2010A&A...520A..84P

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. 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 ∼15x13pc² field obtained with the IRAM 30m telescope. 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 ≃3.3pc and a thickness of ≃0.41pc, is about 1.9x10⁴M_☉. The total kinetic energy of the expanding shells is estimated to be ∼10⁴⁹erg, 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 ∼5km/s, resulting in an age of ∼3-7x10⁵ years. In addition, on larger (∼35x50pc²) 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 likely related to the mechanism which triggers the starburst in W49A.