J. Steinacker

The Ubiquity of Micrometer-Sized Dust Grains in the Dense Interstellar Medium

Dust to Dust Recently, the so-called coreshine effect was identified in a nearby interstellar cloud. The coreshine effect refers to the scattering of mid-infrared light by micron-sized dust grains in the densest regions of molecular clouds, the places where stars and planets are known to form. Using data from the Spitzer telescope, Pagani et al. (p. 1622) now show that, rather than being limited to one single molecular cloud, the coreshine effect is common all over our galaxy, but is not universal and could be used to learn about the properties of star-forming cores and the dust therein. The light scattered by small dust grains can tell us about the properties of star-forming regions in our Galaxy. Cold molecular clouds are the birthplaces of stars and planets, where dense cores of gas collapse to form protostars. The dust mixed in these clouds is thought to be made of grains of an average size of 0.1 micrometer. We report the widespread detection of the coreshine effect as a direct sign of the existence of grown, micrometer-sized dust grains. This effect is seen in half of the cores we have analyzed in our survey, spanning all Galactic longitudes, and is dominated by changes in the internal properties and local environment of the cores, implying that the coreshine effect can be used to constrain fundamental core properties such as the three-dimensional density structure and ages and also the grain characteristics themselves.

The 2D continuum radiative transfer problem : Benchmark results for disk configurations

We present benchmark problems and solutions for the continuum radiative transfer (RT) in a 2D disk configuration. The reliability of three Monte-Carlo and two grid-based codes is tested by comparing their results for a set of well-defined cases which differ for optical depth and viewing angle. For all the configurations, the overall shape of the resulting temperature and spectral energy distribution is well reproduced. The solutions we provide can be used for the verification of other RT codes. We also point out the advantages and disadvantages of the various numerical techniques applied to solve the RT problem.

Three-Dimensional Dust Radiative Transfer*

Cosmic dust is present in many astrophysical objects, and recent observations across the electromagnetic spectrum show that the dust distribution is often strongly three-dimensional (3D). Dust grains are effective in absorbing and scattering ultraviolet (UV)/optical radiation, and they re-emit the absorbed energy at infrared wavelengths. Understanding the intrinsic properties of these objects, including the dust itself, therefore requires 3D dust radiative transfer (RT) calculations. Unfortunately, the 3D dust RT problem is nonlocal and nonlinear, which makes it one of the hardest challenges in computational astrophysics. Nevertheless, significant progress has been made in the past decade, with an increasing number of codes capable of dealing with the complete 3D dust RT problem. We discuss the complexity of this problem, the two most successful solution techniques [ray-tracing (RayT) and Monte Carlo (MC)], and the state of the art in modeling observational data using 3D dust RT codes. We end with an outlook on the bright future of this field.

Dust-temperature of an isolated star-forming cloud: Herschel observations of the Bok globule CB244

We present Herschel observations of the isolated, low-mass star-forming Bok globule CB244. It contains two cold sources, a low-mass Class 0 protostar and a starless core, which is likely to be prestellar in nature, separated by 90 �� (∼18 000 AU). The Herschel data sample the peak of the Planck spectrum for these sources, and are therefore ideal for dust-temperature and column density modeling. With these data and a near-IR extinction map, the MIPS 70 μm mosaic, the SCUBA 850 μm map, and the IRAM 1.3 mm map, we model the dust-temperature and column density of CB 244 and present the first measured dust-temperature map of an entire starforming molecular cloud. We find that the column-averaged dust-temperature near the protostar is ∼17.7 K, while for the starless core it is ∼10.6 K, and that the effect of external heating causes the cloud dust-temperature to rise to ∼17 K where the hydrogen column density drops below 10 21 cm −2 . The total hydrogen mass of CB 244 (assuming a distance of 200 pc) is 15 ± 5 M� . The mass of the

3D continuum radiative transfer in complex dust configurations around stellar objects and active galactic nuclei. I. Computational methods and capabilities

We present the new grid-based code SteinRay which has been developed to solve the full 3D continuum radiative transfer problem generally arising in the analysis of star-forming regions, matter around evolved stars, starburst galaxies, or tori around active galactic nuclei. The program calculates the intensity emerging from these complicated structures using a combination of step-size controlled ray-tracing and adaptive multi-wavelength photon transport grids. Along with a 2nd order finite differencing approach, the grids are optimized to reduce the discretization error and provide global error control. The full wavelength-dependent problem is solved without any flux approximation, and for arbitrary scattering properties of the dust. The program is designed to provide spatially resolved images, visibilities, and spectra of complex dust distributions even without any symmetry for wavelengths ranging from the UV to FIR and allows for multiple internal and external sources. In this paper, the algorithm is described and the capabilities of the code are illustrated by the treatment of 1D and 3D test cases. Analyzing the properties of typical cosmic dust, we discuss the wavelength range for which the time-consuming solution on adaptive grids can be omitted. The temperature is calculated self-consistently using standard accelerated Λ -iteration.

From high-mass starless cores to high-mass protostellar objects

Aims. Our aim is to understand the evolutionary sequence of high-mass star formation from the earliest evolutionary stage of h ighmass starless cores, via high-mass cores with embedded low- to intermediate-mass objects, to finally high-mass protost ellar objects. Methods. Herschel far-infrared PACS and SPIRE observations are combined with existing data at longer and shorter wavelengths to characterize the spectral and physical evolution of massive star-forming regions. Results. The new Herschel images spectacularly show the evolution of the youngest and cold high-mass star-forming regions from mid-infrared shadows on the Wien-side of the spectral energy distribution (SED), via structures almost lost in the back ground emission around 100� m, to strong emission sources at the Rayleigh-Jeans tail. Fi ts of the SEDs for four exemplary regions covering evolutionary stages from high-mass starless cores to high-mass protostellar objects reveal that the youngest regions can be fitted by single-component black-bodies with temperatures on the order of 17 K. More evolved regions show mid-infrared excess emission from an additional warmer component, which however barely contributes to the total luminosities for the youngest regions. E xceptionally low values of the ratio between bolometric and submm luminosity additionally support the youth of the infrared-dark sou rces. Conclusions. The Herschel observations reveal the spectral and physical properties o f young high-mass star-forming regions in detail. The data clearly outline the evolutionary sequence in the images and SEDs. Future work on larger samples as well as incorporating full radiative transfer calculations will characterize th e physical nature at the onset of massive star formation in even more depth.

Efficient integration of intensity functions on the unit sphere

Abstract To integrate peaking intensity functions over all ray directions, commonly occuring in radiative transfer calculations, we present efficient quadrature formulae by calculating appropriate nodes and weights. Instead of product formulae using univariate quadrature rules we construct multivariate quadrature formulae for the sphere. Due to the fact that there is no Gaussian quadrature for the unit sphere for grid point numbers of interest, approximate grids and corresponding weights have to be calculated. Using a special Metropolis algorithm, we minimize the potential energy of an N -charged particle distribution on the sphere and discuss the resulting, nearly isotropically distributed configurations. We find that the vertices of the cube and pentagon dodecahedron are not the optimal distribution, although they have as Platonian bodies equally distributed vertices. The algorithm finds even high-resolving grids ( N ~ 1000) with moderate computational effort (4 h on a 30 MFlop workstation). The corresponding weights of the quadrature rule are obtained by evaluating special Gegenbauer polynomials at products of the nodes and inverting the resulting symmetric matrix by Cholesky-decomposition. Thus we get very precise quadrature rules (with a relative error of the order 10 −12 ) though the weights are not equal.

Dust properties inside molecular clouds from coreshine modeling and observations

Context. Using observations to deduce dust properties, grain-size distribution, and physical conditions in molecular clouds is a highly degenerate problem.Aims. The coreshine phenomenon, a scattering process at 3.6 and 4.5 μm that dominates absorption, has revealed its ability to explore the densest parts of clouds. We use this effect to constrain the dust parameters. The goal is to investigate to what extent grain growth (at constant dust mass) inside molecular clouds is able to explain the coreshine observations. We aim to find dust models that can explain a sample of Spitzer coreshine data. We also examine the consistency with near-infrared data we obtained for a few clouds.Methods. We selected four regions with a very high occurrence of coreshine cases: Taurus-Perseus, Cepheus, Chameleon, and L183/L134. We built a grid of dust models and investigated the key parameters to reproduce the general trend of surface brightnesses and intensity ratios of both coreshine and near-infrared observations with the help of a 3D Monte Carlo radiative transfer code. The grid parameters allowed us to investigate the effect of coagulation upon spherical grains up to 5 μm in size derived from the DustEm diffuse interstellar medium grains. Fluffiness (porosity or fractal degree), ices, and a handful of classical grain-size distributions were also tested. We used the near- and mostly mid-infrared intensity ratios as strong discriminants between dust models.Results. The determination of the background-field intensity at each wavelength is a key issue. In particular, an especially strong background field explains why we do not see coreshine in the Galactic plane at 3.6 and 4.5 μm. For starless cores, where detected, the observed 4.5 μm/3.6 μm coreshine intensity ratio is always lower than ~0.5, which is also what we find in the models for the Taurus-Perseus and L183 directions. Embedded sources can lead to higher fluxes (up to four times higher than the strongest starless core fluxes) and higher coreshine ratios (from 0.5 to 1.1 in our selected sample). Normal interstellar radiation-field conditions are sufficient to find suitable grain models at all wavelengths for starless cores. The standard interstellar grains are not able to reproduce observations and, because of the multiwavelength approach, only a few grain types meet the criteria set by the data. Porosity does not affect the flux ratios, while the fractal dimension helps to explain coreshine ratios, but does not seem able to reproduce near-infrared observations without a mix of other grain types.Conclusions. Combined near- and mid-infrared wavelengths confirm the potential of revealing the nature and size distribution of dust grains. Careful assessment of the environmental parameters (interstellar and background fields, embedded or nearby reddened sources) is required to validate this new diagnostic.

Scattering from dust in molecular clouds: Constraining the dust grain size distribution through near-infrared cloudshine and infrared coreshine

Context. The largest grains (0.5-1 micron) in the interstellar size distribution are efficient in scattering near- and mid-infrared radiation. These wavelengths are therefore particularly well suited to probe the still uncertain high-end of the size distribution. Aims. We investigate the change in appearance of a typical low-mass molecular core from the Ks (2.2 micron) band to the Spitzer IRAC 3.6 and 8 micron bands, and compare with model calculations, which include variations of the grain size distribution. Methods. We combine Spitzer IRAC and ground-based near-infrared observations to characterize the scattered light observed at the near- and mid-infrared wavelengths from the core L260. Using a spherical symmetric model core, we perform radiative transfer calculations to study the impact of various dust size distributions on the intensity profiles across the core. Results. The observed scattered light patterns in the Ks and 3.6 micron bands are found to be similar. By comparison with radiative transfer models the two profiles place constraints on the relative abundance of small and large (more than 0.25 micron) dust grains. The scattered light profiles are found to be inconsistent with an interstellar silicate grain distribution extending only to 0.25 micron and large grains are needed to reach the observed fluxes and the flux ratios. The shape of the Ks band surface brightness profile limits the largest grains to 1-1.5 micron. Conclusions. In addition to observing coreshine in the Spitzer IRAC channels, the combination with ground-based near-infrared observations are suited to constrain the properties of large grains in cores.

Ray Tracing for Complex Astrophysical High-opacity Structures

We present a ray-tracing technique for radiative transfer modeling of complex three-dimensional (3D) structures that include dense regions of high optical depth, such as that in dense molecular clouds, circumstellar disks, envelopes of evolved stars, and dust tori around active galactic nuclei. The corresponding continuum radiative transfer problem is described, and the numerical requirements for inverse 3D density and temperature modeling are defined. We introduce a relative intensity and transform the radiative transfer equation along the rays to solve machine precision problems and to relax strong gradients in the source term. For the optically thick regions where common ray tracers are forced to perform small trace steps, we give two criteria for making use of a simple approximative solver crossing the optically thick region quickly. Using an example of a density structure with optical depth changes of 6 orders of magnitude and sharp temperature variations, we demonstrate the accuracy of the proposed scheme using a common fifth-order Runge-Kutta ray tracer with adaptive step-size control. In our test case, the gain in computational speed is about a factor of 870. The method is applied in order to calculate the temperature distribution within a massive molecular cloud core for different boundary conditions for the radiation field.