Astrophysics Science Division Colloquium Series
Schedule: October - December 2006

Astrophysics Science Division Colloquium Series
Schedule: October - December 2006

Through the courtesy of the speakers since 2004, most presentations are available on line.

Future schedules:

Past schedules:

  • 2006, Second Quarter
  • 2006, First Quarter
  • 2005, Fourth Quarter
  • 2005, Third Quarter
  • 2005, Second Quarter
  • 2005, First Quarter
  • 2004, Fourth Quarter
  • 2004, Third Quarter
  • 2004, Second Quarter
  • 2004, First Quarter
  • 2003, Fourth Quarter

  • Time: 3:45 pm (Meet the Speaker at 3:30 pm) - Location: Bldg 21, Room 183 - unless otherwise noted.
    To view the abstract of a seminar, click on the title.


    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
    1 2 3 4 5 6 7
    8 9 10 11 12 John Wise (Stanford) - Resolving the Formation of Protogalaxies 13 14
    15 16 17 18 19 Carter Hall (U. Maryland) - Searching for double beta decay with the Enriched Xenon Observatory 20 21
    22 23 24 Max Lyutikov (UBC) - Radiation transfer in neutron star magnetospheres: the binary pulsar and magnetars 25 26 27 28
    29 30 31 James Reeves (GSFC/JHU) - An Overview of Suzaku Results on Active Galaxies


    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
        1 2 3 4
    5 6 7 8 9 10 11
    12 13 14 Enrico Ramirez-Ruiz (IAS, Princeton) - Triggering Short Gamma-Ray Bursts 15 16 17 18
    19 20 21 Jason Link (GSFC) - Radio Detection of Ultra-High Energy Neutrinos with the ANITA Experiment 22 23 24 25
    26 27 28 No Colloquium 29 30


    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
          1 2
    3 4 5 Howard Bond (STScI) - HST Observations of the Light Echo Around V838 Monocerotis 6 7 8 9
    10 11 12 Leisa K. Townsley (Penn State) - Not Your Grandmother's HII Regions: An X-ray Tour of Massive Star-forming Regions 13 14 15 16
    17 18 19 No Colloquium 20 21 22 23
    24 25 26 No Colloquium 27 28 29 30

    Resolving the Formation of Protogalaxies

    John Wise


    Thursday, October 12, 2006

    Time - 3:45pm; Location - Bldg 2/Rm 8


    Numerous cosmological hydrodynamic studies have addressed the formation of galaxies. Here we expand the standard model of galaxy formation to include molecular hydrogen cooling and primordial stellar feedback with a suite of cosmological Eulerian adaptive mesh refinement simulations that resolve the Jeans length by at least 16 cells. We gradually introduce molecular hydrogen cooling, radiative transfer, metal enrichment, and radiative backgrounds to determine the importance of each process. In simulations that consider the standard galaxy formation model with only hydrogen and helium cooling, gravitationally unstable central objects with masses more than 10^5 solar masses within a radius of 1 pc form within dark matter halos ~10^8 solar masses. These cores do not fragment down in sub-solar scales and could form a massive black hole. We also observe that no rotationally supported disk forms before this central collapse. Then we introduce molecular hydrogen cooling while suppressing the residual electron fraction in order to restrict our focus to protogalaxies. Here protogalactic halos with masses greater than ~8 x 10^6 solar masses at z ~ 20 can cool and collapse. Due to the exponential nature of Press-Schechter formalism, this corresponds to an order of magnitude increase in protogalaxy number density at z = 20, and galaxy formation may start earlier than previously thought. Next we consider radiative feedback from primordial stars using time dependent adaptive ray tracing that is solved self-consistently with the hydrodynamics, chemistry, and radiative cooling. This technique retains the time derivative and is photon conserving. We follow more than 20 primordial stars as they photo-ionize their host halos and the surrounding few kpc and expel all baryons from their dark matter halos. The increased electron fraction in the relic HII regions of primordial stars causes molecular hydrogen to become more abundant and increases the number of primordial stars by a factor of a few. The dynamical and thermal feedback from primordial stars affect the protogalaxy in several ways, e.g. decreasing the baryon content, skewing the angular momentum distribution to higher values, increasing the temperature of accreting gas, and enriching the IGM with the first metals. Our results highlight the importance of the inclusion of primordial stars and molecular hydrogen cooling in high redshift galaxy formation models.

    Searching for double beta decay with the Enriched Xenon Observatory

    Carter Hall

    Department of Physics, University of Maryland

    Thursday, October 19, 2006

    Time - 11am; Location - Bldg 2/Rm 8


    Neutrinoless double beta decay has recently become a top priority for the global experimental neutrino physics program. Double beta decay has the potential to resolve the scale of the neutrino mass spectrum, and is also the only practical tool we have for understanding the particle/anti- particle nature of the neutrino. The Enriched Xenon Observatory (EXO) collaboration is developing sensitive searches for the double beta decay of Xenon-136. Our first experiment, EXO-200, will be the largest double beta decay experiment ever attempted by an order-of-magnitude, and is rapidly being constructed. We are also pursuing R&D to realize a system to tag the daughter barium nucleus of the decay using the techniques of single-ion spectroscopy. This seminar will summarize the current status of our work.

    Radiation transfer in neutron star magnetospheres: the binary pulsar and magnetars

    Maxim Lyutikov

    University of British Columbia

    Tuesday, October 24, 2006


    This talk will be separated into two parts.

    First, I will discuss eclipses in the binary pulsar system PSR J0737-3039A/B, where the faster pulsar A is eclipsed once per orbit. A simple model of eclipses based on synchrotron absorption on closed field lines of the slower pulsar B reproduces the complicated observed light curve down to intricate details and provides a direct proof of the long-standing assumption of dipolar magnetic fields of neutron stars.

    In a second part I will describe a model of non-thermal emission from magetars due to resonant cyclotron scattering of surface radiation in their magnetospheres. Applying the model to anomalous X-ray pulsar 1E 1048.1--5937 gives a fit just as good as less physically motivated "black body plus powerlaw" spectra and allows one to determine density and temperature of the magnetospheric plasma.

    An Overview of Suzaku Results on Active Galaxies

    James Reeves

    GSFC / Johns Hopkins University

    Tuesday, October 31, 2006


    I give an overview of early observational results from Suzaku on AGN. One of the primary goals of the AGN program was to utilize the excellent broad high energy bandpass of Suzaku in order to accurately study the iron K line and Compton reflection component in many AGN, while removing the ambiguity present in modeling the iron line profile. The Suzaku data show that both narrow and broad velocity components of the iron line are present in many Seyfert galaxies, while the Compton reflection component is detected with the HXD in several AGN. The observations show that at least part of the iron line and reflection hump appears to originate from the innermost accretion disk, with the narrow iron line core likely arising from distant matter (such as the molecular torus). Studies of the broad band spectral variability with Suzaku is revealing that in many AGN, the emission is least variable at higher energies. This may be attributed to the reflection component appearing to be less variable than the primary continuum emission. Gravitational light bending from near a Kerr black hole offers one explanation for the relative constancy of the reflection spectrum, although dilution of the variability by a distant reprocessor may be important in some AGN.

    Triggering Short Gamma-Ray Bursts

    Enrico Ramirez-Ruiz

    IAS, Princeton

    Tuesday, November 14, 2006


    Although they were discovered more than 30 years ago, short gamma-ray bursts are still a mystery. All that we can be confident about is that they involve compact objects and relativistic plasma. Current ideas and prospects are briefly reviewed. There are, fortunately, several new observations that could help clarify the issues.

    Radio Detection of Ultra-High Energy Neutrinos with the ANITA Experiment

    Jason Link


    Tuesday, November 21, 2006


    The ANITA (Antarctic Impulsive Transient Antenna) is an Antarctic NASA long duration balloon mission which will 'listen' for the impulsive radio chirp of ultra-high energy neutrinos interacting in the ice sheet. The payload is poised to launch any day now from Mc Murdo base in Antarctica. In this colloquium I'll discuss ultra-high energy neutrinos, Askaryan pulses, ANITA instrumentation and hardware, scientific results from the prototype ANITA-LITE flight, Antarctic penguins, and expected results for the full ANITA flight.

    Hubble Space Telescope Observations of the Light Echo Around V838 Monocerotis

    Howard E. Bond

    Space Telescope Science Institute

    Tuesday, December 5, 2006


    A "light echo" is one of the rarest and most beautiful phenomena in astronomy. The most spectacular light echo in history is occurring now around the previously unknown star V838 Monocerotis, and is being imaged regularly by the Hubble Space Telescope. A light echo is created when light from a sudden stellar brightening spreads out into space and illuminates nearby interstellar dust. Because of the detour that the light takes in going out to the dust, scattering off it, and then traveling to the Earth, the light arrives months or years after the light from the star itself. The light echo around V838 Mon, which had a sudden outburst in early 2002, leads to a direct geometric determination of the distance to the star through a novel technique based on polarimetric imaging. The resulting distance of 20,000 light-years shows that V838 Mon was temporarily one of the brightest stars in the entire Milky Way. Its outburst was of a type not seen before, thus posing a severe puzzle to stellar theoreticians. The puzzle is deepened by our recent discovery that V838 Mon belongs to a very young stellar cluster, lying at the outer edge of our galaxy. Perhaps more important to the non-specialist, however, the images of the light echo are among the most stunning obtained to date by the Hubble Space Telescope.

    Not Your Grandmother's HII Regions: An X-ray Tour of Massive Star-forming Regions

    Leisa K. Townsley

    Penn State University

    Tuesday, December 12, 2006


    Chandra and XMM-Newton are providing remarkable new views of massive star-forming regions, revealing all stages in the life cycle of high-mass stars and their effects on their surroundings. We will tour several such regions, highlighting physical processes that characterize the life of a cluster of massive stars, from deeply-embedded cores too young to have established an HII region to superbubbles so large that they shape our views of galaxies. Along the way we see that X-ray observations reveal hundreds of pre-main sequence stars accompanying the massive stars that power great HII region complexes. The most massive stars themselves are often anomalously hard X-ray emitters; this may be a new indicator of close binarity. These complexes are sometimes suffused by diffuse X-ray structures, signatures of multi-million-degree plasmas created by fast O-star winds. In older regions we see the X-ray remains of the deaths of massive stars that stayed close to their birthplaces, exploding as cavity supernovae within the superbubbles that these clusters created.

    Jerry Bonnell

    NASA Logo, National Aeronautics and Space Administration