in Astronomy & Astrophysics. HYPERION is being prepared for release at the start of 2012, but you can already sign up to the mailing list at http://www.hyperion-rt.org to be informed once it is available for downloa

Computation of Light Scattering in Young Stellar Objects

A Monte Carlo light scattering code incorporating aligned non-spherical particles is described. The major effects on the flux distribution, linear polarisation and circular polarisation are presented, with emphasis on the application to Young Stellar Objects (YSOs). The need for models with non-spherical particles in order to successfully model polarisation data is reviewed. The ability of this type of model to map magnetic field structure in embedded YSOs is described. The possible application to the question of the origin of biomolecular homochirality via UV circular polarisation in star forming regions is also briefly discussed.Comment: Accepted by The Journal of Quantitative Spectroscopy and Radiative Transfer. Replaced version corrects an error in the definition of the sense of Cpol in the published version and other minor errors found at the proof stag

3-D Models of Embedded High-Mass Stars: Effects of a Clumpy Circumstellar Medium

We use 3-D radiative transfer models to show the effects of clumpy circumstellar material on the observed infrared colors of high mass stars embedded in molecular clouds. We highlight differences between 3-D clumpy and 1-D smooth models which can affect the interpretation of data. We discuss several important properties of the emergent spectral energy distribution (SED): More near-infrared light (scattered and direct from the central source) can escape than in smooth 1-D models. The near- and mid-infrared SED of the same object can vary significantly with viewing angle, depending on the clump geometry along the sightline. Even the wavelength-integrated flux can vary with angle by more than a factor of two. Objects with the same average circumstellar dust distribution can have very different near-and mid-IR SEDs depending on the clump geometry and the proximity of the most massive clump to the central source. Although clumpiness can cause similar objects to have very different SEDs, there are some observable trends. Near- and mid-infrared colors are sensitive to the weighted average distance of clumps from the central source and to the magnitude of clumpy density variations (smooth-to-clumpy ratio). Far-infrared emission remains a robust measure of the total dust mass. We present simulated SEDs, colors, and images for 2MASS and Spitzer filters. We compare to observations of some UCHII regions and find that 3-D clumpy models fit better than smooth models. In particular, clumpy models with fractal dimensions in the range 2.3-2.8, smooth to clumpy ratios of <50%, and density distributions with shallow average radial density profiles fit the SEDs best.Comment: accepted to ApJ; version with full-res figures: http://www.astro.virginia.edu/~ri3e/clumpy3d.pd

Efficient Monte Carlo methods for continuum radiative transfer

We discuss the efficiency of Monte Carlo methods in solving continuum radiative transfer problems. The sampling of the radiation field and convergence of dust temperature calculations in the case of optically thick clouds are both studied. For spherically symmetric clouds we find that the computational cost of Monte Carlo simulations can be reduced, in some cases by orders of magnitude, with simple importance weighting schemes. This is particularly true for models consisting of cells of different sizes for which the run times would otherwise be determined by the size of the smallest cell. We present a new idea of extending importance weighting to scattered photons. This is found to be useful in calculations of scattered flux and could be important for three-dimensional models when observed intensity is needed only for one general direction of observations. Convergence of dust temperature calculations is studied for models with optical depths 10-10000. We examine acceleration methods where radiative interactions inside a cell or between neighbouring cells are treated explicitly. In optically thick clouds with strong self-coupling between dust temperatures the run times can be reduced by more than one order of magnitude. The use of a reference field was also examined. This eliminates the need for repeating simulation of constant sources (e.g., background radiation) after the first iteration and significantly reduces sampling errors. The applicability of the methods for three-dimensional models is discussed.Comment: submitted to A&A, 19 page

Radio Astronomical Polarimetry and the Lorentz Group

In radio astronomy the polarimetric properties of radiation are often modified during propagation and reception. Effects such as Faraday rotation, receiver cross-talk, and differential amplification act to change the state of polarized radiation. A general description of such transformations is useful for the investigation of these effects and for the interpretation and calibration of polarimetric observations. Such a description is provided by the Lorentz group, which is intimately related to the transformation properties of polarized radiation. In this paper the transformations that commonly arise in radio astronomy are analyzed in the context of this group. This analysis is then used to construct a model for the propagation and reception of radio waves. The implications of this model for radio astronomical polarimetry are discussed.Comment: 10 pages, accepted for publication in Astrophysical Journa

Testing for Chaos in Deterministic Systems with Noise

Recently, we introduced a new test for distinguishing regular from chaotic dynamics in deterministic dynamical systems and argued that the test had certain advantages over the traditional test for chaos using the maximal Lyapunov exponent. In this paper, we investigate the capability of the test to cope with moderate amounts of noisy data. Comparisons are made between an improved version of our test and both the ``tangent space'' and ``direct method'' for computing the maximal Lyapunov exponent. The evidence of numerical experiments, ranging from the logistic map to an eight-dimensional Lorenz system of differential equations (the Lorenz 96 system), suggests that our method is superior to tangent space methods and that it compares very favourably with direct methods

Mid-infrared interferometry of the massive young stellar object NGC3603 - IRS 9A

We present observations and models for one of these MYSO candidates, NGC3603 IRS 9A. Our goal is to investigate with infrared interferometry the structure of IRS 9A on scales as small as 200AU, exploiting the fact that a cluster of O and B stars has blown away much of the obscuring foreground dust and gas. Observations in the N-band were carried out with the MIDI beam combiner attached to the VLTI. Additional interferometric observations which probe the structure of IRS 9A on larger scales were performed with an aperture mask installed in the T-ReCS instrument of Gemini South. The spectral energy distribution (SED) is constrained by the MIDI N-band spectrum and by data from the Spitzer Space Telescope. Our efforts to model the structure and SED of IRS 9A range from simple geometrical models of the brightness distribution to one- and two-dimensional radiative transfer computations. The target is resolved by T-ReCS, with an equivalent (elliptical) Gaussian width of 330mas by 280mas (2300 AU by 2000 AU). Despite this fact, a warm compact unresolved component was detected by MIDI which is possibly associated with the inner regions of a flattened dust distribution. Based on our interferometric data, no sign of multiplicity was found on scales between about 200AU and 700AU projected separation. A geometric model consisting of a warm (1000 K) ring (400 AU diameter) and a cool (140 K) large envelope provides a good fit to the data. No single model fitting all visibility and photometric data could be found, with disk models performing better than spherical models. While the data are clearly inconsistent with a spherical dust distribution they are insufficient to prove the existence of a disk but rather hint at a more complex dust distribution.Comment: 8 pages, 11 figures. Accepted for publication in A&

A High Mass Dusty Disk Candidate: The Case of IRAS 18151-1208

Many questions remain regarding the properties of disks around massive prototstars. Here we present the observations of a high mass protostellar object including an elongated dust continuum structure perpendicular to the outflow. Submillimeter Array 230 GHz line and continuum observations of the high mass protostellar object IRAS 18151-1208 along with single dish IRAM 30m observations afford us high spatial resolution (0.8") as well as recovery of the extended emission that gets filtered out by the interferometer. The observations of 12CO confirm the outflow direction to be in the southeast-northwest direction, and the 1.3 mm continuum exhibits an elongation in the direction perpendicular to the outflow. We model the physical parameters of the elongated structure by simultaneously fitting the observed spectral energy distribution (SED) and the brightness profile along the major axis using the 3D Radiative Transfer code MC3D. Assuming a density profile similar to that of a low mass disk, we can also reproduce the observations of this high mass protostellar object. This is achieved by using the same density distribution and flaring parameters as were used in the low mass case, and scaling up the size parameters that successfully modeled the circumstellar disk of several T Tauri stars. We also calculate that a region within the inner 30 AU of such a high mass disk is stable under the Toomre criterion. While we do not rule out other scenarios, we show here that the observations in the high mass regime are consistent with a scaled up version of a low mass disk. Implications on high mass star formation are discussed.Comment: 14 pages, 11 figures, accepted for publication in Ap

The circumstellar disc in the Bok globule CB 26: Multi-wavelength observations and modelling of the dust disc and envelope

Circumstellar discs are expected to be the nursery of planets. Grain growth within such discs is the first step in the planet formation process. The Bok globule CB 26 harbours such a young disc. We present a detailed model of the edge-on circumstellar disc and its envelope in the Bok globule CB 26. The model is based on HST near-infrared maps in the I, J, H, and K bands, OVRO and SMA radio maps at 1.1mm, 1.3mm and 2.7mm, and the spectral energy distribution (SED) from 0.9 microns to 3mm. New photometric and spectroscopic data from the Spitzer Space Telescope and the Caltech Submilimeter Observatory have been obtained and are part of our analysis. Using the self-consistent radiative transfer code MC3D, the model we construct is able to discriminate parameter sets and dust properties of both its parts, namely envelope and disc. We find that the disc has an inner hole with a radius of 45 +/- 5 AU. Based on a dust model including silicate and graphite the maximum grain size needed to reproduce the spectral millimetre index is 2.5 microns. Features seen in the near-infrared images, dominated by scattered light, can be described as a result of a rotating envelope. Successful employment of ISM dust in both the disc and envelope hint that grain growth may not yet play a significant role for the appearance of this system. A larger inner hole gives rise to the assumption that CB 26 is a circumbinary disc.Comment: 18 pages, 15 figures, Accepted for publication in A&

Accretion disks around massive stars: Hydrodynamic structure, stability and dust sublimation

We investigate the structure of accretion disks around massive protostar applying steady state models of thin disks. The thin disk equations are solved with proper opacities for dust and gas taking into account the huge temperature variation along the disk. We explore a wide parameter range concerning stellar mass, accretion rate, and viscosity parameter \alpha . The most essential finding is a very high temperature of the inner disk. For e.g. a 10 M_sun protostar and an accretion rate of 10^-4 M_sun/yr, the disk midplane temperature may reach almost 10^5 K. The disk luminosity in this case is about 10^4 L_sun and, thus, potentially higher than that of a massive protostar. We motivate our disk model with similarly hot disks around compact stars. We calculate a dust sublimation radius by turbulent disk self-heating of more than 10AU, a radius, which is 3 times larger than caused by stellar irradiation. We discuss implications of this result on the flashlight effect and the consequences for the radiation pressure of the central star. In difference to disks around low mass protostars our models suggest rather high values for the disk turbulence parameter \alpha close to unity. However, disk stability to fragmentation due to thermal effects and gravitational instability would require a lower \alpha value. For \alpha = 0.1 we find stable disks out to 80AU. Essentially, our model allows to compare the outer disk to some of the observed massive protostellar disk sources, and from that, extrapolate on the disk structure close to the star which is yet impossible to observe.Comment: 23 pages, 8 figures, accepted by ApJ, For full version see http://www.mpia.de/homes/vaidya