SHAO Astrophysics Colloquium
Title: The Origin of the Astrophysical r-Process
Speaker: Timothy C. Beers (Univ. of Notre Dame)
Time: 3:00 pm, May 9 (Thursday)
Location: Lecture Hall, 3rd floor
Abstract: There are presently some 25 highly r-process-element-enhanced metal-poor (r-II) stars known in the Galactic halo, roughly twenty-five years after their first recognition. These stars exhibit enhancements of their r-process-element to iron ratios, relative to Solar ratios, by a factor of 10 to 100+ ([r-element/Fe] > +1.0). Despite their very low metallicities ([Fe/H < –2.0), these stars exhibit an apparently universal [r-element/Fe] pattern that is very well-matched to the Solar r-process pattern. As such, they have long been thought to provide fundamental information on the likely astrophysical site of the r-process. We describe a comparison of the observed properties of halo r-II stars with the remarkable recent detection of a large sample of r-II stars identified in the Ultra Faint Dwarf (UFD) galaxy Reticulum-II, and suggest that the UFD environment is the natural birthplace of essentially all r-II stars. This hypothesis has received support from the identification of lanthanide signatures in photometric and spectroscopic observations of the kilonova associated with the LIGO/Virgo neutron star merger discovery. A new large-scale effort to dramatically increase the numbers of recognized r-II stars (from ~25-~100-150) is now underway; current results will be reported on, including the identification of numerous new bright r-II stars, severa
Special Astrophysics Colloquium
Title: Why do active galactic nuclei vary?
Speaker: Christopher S. Reynolds (University of Cambridge)
Time: 9:00 am, May 9 (Thursday)
Location: Lecture Hall, 3rd floor
Abstract: One of the most basic characteristics of active galactic nuclei, or indeed any accreting system, is time variability. Yet the physics underlying much of the observed variability is still quite mysterious. I will discuss recent progress in our theoretical understanding of variability processes within accretion disks. We find that observed variability is NOT a trivial consequence of the accretion disk being turbulent, and instead we find that quasi-periodic dynamo processes and thermal instability within the disk are an essential ingredient. I shall discuss how these insights unify together our view of AGN with other accreting objects.
Title: Recent Advances in X-ray Observational Studies of Luminous Black Holes
Speaker: Andy Fabian (University of Cambridge)
Time: 10:30 am, May 9 (Thursday)
Location: Lecture Hall, 3rd floor
Abstract: New X-ray instruments on NuSTAR and NICER, together with XMM and other satellites are enabling much progress to made in observational studies of luminous accreting black holes. Recent work on reflection and reverberation will be reviewed, including both Active Galactic Nuclei and stellar-mass black hole systems. We are now able to map out much of the innermost accretion flow, measuring black hole spins and disk densities.
