On the importance of scattering at 8 μm: Brighter than you think

Abstract

Context. Extinction and emission of dust models need for observational constraints to be validated. The coreshine phenomenon has already shown the importance of scattering in the 3 to 5 µm range and its ability to validate dust properties for dense cores. Aims. We want to investigate whether scattering can also play a role at longer wavelengths and to place even tighter constraints on the dust properties. Methods. We analyze the inversion of the Spitzer 8 µm map of the dense molecular cloud L183, to examine the importance of scattering as a potential contributor to the line-of-sight extinction.Results. The column density deduced from the inversion of the 8 µm map, when we neglect scattering, disagrees with all the other column density measurements of the same region. Modeling confirms that scattering at 8 µm is not negligible with an intensity of several hundred kJy sr −1 . This demonstrates the need of efficiently scattering dust grains at mid-infrared wavelengths up to 8 µm. Coagulated aggregates are good candidates and might also explain the discrepancy at high extinction between E(J − K) and τ 9.7 toward dense molecular clouds. Further investigation requires considering efficiently scattering dust grains including ices as realistic dust models.

Indirect evidence of significant grain growth in young protostellar envelopes from polarized dust emission

“…In molecular clouds and dense cold cores, the observed decrease in the mid-IR to far-IR emission ratio (Stepnik et al 2003;Flagey et al 2009), the increase in the dust far-IR/millimeter opacity and spectral index (Martin et al 2012;Bracco et al 2017), and the increased scattering efficiency from the near-to mid-IR (Pagani et al 2010) point out to an evolution of dust properties in dense regions. It was suggested that these observations could be explained if dense structures contain aliphatic-rich ice-mantled grains (Jones et al 2013;Ysard et al 2016), or already evolved micrometer-sized grains (Steinacker et al 2010;Lefèvre et al 2016). A rather unexplored step on the evolutionary sequence lead-ing to the formation of planets is the protostellar stage, and even more so the main accretion phase (Class 0 protostars, André et al 2000).…”

Dust evolution in pre-stellar cores

“…2.2). While we have already found a reasonable solution to model L183 in the NIR and MIR [6], the NIKA2 data will help to derive the best grain composition thanks to SIGMA and 3D radiative transfer modeling. Figure 2.…”

“…The first step of our approach is to build a 3D density model of the cloud independently from scattered light or degenerated emission process. Our density model relies on dust extinction as measured by reddening in the NIR, and the PSC densities are deduced from molecular observations of N 2 H + and N 2 D + [6,13], assuming thermalized gas and dust and rotational symmetry 1 (see Fig. 1).…”

“…A V maps of L183 with color scale in magnitudes. The 3D density model is based on NIR extinction of background stars and N 2 H + and N 2 D + observations for the PSC [6]. From left to right: facing the cloud, from above, from the side.…”

“…Figure 2. Herschel observations on left column at 250 µm, 350 µm, and 500 µm and corresponding modeled maps on right side using the dust model and density profile built from MIR scattering [6]. Color scale is in MJy/sr, box size is 40 .…”