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This Week at Physics

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Events on Monday, September 26th, 2016

Graduate Introductory Seminar
Atomic, Molecular, and Optical Physics
Time: 12:00 pm
Place: 5280 Chamberlin Hall
Speaker: Lin, Lawler, Saffman, Walker, Yavuz
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Plasma Physics (Physics/ECE/NE 922) Seminar
Turbulent Dynamo Amplification of Magnetic Fields in Laser-Produced Plasmas
Time: 12:00 pm
Place: 2241 Chamberlin Hall
Speaker: Petros Tzeferacos, University of Chicago
Abstract: Magnetic fields are ubiquitous in the Universe, as revealed by diffuse radio-synchrotron emission and Faraday rotation observations, with strengths from a few nG to tens of μG. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter in the Universe. The standard model for the origin of these intergalactic magnetic fields is through the amplification of seed fields via turbulent dynamo to the level consistent with current observations. We have conceived and conducted a series of experiments using high-power laser facilities to study the amplification of magnetic fields via turbulence. In these experiments, we characterize the properties of the fluid and the magnetic field turbulence using a comprehensive suite of plasma and magnetic field diagnostics. We describe the large-scale 3D simulations we performed with the radiation-MHD code FLASH on ANL's Mira to help design and interpret the experiments. We then discuss the results of the experiments, which indicate magnetic Reynolds numbers above the expected dynamo threshold are achieved and seed magnetic fields produced by the Biermann battery mechanism are amplified by turbulent dynamo. We relate our findings to processes occurring in galaxy clusters.
Host: Cary Forest
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Astronomy Lunch Talk
Formation of low-mass helium white dwarfs orbiting millisecond pulsars
Time: 12:00 pm
Place: 4421 Sterling
Speaker: Alina Istrate, UW-Milwaukee
Abstract: Millisecond pulsars (MSPs) belong to a class of radio pulsars characterized by high rotational spin rates and low magnetic fields. These neutron stars are believed to be the end-product of binary evolution, i.e. the recycling scenario, in which an old neutron star accretes matter and angular momentum from a close stellar companion for an extended period of time, while being observable as an X-ray binary.

A handful of MSPs are detected in very compact orbits (orbital periods between 2 − 9 hr), around low-mass He WDs with masses below 0.25 Msun and surface gravities 5 < log g < 7, the so-called extremely low-mass helium white dwarfs (ELM WDs).

Today we know of the existence of more than 80 ELM WDs, with the majority of them in a binary system with a more massive CO WD. The increasing number of discovered ELM WDs reveals that they are formed in different environments, from the Galactic disk to open and globular clusters. ELM WDs are most likely the result of binary evolution as they cannot be formed from single stars within a Hubble time. The new wealth of data raises questions regarding the puzzling presence of metals in the atmospheres of young bloated ELM proto-WDs and the newly discovered pulsations in three ELM proto-WDs. The current available evolutionary models fail to explain such properties.
In this talk, I will present the latest theoretical efforts regarding the formation and evolution of ELM WDs.
Host: Astronomy Dept.
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