JANUARY 28 - FEBRUARY 1, 2008

MONDAY, JANUARY 28

12 noon: Solar, Stellar, and Planetary Sciences Division Seminar. "Turbulence-Driven Polar Winds from T Tauri Stars Energized by Magnetospheric Accretion," Dr. Steven R. Cranmer, CfA. Pratt Conference Room.

Abstract: Pre-main-sequence stars are observed to be surrounded by both accretion flows and some kind of wind or jet-like outflow. There are several possible explanations of how and where the outflows arise, including disk winds, X-winds, impulsive (CME-like) ejections, and "true" stellar winds. Whatever their origin, the outflows produce observational diagnostics that indicate mass loss rates exceeding the Sun's present mass loss rate by factors of a thousand to a million. Recent work by Matt and Pudritz has suggested that if there is a stellar wind with a mass loss rate about 10% of the accretion rate, the wind can carry away enough angular momentum to keep T Tauri stars from being spun up unrealistically by the accretion. In this talk, I will present a preliminary set of theoretical models of accretion-driven winds from the polar regions of T Tauri stars. These models are based on recently published self-consistent simulations of the Sun's coronal heating and wind acceleration. In addition to the convection-driven MHD turbulence (which dominates in the solar case), I add a source of wave energy at the photosphere that is driven by the impact of plasma in "neighboring" flux tubes undergoing magnetospheric accretion. This added energy is enough to produce large mass loss rates, but it is still uncertain whether it is enough to solve the T Tauri angular momentum problem.

3:00 pm: Joint CfA/MIT Seminar on Neutron Stars and Supernova Remnants. "Two-Fluid Models of Cosmic-Ray Modified Radiative and Adiabatic Shocks," Alex Wagner, University of Leeds. Phillips Auditorium.

Abstract: Shocks occur in almost every environment of the diffuse universe, and within these environments, the cosmic ray pressure is often comparable to the thermal pressure. In supernova remnants, optical and UV spectra of shocked gas contain powerful diagnostics to test shock models at the global and kinetic level. Most shock models for this purpose have neglected cosmic rays, even though the theory of diffusive shock acceleration of ions predicts significant modifications to the shock structure. The shock models have also often assumed a steady flow, despite the fact that shocks in which cooling has become dynamically important (radiative shocks), are subject to the thermal instability. To investigate the effects of the thermal instability and the role of cosmic rays in radiative and adiabatic shocks, we have performed time-dependent numerical simulations using the two-fluid approximation. The cosmic rays are governed by a fluid equation and their pressure contributes to the momentum and energy balance in the flow. I will present a selection of such models and show an application to an adiabatic shock in Tycho's supernova remnant.

THURSDAY, JANUARY 31

11:00 am: Institute for Theory and Computation (ITC) Colloquium. "Dynamics of Accretion Flows Irradiated by a Quasar," Dr. Daniel Proga, University of Nevada, Las Vegas. Pratt Conference Room.

4:00 pm: Colloquium. "Studies of Active Galaxies in the GLAST Era," Dr. Greg Madejski, Stanford/KIPAC. (Anyone wishing to meet with the speaker should contact the host, Dr. Dan Schwartz, ext. 5-7232.) Preceded by tea at 3:30 pm. Phillips Auditorium.

Abstract: Active galaxies, presumably powered by flow of galaxian matter onto supermassive black holes, are amongst the most luminous celestial sources. Their isotropic radiation is sometimes accompanied by Doppler-boosted emission from a relativistic jet. The spectra emitted by such jets often extend from the lowest accessible radio frequencies to the most energetic (TeV) observable gamma-rays. This presentation will cover the new results that are inferred from current X-ray and soft gamma-ray observations of jet-dominated AGN, and will present the prospects for understanding their structure with the launch of the Gamma-ray Large Area Space Telescope, slated for launch in May 2008.

SAO HUMAN RESOURCES NOTES

Leave Donations Requested

Due to a medical emergency, Joyce Janjigian of the Solar, Stellar and Planetary Science Division has been approved as a leave donor recipient in the Leave Transfer Program. Any SAO Trust Fund employee who wishes to donate annual leave to Joyce should complete a Leave Donor Form and send it to MS 17.

Under the Leave Transfer Program, Trust Fund employees may only donate annual leave to other Trust Fund employees and Federal employees may donate annual leave only to other Federal employees. If you should have any questions or are not able to access the form, please contact Anne LeBlanc, ext. 5-7371.

Job Vacancies

An online listing of all current SAO job openings is available. For more information, please call Human Resources, ext. 5-7371, or view your nearest bulletin board.