Wednesday Colloquium


"Surprises from the Crab"

Mitch Begelman (JILA, University of Colorado, Boulder)

The unexpected discovery of gamma-ray flares from the Crab Nebula may have surprising implications for plasma astrophysics. Standard particle acceleration mechanisms cannot account for the energies of the flaring photons. Instead, these observations point toward an acceleration process involving rapid destruction of magnetic field through reconnection. I will discuss the extreme particle acceleration process that may lead to the flares, and the likely role of current-driven instabilities in triggering reconnection in the Crab and elsewhere.


"The Local Group as a springboard to the early Universe: the case of Fornax dSph"

Andres del Pino Molina (Instituto de Astrofísica de Canarias)

: I will present the full Star Formation History as a function of radius of the Fornax dSph galaxy as well as some preliminary results on the spatial distribution of the stellar populations. We found significant differences in the populations as a function of the galactocentric radius, in addition to strong asymetries in the distribution of the youngest stellar populations which may be the result of interactions between Fornax and other systems. This study is based on FORS1@VLT photometry as deep as I~24.5 and the IAC-star, IAC-pop and MinnIAC codes.


"The Classical Cepheids one hundred years after discovery of the Period - Luminosity relation"

Wojciech Dziembowski (NCAC, Warsaw, OAUW )

I will begin with a review of various applications of Cepheids in astronomy and the road to understanding of their pulsation. New data from the OGLE project significantly expanded data base on Cepheids in the Magellanic Clouds. Some of these data present an intriguing challenge to the stellar pulsation theory.


"Eddington capture sphere around luminous stars"

Maciej Wielgus (Warsaw University of Technology)

The phenomena related to test particle motion in the vicinity of a compact luminous star in Schwarzschild metric will be discussed. It will be shown that the corresponding equations of motion display a critical point, located between the stellar surface and infinity for mildly super-Eddington luminosities. The stability of the equilibrium state in the critical point will be discussed. The term “Eddington capture sphere” (ECS) refers to the set of such critical points, which form a sphere that separates super-Eddington region inside it from the sub-Eddington region outside. Particles infalling onto the star are likely to be trapped on the ECS, which may have a substantial impact on the accretion. Toy model of the oscillatory behavior in optically thin accretion, related to the periodic appearance and disappearance of the ECS will be presented.