Jan 31

"Dark Matter"
Katherine Freese, University of Michigan (TCC Visiting Speaker)

All the matter and energy of our daily experience -- tables, chairs, people, air, planets, etc -- constitute only 4% of the total content of the Universe. The majority of the Universe resides in the Dark Side: Dark Matter and Dark Energy. This talk will examine the dark matter that comprises most of the mass of the Milky Way and all other galaxies. I will begin by reviewing the observational evidence for dark matter and then turn to what we think it is. Its existence was first noticed in clusters in the 1930s, and yet its nature remains a mystery still today. I will discuss proposed candidates for the dark matter, which is probably made of some new kind of fundamental particle. The best motivated dark matter particles are Weakly Interacting Massive Particles, or WIMPs, such as supersymmetric particles. A great deal of excitement currently pervades this field because of current and upcoming experiments that are finding hints of dark matter in the form of otherwise unexplained events. We first made these predictions twenty years ago, and it is very exciting that more and more signals are emerging that may in fact be signatures of dark matter detection. These particles have been powerful motivation for the LHC at CERN, the underground experiments such as XENON or CDMS, satellites such as FERMI or PAMELA, and neutrino detectors such as ICECUBE at the South Pole. The current status will be discussed. Dark Stars provide another potential avenue for discovery: we found that the first stars to form in the universe may be powered by WIMP dark matter heating rather than by fusion (a new phase of stellar evolution). Dark Stars may be detectable as well in the upcoming James Webb Space Telescope. Perhaps in the near future the dark matter mystery will be solved.

Feb 7

"Formation of Icy Planetesimals in the Primitive Nebula: Implications for the Composition of Gas Giant Planets, Satellites, and Comets"
Olivier Mousis, University of Franche-Comte and Besancon Observatory (Tinsley Scholar)

Formation scenarios of the solar nebula invoke two main reservoirs of water ice that may have taken part concurrently into the production of solids. In the first reservoir, which is located within the heliocentric distance of 30 AU, water ice infalling from the Interstellar Medium (ISM) initially vaporized into the hot inner part of the disk and condensed in its crystalline form during the cooling of the solar nebula. The second reservoir, located at larger heliocentric distances, is composed of water ice originating from ISM that did not suffer from vaporization when entering into the disk. In this reservoir, water ice remained mainly in its amorphous form. From these considerations, we discuss here the trapping conditions of volatiles in planetesimals produced within the outer solar nebula and their implications for the origin and composition of gas giant planets, their surrounding satellite systems and comets. In particular, we show that the formation of icy planetesimals agglomerated from clathrate hydrates in the solar nebula can explain in a consistent manner the volatiles enrichments measured at Jupiter and Saturn, as well as the composition of Titan's atmosphere.

Feb 28

"The Quest for Type Ia Supernova Progenitors"
Carles Badenes, Tel-Aviv University/Weizmann Institute

The identity of the progenitor systems of Type Ia Supernovae (SN Ia) has become one of the key open questions in stellar astrophysics. I will review the state of the art in SN Ia progenitors and point out the main reasons why this has remained an unsolved issue for so long. I will then describe some recent advances in the study of two kinds of objects - supernova remnants (SNRs) and binary white dwarfs (BWDs) - that are shedding new light on key aspects of the problem. SNRs and BWDs are much closer to Earth than the extragalactic SNe that are routinely discovered by automated SN searches, and they provide a radically different perspective of the SN Ia phenomenon. I will conclude by outlining some promising avenues for future research.

Mar 14

"When the Moon was a Cloud - The Creation Myth"
Kaveh Pahlevan, Yale University, Dept. of Geology and Geophysics

In 1975-76, the idea arose that the Moon was formed as a by-product of a collision between the proto-Earth and another planet-sized body. Although largely ignored for nearly a decade, the giant impact emerged as the leading hypothesis at a famous conference on the Origin of the Moon held in Kona, Hawaii in 1984. Since then, this hypothesis has been developed via 1) numerical simulations of the impact event 2) theoretical calculations of the post-impact disk evolution and 3) precise measurements of the isotopic composition of the Apollo samples, the putative products of this evolution. I will discuss how isotopes can be used as tracers to reveal the details of lunar formation, and present models that allow the isotopic data to be interpreted in terms of the formation process. I will end by speculating about two ways in which the science of lunar origin can benefit from studies of extra-solar systems.

[Kaveh Pahlevan is a Bateman Postdoctoral Fellow at the Department of Geology and Geophysics at Yale University. He is interested in the origin of terrestrial planets and satellites. For his thesis work, under the direction of Dave Stevenson at Caltech, he studied isotopic exchange during the moon-forming impact.]

Mar 28

"Molecular Biophysics of Habitability"
John Scalo, University of Texas at Austin

This talk will outline the areas of physics that are (and are not) required for a viable theory for the origin of molecular functionality in exoplanet environments--quantum theory; N-body and rigid body dynamics; electrostatics; Langevin, Fokker-Planck and other stochastic models; fluid dynamics; and radiative transfer. I will try to avoid the need for audience biochemistry background by using astronomical analogies. The emphasis will be on the N-body problem that is the analog of the "protein folding problem" for RNA, but adapted to the origin of folding and functionality on early Earth and exoworlds. The focus on RNA-like worlds is motivated by the fact that a population of interacting RNA molecules remains by far the only viable quantitative scenario for the transition to life on Earth, supported by the discovery of vast RNA functional versatility, in vitro experiments starting with random sequences, and theoretical considerations. I will try to briefly cover a number of basic points. (a) Contrary to popular assumptions, energy sources beyond thermal fluctuations are probably unimportant for the transition to functionality. (b) Starting from the atomic level, a coarse-graining approach can circumvent the computational barrier to following tertiary folding of oligomeric systems for relevant (long) timescales, a technique that is useful only for theories of unevolved protobiomolecules. (c) Using this coarse-grained picture, a number of simple key length, time, and energy scales can be used to understand the physics of nucleic acid polymers on RNA exoworlds, and the results lead to rather severe observable consequences for detection of exoplanets in the "habitable zone." I show that the resulting folding timescales agree with existing experimental data on natural RNA molecules. (d) A key problem is the structure of an ion atmosphere around systems of cylinder-like helices, which neutralizes and shields the strong negative charges of nucleic acid polymers. I contrast the problem with that of stellar atmospheres or baryonic correlations in dark matter halos. (d) The model clarifies the fundamental significance of the polarizability of water for a transition to functional polymers, even though the thermal limits imposed by functionality greatly reduce the range of allowable water temperatures. The results are easy to generalize to protein folding and membrane stability, which, however, almost certainly must follow a world of RNA-like polymeric populations.

Apr 4

"The Sources of Reionization"
Andreas Pawlik, University of Texas at Austin

The reionization of the Universe, that is the transformation of the cosmic hydrogen from its initial neutral to its present highly ionized state a few hundred million years after the Big Bang, is a watershed event in the history of the formation and evolution of galaxies. In the next few years, upcoming telescopes such as the Low Frequency Array and the James Webb Space Telescopes will enable, for the first time, to directly probe galaxy formation during reionization and allow to address many of the open question that define the current frontier of the field. The perhaps single most important question concerns the nature of the sources that reionized the Universe. Present observations of high-redshift galaxies may already hold the answer to this question, but their interpretation is complicated by uncertainties in the rates at which the interstellar and intergalactic gas recombine. Cosmological simulations of galaxy formation and reionization are powerful tools for constraining these and other key quantities. In this talk I will present results from both large-scale cosmological simulations of reionization and zoomed cosmological simulations of the assembly of the first galaxies to investigate the role and nature of galaxy formation during reionization. I will discuss current constraints on the capability of observed galaxies to reionize the Universe.