Talks & Events
Special Seminars: 2009
"Supernovae, explosions, and big computers"
Accretion, Feedback, and Tidal Interaction Mechanisms in our Local Neighborhood
Characterizing the infall and outflow of gas and metals in galaxies is crucial for understanding the evolution of galaxies and the intergalactic medium. The study of these phenomena in local galaxies provides the most robust diagnostic tests for models of tidal interaction, galactic winds, and accretion of primordial material onto
galaxies, which are important for piecing together the history of matter and metals in the Universe. The Magellanic System with its high-velocity complexes connected to the Magellanic Clouds provides a laboratory where the techniques of gas-phase absorption line spectroscopy can be used to study the gas toward many individual stars and QSOs. Here I will present results on the Magellanic Clouds that have changed our views of these galaxies.
Enhancing Diversity in Astronomy in the Coming Decade
We briefly review the current status of ethnic minorities in the physical sciences: The underrepresentation of Black-, Hispanic-, and Native-Americans is an order of magnitude problem. We then describe in detail the Fisk-Vanderbilt Masters-to-PhD Bridge program as a successful model for effective partnerships with minority-serving institutions toward addressing this problem. The program couples targeted recruitment with active retention strategies, and is built upon a clearly defined structure that is flexible enough to address individual student needs while maintaining clearly communicated baseline standards for student performance. Students are deliberately prepared to transition into the Vanderbilt PhD program through active involvement in research experiences with future PhD advisers, coursework that demonstrates competency in core PhD subject areas, and frequent interactions with joint mentoring committees. Since 2004 the program has admitted 31 underrepresented minority students (60% female), with a retention rate of 94%. Recent research indicates that minority students are 50% more likely than non-minority students to seek a Masters degree en route to the PhD. In essence, the Bridge program builds upon this increasingly important pathway, with a dedicated mentoring process designed to ensure that the Masters-to-PhD transition is a successful one. Finally, we discuss larger demographic trends among minorities in higher education, and suggest promising approaches for significantly broadening participation of minorities in the coming decade
The evolution of the cosmic UV background photoionisation rate
We present a new determination of the hydrogen photoionisation rate due to the metagalactic UV radiation field, between redshifts z~2 and z~5. We have developed a novel and unbiased method to estimate this quantity, exploiting the distribution properties of the strength of the Proximity Effect in samples of high-quality quasar absorption line spectra. Our new method overcomes the systematic bias towards too high values of the UVB, inherent in the standard analysis technique of the proximity effect. We have applied our method to a sample of 40 high S/N, high-resolution quasar spectra taken with the ESO-VLT, finding that there is at best marginal evidence for a decrease of the UVB intensity between z~2 and z~4. With the same method we now have analysed a sample of 2000 quasar spectra from the SDSS, extending our redshift range up almost z~5 and greatly improving the statistical sampling rate. With this dataset we can estimate the UVB photoionisation rate independently for several redshift bins. We find that the UVB remains approximately constant up to the highest redshifts covered. Comparing our results with synthesis models of the UVB from the population properties of possible UV sources, we conclude that quasars do not contribute significantly to the UVB at and above a redshift of z~3. Our measurements are in excellent agreement with UV light from star-forming galaxies dominating the UVB around z~3-3.5, but current surveys appear to fall short of uncovering the required UV emissivity to account for a flat UVB at z>~4.
The Interstellar Boundary Explorer (IBEX): Mapping the boundary between the Sun and the cosmos with energetic neutral atoms
The Interstellar Boundary Explorer (IBEX) spacecraft has just completed the first skymaps of the heliosphere boundaries. The contact region between the solar wind and interstellar medium is imaged in maps of energetic neutral atoms (ENAs) in the energy range 10 eV to 6 keV, formed when energetic ions charge-exchange with interstellar neutrals. The spectra of the ENA emissions provide data on the energy distribution of the underlying ions from which they are formed. Seen in these maps are the effects of the heliosphere nose-tail morphology, where the nose is directed near the galactic center and the tail is directed towards Taurus and Orion. The flat spectra at high ecliptic latitudes follow the latitudinal dependence of the solar wind energy flux. IBEX has also made the first in situ measurement of interstellar oxygen atoms. Most surprising of all, a remarkable Ribbon of ENA emission arcs 300 degrees across the sky. The Ribbon traces the region where the interstellar magnetic field is perpendicular to the sightline (as viewed from the Sun). Forces resulting from the motion of charged particles through magnetic fields in the outer heliosphere appear to explain the Ribbon structure. makahiki > imprint 101509_ibex_abstract.
Summary and Recommendations from Women in Astronomy 2009
High Resolution X-Ray spectroscopy and Theoretical Interpretation of SN 1996cr
SN1996cr is the one of the five closest SNe to explode in the past 30 yr, yet lay undiscovered for many years. Due to its fortuitous location in the Circinus Galaxy at 3.7 Mpc, we have a wealth of serendipitous archival data available to piece together its early evolution. In addition to this, our team was awarded a 500,000 sec Chandra HETG observation of the SN, which was completed in late 2008-early 2009. In order to interpret this data, we have carried out hydrodynamic simulations, followed by computations of non-equilibrium ionization simulated spectra that can be directly compared with the observations. These calculations allow us to infer the evolution of the SN shock wave, and the abundances of the ejecta and surrounding medium to reasonable accuracy. The data imply that SN 1996cr exploded in a low-density medium before interacting with a dense shell of material about 0.03pc away from the star. In this talk we will discuss in detail the data, the abundance calculations and its theoretical implications. In particular we will show how these calculations allow us to probe the stellar mass loss in the very last (0.1%) phases of a massive star's life. SN 1996cr seems to be one of several recently discovered SNe whose emission reveals evidence of abrupt mass-loss in the very last stages of a massive star's life.