הרצאות לזכרו של ג'ון בקל
Microlensing Planet Searches On a Cusp
50 Years after it was first proposed as a planet search technique,
microlensing has developed into a powerful probe of planet populations that are inaccessible to other methods. Microlensing has already given us the first frequency measurements of free-floating planets, and of bound planets beyond the snow line, as well as the first true solar system analogs, i.e., multi-giant systems beyond the snow line. Even its detections of more prosaic binaries have yielded startling results. Microlensing planet searches are now entering a new era of "second generation surveys", in which the rate of planet detection is rising rapidly. I review accomplishments to date and then discuss the observational prospects as well as theoretical challenges of this new era.
The Formation of Massive Stars
John Bahcall was one of the greatest astrophysicists of the last 50 years. His impact extended well beyond his research, through his mentoring of generations of astrophysicists and his influence on science policy. His work touched many fields, including the study of supernovae, most of which result from the explosion of stars with a mass more than about 10 times the mass of the Sun. Such stars are responsible for creating most of the heavy elements in the universe, for governing the evolution of galaxies, and quite possibly for re-ionizing the universe a few hundred million years after the Big Bang. The formation of these stars can be understood as an extension of the theory of low-mass star formation, generalized to include the effects of interstellar turbulence. However, a major problem must be overcome: For massive stars, the outward force due to radiation pressure exceeds the inward force due to gravity; how can gas accrete onto the protostar in that case? Circumstellar disks, outflow cavities, and radiative Rayleigh-Taylor instabilities all contribute to the solution of this problem. These conclusions are validated by means of 3D radiation-hydrodynamic simulations of high-mass star formation. Massive stars are powerful sources of ionizing radiation, which has a significant impact on star formation.
Impressively Unimpressive: the Tiniest of Galaxies
Galaxies are objects where normal, baryonic matter has condensed at the centers of dark matter mass concentrations and has been turned into stars. There is a well established, and broadly understood, upper limit for how many stars can make up a single galaxy. Yet, how few stars a galaxy can have is not at all understood. And we now know galaxies that have 100 times fewer stars than the smallest ones known only 5 years ago. The tiniest galaxies are proving a fabulous laboratory for understanding how galaxies form and at the same time provide a unique opportunity to constrain the physical properties of dark matter.
Formation of Massive Galaxies
The physics and sequence of events behind the formation of galaxies, the remarkable structures in which most of the stars in the universe reside, has remained a long-standing puzzle. Now that we have a quite definite cosmological model, providing us with a quantitative picture of how perturbations grew from very low amplitude fluctuations, we can perform the forward modeling of representative pieces of the universe using standard physical processes, to see how well our computer simulations match real, locally observed galaxies. Finally, we can employ large ground and space based telescopes to use the universe as a time machine – directly observing the past history of our light-cone, and comparing with our computed evolutionary tracks. I will present the coherent and plausible picture that emerges and that leads naturally to the mass, size, scale and epoch of galaxy formation.
Many of us think that the heavens are serene and calm. However, most of the interesting things in the universe starting from the birth of the Universe happen during explosions. The most notable cosmic explosions are supernovae which mark the death of massive stars. During the explosion heavy elements (critical for our existence) are created. Cosmic explosions, thanks to their brilliance, allow astronomers to probe the young Universe and even infer the existence of dark energy. Over the past decade astronomers have uncovered new types of cosmic explosions. The speaker will summarize these new developments in the field of cosmic explosions (namely gamma-ray bursts) and proceed to speculate on yet new classes of cosmic explosions. Such a discussion is timely given the imminent commissioning of wide angle optical and radio surveys of the Sky.
The new standard model of cosmology
Key Questions about Supermassive Black Holes in Galaxies
Recent data indicates that almost all galaxies possess a supermassive black hole at their center. When gas accretes onto such black holes it heats-up and shines, resulting in the appearance of a bright quasar. The earliest quasars are found to exist only a billion years after the big bang. I will describe recent observations of both the nearest and the most distant supermassive black holes in the universe. The formation and evolution of the black hole population can be described in the context of popular models for galaxy formation. I will describe the key questions that drive current research on supermassive black holes and present theoretical work on the radiative and hydrodynamic effects that quasars have on their cosmic habitat. Within the coming decade it would be possible to test general relativity by monitoring and possibly even imaging the polarized emission from hot spots around the black hole in the center of our Galaxy, SgrA*.