Context. The properties of dust grains, in particular their size
distribution, are expected to differ from the interstellar medium to the
high-density regions within molecular clouds. Aims. We measure the mid-infrared
extinction law produced by dense material in molecular cloud cores. Since the
extinction at these wavelengths is caused by dust, the extinction law in cores
should depart from that found in low-density environments if the dust grains
have different properties. Methods. We use the unbiased LINES method to measure
the slope of the reddening vectors in color-color diagrams. We derive the
mid-infrared extinction law toward the dense cores B59 and FeSt 1-457 in the
Pipe Nebula over a range of visual extinction between 10 and 50 magnitudes,
using a combination of Spitzer/IRAC, and ESO NTT/VLT data. Results. The
mid-infrared extinction law in both cores departs significantly from a
power-law between 3.6 and 8 micron, suggesting that these cores contain dust
with a considerable fraction of large dust grains. We find no evidence for a
dependence of the extinction law with column density up to 50 magnitudes of
visual extinction in these cores, and no evidence for a variation between our
result and those for other clouds at lower column densities reported elsewhere
in the literature. This suggests that either large grains are present even in
low column density regions, or that the existing dust models need to be revised
at mid-infrared wavelengths. We find a small but significant difference in the
extinction law of the two cores, that we tentatively associate with the onset
of star formation in B59.Comment: 8 pages, 6 figures. Accepted to A&
