2011A&A...531A..26B

High-mass star formation has been a very active field over the past decade; however, most studies have targeted regions of luminosities between 10⁴ and 10⁵L_☉. In contrast to that, the highest mass stars reside in clusters exceeding 10⁵ or even 10⁶L_☉. We want to study the physical conditions associated with the formation of the highest mass stars. To do this, we selected the W31 star-forming complex with a total luminosity of ∼6x10⁶L_☉ (comprised of at least two subregions) for a multiwavelength spectral line and continuum study covering wavelengths from the near- and midinfrared via (sub)mm wavelength observations to radio data in the cm regime. While the overall structure is similar among the multiwavelength continuum data, there are several intriguing differences. The 24µm emission stemming largely from small dust grains tightly follows the spatial structure of the cm emission tracing the ionized free-free emission. As a result, warm dust resides in regions that are spatially associated with the ionized hot gas (∼10⁴K) of the HII regions. Furthermore, we find several evolutionary stages within the same complexes, ranging from infrared-observable clusters, via deeply embedded regions associated with active star formation traced by 24µm and cm emission, to at least one high-mass gas clump devoid of any such signature. The ¹³CO(2-1) and C¹⁸O(2-1) spectral line observations reveal kinematic breadth in the entire region with a total velocity range of approximately 90km/s. Kinematic and turbulent structures are set into context. While the average virial mass ratio for W31 is close to unity, the line width analysis indicates large-scale evolutionary differences between the southern and northern subregions (G10.2-0.3 and G10.3-0.1) of the whole W31 complex. A color-color analysis of the IRAC data also shows that the class II sources are broadly distributed throughout the entire complex, whereas the Class 0/I sources are more tightly associated with the active high-mass star-forming regions. The clump mass function - tracing cluster scales and not scales of individual stars - derived from the 875µm continuum data has a slope of 1.5±0.3, consistent with previous cloud mass functions. The highest mass and luminous stars form in highly structured and complex regions with multiple events of star formation that do not always occur simultaneously but in a sequential fashion. Warm dust and ionized gas can spatially coexist, and high-mass starless cores with low-turbulence gas components can reside in the direct neighborhood of active star-forming clumps with broad linewidths.