Molecular cloud structure and star-formation seen by Herschel
F. Motte, P. Andre et al.
Interstellar turbulence plays a major role during the formation process of molecular clouds and stars. Recent MHD models show that large-scale colliding HI flows form molecular clouds, and in unison dense structures at stagnation points that can collapse to form stars/clusters. At the same time, feedback from already formed stars (outflows from low- and high-mass protostars, UV-radiation from massive stars) inject turbulence on smaller scales.
Studies using structure characterizing tools (e.g. power spectra or Delta-variance), and filament-size measurements in Herschel continuum maps show that there are characteristic spatial scales in molecular clouds. Probability density functions (PDFs) determined in high-mass star forming regions from Herschel column density maps indicate a deviation from the log-normal form at characteristic extinction values signifying probably the threshold of star-formation.
The new Herschel far-infrared maps (70, 160, 250, 350, 500 micron) obtained within various photometric programs (HOBYS, Gould Belt, Hi-GAL) allow for the first time to derive (i) a very large set of column density maps that allow to precisely determine a possible star-formation threshold, and (ii) a complete catalog of young stellar objects in order to determine the star-formation rate. Then, the 'Kennicutt-Schmidt' law, relating the star-formation rate and the surface density, determined for a sample of galaxies, can be determined from Galactic molecular clouds.
Combining Herschel continuum data with (sub)-mm molecular line data is essential to study the velocity structure of the clouds and to compare with numerical models. Molecular line data allow to derive the Mach-number, and thus to characterize the turbulence state of a cloud. Preliminary results to which extent UV-radiation triggers further star-formation, extrapolated from studies in the Rosette, indicate that star-formation in GMCs will happen in any case, mainly determined by the large-scale turbulence that defined a primordial density structure. The influence of UV-radiation is only local, visible for example in the formation of pillars and globules in the HII-region/molecular cloud interface.