X-ray Spectral Measurements of the Most Massive Stars: Stellar Wind Mass-Loss Rates and Shock Physics
David Cohen, Swarthmore College
Wed., Feb. 20, 2013, 4:30 PM
The most massive stars in the Galaxy burn brightly, live very short lives, and explode as supernovae. The intensity of the light they emit throughout their lives drives an outflow of material from these massive stars' surfaces. These so-called stellar winds inject momentum, energy, and nuclear-processed matter into the Galactic environment. And they also are strong enough that they can appreciably reduce the mass of a star over the course of its life, affecting supernova statistics and properties. I will discuss how my research group has developed a technique for using high-resolution X-ray spectroscopy to measure the wind mass-loss rates of some of the most massive and luminous stars in the Galaxy. Along the way, we are able to test models of shock-heating and X-ray production in massive star winds and also to constrain the degree of wind clumping and porosity.
Wed., Feb. 27, 2013, 4:30 PM
The concept that black holes behave as thermodynamic objects was first realized after the formulation of the laws of black hole mechanics by Bardeen, Carter and Hawking during the 1970's. Since then, black hole temperature and entropy have provided an ample testing bed for most current competing theories of quantum gravity. The widely accepted forms of these thermodynamic quantities are:
T_H = hbar*kappa/2pi (Hawking Temperature)
S_BH = A/(4*hbar*G) (Bekenstein-Hawking Entropy)
where A is the black hole surface area, kappa the black hole surface gravity, hbar is Planck's constant and G is Newton's constant. It is widely believed that any viable ultraviolet completion of general relativity (quantum gravity) should reproduce some variant of the above equations. To date there is a plethora of different approaches for arriving at these formulae, with string theories and loop quantum gravity the predominant competitors. However, no candidate approach has ever yielded a complete formulation of a possible quantum gravity theory and there seems to be no clear consensus which approach to prefer over the other.
In this talk we will outline the problem of quantum gravity, why black holes behave as thermodynamic objects and why they are a useful tool in the study of a yet unsolved problem. In particular we will highlight the AdS/CFT approach pioneered in string theories, for studying two dimensional near black hole horizon quantum conformal field theories relevant to four dimensional black hole thermodynamics.
Kate Jones-Smith, Reed College
Fri., Mar. 1, 2013, 12:30 PM
According to the current cosmological paradigm, the universe is dominated by a mysterious form of 'dark energy', about which very little is known. This dark energy reveals itself indirectly through astrophysical observations, which are bolstered by a number of theoretical considerations. Many models of dark energy have been proposed, but the astrophysical observations alone are not precise enough to distinguish among them. One class of models that has become popular in recent years is the so-called chameleon scalar field, which can actually be constrained with terrestrial/laboratory experiments. In this talk I will show that the chameleon model of dark energy obeys an electrostatic analogy, and describe how the analogy might be used in experiments to increase their range of sensitivity, thereby opening up the exciting possibility of detecting dark energy in the lab.
Nelia Mann, Reed College
Wed., Mar. 6, 2013, 4:30 PM
The techniques of old quantization were successfully used as an approximation to the real world for many years before the advent of modern quantum mechanics, but are often forgotten or neglected today. I will review the application of this method to simple quantum mechanical systems with known analytic results, and show that old quantization gives useful information. I will then discuss its application to a pair of more sophisticated systems (a logarithmic potential and a Yukawa potential) where analytic solutions are not possible, and compare the results with those obtained using numerical methods. In all cases old quantization provides good predictive power for the main features of the true quantum mechanical problem, and thus can serve as a "back-of-the-envelope" technique for easily providing rough, qualitative information about the system.
Tristan Smith, Lawrence Berkeley National Laboratory and the University of California
Fri., Mar. 8, 2013, 12:30 PM
In the short span of 40 years, cosmology has transformed from a purely theoretical field to one overflowing with increasingly precise data. As a result, our picture of how the universe came into being and how it evolves has come into near-perfect focus: it seems as though, after thousands of years of thought, we may be a few short decades away from understanding the true nature of our ultimate origins. Although correct in certain respects, this sense of understanding may not be as founded as we would hope. Contained within our current picture of the universe are several ideas, which are no more than place-holders waiting for a deeper level of understanding. By far the most puzzling of these place-holders is what we generally refer to as "dark energy": its existence is beyond a doubt and it fundamentally challenges long-held ideas about the fate of the universe and many basic principles upon which all of physics is based. I will introduce the "problem" of dark energy and focus on work I have done in order to explore its fundamental nature using cosmological and astrophysical observations.
Wynter Duncanson, Harvard University
Wed., April 17, 2013, 4:30 PM
Ultrasonic imaging is one of the primary means of non-invasively visualizing structures within the human body. Similarly, seismic imaging remains the dominant means of imaging the subsurface geology of oil reservoirs. Both these acoustic imaging techniques can be improved through the addition of a contrast agent. The most suitable contrast agents are gas bubbles; gas bubbles provide the contrast to strongly reflect or scatter sound. However, gas bubbles must be stabilized against dissolution; yet, conventional fabrication techniques do not provide control of their physicochemical characteristics to enable this. I will describe the assembly of stabilized bubbles using droplet microfluidics; this technology provides precise control over the composition of the gas core and a stabilizing shell. I design new types of bubbles by integrating various stimulus-responsive materials into their structure; these 'smart' bubbles adapt to changes in their environment. Moreover, I can easily tune the mechanical properties of the shell to enable bubble deformation and passage through the pore space and I demonstrate methods to accomplish and measure this. In addition, I can create bubble structures that withstand large hydrostatic pressures making it possible to use them as contrast agents in oil reservoir applications. I envisage these new 'smart' bubble contrast agents will lead to improved and more sensitive acoustic imaging, both in the human body and in oil reservoirs.
Further information is available on Dr. Duncanson's website.
David Schaffner, Swarthmore College
Fri., April 26, 2013, 12:30 PM