Fall 2002 Astronomy Colloquia - University of Texas at Austin
September 3, 2002
University of Texas at Austin, Physics Dept.
"Direct Determination of the Mass of the Electron Antineutrino"
September 10, 2002
Institut d' Astrophysique Spatiale, Orsay
"Dust and CO Emission from High z Quasars"
The relation between the growth of the central black hole and the formation of the bulge stars is a fundamental issue for the formation and evolution of galaxies. Probing starburst activity in the host galaxies of high redshift quasars has therefore become a key area of observational cosmology. I will report on recent studies based on pointed (sub)millimeter observations of optically selected quasars. First, I will report observations at 1.2 mm of redshift 1.8 < z < 5.5 radio quiet quasars using MAMBO at the IRAM 30-meter telelescope and describe the main results of these studies. Second, I will review our current knowledge of the molecular gas in these sources and report recent detections of CO in high z quasars obtained with the Plateau de Bure interferometer. I will conclude with a presentation of how this field of research will evolve, with a particular emphasis on the perspectives offered by the Atacama Large Millimeter Array.
September 17, 2002
University of Texas at Austin
"Planetary Nebulae as Agents of Galactic Chemical Evolution"
Planetary nebulae represent the final stages of envelope loss for highly evolved stars of about 1-8 solar masses. Since the material constituting the nebula was once the envelope of an AGB star, these stellar outflows may contain material whose elemental composition has been altered by nucleosynthesis and convective dredge-up. Although astronomers tend to think of massive stars and supernovae as the primary drivers of cosmic chemical evolution, many of the heavier, trans-iron, nuclei are synthesized by slow neutron-capture (the s-process) in these lower-mass stars.
Recent advances in astronomical instrumentation have enabled us to directly observe some of the more abundant n-capture elements in planetary nebulae. I will report on recent ultraviolet, optical, and infrared detections of several elements in atomic number range 30 < Z < 40 and summarize the evidence for s-process enrichment in planetary nebulae. While many planetary nebulae are carbon-rich, in exceptional cases (e.g. low metallicity) they can also be sources of primary oxygen. I will also comment briefly on other constituents of planetary nebulae, particularly their molecular hydrogen content as observed in the infrared and ultraviolet.
September 24, 2002
Carnegie Institution of Washington
"Gas and Ice Giant Planet Formation"
Two very different mechanisms have been proposed for the formation of the gas and ice giant planets. The conventional explanation for the formation of gas giant planets, core accretion, presumes that a gaseous envelope collapses upon a roughly 10 Earth-mass, solid core that was formed by the collisional accumulation of planetary embryos orbiting in a gaseous disk. The more radical explanation, disk instability, hypothesizes that the gaseous portion of protoplanetary disks undergoes a gravitational instability, leading to the formation of self-gravitating clumps, within which dust grains coagulate and settle to form cores. Core accretion appears to require several million years or more to form a gas giant planet, implying that only long-lived disks would form gas giants. Disk instability, on the other hand, is so rapid (thousands of years), that gas giants could form in even the shortest-lived disks. Core accretion has severe difficulty in explaining the formation of the ice giant planets, unless two extra protoplanets are formed in the gas giant planet region and thereafter migrate outward. Recently, an alternative mechanism for ice giant planet formation has been proposed, based on observations of protoplanetary disks in the Orion nebula cluster: disk instability leading to the formation of four gas giant protoplanets with cores, followed by photoevaporation of the disk and gaseous envelopes of the protoplanets outside about 10 AU by a nearby OB star, producing ice giants. In this scenario, Jupiter survives unscathed, while Saturn is a transitional planet. These two basic mechanisms have very different predictions for gas and ice giant extrasolar planets, both in terms of their frequency and epoch of formation, suggesting a number of astronomical tests which could determine the dominant mechanism for giant planet formation.
October 1, 2002
University of Texas at Austin, Physics Dept.
October 8, 2002
California Institute of Technology
"Precision Measurement of Neutron Star Radii"
Neutron stars are the most compact objects known to exist in the universe. Their masses have been measured to exquisite precision, but there are no precise radius measurements. This stands in the way of making useful statements about the equation of state of neutron star matter. I will describe recent developments in the theory of neutron star emission in low mass X-ray binaries, and recent observations of these systems with the X-ray observatories Chandra and XMM, which lead us to an observational path forward to precision neutron star radius measurements.
October 15, 2002
University of Wisconsin
"A Twelve Step Program to the Primordial Helium Abundance"
The primordial abundance of helium is one of the fundamental constraints on models of Big Bang nucleosynthesis and the evolution of the early Universe. However, accuracies of order one percent are necessary to provide meaningful constraints! I will discuss the main steps necessary to achieve this level of accuracy and the principal sources of uncertainties remaining in the estimating the primordial helium abundance. I will then compare how the results from helium compare to the results from deuterium and from studies of the spatial variation of the cosmic microwave background.
October 21, 2002
New York University
"The Polarization of the Cosmic Microwave Background"
The anisotropies in the cosmic microwave background are expected to be partially polarized. A detection of this polarization should provide a wealth of information about our universe at both early and late times. Polarization will also provide the basis for several consistency check of our cosmological model. In this talk I will review the physical mechanism that is expected to induce the polarization and discuss the different effects that leave their imprint in the polarization signal.
October 29, 2002
University of Texas at Austin
"From Molecular Cores to Planet-forming Disks with SIRTF"
The SIRTF mission and the Legacy programs will be introduced, followed by a more detailed discussion of the Legacy program on star formation and protoplanetary disk evolution.
These topics will be placed in the context of recent advances in these areas, and prospects for post-SIRTF studies will be briefly addressed.
November 5, 2002
NASA/Ames Research Center
Modern theories of star and planet formation and of the orbital stability of planetary systems are described and used to discuss possible characteristics of undiscovered planetary systems. The most detailed models of planetary growth are based upon observations of planets and smaller bodies within our own Solar System and of young stars and their environments. Terrestrial planets are believed to grow via pairwise accretion until the spacing of planetary orbits becomes large enough that the configuration is stable for the age of the system.
Giant planets begin their growth as do terrestrial planets, but they become massive enough that they are able to accumulate substantial amounts of gas before the protoplanetary disk dissipates. These models predict that rocky planets should form in orbit about most single stars. It is uncertain whether or not gas giant planet formation is common, because most protoplanetary disks may dissipate before solid planetary cores can grow large enough to gravitationally trap substantial quantities of gas. A potential hazard to planetary systems is radial decay of planetary orbits resulting from interactions with material within the disk. Planets more massive than Earth have the potential to decay the fastest, and may be able to sweep up smaller planets in their path. The implications of the giant planets found in recent radial velocity searches for the abundances of habitable planets are discussed, and the methods that are being used and planned for detecting and characterizing extrasolar planets are reviewed.
November 12, 2002
Pusan National University, Korea
"Cosmic Rays in the Large Scale Structure"
In analogy to the interstellar medium of our Galaxy, the intergalactic medium may be filled with energetic particles and permeated by magnetic fields. Recent observations indicate that cosmic rays may be energetically as important as the hot gas in clusters of galaxies and magnetic fields up to several tens of microgauss may exist in the intracluster medium.
Here we consider two possible origins of cosmic rays:
1) accretion shocks and merger shocks which form due to the flow motions associated with large scale structure formation can act as efficient acceleration sites of cosmic rays via the first-order Fermi process.
2) cosmic-ray particles and magnetic fields can be ejected from black holes at the centers of AGNs.
We will present the results of cosmological hydrodynamic simulations which incorporate the evolution of cosmic rays, and discuss the dynamical importance of cosmic rays in clusters of galaxies and large scale structure.
November 19, 2002
Lawrence Livermore National Laboratory
"Rulers of the Andes, Children of the Sun"
As children of the sun god, Inti, the Inca ruled their empire. Their claim to this position was reinforced through mythology, and ceremonies involving a system of solar markers around the horizon of Cuzco. The remains of such solar markers have now been found, giving flesh to early Spanish accounts but many questions remain. This system required the support of other observations. Who made them and how? More importantly, how was this sky-watching activity used to support the Inca imperial system? By 1572 the last legitimate heir to the Inca crown had been executed, and many of the important shrines had been desecrated, or destroyed. A great deal of information on Inca social structure, ceremonial activity, and belief is lost. Still, through ethnohistoric accounts, and archaeological fieldwork, it is possible to piece together the sky-watching practices of the Inca, and understand some of its use in organizing their empire.