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2018-2019 Schedule

Oct 11th:
Speaker: Vladimir Zhdankin (Princeton)
Title: Nonthermal Particle Energization in Relativistic Plasma Turbulence
Abstract:  I will describe my recent work on driven turbulence in relativistic collisionless plasmas, relevant for a broad range of high-energy astrophysical systems (such as pulsar wind nebulae, black-hole accretion flows and jets). I will present results from particle-in-cell (PIC) simulations, which provide a first-principles approach to studying turbulence statistics in great detail. One main outcome is the confirmation that turbulence can be a viable and efficient astrophysical particle accelerator, producing nonthermal energy distributions with extended power laws, which supports theoretical ideas that have been debated for decades. I will also discuss intriguing results on electron-ion energy partition, showing that the dissipation of turbulence naturally produces a two-temperature plasma (with ions much hotter than electrons, as required by models of radiatively inefficient accretion flows).

Oct 18th:
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Oct 25th:
Speaker: Henrik Latter (DAMTP, Cambridge)
Title: Gravitoturbulence and magnetic fields in young protostellar disks
Abstract: In its early stages, a protostellar disk is sufficiently massive to undergo gravitational instability. Meanwhile it may be ionised to a degree that allows some coupling between any latent magnetic field and the gas, and possibly the emergence of the magnetorotational instability. The mutual interactions between these instabilities could profoundly influence several important processes, such as accretion variability, outbursts, fragmentation, and dynamo action. In this talk, I will present recent simulations in vertically stratified shearing boxes that incorporate self-gravity and MHD and hence explore aspects of this 'mixed' regime.

Nov 1st:
Speaker: Jing Luan (IAS)
Title: Oscillations and Outbursts of White Dwarfs
Abstract: As established by ground based surveys, white dwarfs with hydrogen atmospheres pulsate as they cool across the temperature range, 12,500 K < Teff < 10,800 K. Known as DAVs or ZZ Ceti stars, their oscillations are attributed to overstable g-modes excited by convective driving. The effective temperature at the blue edge of the instability strip is slightly lower than that at which a surface convection zone appears. The temperature at the red edge is a two-decade old puzzle. Recently, the Kepler space telescope discovered a number of cool DAVs exhibiting sporadic outbursts separated by days, each lasting several hours, and releasing ~10^33-10^34 erg. We provide quantitative explanations for both the red edge and the outbursts. The minimal frequency for overstable modes rises abruptly near the red edge. Although high frequency overstable modes exist below the red edge, their photometric amplitudes are generally too small to be detected by ground based observations. Nevertheless, these overstable parent modes can manifest themselves through nonlinear mode couplings to damped daughter modes which generate limit cycles giving rise to photometric outbursts.

November 8th:
Speaker: Sean Ressler (UC Berkeley)
Title: Origin Stories: Understanding the Inner Accretion Flow of Sagittarius A* by Understanding Its Source
Abstract: Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, provides a unique laboratory for studying low-luminosity active galactic nuclei across a range of dynamical scales due to its proximity and the corresponding wealth of observational data. Here I present 3D MHD simulations that track accretion from the winds of the 30 Wolf-Rayet stars orbiting at ~ parsec scales all the way down to a few hundred Schwarzschild radii of the central black hole. The main focus of my talk will be the result that, in the same simulation, we are able to reproduce the large rotation measures of the galactic center magnetar and Sgr A* as well as the total diffuse X-ray luminosity observed by Chandra.  I will discuss the plausibility of this scenario and make predictions about 1) future detections of galactic center pulsars and 2) the time variability of the rotation measure of Sgr A* going forward.  Finally, I will present preliminary results on the flow structure at the innermost radii of the simulation and their implications for the horizon-scale, general relativistic magneto-hydrodynamic (GRMHD) simulations that are often used to interpret the multi-frequency emission of Sgr A*.

November 15th:
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November 29th:
Speaker: Hayk Hakobyan (Princeton)
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December 6th:
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December 13th:
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January 17th:
Speaker: David Vartanyan (Princeton)
Title: Revival of the Fittest: Exploding Core-Collapse Supernovae
Abstract: Fifty years have lapsed since the early simulations of Colgate and White identified neutrino heating as key to powering core-collapse supernovae (CCSNe). Yet, for over half a century, the mechanism for producing a robust CCSN explosion has endured as a scientific mystery. Outcome - explosion or dud - depends sensitively on the progenitor structure, the neutrino-matter microphysics, and macrophysical properties (e.g., rotation and velocity perturbations). I will present recently published results, using our multidimensional hydro/radiative transfer code FORNAX, of one of the first 3D simulations of a CCSN progenitor with detailed microphysics and state of the art neutrino transport. Our model explodes within 100 milliseconds, and is estimated to accumulate energy at a rate of 0.5 Bethe (10^50 erg) over 2 seconds. The vigorous explosion highlights the crucial dependence on input physics and illustrates recent communal progress on understanding CCSNe.

January 24th:
Speaker: Luke Bouma (Princeton)
Title: The Early Arrival of WASP-4b
Abstract: The Transiting Exoplanet Survey Satellite (TESS) has begun to monitor the brightness of many stars.  I will give an update on the current status of TESS, and discuss a recent search for timing anomalies in hot Jupiter systems.  One
object surprised us: the hot Jupiter WASP-4b transited 81.6 +/- 11.7 seconds earlier than expected, based on data stretching back to 2007.  Its orbital period also appears to be shrinking, by 12.6 +/- 1.2 milliseconds per year. The period change could be caused by tidal decay, apsidal precession, or the gravitational influence of a third body.  Any of these possibilities would be interesting, for reasons that I will detail.  One tenth of the TESS Prime Mission's data have been released, and the mission could be extended. In this light, I will close with some expectations for future explorations.

January 31st:
Speaker: Chang-Goo Kim (Princeton) 
Title: Star-Forming ISM: a Fierce Battlefield between Gravity and Feedback
Abstract: Stars are one end of the cosmic baryon evolution, but they are another beginning of the evolution. Gravity successfully beats its opponents to form stars. At the very end of this chain, gravity meets the most significant opponent, stellar feedback, which feeds prodigious amounts of energy created by nuclear reactions within stars back to their birthplace, the interstellar medium (ISM). The star-forming ISM is thus the heart of the cosmic evolution where a battle between gravity and feedback is continuing. In this talk, I will describe recent achievements in modeling such a fierce battlefield inviting essential forces. In the recently developed numerical framework, called TIGRESS, the ISM evolution in a portion of galactic disks is directly simulated by solving MHD equations with self-gravity including the most important forms of stellar feedback, supernova and radiation. With the help of realistic simulations, I will answer challenging questions in the study of star formation and ISM: (1) how does feedback control star formation rates? (2) how does feedback drive galactic winds and fountains? and (3) how does the turbulent, magnetized ISM look like in dust polarization observations? These answers will guide us to develop physics-based subgrid models for a predictive galaxy formation theory. Also, the ability to create realistic polarized maps is critical to access the feasibility of future cosmic microwave background missions searching for primordial gravitational waves. I will also highlight a few undergoing extensions of the TIGRESS framework with radiation transfers of FUV and EUV radiation, spiral arms, and ICM winds.

February 28th: 
Speaker: Ivanna Escala (Caltech)
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March 14th: 
Speaker: Jia Liu (Princeton)
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March 21st: 
Speaker: Sean Johnson (Princeton)
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April 4th: 
Speaker: Eliot Quataert (UC Berkeley)
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April 11th: 
Speaker: Chuanfei Dong (Princeton)
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April 18th: 
Speaker: Brandon Hensley (Princeton)
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April 25th: 
Speaker: Kris Pardo (Princeton)
Title: Implications for the Stochastic Gravitational Wave Background from a Massive Quasar-binary
Abstract: The collective inspirals of very close-separation supermassive black holes (SMBHs) are expected to produce a stochastic gravitational wave background (GWB) at nHz frequencies, which is accessible to pulsar timing arrays. However, we have yet to detect this background or find any SMBH binaries that are sufficiently massive or at small enough separations to contribute appreciably to the GWB. Using Hubble Space Telescope imaging, we have identified a pair of rapidly-growing supermassive black holes (M>4.e8 solar masses) at z=0.2 that are separated by only 400 parsecs. While this pair is not currently in the GW-emission regime, it points to a population of SMBH pairs that should have merged by today, unless there is a final parsec problem. Using estimates for the number density of such sources, we place limits on the expected GWB amplitude. I'll discuss the issues surrounding these calculations and how we can expect to learn more about the GWB and SMBH dynamics as we search for more of these SMBH pairs.

May 2nd
Speaker: Ewine van Dishoeck (Leiden Observatory)
Title: Some recent results on protoplanetary disks with ALMA
Abstract: Thanks to ALMA, the study of protoplanetary disks is undergoing a
revolution, with a wide variety of results being reported on
statistics of disks, fascinating substructures in dust and gas, and
surprisingly weak CO emission. This informal talk will present a few
recent results from our group and collaborators on ALMA observations
and associated models of disks. First, the latest statistics on disk
masses in nearby star-forming regions will be presented, including low
vs high mass regions, with implications for disk evolution. Second,
the differences in sizes of gas vs dust disks will be discussed. Do
the much smaller dust disks indeed provide evidence for radial drift
of mm-sized dust grains as often claimed? How do the large disks
imaged by the DSHARP team fit into the overall disk population?

Finally, the last part of the talk will provide a brief update of the
IAU and its new strategic plan, and discuss the IAU 100 years
centennial activities. See www.iau.org and www.iau-100.org.


May 9th: 
Speaker: Elena Murchikova (IAS)
Title: Discovery of a Cool Accretion Disk around the Galactic Center Black Hole Sagittarius A*
Abstract: There is a supermassive black hole of mass 4x10^6 Msun at the center of the Milky Way Galaxy. A large reservoir of hot (~10^7 K) and cooler (10^2 - 10^4 K) gas surrounds it within few pc. While the amount of hot gas in the accretion zone of the black hole, i.e. within 10^5 Schwarzschild radii, is constrained by X-ray observations, the amount of the cool gas remained unconstrained. One possible way to search for this gas is to look at hydrogen recombination lines. I we report imaging of the 10^4 K ionized gas disk within 2x10^4 Schwarzschild radii, using the 1.3 mm line of hydrogen H30alpha. The emission is double peaked with a line spanning full width ~2,200 km/s. The total line is centered on the radio source Sgr A*, with the red-shifted the blue-shifted sides straddling the black hole. We interpret these as a rotating disk with mass ~10^-5 - 10^-4 Msun and with mean hydrogen density ~10^5-10^6 cm^-3. The estimate is sensitive to the assumed geometry.

May 16th: 
Speaker: Robyn Sanderson (UPenn)
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May 23rd: 
Speaker: Melinda Soares-Furtado (Princeton)
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May 30th: 
Speaker: Rocio Kiman (AMNH)
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