FRAGMENTATION AND KINEMATICS OF THE W49N CLOUD CORE

A multitransitional study of the W49N molecular cloud core has been carried out in seven rotational lines of CS and (CS)-S-34, ranging in excitation from J = 3-2 to J = 10-9. The cloud core has been found to consist primarily of three dense molecular clumps, of size 0.4-1.0 pc, located approximately along an axis at P.A. 560 on the plane of the sky. The clumps' local densities are 2-6 x 10(6) cm-3, and their masses a few 10(4) M. each, yielding a total core mass of almost-equal-to 10(5) M.. The two outer clumps are receding from us at an LSR velocity of 12 km s-1, but the central clump is instead moving away at only 4 km s-1, indicating that two different molecular clouds are present toward W49A. Absorption studies place the 12 km s-1 gas closer to us than the 4 km s-1 gas, implying that these two molecular clouds are converging upon each other. In addition, the central CS clump is found to be spinning rapidly (with a gradient of 3 km s-1 pc-1) in a direction counter both to Galactic rotation, and to the direction inferred earlier from recombination line observations of the W49A compact H II regions. The hypothesis of global molecular cloud collapse, which was inferred for W49N from the recombination line observations, is thus found to be inconsistent with our new molecular line data. Instead, the molecular gas kinematics suggest that the enhanced 0-star formation rate in W49N has been triggered by a cloud-cloud collision, which we hypothesize has resulted from orbit crowding in our Galaxy's Sagittarius spiral arm.