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

Abstract

Context. The largest grains (0.5−1 μm) 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 μm) 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 μm 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 μm) dust grains. The scattered light profiles are found to be inconsistent with an interstellar silicate grain distribution extending only to 0.25 μm 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 μm. 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.

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