Speaker
Peter Scicluna
(ESO)
Description
The spatial distribution of dust has significant influence on the observed extinction curve. As a result, when observing anisotropic, clumpy or turbulent environments the aperture may contain a wealth of unresolved structure. Hence, observations only probe an effective optical depth and effective extinction curve. In order to elucidate and quantify the impact of unresolved structure we perform radiative transfer modelling of various dust geometries. We find that stars embedded in homogeneous, optically-thin unresolved spherical shells have steeper extinction curves than that of the input dust cross-sections, which would be recovered with pencil-beam observations. This is in agreement with previous studies of stars in homogeneous clouds. Similarly, the effective optical depth is lower than that of a pencil beam due to the collection of scattered light, resulting in an under-estimate of the dust column density from extinction measurements. Equal mass shells composed of optically thick clumps, on the other hand, are shown to have significantly flatter extinction than the cross-sections. The degree of flattening is determined by the wavelength at which the clumps become optically thick and the clump distribution, which determines the scattered light. Our models indicate that this leads to a log-normal distribution of R_V for different lines-of-sight into a clumpy shell. We expand our investigation to clumpy debris disc and molecular cloud analogues, with similar findings. We also show that, due to optical depth effects, the strength of the 2200AA feature can be significantly different in clumpy media. This affirms that changes in the extinction observed toward embedded objects are not robust probes of changes in dust composition. Hence, studies which use extinction to explore dust must make use of background sources or sufficiently high resolution observations to resolve out circumstellar structure. For typical seeing limited observations, these effects start to become significant when observing in Orion, and become stronger with increasing distance.
Author
Peter Scicluna
(ESO)