Forum Schedule Spring 2018

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

Date Speaker Topic (click down-arrow to see abstract)
Jan 19 Dong Ruobing
University of Arizona
host: Zhaohuan Zhu
Observational Planet Formation

Planets form in gaseous protoplanetary disks surrounding newborn stars. As such, the most direct way to learn how they form from observations, is to directly watch them forming in disks. In the past, this was very difficult due to a lack of observational capabilities; as such, planet formation was largely a subject of pure theoretical astrophysics. Now, thanks to a fleet of new instruments with unprecedented resolving power that have come online recently, we have just started to unveil features in resolve images of protoplanetary disks, such as gaps and spiral arms, that are most likely associated with embedded (unseen) planets. By comparing observations with theoretical models of planet-disk interactions, the masses and orbits of these still forming planets may be constrained. Such planets help us to directly test various planet formation models. This marks the onset of a new field — observational planet formation. I will review the current status of this field, highlight some of the latest major developments, and discuss where this field is heading.

Jan 26 Greg Salvesen
University of California, Santa Barbara
host: Rebecca Martin
The Role of Magnetic Fields in Black Hole Accretion Disks

Magnetic fields are known to be fundamental to the underlying process of disk accretion onto black holes. For some black hole X-ray binaries, observations suggest that strong magnetic fields may be playing an active role in the characteristic phenomena from these systems, such as powerful outflows and dramatic cycles of brightening and dimming. These observations motivate the effort to understand how magnetic fields influence accretion disk physics, which is generally not accounted for when interpreting observations. In this talk, I will highlight results from recent numerical simulations of moderately-to-strongly magnetized accretion disks and discuss the implications of disk magnetization on black hole spin measurements.

Feb 5 (Monday 12:00PM) Bin-Bin Zhang
Nanjing University
host: Bing Zhang
A Tale of Two Bursts

In the talk I will review our recent work on the most remarkable Gamma-Ray Bursts (GRBs) in the past two years: GRB 160625B and GRB 170817A. GRB 160625B was an extraordinarily bright burst with three isolated episodes separated by long quiescent intervals. By conducting detailed time-resolved spectral analysis in each episode, we found that the spectral properties of the first two sub-bursts are distinctly different, allowing us to for the first time observe a clear transition from thermal to non-thermal radiation between well-separated emission episodes within one single GRB. Such a transition is a strong indication of the change of jet composition from a fireball to a Poynting-flux-dominated jet. GRB 170817A was the first confirmed short GRB associated with gravitational event GW170817. By performing detailed analysis on the prompt emission data, we show that the fluence and spectral peak energy of this sGRB fall into the lower portion of the distributions of known sGRBs. Its peak isotropic luminosity is abnormally low. The estimated event rate density above this luminosity is at least ~190 Gpc^-3yr^-1, which is close to but still below the double neutron star merger event rate density. The physical implications of the data including the jet structure will also be discussed.

Feb 9 Jonti Horner
University of Southern Queensland
host: Rebecca Martin
Exoplanets and life elsewhere: Which ExoEarths should we search for life?

In the coming years, we will begin to discover the first truly Earth-like planets orbiting distant stars. At that point, the race will be on to search for evidence of life beyond the Solar system. However, the observations to search for any such life will be hugely challenging - and it is very unlikely that astronomers will be able to focus on more than a few of the most promising targets from the plethora of planets we are sure to discover. So how should we work out where to look? What factors could come together to render one exoplanet more (or less) suitable as a target for the search for life? In this talk, I discuss the search for planets orbiting other stars, then move on to talk about the various factors that have been proposed that could impact the habitability of and ‘Earth-like' planets we find.

Feb 16 Subo Dong
Kavli Institute for Astronomy and Astrophysics, China
host: Bing Zhang
Direct collision of white dwarfs as a major channel for type Ia supernova explosions

The explosion mechanism of type Ia supernovae (SN Ia) is unknown. We argue that the direct white dwarf-white dwarf collisions in the field multiple stellar systems is the most promising mechanism to explain SNe Ia. The collision model explains several robust observational features across the entire SN Ia population. It also predicts the bi-modal Ni56 distributions in the ejecta, and we have discovered from the nebular-phase spectra of SNe Ia that such bi-modality is common, providing direct support of the collision model. I will discuss key observational tests on the collision model enabled by the All-Sky Automated Survey for SuperNovae (ASAS-SN) — the most productive bright supernova survey since 2015.

Feb 23

Mar 2 G P Das
Department of Materials Science, Indian Association for the Cultivation of Science
Jadavpur, Kolkata, INDIA
host: Ashkan Salamat
2D Materials for Energy Storage: an Emerging Scenario

In this talk, I shall start with an overview of the essential requirements for storage of renewable energies, from the point of view of a materials scientist. I shall then focus on some nanostructures, in particular carbon based nanostructures, that can be used for efficient storage of hydrogen and also as metal-free catalysts for low temperature fuel cells. First principles density functional based simulation prove extremely handy in designing such novel materials with desired combination of properties. After presenting some case studies, I shall wrap up by arguing how materials scientists, who sometime might sound futuristic, can play a crucial role in building up a sustainable energy economy.

Mar 9 (10:00AM) Rebecca Nealon
University of Leicester, UK
host: Rebecca Martin
Misaligned Black Hole Accretion Discs

Accretion discs around rotating black holes are often assumed to be aligned with the black hole spin. Relaxing this assumption by inclining the disc means that the rotation of the black hole is able to affect the disc evolution, leading to previously unexplored structures in the disc. I will present three-dimensional simulations investigating the evolution of such tilted discs at a range of inclinations. In the small inclination case we identify an oscillatory disc profile, in agreement with previous analytical predictions. In the high inclination case we confirm disc breaking and tearing, where the disc is torn in independently precessing rings of gas. I will then consider applications of these structures to observational features, considering the currently unexplained phenomenon of quasi-periodic oscillations. Turning to geometrically thick accretion flows we consider a torus (with a donut shaped profile), such as would form from a tidal disruption event. Simulating a torus in three-dimensions is difficult as they may be unstable to the non-axisymmetric hydrodynamic Papaloizou-Pringle instability. I will present our simulations of an idealised, circularised torus that has similar parameters to a previously simulated tidal disruption remnant and show that i) it is unstable to this instability and ii) this instability is capable of driving angular momentum transfer. Our simulations imply that the initial accretion rate (and hence lightcurve) of tidal disruption events​ ​may be constrained by this instability rather than the magneto-rotational instability in the case of extremely weak initial magnetic​ fields.

Mar 9 Mitch Begelman
University of Colorado, Boulder
host: Daniel Proga
Kinetic Modeling of Particle Acceleration in Relativistic Reconnection and Turbulence

Nonthermal acceleration of relativistic particles is a key element in the energetics and observational appearance of systems as diverse as blazar jets, accretion disk coronae, gamma-ray bursts and pulsar wind nebulae. Recently, it has become feasible to carry out kinetic simulations of relativistic plasmas that are large enough to study particle acceleration in detail. I will discuss our particle-in-cell simulations of reconnection and turbulence in collisionless pair and electron-ion plasmas, which have revealed surprising systematic relationships between the particle spectrum and plasma magnetization (the ratio of magnetic to particle energy). We have also identified a "linear accelerator" mode of particle acceleration that may be responsible for very hard, rapid gamma-ray flares in the Crab Nebula and blazars.

Mar 16

Mar 23 Ken Nagamine
Osaka University
host: Bing Zhang
Direct Collapse to Supermassive Black Hole Seeds with Radiation Transfer: Isolated & Cosmological Halos

Direct collapse of a gas sphere is an attractive scenario of massive black hole formation. We model direct collapse of a primordial gas within dark matter halos in the presence of radiative transfer. High-resolution zoom-in numerical simulations of gravitational collapse have been performed in both isolated halos and a cosmological framework, down to the formation of the photosphere and growth of the central object. Radiative transfer has been implemented in the flux-limited diffusion (FLD) approximation, and adiabatic models have been run for comparison. We find that (1) the FLD flow forms an irregular central structure with dynamically insignificant rotation, and does not exhibit fragmentation. This is contrary to adiabatic flow which forms an asymmetric, geometrically-thick disk that drives a pair of strong spiral shocks, subject to Kelvin-Helmholtz shear instability, forming fragments, which tend to merge with the central disk; (2) the growing central core in the FLD flow quickly reaches a core mass of ~10 Msun and a highly variable luminosity of order of 10^38-10^39 erg/s, comparable to the Eddington luminosity. It experiences massive recurrent outflows driven by radiation force and thermal pressure gradients, which form dense expanding shells, mixing with the accretion flow and transferring the angular momentum outwards; and (3) the interplay between radiation and thermal pressure gradients and gravity, subject to the massive accretion rate, results in photosphere of radius ~10 AU, much larger than that of a protostar. Overall, the inclusion of radiative transfer reveals complex early stages of formation and growth of the central structure in direct collapse scenario of massive black hole formation. For more details, please see arXiv:1803.03278 and 1801.08545.

Mar 30 Spring Break

Apr 6

Apr 13

Apr 20 Daniel Wolf Savin
Columbia University
host: Bernard Zygelman
Laboratory Astrophysics Studies along the Cosmic Cycle of Gas

Tracing the evolution of baryonic matter from atoms in space to stars such as our Sun hinges on an accurate understanding of the underlying physics controlling the properties of the gas at every step along this pathway. Here I will explain some of the key epochs in this cosmic cycle of gas and highlight our laboratory studies into the underlying atomic, molecular, and plasma physics which control the observed properties of the gas.

Apr 27 Qingyang Hu
High Pressure Synergetic Consortium at the Advanced Photon Source
host: Qiang Zhu

May 4 Tao Cheng
California Institute of Technology
host: Qiang Zhu

May 11

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