List of Past CTC Theory Lunches : 01-Sep-2012 to 31-Dec-2012

Date:   Monday 10-Sep-2012
Speaker:   Sebastian Hoenig (UCSB)
Title:  The Dusty Heart of Active Galaxies - Dissecting the nucleus with high-angular resolution observations in the IR

Over the last years we have made significant progress in our understanding of the dusty environment around AGN -- commonly referred to as "dust torus", one of the cornerstones of AGN unification. A good part of this progress has been made possible by the high spatial resolution capabilities of the VLT and VLTI in the infrared. We are now able to not only resolve the dusty environment but constrain the distribution of the material that is believed to eventually accrete onto the supermassive black hole. I will give an overview of some of our most recent results involving small samples of AGN as well as individual objects using VISIR, MIDI, and the Keck interferometer together with 3D clumpy torus models.

Date:   Monday 17-Sep-2012
Speaker:   John Cannizzo (UMBC/Goddard)
Title:  The Thermal-Viscous Limit Cycle Model in Dwarf Novae

I'll discuss a few interesting results presented at the recent conference in Warsaw to honor Joe Smak: "Accretion flow instabilities: 30 years of the thermal-viscous disc instability model", and then give my own talk in which I (i) present a historical overview of the model, from its inception to the present, (ii) discuss the development of different aspects of the model and their connection with observation, and (iii) results from Kepler of several short period dwarf novae which now have amassed > 2 yrs of data, and the implications of these high fidelity data for accretion disks in general.

Date:   Monday 24-Sep-2012
Speaker:   Jonathan Trump (UC Santa Cruz)
Title:  Connecting AGN Accretion and Obscuration to Host Galaxy Evolution

I will present a co-evolutionary picture linking AGNs and their host galaxies, using observations from the SDSS, COSMOS, and CANDELS surveys to constrain different theoretical models of AGN/galaxy growth. In particular AGNs can be unified by their accretion rate and obscuration, and each unification parameter correlates to various host galaxy properties. Rapidly accreting AGNs tend to lie in disturbed galaxies with merger signatures (when obscured), or bulge-dominated galaxies with recent starbursts (when unobscured). In contrast weakly accreting AGNs are primarily found in undisturbed disks. New IR spectroscopy in CANDELS suggests that at z~2, secular fueling of low-level AGNs is common even to low stellar masses (M*~10^9 M_sun). Meanwhile mergers or violent disk instabilities are necessary only to explain the most powerful AGNs. I will conclude by synthesizing the varied observations into a toy "unified model" for AGN/galaxy coevolution, linking black hole accretion to the rise and quenching of galaxy star formation.

Date:   Monday 01-Oct-2012
Speaker:   Marcio Melendez (UMD)
Title:  Massive molecular outflows observed by Herschel-PACS

Mass outflows driven by stars and active galactic nuclei are a key element in many current models of galaxy evolution. They may produce the observed black hole-galaxy mass relation and regulate and quench both star formation in the host galaxy and black hole accretion. I will present the detection of massive molecular outflows, traced by the hydroxyl molecule (OH), in the far-infrared spectra of ULIRGs/AGNs obtained with Herschel-PACS. I will address the issue of the energy source (AGN or starburst) of these outflows and present evidence that higher AGN luminosity have outflows with higher terminal velocities.

Date:   Monday 08-Oct-2012
Speaker:   Jason Li (Princeton)
Title:  Resistivity and Dissipation in Pulsar Magnetospheres

Current models of pulsar magnetospheres typically assume either a complete absence of plasma or abundant ideal plasma filling the magnetosphere. The latter condition is thought to be closer to reality, but we know of a number of pulsars in which the ideal conditions break down, resulting in dissipation and high-energy emission. In this work we formulate a resistive force-free scheme that allows us to consider the effects of resistive plasma and accelerating fields on the magnetospheric structure. We run numerical simulations to construct a family of resistive solutions that smoothly bridges the gap between the vacuum and the force-free magnetosphere solutions. We further provide a self-consistent model for the spin-down of intermittent pulsars, pulsars which appear to transition between radio-loud and radio-quiet states with different spin-down rates. Finally, we present models for high-energy emission from reconnecting current sheets in Gamma-ray pulsars.

Date:   Monday 15-Oct-2012
Speaker:   Francesco Tombesi (UMD/NASA Goddard)
Title:  X-ray evidence for accretion disk outflows in local AGNs and their role on feedback

X-ray evidence for massive, highly ionized, ultra-fast outflows (UFOs) has been recently reported in a number of AGNs through the detection of blue-shifted Fe XXV/XXVI absorption lines. We present the results of a comprehensive spectral analysis of a large sample of 42 local Seyferts and 5 radio galaxies observed with XMM-Newton and Suzaku. We find that UFOs are common phenomena, being present in >40% of the sources, and that they are not very collimated. Their outflow velocities are mildly-relativistic, in the range ~0.03-0.3c. Their location is constrained at sub-pc scales from the central black hole, consistent with the launching region of accretion disk winds/outflows. The mass outflow rate is in the interval ~0.01-1Msun/yr and the associated mechanical power is high, in the range ~10^43-10^45 erg/s, comparable to that of jets in radio-loud sources. Therefore, UFOs are capable to provide a significant contribution to the AGN cosmological feedback and their study can provide important clues on the connection between accretion disks, winds and jets.

Date:   Monday 22-Oct-2012
Speaker:   George Younes (NASA/Marshall/USRA)
Title:  Multiwavelength Analysis as a Probe of Accretion and Radiative Processes in LINERs

This talk focuses on the faint end of the active galactic nuclei (AGN) luminosity function in the nearby universe, mainly populated by Low Ionization Nuclear Emission-line Regions (LINERs). LINERs are characterized by nuclear optical spectra dominated by emission lines from low-ionized species (e.g., O I). These LINERs exhibit bolometric luminosities at least two orders of magnitude lower than classical luminous AGN (Seyferts and quasars). Three possibilities have been invoked as the dominant source of power in these sources; (1) a geometrically thin optically thick accretion disk, similar to the one believed to exist in AGN, (2) a radiatively inefficient accretion flow (RIAF), and finally (3) a jet-only model. I will tackle the problem of accretion and radiative processes in a well defined optically selected sample of LINERs showing a definite detection of broad H\alpha emission (LINER 1s). I will give the X-ray and multiwavelength properties of these LINER 1s and show how they compare to AGN and to their less massive counter-parts, galactic black-hole X-ray binaries.

Date:   Monday 29-Oct-2012
Speaker:   Mike Kesden (NYU)
Title:  Tidal Disruption by Spinning Supermassive Black Holes

Supermassive black holes reside at the centers of most large galaxies, including our own Milky Way. Stars can get scattered onto orbits where the tidal force exerted by the black hole exceeds the star's self-gravity, leading to the star's tidal disruption. Some of the debris from the disrupted star will be accreted by the black hole, releasing gravitational potential energy that can power a bright electromagnetic flare. If the tidal radius at which the star is disrupted is close enough to the black hole's event horizon, general relativity will affect both the rate at which these tidal disruption events occur and the light curves for individual events. I will present semi-analytic models of stellar tidal disruption that allow these relativistic effects to be quantified, and will summarize current and future observational efforts to detect tidal disruption events.

Date:   Monday 05-Nov-2012
Speaker:   Wade Henning (NASA/GSFC)
Title:  Exploring the Extremes of Tidal Activity in Terrestrial Class Exoplanets

Among the population of extrasolar planets may exist rocky bodies with orbital configurations that give rise to extreme tidal heating, potentially far in excess of tidal examples seen in our solar system. The recipe for such worlds begins with close proximity to a massive host, and a strong and durable orbital perturbation to counteract rapid tidal circularization. Extreme tidal cases may be able to produce magma oceans, or magma slush mantles at high melt fractions. While tidal heat may often exceed radiogenic heat production, it is far less likely to exceed insolation, and thus may have limited surface temperature expression, even while causing large scale internal melting. Observations will have difficulty detecting tidal surface temperature enhancements given a few percent uncertainty in albedo, thus confirmation of tidal activity may require spectral detection of hotspots or searches for high volcanic gas concentrations including sulfur dioxide or hydrogen sulfide. Tides have the greatest impact around dim stars and nonluminous hosts, and will only alter the habitable zones for smaller M dwarf stars, however tidal activity is shown to be generally detrimental to habitable zone widths. Multiple viscoelastic models are explored, including the Maxwell and Burgers rheologies, and applied to both solar system and extrasolar objects. Recent work is now extending these models to multilayer inhomogeneous bodies such as detailed investigations of the solid tidal dissipation in the early Earth-Moon system during the era of magma ocean crystallization, as well as to the evolution of ice-silicate hybrid superEarth exoplanets.

Date:   Monday 12-Nov-2012
Speaker:   Greg Snyder (Harvard)
Title:  Modeling Diagnostics of Quasars and Starbursts

Numerous uncertain processes act on or within galaxies. I will discuss our efforts to combine hydrodynamical merger simulations with dust radiative transfer to constrain such issues, including the link between active galactic nuclei (AGN) and mid-infrared flux in ultra-luminous infrared galaxies (ULIRGs). Focusing on the effects of galaxy dust, in this work we evaluate diagnostics used to disentangle AGN emission and star formation, such as 9.7 micron silicate absorption, PAH emission, and SED slope as a function of time, dust model, and AGN strength in a small number of realizations meant to bracket the properties of ULIRGs. Although generally consistent with previous interpretations, our results imply none of these indicators unambiguously estimate the AGN luminosity fraction in all cases. For a wide range of obscuration, a JWST-accessible combination of these mid-infrared features can simultaneously constrain the AGN fraction and dust grain model. I will also discuss ongoing efforts to model galaxy observations from ever-larger cosmological simulations of galaxy formation.

Date:   Monday 19-Nov-2012
Speaker:   Nicholas Stone (Harvard)
Title:  Compression, Disruption, and Precession in Strong Tidal Encounters

The tidal disruption of a star by a supermassive black hole (SMBH) produces a luminous, multiwavelength flare that can probe the nuclei of quiescent galaxies. I will discuss a new analytic model we have recently introduced to study the tidal free fall phase of these events, when the disrupted star undergoes homologous vertical collapse. Using this model, we corrected a longstanding error in the literature and investigated the role of stellar spin in tidal disruption events. I will also discuss two ways in which general relativity can leave its imprint on tidal disruption flares: gravitational wave emission from the vertical bounce of the disrupted star, and the precession of accretion disks.

Date:   Monday 26-Nov-2012
Speaker:   Diego Munoz (Harvard)
Title:  The Moving-mesh Code Arepo: Applications to Circumstellar Disks

In the past few years, high-sensitivity, high-resolution, sub-millimeter imaging has provided robust statistics on disks in young binary systems, showing that both wide and close binaries possess substantial gas disks. Whether these disks have interacted or will interact during their lifetimes remains an open question. I will describe the basics of the novel, moving-mesh scheme AREPO (Springel 2010), and illustrate some of its applications to the modeling of binary star+disk systems. As a Lagrangian/Eulerian code, Arepo combines some properties of grid-based and particle-based codes, making it a promising tool for high-Mach number, geometrically complex astrophysical flows. I will show results of the first simulations of self-gravitating disk-disk interaction on a moving mesh, focusing on massive young binary systems.

Date:   Monday 03-Dec-2012
Speaker:   Aleksander (Olek) Sadowski (Harvard)
Title:  Outflows of mass and energy from radiatively inefficient BH accretion disks

BH accretion disks are expected to produce outflows at very low accretion rates. As a result, the inward accretion rate drops significantly between the outer edge of the disk and the horizon. Outflowing mass and energy affect the evolution of galaxies hosting supermassive BHs leading to the BH feedback effect. Things become even more interesting when BHs are rotating, depositing their rotational energy and driving relativistic jet-like outflows. In this talk I will discuss how long-duration GRMHD simulations of accretion disks help us understand this phenomenon.

Date:   Monday 10-Dec-2012
Speaker:   Andrey Timokhin (NASA Goddard)
Title:  Radio pulsars: electrical generators in the sky

Radio pulsars are celestial electrical generators directly accelerating charged particles and producing abundant electron-positron plasma in their magnetospheres. They are fascinating astronomical sources shining over the whole electromagnetic spectrum, from radio to the highest energy gamma-rays. Although they belong to "classical old-fashioned" astronomical objects discovered a long time ago, there is still no quantitative theoretical model which could explain even the most fundamental of their observational properties. In this talk I will briefly overview the current state of pulsar models and recent progress in the field. I will argue that pulsars are much more complex objects than was assumed before.

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