W49A: a starburst triggered by expanding shells

¹ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany e-mail: tcpeng@mpifr-bonn.mpg.de
² SRON Netherlands Institute for Space Research, 9747 AD Groningen, The Netherlands
³ INAF Osservatorio Astrofisico di Arcetri, 50125 Firenze, Italy 


Received: 11 May 2010
Accepted: 21 July 2010

Abstract

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 . 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.