Multi-wavelength observations and modelling of a quiescent cloud LDN1512

Abstract

Context. Light scattering at near-infrared (NIR) wavelengths has been used to study the optical properties of the interstellar dust grains, but these studies are limited by the assumptions on the strength of the radiation field. On the other hand, thermal dust emission can be used to constrain the properties of the radiation field, although this is hampered by uncertainty about the dust emissivity. Aims. Combining light scattering and emission studies allows us to probe the properties of the dust grains in detail. We wish to study if current dust models allow us to model a molecular cloud simultaneously in the NIR and far-infrared (FIR) wavelengths and compare the results with observations. Our aim is to place constraints on the properties of the dust grains and the strength of the radiation field. Methods. We present computations of dust emission and scattered light of a quiescent molecular cloud LDN1512. We use NIR observations covering the J, H, and KS bands, and FIR observations between 250 and 500 μm from the Herschel space telescope. We constructed radiative transfer models for LDN1512 that include an anisotropic radiation field and a three-dimensional cloud model. Results. We are able to reproduce the observed FIR observations, with a radiation field derived from the DIRBE observations, with all of the tested dust models. However, with the same density distribution and the assumed radiation field, the models fail to reproduce the observed NIR scattering in all cases except for models that take into account dust evolution via coagulation and mantle formation. The intensity from the diffuse interstellar medium like, dust models can be increased to match the observed one by reducing the derived density, increasing the intensity of the background sky and the strength of the radiation field between factors from two to three. We find that the column densities derived from our radiative transfer modelling can differ by a factor of up to two, compared to the column densities derived from the observations with modified blackbody fits. The discrepancy in the column densities is likely caused because of a temperature difference between a modified blackbody fit and the real spectra. The difference between the fitted temperature and the true temperature could be as high as ΔT = +1.5 K. Conclusions. We show that the observed dust emission can be reproduced with several different assumptions about the properties of the dust grains. However, in order to reproduce the observed scattered surface brightness, dust evolution must be taken into account.