Walborn, N. R.
Hot and Cool: Bridging Gaps in Massive Star Evolution ASP Conference Series Vol. 425, Proceedings of a Workshop held at the California Institute of Technology, Pasadena, California, 10-12 November 2008. Edited by Claus Lietherer, Philip Bennett, Pat Morris, Jacco van Loon. San Francisco: Astronomical Society of the Pacific, 2010., p.45
The correlated stellar and nebular properties of coeval massive young clusters as a function of age are discussed. The recently investigated object Westerlund 1 is an essential addition to the standard sequence, which is so far unique in the solar vicinity. The masses of its most evolved population are just at or below the Humphreys-Davidson Limit, resulting in a fully populated supergiant sequence across the entire Hertzsprung-Russell Diagram. In contrast, the comparably rich (albeit more extended) Scorpius OB1 association has no supergiants later than type B1.5; its most massive evolved stars must be just above the HD Limit. These two groups define the location of the Limit, to within the uncertainties introduced by apparent ranges in the masses of their evolving stars, and by possible metallicity differences. In two-stage starbursts, two sequential cluster age phases with a difference of 1-2 Myr coexist. On the basis of detailed age calibrations in the Large Magellanic Cloud, the morphology of giant H II regions (or their absence) accurately estimates the ages of extended regions in starburst galaxies.