April 29, 2016 3:30PM BPB-217. Alexandre Lazarian. Turbulent reconnection and particle acceleration.

I shall explain how turbulence makes magnetic reconnection fast and will compare this approach with other ideas in the field, e.g. related to tearing instability. I shall discuss both non-relativistic and relativistic reconnection. I shall demonstrate that turbulent reconnection inevitably leads to the particle acceleration. Finally, I shall discuss the implications of the process for the famous ICMART model by Zhang & Yan.

April 27, 2016 12:00PM BPB-250. Rolf Kuiper. The Formation and Feedbak of the Most Massive Stars.

The formation of massive stars yields strong feedback effects onto its host core and stellar cluster environment via protostellar outflows, radiation heating, radiation pressure, ionization, stellar winds and mass loss, and supernova (in chronological order). From a theoretical point of view, the question arises, how a massive protostar is able to accrete its mass up to the observed upper mass limit despite its strong radiation pressure feedback. In this talk, I present results of several series of self-gravitating radiation-hydrodynamics simulations of core collapse towards high-mass star formation, including the effects of radiation heating, radiation pressure, protostellar outflows, and ionization. We propose a solution to the radiation pressure problem in the formation of massive stars via the so-called flashlight effect: due to the formation of an optically thick accretion disk, the thermal radiative flux becomes strongly anisotropic and preferentially escapes through the disk's atmosphere, i.e. perpendicular to the sustained accretion through the disk's mid-plane. Furthermore, including feedback of early protostellar outflows yields a large scale anisotropy, which extends the disk's flashlight effect from the few hundred AU scale of the circumstellar disk to a core's flashlight effect up to a 0.1-parsec scale. This core’s flashlight effect allows core gas to accrete on the disk for longer, in the same way that the disk’s flashlight effect allows disk gas to accrete on the star for longer. In summary, I will demonstrate straight-forward mechanisms, which allows the formation of the most massive stars known in the present-day universe despite of their strong feedback via outflows and radiation. The basic theory of star formation herein is just a scaled-up version of low-mass star formation, i.e. based on accretion from core to disk to protostellar scales.

April 22, 2016 3:30 BPB-217. Niel Brandt. A Good Hard Look at Growing Supermassive Black Holes in the Distant Universe.

Sensitive cosmic X-ray surveys with the Chandra, XMM-Newton, and NuSTAR observatories have revolutionized our ability to find and study distant active galactic nuclei (AGNs), the main sites of supermassive black hole growth in the Universe. I will describe some of the resulting discoveries about the demographics, physics, and ecology of AGNs. Topics covered will include the utility of deep X-ray plus multiwavelength surveys for investigating distant AGNs; evolution constraints for the typical AGNs of the distant Universe; the cosmic balance of power between supermassive black holes and stars; interactions between AGNs and their hosting galaxies; and the AGN content of newly forming galaxies. I will end by discussing some key outstanding questions and new observations and missions that aim to answer them.

April 15, 2016 3:30 BPB-217. Timothy Strobel. Beyond thermodynamic stability: synthetic pathways to new materials with exceptional properties.

Multiple allotropes and/or chemical compounds can be formed under various pressure / temperature conditions, and some of these could remain metastable under standard conditions for time scales as long as the age of the universe (in fact it is estimated that 50% of all known inorganic compounds are metastable ones!). But the number of known allotropes/compounds pales in comparison with the number of hypothetical ones with energetic feasibility. For any given thermodynamic state, thousands of energetically competitive structures are plausible, a subset of which will exhibit mechanical stability. Further subsets of these structures offer enticing physical properties that differ from those of thermodynamic ground states. Here we delineate thermodynamic and kinetic synthesis methods and discuss strategies and examples for accessing these states experimentally. In particular, we discuss successful experimental realization of new forms of silicon and carbon.

April 8, 2016 3:30 BPB-217. Richard de Grijs. Not-so-simple stellar populations in nearby, resolved massive star clusters.

Until about a decade ago, star clusters were considered "simple" stellar populations: all stars in a cluster were thought to have similar ages and the same metallicity. Only the individual stellar masses were thought to vary, in essence conforming to a "universal" initial mass function. Over the past decade, this situation has changed dramatically. Yet, at the same time, star clusters are among the brightest stellar population components and, as such, they are visible out to much greater distances than individual stars, even the brightest, so that understanding the intricacies of star cluster composition and their evolution is imperative for understanding stellar populations and the evolution of galaxies as a whole. I will discuss my group's recent progress in this context, with particular emphasis on the properties and importance of binary systems, the effects of rapid stellar rotation, and the presence of multiple populations in Local Group star clusters across the full age range. Our most recent results imply a reverse paradigm shift, back to the old simple stellar population picture for at least some intermediate- age (~2 Gyr-old) star clusters, which opens up exciting avenues for future research efforts.