Grain size limits derived from 3.6 μm and 4.5 μm coreshine ^⋆

¹ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
e-mail: stein@mpia.de
² CNRS, IPAG, 38000 Grenoble, France
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Gemini Observatory, Casilla 603, La Serena, Chile
⁵ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁶ Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
⁷ Department of Physics, PO Box 64, University of Helsinki, 00014 Helsinki, Finland
⁸ Finnish Centre for Astronomy with ESO, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁹ LERMA & UMR8112 du CNRS, Observatoire de Paris, 61, Av. de l’Observatoire, 75014 Paris, France
¹⁰ Space Telescope Science Institute, Baltimore, MD 21218, USA

Received: 28 November 2014
Accepted: 17 June 2015

Abstract

Context. Recently discovered scattered light from molecular cloud cores in the wavelength range 3–5 μm (called “coreshine”) seems to indicate the presence of grains with sizes above 0.5 μm.

Aims. We aim to analyze 3.6 and 4.5 μm coreshine from molecular cloud cores to probe the largest grains in the size distribution.

Methods. We analyzed dedicated deep Cycle 9 Spitzer IRAC observations in the 3.6 and 4.5 μm bands for a sample of 10 low-mass cores. We used a new modeling approach based on a combination of ratios of the two background- and foreground-subtracted surface brightnesses and observed limits of the optical depth. The dust grains were modeled as ice-coated silicate and carbonaceous spheres. We discuss the impact of local radiation fields with a spectral slope differing from what is seen in the DIRBE allsky maps.

Results. For the cores L260, ecc806, L1262, L1517A, L1512, and L1544, the model reproduces the data with maximum grain sizes around 0.9, 0.5, 0.65, 1.5, 0.6, and >1.5 μm, respectively. The maximum coreshine intensities of L1506C, L1439, and L1498 in the individual bands require smaller maximum grain sizes than derived from the observed distribution of band ratios. Additional isotropic local radiation fields with a spectral shape differing from the DIRBE map shape do not remove this discrepancy. In the case of Rho Oph 9, we were unable to reliably disentangle the coreshine emission from background variations and the strong local PAH emission.

Conclusions. Considering surface brightness ratios in the 3.6 and 4.5 μm bands across a molecular cloud core is an effective method of disentangling the complex interplay of structure and opacities when used in combination with observed limits of the optical depth.