The Structure of the W49A Molecular Cloud Complex: Burst of Star Formation in the 10⁵ M_⊙ Core

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We have observed the CS (J = 1-0), C³⁴S (J = 1-0) and H51α emission toward the W49A molecular cloud complex in an area of 3'x 2′ (α x δ) with an angular resolution of 33″. The CS emitting region is 100″ x 80″ or 6.7 pc x 5.4 pc (α x δ) at the half maximum level. Although the CO emission is self-absorbed due to the foreground cold gas, the CS optical depth of the foreground gas is found to be small. Therefore, the two CS peaks at V_LSR = 4 km s⁻¹ and 12 km s⁻¹ imply the presence of two dense molecular clouds toward W49A. The brighter 12 km s⁻¹ cloud peaks 35″ southeast of W49A IRS, the infrared and H₂O/OH maser sources associated with the compact H II region, while the 4 km s⁻¹ cloud has a peak at W49A IRS. The hydrogen column density through the c³⁴S emitting region is (0.3-1.7) x 10²⁴ cm⁻². The estimated core mass of the W49A molecular cloud is (0.5-2.5) x 10⁴ M_⊙. This mass is closely packed in a small region of 3.4 pc in diameter, and is about an order of magnitude larger than the virial mass of the system. The massive core will collapse within 10 years unless there is some special supporting mechanism. There was a sudden increase in the star formation rate 10⁴– 10⁵ years ago, suggesting a triggered burst of star formation in the core of W49A. The collision of two velocity clouds might have triggered the formation of this massive core and the burst of star formation.