List of Past CTC Theory Lunches : 01-Jan-2015 to 01-Jun-2015

Date:   Monday 30-Mar-2014
Speaker:   John Cannizzo (Goddard Space Flight Center)
Title:  "Clues to the evolution of helium WD-WD binaries from the PTF"

The study of AM CVn stars - semi-detached He WD-WD binaries - has exploded in recent years thanks to long term light curves obtained by the Palomar Transient Factory. Systems are seen with binary periods ranging from about 5 minutes to about an hour. AM CVn stars are similar to dwarf novae in that they can undergo accretion disk outbursts. Systems with high dM/dt have steady disks in permanent outburst, whereas for very low dM/dt systems the disks are too cool to have outbursts. Disk instability theory gives a specific prediction for the zone of instability, therefore by matching the observed zone with the theoretical one we constrain dM/dt(P_orb), the rate of mass transfer versus orbital period. The inferred relation is consistent with expectations from stellar evolution. One also has predictions for the recurrence time for outbursts and outburst duration versus P_orb which can be compared to observations.

Date:   Monday 09-Feb-2015
Speaker:   Ferah Munshi (U. of Oklahoma)
Title:  The 40 Thieves: A Novel High Resolution Dwarf Galaxy Simulation

I will present results from a fully cosmological, extremely high-resolution LCDM simulation of a cosmological sheet of isolated dwarf galaxies utilizing the new N-body + SPH code ChaNGa. The present day virial masses of these dwarfs range from Mvir=3.3x10^9 – 1.3x10^10 for halos which form stars. The simulation includes a blastwave scheme for supernova feedback, a novel scheme for implementing early stellar feedback, metal-dependent radiative cooling, a scheme for the turbulent diffusion of metals and thermal energy, as well as a uniform UV background. The dwarfs show excellent agreement with the present-day Stellar to Halo Mass (SMHM) relationship, have realistic blue colors and low star formation rates, in addition to realistic baryon fractions, colors and metallicities. Their cumulative star formation histories also show excellent agreement with local group dwarfs, especially when observational corrections are applied. I will show the affects of baryon physics on both the dark matter profiles and virial masses in addition to studying the contribution of a bursty star formation history to variations in the H-alpha to UV SFR ratio found in dwarfs.

Date:   Monday 29-Feb-2015
Speaker:   Robyn Sanderson (Columbia University)
Title:  Inferring the Galactic gravitational potential with Gaia and friends

Abstract: Tidal streams can be used to infer the Galactic gravitational potential thanks to the common origin of the stars in each one. I will present a method for doing so by maximizing the information content (i.e. clumpiness) of the action space of the stream stars. This statistical approach eliminates the need to assign stars to particular streams, allowing us to use the entire accreted stellar halo simultaneously. Using a toy model of the stellar halo in a known potential, and including updated error models for Gaia, I will show how ground-based spectroscopic follow-up of faint halo stars and/or precise distance estimates for halo RR Lyrae will improve our ability to measure the gravitational potential when combined with the Gaia catalog. By fitting a spherical NFW potential to streams in a cosmologically simulated halo, I will illustrate how oversimplification of the potential model affects fit results.

Date:   Monday 02-Mar-2015
Speaker:   Roseanne Cheng (Johns Hopkins)
Title:  The Accretion Problem in Tidal Disruption Events

In the classic picture of a tidal disruption event, debris from a disrupted star forms a disk quickly around the black hole while generating flares and/or jets. As it stands there are inconsistencies with observed candidates and classical theory: observed peak luminosities are 2--3 orders of magnitude below expectations and observed color temperatures are ~1 order of magnitude cooler than expectations. Furthermore, it is not well-understood how the stellar debris joins the accretion flow in the theory. A summary will be given of the current state of the art as well as our recent work which investigates the issue of disk formation by combining a post-Newtonian hydrodynamics simulation of the star itself as it is torn apart with a fully general relativistic hydrodynamics simulation of the subsequent motion of the stellar debris as it orbits the black hole. Choice of parameters and the importance of a detailed relativistic calculation of the disrupted star as initial conditions in a global simulation will be discussed. Finally, we consider how our results can be applied to recently observed tidal disruption candidates.

Date:   Monday 09-Mar-2015
Speaker:   Hotaka Shiokawa (Johns Hopkins)
Title:  General Relativistic Hydrodynamic Simulation of Accretion Flow from a Stellar Tidal Disruption

When a star comes close enough to a black hole, the strong tidal force tears off a significant amount of mass from the star and possibly tears apart the star completely. Its matter is then redistributed into either unbound or highly elliptical orbits. Subsequent rain back of the bound material is thought to ultimately form an accretion disk. A key unsolved problem is how the highly elliptical orbits of the bound matter are circularized so that the matter can form an accretion disk. No generally applicable mechanism has yet been identified to accomplish this. Combining a relativistic hydrodynamic simulation of the stellar disruption with a relativistic hydrodynamics simulation of the subsequent debris motion, we track the evolution of such a system until 80% of the stellar mass bound to the black hole has settled into an accretion flow. I will present detailed dynamics of the flow in our simulation and its discrepancy between previously envisioned disk formation scenarios.

Date:   Tuesday 19-May-2015
Speaker:   Kyuseok Oh (ETH Zürich)
Title:  "A new catalogue of type 1 AGN and its implication on the AGN unified model"

We have newly discovered a substantial number of type 1 active galactic nuclei (AGN) featuring weak broad-line regions (BLRs) at z < 0.2 from detailed analysis of galaxy spectra in the Sloan Digital Sky Survey Data Release 7. These objects predominantly show a stellar continuum but also a broad H-alpha emission line, indicating the presence of a low-luminosity AGN oriented so that we are viewing the central engine directly without significant obscuration. These accreting black holes have previously eluded detection due to their weak nature. The new BLR AGNs we found increased the number of known type 1 AGNs by 49%. Some of these new BLR AGNs were detected at the Chandra X-ray Observatory, and their X-ray properties confirm that they are indeed type 1 AGN. Based on our new and more complete catalogue of type 1 AGNs, we derived the type 1 fraction of AGNs as a function of [OIII] λ5007 emission luminosity and explored the possible dilution effect on the obscured AGN due to star-formation. The new type 1 AGN fraction shows much more complex behavior with respect to black hole mass and bolometric luminosity than suggested by the existing receding torus model. The type 1 AGN fraction is sensitive to both of these factors, and there seems to be a sweet spot (ridge) in the diagram of black hole mass and bolometric luminosity. Furthermore, we present a hint that the Eddington ratio plays a role in determining the opening angles.

Date:   Tuesday 26-May-2015
Speaker:   David Tsang (McGill University)
Title:  "The Principle of Stationary Nonconservative Action: Basic Formalism and Numerical Applications"

I will briefly outline the nonconservative variational principle that we have recently developed, which allows for nonconservative processes to be generically modelled with an action. This new variational principle allows for the effect of "inaccessible" degrees of freedom to be incorporated into the action, allowing for nonconservative physics to naturally arise. I will demonstrate how this allows for the formulation of nonconservative variational integrators: numerical integrators with the Noether charge (energy, angular momentum etc) accuracy properties of symplectic integrators but for systems that have dissipation or other nonconservative physics (e.g. tides, radiation reaction, drag).

Date:   Monday 08-Jun-2015
Speaker:   Sebastien Guillot (Santiago, Chile)
Title:  "Combining Neutron Star Observations to Constrain the Nuclear Physics of Dense Matter"

Neutron stars are wonderful objects to understand the behaviour of dense matter at and above nuclear density. Different sub-classes of neutron stars offer a variety of methods to probe matter at densities that cannot currently be attained in laboratories. With the current generation of observatories at all wavelengths, high precision measurements of the neutron physical properties (e.g. masses Mns, or radius Rns, or Mns/Rns) are now available and improving our understanding of dense matter. Because it is important to employ systematically independent approaches, I will present two of these methods, addressing their advantages and drawbacks. Individual neutron stars cannot place stringent constraints on the dense matter equation of state, and their observations need to be combined in a statistically consistent manner. I will discuss how combining neutron star observations can help us determine the dense matter equation of state and better understand nuclear physics at extreme densities.

Date:   Monday 15-Jun-2015
Speaker:   Cecilia Chirenti (Santo Andr&eacute;)
Title:  "Neutron star oscillations and gravitational waves"

Isolated neutron stars can also be promising sources of gravitational waves for future detectors. More recent studies of neutron star oscillations have included new ingredients like realistic equations of state, differential rotation and magnetic fields, among others. Rotational instabilities like the CFS instability can also play an important role. I will discuss some of these ingredients and their relevance for the future detection of gravitational waves.

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