* Astronomy

Title: Formation of stars and clusters over cosmological time
Author: Bruce G. Elmegreen

The concept that stars form in the modern era began some 60 years ago with the key observation of expanding OB associations. Now we see that these associations are an intermediate scale in a cascade of hierarchical structures that begins on the ambient Jeans length close to a kiloparsec in size and continues down to the interiors of clusters, perhaps even to binary and multiple stellar systems. The origin of this structure lies with the dynamical nature of cloud and star formation, driven by supersonic turbulence and interstellar gravity. Dynamical star formation is relatively fast compared to the timescale for cosmic accretion, and then the star formation rate keeps up with the accretion rate, leading to a sequence of near-equilibrium states during galaxy formation and evolution. Dynamical star formation also helps to explain the formation of bound clusters, which require a local efficiency that exceeds the average by more than an order of magnitude. Efficiency increases with density in a hierarchically structured gas. Cluster formation should vary with environment as the relative degree of cloud self-binding varies, and this depends on the ratio of the interstellar velocity dispersion to the galaxy rotation speed. As this ratio increases, galaxies become more clumpy, thicker, and have more tightly bound star-forming regions. The formation of old globular clusters is understood in this context, with the metal-rich and metal-poor globulars forming in high-mass and low-mass galaxies, respectively, because of the galactic mass-metallicity relation. Metal-rich globulars remain in the disks and bulge regions where they formed, while metal-poor globulars get captured as parts of satellite galaxies and end up in today's spiral galaxy halos. Blue globulars in the disk could have formed very early when the whole Milky Way had a low mass.

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Title: Gone With the Wind: Where is the Missing Stellar Wind Energy from Massive Star Clusters?
Author: Anna L. Rosen, Laura A. Lopez, Mark R. Krumholz, Enrico Ramirez-Ruiz

Star clusters larger than ~10^3 solar masses contain multiple hot stars that launch fast stellar winds. The integrated kinetic energy carried by these winds is comparable to that delivered by supernova explosions, suggesting that at early times winds could be an important form of feedback on the surrounding cold material from which the star cluster formed. However, the interaction of these winds with the surrounding clumpy, turbulent, cold gas is complex and poorly understood. Here we investigate this problem via an accounting exercise: we use empirically determined properties of four well-studied massive star clusters to determine where the energy injected by stellar winds ultimately ends up. We consider a range of kinetic energy loss channels, including radiative cooling, mechanical work on the cold interstellar medium, thermal conduction, heating of dust via collisions by the hot gas, and bulk advection of thermal energy by the hot gas. We show that, for at least some of the clusters, none of these channels can account for more than a small fraction of the injected energy. We suggest that turbulent mixing at the hot-cold interface or physical leakage of the hot gas from the HII region can efficiently remove the kinetic energy injected by the massive stars in young star clusters. Even for the clusters where we are able to account for all the injected kinetic energy, we show that our accounting sets strong constraints on the importance of stellar winds as a mechanism for feedback on the cold interstellar medium.

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