List of Past CTC Theory Lunches : 01-Jan-2002 to 01-Jun-2002

Date:   Tuesday 22-Jan-02
Speaker:   Drake Deming
Title:   Radiative-Hydrodynamic Modeling of the Splashback Phase of the Shoemaker-Levy 9 Impacts

Date:   Monday 11-Feb-02
Speaker:   Stephen O'Sullivan
Title:   Numerical Simulations of Steady and Pulsed Non-Adiabatic Magnetised Jets From Young Stars

Date:   Tuesday 19-Feb-02
Speaker:   Patricia Boyd
Title:  The Long Term Light Curves of X-ray Binaries Contain Simultaneous Periodic and Random Components

LMC X-3 and Cyg X-2 show large amplitude fluctuations in their X-ray intensity that have been attributed to the precession of a warped accretion disk. Cyg X-3 displays high amplitude, apparently non-periodic oscillations. We have studied these three very different systems using RXTE ASM data and time-domain analysis techniques.

The intensity excursions for all three systems are found to be well described by the product of a constant period multiplied by a "random" integer. The long term variations in Cyg X-2 can be characterized by excursions whose durations are integer multiples of the 9.8 day orbital period, including one essentially identical to the reported "period" of 78 days. Cyg X-3 can be characterized in terms of integer multiples of a 70 day fundamental period, unrelated to the 4.8 hour orbital period, but closer to the ~60 day reported precession period of the relativistic jet inferred from recent radio observations.

The long term excursions of LMC X-3 are related to each other by rational fractions, suggesting the characteristic time scale is actually shorter than any observed excursion to date. A fundamental ~10.6 day period is postulated, integer multiples of which describe virtually all long-term excursions seen in LMC X-3.

We explore the 2-dimensional phase space evolution of the light curves using a natural embedding and find that all three systems possess two centers of rotation in phase space. Trajectories orbit about these regions, one of which is low luminosity and the other high luminosity.

The implications of this behavior are discussed by comparing some general predictions of recent accretion disk dynamical models to the results found here, and by comparing the phase space evolution to a dynamical system which displays the same basic characteristics.

Date:   Friday 22-Mar-2002
Speaker:   Steve Desch
Title:   Is Layered Accretion Relevant to Protoplanetary Disk Evolution?

Date:   Tuesday 2-Apr-2002
Speaker:   Kouichi Hirotani
Title:  Particle-acceleration Mechanism in Outer Magnetospheres: Pulsed GeV-TeV Spectrum of the Crab Pulsar

We investigate a stationary pair production cascade in the outer magnetosphere of a spinning neutron star. Migratory electrons and/or positrons are accelerated by the electric field due to charge depletion, and emit gamma-rays via curvature radiation and inverse-Compton scatterings. Some of the gamma-rays collide with the soft photons to materialize as pairs in the potential gap. The replenished charges partially screen the electric field, which is self-consistently solved together with the distribution functions of particles and gamma-rays. If no current is injected from outside, the gap is located around the null surface, where the local Goldreich-Julian charge density vanishes. However, we first find that the gap position shifts outwards (or inwards) when particles are injected at the inner (or outer) boundary. Applying the theory to the Crab pulsar, we demonstrate that the pulsed TeV flux does not exceed the observed upper limit for moderate infrared photon density and that the gap should be located near to or outside of the null surface so that the pulsed GeV spectrum may be explained. To constrain the accelerator position and the magnetic inclination more accurately, we need a more detailed pulsed spectrum above 10 GeV.

References: Hirotani, Shibata 2001, ApJ 588, 216 (Crab); Hirotani, Shibata 2002, ApJ 564, 369 (B1055-52); Hirotani, Shibata 2001, MNRAS 325, 1228 (other 7 pulsars)

Date:   Tuesday 9-Apr-2002
Speaker:   Rob Olling
Title:  Size, Mass and Shape of the Milky Way: The Case for a Leaner Milky Way

The shape of the Milky Way's dark halo has been determined using two independent methods. First, we have compared the amount of dark matter close to the Galactic plane as inferred from stellar kinematics to the predictions of the different dark halo models. Second, we have compared the thickness of the Galaxy's gas layer to the predictions of dark halos models with a wide range in halo shape. Combining these two techniques, we find that the Milky Way has a relatively-round dark halo with shortest-to-longest axial ratio of q=0.75 +/- 0.25. The inferred halo flattening depends on the Sun's distance from the Galactic centre (R_0), the Galactic rotation speed at the Solar circle (\Theta_0), and the stellar column density in the Solar neighbourhood. We find that an internally consistent Milky Way model can only be constructed for a very limited range in parameters.

If the dark halo is oblate, we infer \Theta_0 \la 190 km/s. Models that use the IAU-sanctioned Galactic constants of 8.5 kpc and 220 km/s REQUIRE a substantially prolate dark matter halo. The distance to the Galactic centre can be determined in several ways, but all yield small similarly values for R_0, 7.1 - 7.5 kpc. Low values for R_0 also imply a low Galactic rotation speed: \Theta_0 ~187 km/s.

If the Milky Way's rotation speed is larger than ~170 km/s, two dark matter candidates which require a highly flattened dark matter halo are ruled out: 1) decaying massive neutrinos; and 2) a disk of cold molecular hydrogen.

These results imply that the luminous Milky Way is smaller than hitherto assumed. Such a leaner Galaxy is entirely consistent with ALL observational kinematical constraints, including the determination of R_0 from water maser proper motions (7.2 +/- 0.7 kpc), the reflex motion of the Galactic centre, Cepheid velocities, the HIPPARCOS based value of the Oort limit, and the kinematics of the Local group members. The total mass estimate for the Milky Way is unaffected.

Date:   Tuesday 16-Apr-2002
Speaker:   Jeff Valenti
Title:   Observational Constraints on T Tauri Accretion Models

Date:   Tuesday 30-Apr-2002
Speaker:   Olivier Barnouin-Jha
Title:   Interpreting Craters on Asteroid 433 Eros

Date:   Tuesday 7-May-2002
Speaker:   Michael Cai
Title:   How to Make a Baby Black Hole

Date:   Tuesday 14-May-2002
Speaker:   Kevin Rauch
Title:   Chaos and the Long-term Evolution of the Solar System

Date:   Tuesday 21-May-2002
Speaker:   Nader Haghighipour
Title:   Solid-Body Dynamics in Gravitationally Unstable Protostellar Disks

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