The extinction law for molecular clouds. Case study of B 335

TL;DR

This study measures the extinction law in the B 335 molecular cloud from UV to near-IR, confirming it can be described by a single parameter and linking it to H2 column density, with implications for cloud structure modeling.

Contribution

It introduces a new multicolour method for determining reddening laws and star classifications, and applies it to B 335 to derive extinction parameters and cloud structure insights.

Findings

The extinction law in B 335 is characterized by RV=4.8.

Reddening curves are well reproduced by models assuming silicate grains.

The outer shell density follows an r^-2 profile, with a mass estimate of 2.5 solar masses.

Abstract

We determine the extinction curve from the UV to the near-IR for molecular clouds and investigate whether current models can adequately explain this wavelength dependence of the extinction. The aim is also to interpret the extinction in terms of H2 column density. We applied five different methods, including a new method for simultaneously determining the reddening law and the classification of the background stars. Our method is based on multicolour observations and a grid of model atmospheres. We confirm that the extinction law can be adequately described by a single parameter, RV (the selective to absolute extinction), in accordance with earlier findings. The RV value for B 335 is RV = 4.8. The reddening curve can be accurately reproduced by model calculations. By assuming that all the silicon is bound in silicate grains, we can interpret the reddening in terms of column density, NH = 4.4 (\pm0.5) \times 1021 EI-Ks cm-2, corresponding to NH = 2.3 (\pm0.2) \times 1021 \cdot AV cm-2, close to that of the diffuse ISM, (1.8-2.2) \times 1021 cm-2 . We show that the density of the B 335 globule outer shells can be modelled as an evolved Ebert-Bonnor gas sphere with {\rho} \propto r-2, and estimate the mass of this globule to 2.5 Msun