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 13CO J = 1-0 image, complemented with higher resolution (~ 11”) 13CO J = 2-1 and C18O J = 2-1 images over a ~ 15×13 pc2 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 13CO J = 2-1 and C18O J = 2-1 data. The mass of the averaged expanding shell, which has an inner radius of ≈3.3 pc and a thickness of ≈0.41 pc, is about 1.9×104 M⊙. The total kinetic energy of the expanding shells is estimated to be ~1049 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 ~5 km s-1, resulting in an age of ~3-7×105 years. In addition, on larger (~35×50 pc2) scales, remnants of two gas ejections have been identified in the 13CO J = 1-0 data. Both ejections seem to have the same center as the expanding shells with a total energy of a few times 1050 erg. The main driving mechanism for the gas ejections is unclear, but likely related to the mechanism which triggers the starburst in W49A.
