University of Minnesota
University of Minnesota
Colloquia and Seminars

Astrophysics Colloquium

Fall 2010 - Coordinated by Tom W. Jones
Held in Physics 210 at 3:35pm on Fridays
(unless otherwise noted)

September 17 No Colloquium
September 24 Thesis Defense: Andrew Helton
October 1 No Colloquium — Department Picnic
October 4* Dr. Dan Stern, JPL, Caltech
(Monday) *Joint with cosmology lunchtime seminar — 12:15pm. Physics 435
Title: Cosmic Clocks: Constraining the Equation of State of Dark Energy
Abstract: The nature of the physics driving cosmic acceleration is perhaps the biggest question facing physics today. Huge resources and large collaborations are now being amassed to determine the dark energy equation-of-state parameter, w. The avenues that are now receiving the most attention are supernova searches, weak lensing, baryon acoustic oscillations, and cluster counts. However, none of these will be free from systematics, and it is still not clear which is the most promising approach. Most likely, multiple approaches will be required, and new ideas still need to be explored. One novel suggestion, proposed theoretically by Jimenez & Loeb (2002), is to measure the relative ages of the red envelope of cluster galaxies, essentially using them as cosmic chronometers or standard clocks. This probe is particularly sensitive to the variation of w as a function of redshift. I will discuss the astrophysical underpinnings to this approach, results obtained using public spectra from the SDSS, and more recent results obtained with Keck. Although the new technique faces challenges from astrophysical uncertainties, these are not necessarily any more daunting than those associated with the other techniques.
Host: A. Kovacs
October 8 Dr. Andrew MacFayden, NYU
Title: Extreme Explosions - The Dynamics and Observational Signatures of Gamma-Ray Bursts
Abstract: It is now clear that a subset of massive stars are capable of exploding relativistically. These explosions are observed as gamma-ray bursts (GRBs) with accompanying supernovae. Stellar evolution thus follows an unexpected channel which is related to black hole or, possibly, magnetar formation. I will discuss progress in understanding the dynamics and observational signatures of these explosions. I will present new multi-dimensional simulations and computed light curves of GRB blastwaves in the afterglow phase. I will also present recent work demonstrating the generation and amplification of magnetic fields in astrophysical outflows relevant for models of GRB prompt and afterglow emission.
Host: A. Heger
October 15 Dr. Adwin Boogert, IPAC, Caltech
Title: Interstellar Ices
Abstract: A large fraction of the molecules in dense interstellar and circumstellar media is frozen out on grains. I will give an overview of the detected species and of the role the ices play in astrochemistry. I will focus on the evidence of ice processing around Young Stellar Objects.
Host: A. Kovacs
October 22 Dr. Brian Metzger, Princeton U
Title: The Proto-Magnetar Model for Gamma-Ray Bursts
Abstract: Long duration gamma-ray bursts (GRBs) originate from the deaths of massive stars, but it remains unsettled whether the central engine is an accreting black hole or a rapidly spinning, highly magnetized neutron star (a 'proto-magnetar'). This distinction has been brought into particular focus by recent MHD simulations of core collape of massive, rotating 'collapsar progenitors,' which suggest that powerful magneto-centrifugal outflows from the proto-neutron star may stave off black hole formation entirely. I will present new, detailed calculations of the spin-down evolution of proto-magnetars and use them to construct a self-consistent GRB model to compare with observations and phenomenology. Although the site and mechanism of the prompt emission are still debated, I will present calculations the time-averaged GRB emission predicted by two models, magnetic dissipation and internal shocks. Heating from magnetic dissipation powers ~MeV photon emission via Compton scattering near the photosphere, resulting in a light curve that faithfully tracks the jet power. Internal shocks may occur at larger radii because the Lorentz factor of the jet increases monotonically with time as the magnetar cools, such that fast material released at late times catches up to the slower material released earlier. We conduct an extensive parameter study in the space of magnetar properties in order to examine requisite conditions for the full range of high energy phenomena, from sub-luminous GRBs to highly-energetic bursts discovered by the Fermi satellite. I shall argue that the proto-magnetar model provides natural explanations for the energies, durations, Lorentz factors, and abrupt end to the prompt emission of GRBs.
Host: Y. Qian
October 29 Dr. Mark Halpern, U British Columbia
Title: Having a BLAST in Antarctica, and Measuring Star Formation History with a Submillimeter Camera
Abstract: BLAST, the balloon-borne large aperture submillimeter telescope, was designed to study star formation in our galaxy and, primarily, at high redshifts. Most of the UV, and a fair part of the optical radiation associated with new stars is absorbed by dust within the clouds where the stars form. The dust re-radiates this energy at wavelengths of a few hundred microns. This is presumed to be the origin of the cosmic infrared background, which is the brightest diffuse radiation in the sky after the CMB. The challenge is that the Earth's atmosphere is opaque to this radiation and we had to get above the atmosphere to study it.

I will explain how BLAST has helped to resolve the sources of the CIB. I'll try to give a bit of the flavor of performing a balloon experiment in Antarctica too, even though I recognize that UMN has plenty of experience with both balloons and Antarctic experiments.
Host: S. Hanany
November 5 Dr. Chris Reynolds, U Maryland
Title: New insights into black hole accretion
Abstract: For the past 20 years, the paradigm for black hole accretion has highlighted the central role of magnetohydrodynamic (MHD) turbulence. However, only in recent years have high-resolution simulations started to explore the more subtle aspects of MHD turbulent disks. I will discuss results from a series of simulations focusing on geometrically-thin accretion disks. We show that the low density "corona" of the disk plays a previously unrecognized and important role in shaping the dynamics of the disk. I shall also discuss the discovery of quasi-periodic dynamo cycles operating in the inner regions of our simulated disks, and explore the degree to which these explain low-frequency quasi-periodic oscillations seen in X-ray observations of stellar mass black holes.
Host: T.W. Jones
November 12 Dr. Niklas Karlsson, U Minnesota, Physics
Title: On the Origin of Cosmic Rays
Abstract: The origin of cosmic rays is one of the fundamental problems in astrophysics. The break-through measurements by Victor Hess proved the existence of extraterrestrial radiation. Today, the energy spectrum and composition of cosmic rays are well established up to the TeV energy range - but the origin still eludes us. Because cosmic rays are charged particles they interact with magnetic fields making it virtually impossible to trace their origins. The current belief is that they are accelerated in supernova remnants and winds of massive stars, except for the highest-energy cosmic rays which are proposed to originate in AGN. In this talk I will describe how gamma rays are detected and discuss how indirect measurements of cosmic rays using gamma rays as tracers can help us uncover the origin of cosmic rays. Detailed measurements of gamma-ray spectra of supernova remnants and starburst regions and galaxies allows us to determine the underlying production mechanism and can thus provide the "smoking gun" evidence for very energetic cosmic-ray hadrons. Recent observations of supernova remnants and nearby starburst galaxies with gamma-ray telescopes such as Fermi, H.E.S.S. and VERITAS strongly point toward a hadronic origin. I will review these observations and their implications. The talk will be concluded with an outlook to the future. Next generation ground-based gamma-ray telescopes (AGIS/CTA) will have much improved sensitivity and are expected to provide even stronger evidence for the hadronic origin of gamma rays.
November 19 Dr. Eric Thrane, U Minnesota, Physics
Title: Listening for the Big Bang: Searching for Stochastic Gravitational-wave Backgrounds and Foregrounds
Abstract: Precision microwave astronomy experiments such as Planck are mapping out the microwave afterglow of the Big Bang, which persists as a 2.7 deg K omnidirectional blackbody glow. A similar gravitational-wave afterglow is expected to exist, though there are considerable challenges associated with its detection. Gravitational-wave foregrounds from objects such as magnetars may partly obscure our view of the stochastic background, but such foregrounds are interesting in their own right. In this talk, I review mechanisms for the creation of stochastic gravitational-wave backgrounds and foregrounds, I discuss data-analysis strategies and I consider the prospects of detection.
November 26 No Colloquium — Thanksgiving Holiday
December 3 Dr. Alan McConnachie, HIA/Victoria
Title: Galaxy archaeology in the near field
Abstract: It is on galactic scales that our understanding of the cosmological evolution of matter is most incomplete. Many of the predicted features of galaxies, such as faint satellites and diffuse stellar haloes, are extremely low surface brightness, and so the Milky Way, M31 and M33 are some of the only large galaxies in the Universe and that can provide robust tests of many fundamental aspects of galaxy formation models. The Pan-Andromeda Archaeological Survey (PAndAS) is a large, deep, photometric survey that has surveyed nearly 400 sq.degrees in the surroundings of the M31/M33 sub-group, and which is providing the deepest and most complete panorama of galaxy haloes available. This survey has revealed copious stellar streams and substructures, most notable of which is a giant distortion surrounding M33 which helps constrain the orbital history of the Local Group. Large numbers of new dwarf galaxies have been discovered, allowing the derivation of the luminosity function for an entire satellite system to faint magnitudes, and new discoveries of outer halo globular clusters are providing unexpected handles on the accretion history of our nearest galactic neighbor. Here, I will review recent results from PAndAS, and discuss the future development of this field with an eye on the next generation of astronomical facilities.
Host: E. Skillman
December 10 Dr. Roberta Humphreys, U Minnesota, Astronomy
Title: A Galactic Asymmetry and Interaction with the Bar in the Disk