2005MNRAS.359..589M

We present a spectroscopic study of the shape of the dust-extinction law between 1.0 and 2.2µm towards a set of nine ultracompact Hii regions with A_V≳ 15mag. We find some evidence that the reddening curve may tend to flatten at higher extinctions, but just over half of the sample has extinction consistent with or close to the average for the interstellar medium (ISM). There is no evidence of extinction curves significantly steeper than the standard law, even where water ice is present. Comparing the results to the predictions of a simple extinction model, we suggest that a standard extinction law implies a robust upper limit to the grain-size distribution at around 0.1-0.3µm. Flatter curves are most likely the result of changes in this upper limit, although the effects of flattening as a result of unresolved clumpy extinction cannot be ruled out.