MAY 27 - 31, 2013

WEDNESDAY, MAY 29

11:00 am: Optical and Infrared Astronomy Division Seminar. "Stellar Tidal Disruptions: From Optical to Radio," Sjoert van Velzen, Radboud University Nijmegen. Pratt Conference Room.

Abstract: Very rarely -- about once in 100,000 years -- some stars trajectory will carry it into the narrow region around the super-massive black hole at the center of its galaxy, where a star is shredded by the extreme tidal forces. The accretion of this debris produces a flare of thermal radiation, which peaks in the (far) UV band. Since jets are ubiquitous in accreting systems, we also anticipate the launch of a relativistic jet, producing non-thermal emission at radio and X-ray frequencies. This birth of a relativistic outflow was observed recently for two tidal disruption candidates, discovered by Swift. Yet no unambiguous radio transients have been observed from stellar tidal disruption flares with thermal signatures. The number of flares that have been followed-up by radio telescopes is small, so this discrepancy could be explained by: (i) Doppler boosting, which reduces the jet flux for off-axis observers, (ii) a delay of the radio emission with respect to the time of disruption, (iii) a radio-loud/radio-quiet dichotomy related to different accretion states. We present results from radio follow-up observations of a large number of thermal tidal disruption flares, allowing us to test the hypothesis that all tidal disruptions produce relativistic jets. We also present the first measurement of the rate of optical tidal disruptions, obtained from a systematic search of SDSS multi-epoch imaging data (Stripe 82). We show how this rate can be combined with results from upcoming radio transient surveys to determine what fraction of stellar tidal disruptions are radio-loud. These results will help to complete our understanding of jet formation in compact objects.

11:00 am: Radio and Geoastronomy Division Talk. "Reconnection Diffusion Changing the Star Formation Paradigm," Prof. Alex Lazarian, University of Wisconsin. Room M-340, 160 Concord Avenue.

Abstract: The theory of star formation was developed assuming magnetic flux conservation in highly conductive fluids (Alfven theorem). Thus it usually assumed that to form a star either collecting of matter along magnetic field lines or non-ideal effects of ambipolar diffusion are necessary. However, the above assumption is not true. I shall show that magnetic flux freezing is violated in the perfectly conducting turbulent fluid. My conclusion is based on our analytical model of magnetic reconnection in the presence of weak turbulence. The predictions of this model have been successfully tested numerically and, in a separate development, the deep relation of our reconnection model with the recent developments in the Lagrangian theory of MHD turbulence has been established. On the basis of this I shall show the existence of a new process termed "reconnection diffusion". In my talk I shall show how reconnection diffusion induces flux loss in molecular clouds and accretion disks and provide a comparison of the observational data and the theoretical predictions. In particular, I shall show that the reconnection diffusion can solve the so-called the problem of "magnetic braking catastrophe" for the circumstellar accretion disks.

12:30 pm: High Energy Astrophysics Division Lunch Talk. "Testing the Equivalence Principle 10,000 Times Better on a Sounding Rocket," Dr. James Phillips, CfA. Pratt Conference Room.

Abstract: The equivalence principle is at the heart of gravitation theory. It has been tested with increasing accuracy for many centuries; the present upper limit on difference of acceleration for tested pairs of substances is 2 times 10^-13 g. Theories that unify gravity with the other forces tend to predict a violation, but few predict the magnitude. We propose to test the acceleration difference to 2 times 10^-17 g in an experiment launched into free fall by a sounding rocket. The test masses are dropped (released) eight times, for 120 s each time. Between drops, the test masses are held electrostatically and the entire payload is inverted, which reverses the position of the Earth and leaves most systematic error effects unchanged. The high sensitivity is possible in a short time for several reasons: 1) the SAO laser distance gauges measure to 0.1 pm in 1 s; 2) the position of the structure around the test masses follows that of the test masses by virtue of a servo (but not a drag-free satellite); 3) the test masses are unconstrained during drops, avoiding constraint force imperfections; 4) the position measurement is to a plate that is almost stationary with respect to the test masses, by virtue of the position servo; and 5) there are two cascaded thermal low-pass filters with time constants 1000 times longer than the 120 s drops.

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