The mid-infrared extinction law in the darkest cores of the Pipe Nebula

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 what is found in low-density environments if the dust grains have different properties. Methods. We used the unbiased LINES method to measure the slope of the reddening vectors in color-color diagrams. We derived 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 mag, 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 μm, suggesting that these cores contain dust with a considerable fraction of large dust grains. We find no evidence of any dependence in the extinction law with column density up to 50 mag of visual extinction in these cores, and no evidence of any variation between our result and those for other clouds at lower column densities reported elsewhere in the literature. This suggests either that 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, which we tentatively associate with the onset of star formation in B59.