Forum Schedule

Fridays 3:45pm - 4:45pm BPB-217

Date Speaker Topic (click down-arrow to see abstract)
Aug 28 Remembrance of Professor Lon Spight
host:  Department of Physics & Astronomy

The Department of Physics & Astronomy will host an informal memorial for our recently departed friend and colleague Dr. Lon Spight -- one of the founders of our department and long time host of the Forum.

Sep 4 Daniel Whalen
Los Alamos National Laboratory
host: George Rhee
Finding the First Cosmic Explosions

Primordial stars formed about 200 Myr after the big bang, ending the cosmic dark ages. They were the first great nucelosynthetic engines of the universe and may be the origins of the supermassive black holes found in most massive galaxies today. In spite of their importance to the evolution of the early universe not much is known for certain about the properties of Pop III stars. Until now, observers have looked for the nucleosynthetic imprint of Pop III SNe in the chemical abundances in ancient, dim metal-poor stars to constrain the masses of the SN progenitors. But with the advent of JWST, WFIRST and the 30 m telescopes it may soon be possible to directly observe the explosions themselves in the NIR and thus unambiguously constrain the properties of the first stars. I will present radiation hydrodynamical calculations of the light curves of the first SNe in the universe and discuss strategies for their detection. I will also describe how some may already have been captured in surveys of galaxy cluster lenses such as CLASH, Frontier Fields and GLASS.

Sep 11 Dr. Fan Guo
Los Alamos National Laboratory
host: Bing Zhang
Magnetic Reconnection: A Powerful Cosmic Particle Accelerator

Astrophysical magnetic reconnection sites have long been expected to be sources of high-energy particles. Recent observations of high-energy gamma-ray flares from the Crab nebula and models of gamma-ray bursts and TeV blazars have motivated us to better understand magnetic reconnection and its associated particle acceleration in plasma conditions where the magnetic energy is dominant. We will present fully kinetic particle-in-cell simulations of anti-parallel magnetic reconnection in the highly magnetized regime (the magnetization parameter sigma >> 1.) The magnetic energy is converted efficiently into kinetic energy of nonthermal relativistic particles in a power-law spectrum. For a sufficiently large system and strong magnetic field, the power-law index approaches "p=1". The dominant acceleration mechanism is a first-order Fermi process accomplished through the curvature drift motion of particles in magnetic flux tubes along the electric field induced by fast plasma flows. We will also present an analytical model for the formation of power-law distribution and show the nonthermal distribution may be a common feature of magnetically dominated reconnection.

Sep 18 Mario Livio
Space Telescope Science Institute
host: Rebecca Martin
Type Ia Supernovae: Progenitors and Cosmology

I will review the current status concerning the identification of the progenitor systems of Type Ia supernovae, in light of recent observations and theoretical developments. I will also show how current and future observations can be used to tame evolutionary effects, on the road to better constraints on the nature of dark energy.

Sep 25

Oct 2 Drew Clausen
California Institute of Technology
host: Daniel Proga
Using Tidal Disruption Events to Study Massive Black Holes and Their Environments

When a black hole tidally disrupts a star, accretion of the debris will produce a luminous flare and reveal the presence of a dormant black hole. Emission lines produced when the stellar debris and/or other gas in the black hole's vicinity are photoionized by the accretion flare have considerable diagnostic power. I will discuss models of the emission line spectrum produced in the debris released when evolved stars are tidally disrupted by an intermediate-mass black hole (100-10000 solar masses), and discuss the possibility of using the emission lines to identify such events and constrain the properties of the black hole. While there is some agreement between these models and observations of white dwarf tidal disruption candidates in globular clusters associated with NGC 4472 and NGC 1399, there are also drawbacks to interpreting these sources as tidally disrupted white dwarfs. I will also present results from time-dependent photoionization calculations that model the emission line spectrum produced when ambient, circumnuclear gas is illuminated by a tidal disruption flare. The emission line light curves are consistent with the transient extreme coronal line emitters recently identified in SDSS. These tidal disruption event light echoes can be used to probe the circumnuclear environments of quiescent galaxies and to constrain the extreme UV component of tidal disruption flares.

Oct 9 Kazumi Kashiyama
U C Berkeley
host: Bing Zhang
Searching for newborn black holes and magnetars in the era of multi-messenger astronomy

Stellar-mass black holes and magentars have been identified by their bright non-thermal emissions. It is highly uncertain how these objects are formed, which is a missing link of massive stellar evolution. Newborn black holes and magnetars in collapsing massive stars have been considered as the central engine of e.g., gamma-ray bursts and superluminous supernovae. However, the event rates of such luminous transients are extremely small, say ~ 0.01 % of core-collapse supernovae. On the other hand, the formation rates of black holes and magnetars has been estimated to be possibly as high as ~10% of core-collapse supernovae. Thus, a large fraction of black-hole and magnetar formation would occur with ordinary supernovae or less prominent new transients. The key questions are "What are possible smoking guns of black-hole and mangetar formation?", "Whether they can be detectable by multi-messenger time-domain astronomy in the coming years?", and "What is the optimal observational strategy?". I will present some theoretical modeling of multi-messenger signals from newborn black holes and magnetars in collapsing massive stars and discussing the detectability by using current and future observational facilities.

Oct 16 Jason Steffen
host: Dept. of Physics & Astronomy
Laser Probes of the Dark Sector

The majority of the universe is composed of exotic matter and energy of which we know very little. This "dark sector" comprising dark matter, dark energy, and gravity, is likely to be one of the primary scientific challenges of the 21st century. I report on a series of three laboratory experiments at Fermilab to probe this dark sector. These experiments include searches for dark matter axions, dark energy chameleons, and tests of the very nature of space time.

Oct 23

Oct 26 (Monday 12:00 Noon) Yan-Fei Jiang
Harvard-Smithsonian Center for Astrophysics
host: Daniel Proga
Iron Opacity Bump Changes the Structure and Stability of Black Hole Accretion Disks in AGNs

In the standard thin accretion disk model, electron scattering is usually thought to be the dominant opacity in the radiation pressure dominated inner region of black hole accretion disks. I will show that this is not the case for most AGNs, where the density and temperature ranges in the accretion disks are similar as in the envelopes of massive stars and opacity bump caused by Irons are much more important. Through a series of self-consistent radiation MHD simulations, I will show that the iron opacity bump can significant decrease the growth rate of thermal instability in the standard thin accretion disk model compared to case with dominated electron scattering opacity. It can even stabilize the disk. Viscous instability also does not exist for AGN disks with iron opacity bump. I will also discuss implications of the iron opacity bump on resolving various puzzles between standard thin disk models and observations.

Oct 30 Nevada Day Recess

Nov 6 Duncan Lorimer
University of West Virginia
host:Bing Zhang
Fast Radio Bursts: The Story So Far

I will describe a brief history of discovery and some exciting recent developments in the world of pulsars and fast radio bursts. Pulsars, rapidly rotating highly magnetized neutron stars, were discovered in 1967 and continue to surprise and delight astronomers as powerful probes of fundamental physics and astrophysics. Fast radio bursts are millisecond-duration pulses of currently unknown origin that were discovered in 2007. Both pulsars and fast radio bursts have great promise at probing the universe on large scales and in fundamental ways. I will describe the science opportunities these phenomena present, and discuss the challenges and opportunities presented in their discovery.

Nov 13 Kailash Sahu
Space Telescope Science Institute
host: Rebecca Martin
Detecting Exoplanets, and Isolated, Stellar-Mass Black Holes through Microlensing

This talk consists of two parts. In the first part, I will review how almost all the extra-solar planets discovered so far have been relatively nearby --- within ~500 parsecs from the Sun, and within about 2 AU from their parent stars. I will discuss how microlensing provides insights into the frequency of planets across the Milky Way, and up to several AU from the host stars. In the second part, I will discuss the technique of astrometric microlensing, and two HST programs underway aimed at the first detections of isolated, stellar-mass black holes through this technique.

Nov 20 Bharat Ratra
Kansas State University
host: George Rhee
Dark Energy: constant or time variable? (... and other open questions)

Experiments and observations over the last decade and a half have persuaded cosmologists that (as yet undetected) dark energy is by far the main component of the energy budget of the universe. I review a few simple dark energy models and compare their predictions to observational data, to derive dark energy model-parameter constraints and to test consistency of different data sets. I conclude with a list of open cosmological questions.

Nov 27 Thanksgiving Day Recess

Dec 3 (Thursday 3:45PM) Phil Armitage
CU Boulder
host: Rebecca Martin
New directions in astrophysical accretion

Accretion disks play a critical role in planet formation, and in compact object and black hole astrophysics. I will discuss the implications of recent numerical simulations of astrophysical disks for both classes of system, which challenge long-held assumptions as to how disks work. In protoplanetary disks, the non-ideal physics of the Hall effect appears to dominate disk evolution on planet-forming scales, and this may lead to a bimodal distribution of planetary system properties. In black hole disks I will argue that strongly magnetized disks provide an appealing framework for addressing a range of unexplained observations. I will discuss the obstacles and prospects for developing a theory for the long-term evolution of both planet forming and black hole accretion flows.

Dec 11 Final Examinations

Past forums: Spring '15   Fall '14   Spring '14   Fall '13   Spring '13   Fall '12 Spring '12   Fall '11   Spring '11   Fall '10   Spring '10   Fall '09   Spring '09   Fall '08