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 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&

The molecular gas content of the Pipe Nebula I. Direct evidence of outflow-generated turbulence in B59?

The Pipe Nebula is a molecular cloud hosting the B59 region as its only active star-forming clump. While the particular importance of outflows in active star forming regions is subject of debate, the quiet nature of the gas in B59 makes it a good site to directly see the impact of protostellar feedback on the quiescent dense gas. Using HARP at the JCMT, we mapped the B59 region with the J=3-2 transition of 12CO to study the kinematics and energetics of the outflows, and 13CO and C18O to study the overall dynamics of the ambient cloud, the physical properties of the gas, and the hierarchical structure of the region. The B59 region has a total of 30Msun of cold and quiescent material, mostly gravitationally bound, with narrow line widths throughout. Such low levels of turbulence in non-star-forming sites of B59 are indicative of the intrinsic initial conditions of the cloud. On the other hand, close to the forming protostars the impact of the outflows is observed as a localised increase of both line widths from 0.3 to 1 km/s, and 13CO excitation temperatures by 2-3K. The impact of the outflows is also evident in the low column density material which shows signs of being pushed, shaped and carved by the outflow bow shocks as they pierce their way out of the cloud. Much of this structure is readily apparent in a dendrogram analysis of the cloud. The low mass of B59 together with its intrinsically quiescent gas and small number of protostars, allows the identification of specific regions where the outflows from the embedded sources interact the dense gas. Our study suggests that outflows are an important mechanism for injecting and sustaining supersonic turbulence at sub-parsec size scales. We find that less than half of the outflow energy is deposited as turbulent energy of the gas, however this turbulent energy is sufficient to slow down the collapse of the region.Comment: Accepted for publication in A&