Aims. W49A is a giant molecular cloud which harbors some ofthe most luminous embedded clusters in the Galaxy. However, theexplanation 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 ()13CO J=2-1 and C18O J=2-1images over a pc2 field obtained with the IRAM 30m telescope. Results. Two expanding shells have been identified in themid-infrared images, and confirmed in the position-velocity diagramsmade 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 3.3pc and a thickness of 0.41pc, is about .The total kinetic energy of the expanding shells is estimated to be 1049erg,which is probably provided by a few massive stars, whose radiationpressure and/or strong stellar winds drive the shells. The expandingshells are likely to have a common origin close to the two ultracompactH II regions (source O and source N), and their expansion speed is estimated to be 5kms-1, resulting in an age of 3-years. In addition, on larger (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 1050erg.The main driving mechanism for the gas ejections is unclear, but likelyrelated to the mechanism which triggers the starburst in W49A. Key words: stars formation - H II regions - ISM: bubbles
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