Direct evidence of dust growth in L183 from mid-infrared light scattering

¹ LERMA & UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: stein@mpia.de
³ Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble, Cedex 9, France
⁴ Spitzer Science Center, 1200 E California Blvd, Mail Stop 220-6, Pasadena CA 91125, USA

Received: 6 July 2009
Accepted: 30 November 2009

Abstract

Context. Theoretical arguments suggest that dust grains should grow in the dense cold parts of molecular clouds. Evidence of larger grains has so far been gathered in near/mid infrared extinction and millimeter observations. Interpreting the data is, however, aggravated by the complex interplay of density and dust properties (as well as temperature for thermal emission).

Aims. Direct evidence of larger particles can be derived from scattered mid-infrared (MIR) radiation from a molecular cloud observed in a spectral range where little or no emission from polycyclic aromatic hydrocarbons (PAHs) is expected.

Methods. We present new Spitzer data of L183 in bands that are sensitive and insensitive to PAHs. The visual extinction A_V map derived in a former paper was fitted by a series of 3D Gaussian distributions. For different dust models, we calculate the scattered MIR radiation images of structures that agree with the A_V map and compare them to the Spitzer data.

Results. The Spitzer data of L183 show emission in the 3.6 and 4.5 μm bands, while the 5.8 μm band shows slight absorption. The emission layer of stochastically heated particles should coincide with the layer of strongest scattering of optical interstellar radiation, which is seen as an outer surface on I band images different from the emission region seen in the Spitzer images. Moreover, PAH emission is expected to strongly increase from 4.5 to 5.8 μm, which is not seen. Hence, we interpret this emission to be MIR scattered light from grains located further inside the core, and call it ”coreshine”. Scattered light modeling when assuming interstellar medium dust grains without growth does not reproduce flux measurable by Spitzer. In contrast, models with grains growing with density yield images with a flux and pattern comparable to the Spitzer images in the bands 3.6, 4.5, and 8.0 μm.

Conclusions. There is direct evidence of dust grain growth in the inner part of L183 from the scattered light MIR images seen by Spitzer.