Rapidly rotating second-generation progenitors for the blue hook stars of {\omega} Cen

Horizontal Branch stars belong to an advanced stage in the evolution of the oldest stellar galactic population, occurring either as field halo stars or grouped in globular clusters. The discovery of multiple populations in these clusters, that were previously believed to have single populations gave rise to the currently accepted theory that the hottest horizontal branch members (the blue hook stars, which had late helium-core flash ignition, followed by deep mixing) are the progeny of a helium-rich "second generation" of stars. It is not known why such a supposedly rare event (a late flash followed by mixing) is so common that the blue hook of {\omega} Cen contains \sim 30% of horizontal branch stars 10 , or why the blue hook luminosity range in this massive cluster cannot be reproduced by models. Here we report that the presence of helium core masses up to \sim 0.04 solar masses larger than the core mass resulting from evolution is required to solve the luminosity range problem. We model this by taking into account the dispersion in rotation rates achieved by the progenitors, whose premain sequence accretion disc suffered an early disruption in the dense environment of the cluster's central regions where second-generation stars form. Rotation may also account for frequent late-flash-mixing events in massive globular clusters.Comment: 44 pages, 8 figures, 2 tables in Nature, online june 22, 201

Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters

Recent progress in studies of globular clusters has shown that they are not simple stellar populations, being rather made of multiple generations. Evidence stems both from photometry and spectroscopy. A new paradigm is then arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second parameter problem, it also opens new perspectives about the relation between globular clusters and the halo of our Galaxy, and by extension of all populations with a high specific frequency of globular clusters, such as, e.g., giant elliptical galaxies. We review progress in this area, focusing on the most recent studies. Several points remain to be properly understood, in particular those concerning the nature of the polluters producing the abundance pattern in the clusters and the typical timescale, the range of cluster masses where this phenomenon is active, and the relation between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review

Germany's Post-Wirtschaftswunder Struggle

For many years after World War II, Germany's economic output grew very rapidly—a period commonly referred to as the “Wirtschaftswunder,” or economic miracle. It came to an end in the 1970s. This paper addresses three questions: Why did growth decline after the 1970s? Why did Germany take so long to embark on reforms? What is the economic outlook today? It argues that two factors were responsible for falling growth: the end of the post-World War II economic catch-up and a failure to reform the economy in response to shocks. Because of Germany's political system, economic reforms require broad popular support. However, this support changes constantly due to frequent elections, which makes it difficult for governments to push through ambitious reforms that typically have payoffs stretching beyond electoral cycles. Nonetheless, the depth of the 2001-05 economic downturn and Germany's comparatively poor economic growth performance in Europe have by now forged a general agreement on the need for change. The European Union, notably with its policies that combat state aid and that call for sound fiscal positions, has played an important role in forging this agreement. Accordingly, Germany's economic outlook has by now improved noticeably, although significant challenges remain.Business Economics (2007) 42, 21–28; doi:10.2145/20070303