## List of Past Astronomy Colloquia : 01-Sep-1999 to 31-Dec-1999

Date:   Wednesday 8-Sep-99
Speaker:   Dr. Michael A'Hearn (University of Maryland)
Title:  "Deep Impact"

Date:   Wednesday 15-Sep-99
Speaker:   Dr. Paul Ricker (University of Chicago)
Title:  "Numerical Simulations of Galaxy Cluster Mergers"

Date:   Wednesday 22-Sep-99
Speaker:   Dr. Alice Harding (Goddard Space Flight Center)
Title:  "Magnetars: Neutron Stars in the Extreme"

The recent discovery of long periods and high spin-down rates in both soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) has transformed magnetars, neutron stars having magnetic fields above 10^{14} G, from theoretical fantasy into observational reality. Magnetars are the first known astrophysical sources to derive their primary radiation power from magnetic energy, which substantially exceeds their rotational energy. In addition, the physics of supercritical magnetic fields operates in a more exotic realm than even that of normal pulsars. I will discuss the history of and present observational evidence for magnetars, as well as radiation models and spin-down dynamics of the SGRs and AXPs.

Date:   Wednesday 29-Sep-99
Speaker:   Dr. Tim Miller (Bartol Research Institute)
Title:  "Neutrino Astronomy at the South Pole"

The field of neutrino astronomy has recently sparked great interest, thanks to the discovery of oscillations by the SuperKamiokande experiment. Several other experiments are currently or will soon come on line which hold great promise for interesting results in solar neutrino astronomy. In addition, new detectors are currently under construction that hope to detect much higher energy (~ 1 TeV) neutrinos from much more distant and powerful sources, such as active galactic nuclei and gamma ray bursts. One such telescope, the Antarctic Muon and Neutrino Detector Array (AMANDA), has been operating and under continual expansion at the South Pole for the past several years. I will give a brief overview of neutrino astronomy to explain how AMANDA compares to solar and atmospheric neutrino telescopes, then describe AMANDA's operating principles, potential sources, and current status and results. There will also be a short Antarctic travelogue/slideshow.

Date:   Wednesday 6-Oct-99
Speaker:   Dr. Jim Beatty (Penn State University)
Title:  "The Auger Project: An Observatory for the Highest Energy Cosmic Rays"

Date:   Wednesday 13-Oct-99
Speaker:   No colloquium this week; October Conference
Title:

Date:   Wednesday 20-Oct-99
Speaker:   Dr. Kim Weaver (Goddard Space Flight Center)
Title:  "Mapping the Accretion Structure in Seyfert Galaxies"

The accreting material near black holes in AGN can be studied via spectral features of X-ray reprocessing. I will discuss the status of observations of the accretion flow in Seyfert galaxies, with an emphasis on recent results from spectroscopy with ASCA and RXTE. It is becoming apparent that the accreting regions in AGN are highly complex and time variable. Current practical limits to our knowledge of the geometries of the accretion flow and the surrounding nuclear material will be highlighted.

Date:   Wednesday 27-Oct-99
Speaker:   No colloquium this week due to cancellation by speaker
Title:

Date:   Wednesday 3-Nov-99
Speaker:   Dr. Robin Canup (Southwest Research Institute)
Title:  "An Impact Origin of the Earth/Moon System"

The currently favored theory for the origin of the Earth/Moon system is the giant impact hypothesis. Recent work modeling the formation of the Moon from an impact-generated debris cloud has found that the efficiency of incorporation of disk material into a moon or moons is less than 50 percent. While stability analyses indicate that a single moon is the likely end state of accretion in an impact-generated disk, forming a single Moon with a lunar mass has proved more difficult. Recent hydrodynamic simulations of the impact event with greatly improved numerical resolution predict sufficiently massive debris disks for two impact scenarios: 1) an impact with twice the angular momentum of the current Earth/Moon system or 2) an impact that occurred when Earth had only about 65 percent of its current mass. Terrestrial accretion simulations suggest both scenarios may be plausible, and a newly developed scaling relationship suggests that impacts intermediate between these two cases may also yield the Moon. Current work has also addressed potential resonant interactions between the newly formed Moon and the inner protolunar disk, which may have provided the Moon with its initial ~ 10 degree orbital inclination.

Date:   Wednesday 10-Nov-99
Speaker:   Dr. David E. Smith (Laboratory for Terrestrial Physics)
Title:  "The Gravity and Topography of Mars"

Date:   Wednesday 17-Nov-99
Speaker:   Dr. Sara Heap (Goddard Space Flight Center)
Title:  "The He II Gunn-Peterson Effect"

The ultraviolet spectrum (1145--1720~\AA) of the distant quasar Q~0302--003 ($z=3.286$) was observed at 1.8~\AA\, resolution with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. The spectrum clearly delineates the Gunn-Peterson \ion{He}{2} absorption trough, produced by \ion{He}{2}~\Lya\, along the line of sight over the redshift range $z=2.78 - 3.28$.

We find that near the quasar, \ion{He}{2} \Lya\, absorption is produced by discrete clouds, with no significant diffuse gas; this is attributed to a \ion{He}{2} proximity effect'' in which the quasar fully ionizes He in the diffuse intergalactic medium, but not the He in denser clouds. By two different methods we calculate that the average \ion{He}{2} \Lya\, opacity at $z \approx 3.15$ is $\tau \geq 4.8$. In the Dobrzycki-Bechtold void in the \ion{H}{1} \Lya\, forest near $z=3.18$, the average \ion{He}{2} opacity $\tau=4.47^{+0.48}_{-0.33}$. Such large opacities require the presence of a diffuse gas component as well as a soft UV background spectrum, whose softness parameter, defined as the ratio of the photo-ionization rate in \ion{H}{1} over the one in \ion{He}{2} $S \equiv \Gamma^{\mathrm J}_{\mathrm HI}/\Gamma^{\mathrm J}_{\mathrm HeII}\simeq 800$, indicating a significant stellar contribution. At $z=3.05$, there is a distinct region of high \ion{He}{2} \Lya\, transmission which most likely arises in a region where helium is doubly ionized by a discrete local source, quite possibly an AGN.

At redshifts $z<2.87$, the \ion{He}{2} \Lya\, opacity detected by STIS, $\tau=1.88$, is significantly lower than at $z>3$. Such a reduction in opacity is consistent with Songaila's (1998) report that the hardness of the UV background spectrum increases rapidly from $z =3$ to $z = 2.9$.

Date:   Wednesday 1-Dec-99
Speaker:   Dr. Donald Lamb (University of Chicago)
Title:  "Gamma-Ray Bursts as a Probe of the Very High-Redshift Universe"

Date:   Wednesday 8-Dec-99
Speaker:   Dr. Lee Armus (SIRTF Science Center, Caltech)
Title:  "Ultraluminous Infrared Galaxies in the SIRTF Era"