Terahertz ammonia absorption as a probe of infall in high-mass star forming clumps

Cloud contraction and infall are the fundamental processes of star formation. While blue-skewed line profiles observed in high-mass star forming regions are commonly taken as evidence of infall by an ever increasing number of studies, their interpretation offers many pitfalls. Detecting infall via redshifted absorption in front of continuum sources is a much more direct and reliable method but so far mostly restricted toward absorption in the centimeter toward strong HII regions. Here we present a novel approach by probing absorption of rotational ammonia transitions in front of the strong dust emission of massive star-forming regions. A carefully selected sample of three regions with different stages of evolution is selected to study infall through the evolution of massive star-forming clumps. Redshifted absorption is detected toward all three sources and infall rates between 3-10 x 10(-3) M-circle dot yr(-1) are derived.