L1506C NIKA2 maps at 1.15mm and 2mm and A(J) map

To do so, we used observations of dust emission covering, for the first time, a broad wavelength range, from the far-infrared (FIR) to millimeter range, as well as 3D radiative transfer modeling while also taking into account constraints from extinction data. We used both Herschel FIR observations and complementary millimeter wavelength observations with NIKA2 at the IRAM 30m telescope. Analysis of the spectral energy distributions over the whole spectral range was first performed with modified black body (MBB) fits.

The data were then analyzed with 3D modeling using the radiative transfer code SOC and the latest THEMIS 2 dust models. As additional constraints we used extinction observations from WIRCam at CFHT and from Spitzer. The MBB modeling over the wavelength range of 160um to 2mm revealed that L1506C is fragmented into two low density cores with n_H2_<5x10^4^cm^-3^, independently of the assumed dust opacity, with masses smaller than their Jeans masses.

The dust color temperature (T) drops from 16K in the filament to 11K in the cores, and the spectral index {beta} increases from 1.4 to 1.9, showing a clear T-{beta} anticorrelation and a change in grain properties. To model the source with 3D radiative transfer modeling, grains more evolved than the diffuse interstellar medium (ISM) are required in the densest part. Using grains with sizes up to 0.7um, 50% porosity, and 50% ice content, we were able to reproduce the observed millimeter emission within a 10% residual in the central core region and the near-infrared scattering (coreshine) within 1{sigma}.

Extinction data strongly constrain the dust properties, but the best results we obtained still overestimate the extinction by a factor of more than 2.5 with the current THEMIS 2 dust model. The model N_H2_ has a maximum value of 1.3x10^22^cm^-2^, 1.7 times larger than the one calculated with MBB modeling, and the model central dust temperature is down to 8K. Good fits to the observations were obtained for the transition between diffuse and evolved grains at low density, within the range 1500>=nH>=4500cm^-3^.

When adopting a model with a transition threshold of n_H_=3000cm^-3^, evolved grains account for 50% of the dust content along the line of sight, where AV~4 on the observed extinction map. Grain growth can already happen at a very early stage of star formation, even before the onset of gravitational collapse. It is difficult to precisely constrain the cloud properties with 3D radiative transfer modeling when only using dust emission observations because some of the parameters are degenerate.

Observations of near-infrared extinction and scattering and independent measurements from molecular line data are crucial complements to these data, providing independent constraints and breaking degeneracies in the analysis.