Properties Of Large Grains Research Articles

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.

Dust emission from inhomogeneous interstellar clouds: Radiative transfer in 3D with transiently heated particles

We present a numerical code for continuum radiative transfer that is based on the idea of a `library' describing the relation between the intensity of the local radiation field and the resulting dust emission. With this information and local intensities at a few reference wavelengths, the radiative transfer equation can be integrated through the source and an approximation of the emission spectrum is obtained. Tests with small models for which the radiative transfer problem can be solved directly show that with our method one can easily obtain an accuracy of a few per cent. We show spectra computed from three-dimensional MHD simulations containing up to 128^3 cells. The models represent starless, inhomogeneous interstellar clouds embedded in the normal interstellar radiation field. The intensity ratios between IRAS bands show large variations that follow the filamentary density distribution. The power law index of the spatial power spectrum of the column density map is -2.8. In infrared maps temperature variations increase the power at high spatial frequencies, and the index varies between -2.5 and -2.7. Assuming constant dust properties throughout the cloud, the IRAS ratio decreases in densest cores only by a factor of ~4 compared with the value in diffuse medium. In observations the ratio can decrease twice as much even in optically thinner clouds. This requires that most of the small grains are removed in these regions, and possibly a modification of the properties of large grains.