*Tech = Technological Institute (2145 Sheridan Road)
**Db = Dearborn Observatory (2131 Tech Drive)

Summer Quarter 2016

Date/Time		Visitor	Host
Aug. 5   Tech F227A   11:00am		Evan Schneider     University of Arizona     Simulating Galactic Winds on Supercomputers Using Cholla In an era of increasingly complex computer architectures, codes that take full advantage of available computational power are essential. Leveraging the power of today's supercomputers will allow numerical simulations to continue to provide new theoretical insights across a broad range of topics in astrophysics, including the formation and evolution of galaxies. In this talk, I will discuss the code Cholla, and the particular problem within galaxy evolution that I am tackling with it. Cholla is a new hydrodynamics code that I have written and developed to harness the power of the graphics processing units (GPUs) responsible for much of the speed of modern supercomputers. Using Cholla, I have run a set of extremely high resolution simulations of the interaction between cool and hot gas in multiphase galactic winds. The results of these simulations suggest that mass-loading of the warm and hot phases of galactic winds is a highly efficient process. Additionally, I find that similar amounts of mass and momentum are incorporated into wind phases across a broad range of densities and temperatures. Both findings have broad implications for our current theories regarding wind expulsion and recycling that ideally can be leveraged in the future to improve cosmological simulations.	Claude-André Faucher-Giguére

Spring Quarter 2016

Date/Time		Visitor	Host
Apr. 13   Tech F210   2:00pm		Ethan Vishniac     Johns Hopkins University     Editor-in-Chief of The Astrophysical Journal     Dynamos: How Do They Work? Magnetic fields are ubiquitous in the universe, with significant power on the size of the object that contains them. This includes planets, stars, and the disks of the galaxies. Conventional treatment, using mean field dynamo theory in which magnetic field growth is driven by kinematic helicity, "twistiness", of the underlying fluid turbulence, fails to explain observations of stellar magnetic fields. There are also strong theoretical arguments against its application to realistic systems. I will show how the transport of magnetic helicity in turbulent rotating systems provides a natural mechanism for the rapid growth of large magnetic fields. This leads to a generic prediction for the strength of stellar magnetic fields as a function of rotation rate which matches observations.	Fred Rasio
May 12    Tech F160    11:00am		B-G Andersson     USRA/SOFIA     Status of, and Science With, the Stratospheric Observatory For Infrared Astronomy (SOFIA) The SOFIA project flies a significantly modified Boeing 747SP, carrying a 2.7m telescope, into the stratosphere 3-4 times a week to perform astronomical observations, primarily in the mid- to far-infrared. The observatory is now in the middle of executing Cycle 4 of its general user observations, employing all the first generation instruments, and covering all areas of astronomy from Solar System studies to extra-galactic astronomy. The Cycle 5 Call for Proposals was released at the end of April, offering all first and second generation instruments, including the Far-IR polarimeter HAWC+. I will review the status of the project and provide some science highlights from the first few observing cycles.	Giles Novak
May 23    Tech F160    12:00pm		Jonathan Stern     Max Planck Institute for Astronomy (MPIA)     New Constraints on the Acceleration of AGN-driven Galactic Winds and on the Density     Structure of the CGM A vital ingredient in our theoretical understanding of AGN-driven galactic winds is the nature of the physical mechanism which accelerates them, such as radiation pressure on dust grains or the ram pressure of an expanding hot gas bubble. I will demonstrate that the nature of the acceleration mechanism can be constrained from the ratios of emission lines which originate from the cool (T~10^4K) gas illuminated by the AGN. I will then show that observed line ratios in UV-selected quasars suggest that radiation pressure dominates at all scales (0.1 pc -- 10 kpc). I will demonstrate that this result is apparently in conflict with the large momentum outflow rates measured by many previous studies, and discuss how this conflict might be resolved. In the second part of my talk I will argue for a new phenomenological model for the T~10^4K circum-galactic medium (CGM), where the cool CGM spans a large range of gas densities, and small high-density clouds are hierarchically embedded in large low- density clouds. I will then constrain the relation between gas density and physical size, or the `density structure', using ionic column measurements from the COS-Halos survey of low-redshift ~L* galaxies. I will show that this density structure is inconsistent with self-gravity, and thus its physical origin is unclear. I will also present a 3D model of the cool CGM based on our results, which can provide a benchmark for the CGM structure in hydrodynamic simulations.	Claude-André Faucher-Giguère
Jun. 2    Dearborn 23    3:00pm		Juan Diego Soler Pulido     Service d’Astrophysique, CEA. France     Magneto is Not a Hero: The Galactic Magnetic Field and the Formation of Molecular     Clouds. ESA's Planck satellite has produced the first all-sky map of the polarized emission from dust at submillimetre wavelengths. Compared with earlier ground-based and balloon-borne observations this survey is an immense step forward in sensitivity, coverage, and statistics. It provides new insight into the structure of the Galactic magnetic field, as well as the first statistical characterization of one of the main foregrounds to CMB polarization. I will summarize the main results of the Galactic magnetic field studies by the Planck Consortium, particularly focusing on the correlation between the magnetic field and the column density structures, its relation to previous studies based on the observation of starlight polarization and the Zeeman effect, and its implications for our understanding of the formation of density structures in the interstellar medium.	Giles Novak

Fall Quarter 2015

Date/Time		Visitor	Host
Sept. 15   Tech F160   4:00pm		Adrian Hamers     Leiden Observatory     Secular dynamics of high-order multiple systems     — applications to circumbinary planets and Hot Jupiters We present a new efficient method to study the long-term dynamics of hierarchical multiple systems such as high-multiplicity stellar systems and multiplanet systems, without restrictions on the number of bodies nor on the hierarchical configuration. We apply this method to hierarchical quadruple systems consisting of a hierarchical triple orbited by a fourth body, in different contexts. First, we consider the general dynamics, and we characterise the behavior of the system in terms of a ratio of Kozai-Lidov (KL) time-scales applied to different binary pairs. Second, we apply the method to explain the currently observed lack of transiting circumbinary planets around short-period binaries. Finally, we present preliminary integrations of multiplanet systems, in which secular chaos gives rise to high eccentricities and tidal shrinkage in the innermost planet's orbit, giving a pathway for producing Hot Jupiters. Our new method allows for much faster and longer integrations compared to previous direct N-body integrations, making it an excellent tool for better understanding the conditions for producing Hot Jupiters in multiplanet systems through secular chaos.	Fabio Antonini
Oct. 8   Tech F160   2:00pm		Andrew W. Smith     University of Maryland, College Park and NASA/GSFC     Revealing The Violent Universe: Current Results and The Future of Gamma-ray Astronomy The last 10 years have seen a dramatic explosion of important science results from space based instruments (~0.3-300 GeV) such as Fermi-LAT and AGILE, as well as ground based instruments (~100-5000 GeV) such as VERITAS, HESS, and MAGIC. These instruments have provided an in-depth exploration of particle acceleration in the the most energetic known cosmic sources (supernova remnants, gamma-ray bursts, pulsars, AGN, and the center of our own Galaxy), as well as providing important clues to the mystery of high energy cosmic ray origins. In this talk I will review the most important recent results from these instruments as well as providing a preview of the next generation of both ground and space based gamma-ray observatories. The future for ground based instruments is already being cemented with the completion of the HAWC array in Mexico, as well as the upcoming CTA array planned for both the Northern and Southern Hemispheres. The successor to Fermi-LAT is also in the works, with several mature mission concepts progressing forward already. I will review some of these concepts and provide an outlook for the future of this exciting and dynamic field.	Farhad Yusef-Zadeh
Nov. 11   Tech F160   2:00pm		Tuguldur Sukhbold    University of California Santa Cruz     Core-Collapse Supernovae from Neutrino-driven Explosions Nucleosynthesis, light curves, explosion energies, and remnant masses are calculated for a grid of supernovae resulting from massive stars with solar metallicity and masses between 9-120 solar masses. The full evolution is followed using an adaptive reaction network of up to 2000 nuclei. A novel aspect of the survey is the use of a one-dimensional neutrino transport model for the explosion. This explosion model has been calibrated to give the observed energy for SN 1987A and Crab supernova. As a result of using a calibrated central engine, the final kinetic energy of the supernova is variable and sensitive to the structure of the presupernova star. Many progenitors with extended core structures do not explode, but become black holes, and the masses of exploding stars do not form a simply connected set. The resulting nucleosynthesis agrees reasonably well with the sun provided that a reasonable contribution from Type Ia supernovae is also allowed, but with a deficiency of light s-process isotopes. The resulting neutron star IMF has a mean gravitational mass near 1.4 solar masses. The average black hole mass is about 9 solar masses if only the helium core implodes, and 14 solar masses if the entire presupernova star collapses. Only ~10% of supernovae come from stars over 20 solar masses and some of these are Type Ib or Ic. Some useful systematics of Type IIp light curves are explored.	Ron Taam
Nov. 16   Tech F160   12:00pm		John Tobin    Leiden Observatory     Toward Understanding the Formation of Proto-planetary Disks and Multiple Star Systems Disks and multiple star systems are thought to form early in the star formation process. Conservation of angular momentum enables the formation of protostellar disks; protostellar companions can also form if the disk is gravitationally unstable. However, theory and simulations have suggested that large, massive disk formation may be difficult due to removal of angular momentum by magnetic fields. Observations are now sensitive enough to begin testing these predictions, and Keplerian disks have been discovered around newly formed star systems. However, the numbers of Keplerian disks remain small and the frequency of companion protostars on scales < 150 AU is poorly known. To greatly expand our knowledge on the frequency of disks and multiplicity, I am leading a 264 hour VLA large program to observe all protostars in a single star forming region, the Perseus molecular cloud (d 230 pc, N 80), with a spatial resolution of 15 AU. With these data, we are identifying new protostellar disk candidates and closer multiple systems than have ever been previously identified. We have found evidence for a bimodal distribution of protostar separations, suggesting that multiple formation mechanisms are acting on different scales. Finally, we also see indications of evolution in the separation distribution for younger protostars relative to those that are more-evolved. These results open the door to expanded multiplicity and disk surveys to determine if the trends found in Perseus are applicable to other star forming regions.	Farhad Yusef-Zadeh
Nov. 23   Tech F160   12:00pm		Morgan MacLeod     University of California, Santa Cruz     Common Envelope and the Formation of Close Neutron Star Binaries How do close neutron star binaries form? Many aspects of our understanding of the complex channels through which compact binaries form remain uncertain. This talk focuses on a crucial phase in the formation of close binary pairs of neutron stars: a common envelope episode. During the common envelope phase, one star evolves to engulf its companion in a shared gaseous envelope. Before this envelope is ejected, drag forces pull the two stellar cores into a tighter orbit. But the neutron star can also accrete from the surrounding gas. This talk explores the hydrodynamics of the common envelope phase in order to model the metamorphosis of the neutron star and the binary orbit.	Fabio Antonini