W49A: A starburst triggered by expanding shells

Aims. W49A is a giant molecular cloud which harbors some of the most luminous embedded clusters in the Galaxy. However, the explanation for this starburst-like phenomenon is still under debate.

Methods. We investigated large-scale Spitzer mid-infrared images together with a Galatic Ring Survey (13)CO J = 1-0 image, complemented with higher resolution (similar to 11 '') (13)CO J = 2-1 and C(18)O J = 2-1 images over a similar to 15 x 13 pc(2) field obtained with the IRAM 30 m telescope.

Results. Two expanding shells have been identified in the mid-infrared images, and confirmed in the position-velocity diagrams made from the (13)CO J = 2-1 and C(18)O J = 2-1 data. The mass of the averaged expanding shell, which has an inner radius of approximate to 3.3 pc and a thickness of approximate to 0.41 pc, is about 1.9 x 10(4) M(circle dot). The total kinetic energy of the expanding shells is estimated to be similar to 10(49) erg, which is probably provided by a few massive stars, whose radiation pressure and/or strong stellar winds drive the shells. The expanding shells are likely to have a common origin close to the two ultracompact HII regions (source O and source N), and their expansion speed is estimated to be similar to 5 k ms(-1), resulting in an age of similar to 3-7 x 10(5) years. In addition, on larger (similar to 35 x 50 pc(2)) scales, remnants of two gas ejections have been identified in the (13)CO J = 1-0 data. Both ejections seem to have the same center as the expanding shells with a total energy of a few times 10(50) erg. The main driving mechanism for the gas ejections is unclear, but likely related to the mechanism which triggers the starburst in W49A.