The mid-infrared extinction in molecular clouds Case study of B 335

Abstract

The purpose of the investigation is to probe the dust properties inside a molecular cloud, how particle grow and how the presence of ice coatings may change the overall shape of the extinction curve. Field stars can be used to probe the cloud extinction. By combining multi-colour photometry and IR spectroscopy the spectral class of the star can be determined as can the extinction curve. We determine the reddening curve from 0.35 to 24 \mu m. The water ice band at 3.1 \mu m is weaker (\tau(3.1) = 0.4) than expected from the cloud extinction (AV \approx 10 for the sightline to the most obscured star). On the other hand, the CO ice band at 4.7 \mu m is strong (\tau(4.67) = 0.7) and indicates, that the mass column density of frozen CO is about the same as that of water ice. We show that the reddening curves for the two background stars, for which the silicate band has been measured, can be accurately modelled from the UV to 24 \mu m. These models only include graphite and silicate grains. No need for any additional major grain component to explain the slow decline of the reddening curve beyond the K band. The dust model for the dense part of the cloud has more large grains than for the rim. We propose that the well established shallow reddening curve beyond the K band has two different explanations: larger graphite grains in dense regions and relatively small grains in the diffuse ISM, giving rise to substantially less extinction beyond the K band than previously thought. For the sight line towards the most obscured star, we derive the relation AKs = 0.97 \cdot E(J - Ks), and assuming that all silicon is bound in silicates, N(2 H2 +H) \approx 1.5 \times 10^{21} \cdot AV \approx 9 \times 10^{21} \cdot AKs. For the rim of the cloud we get AKs = 0.51 \cdot E(J - Ks), which is close to recent determinations for the diffuse ISM. The corresponding gas column density is N(2 H2 +H) \approx 2.3 \times 10^{21} \cdot AV \approx 3 \times 10^{22} \cdot AKs.