Spring Quarter 2015

Date/Time		Visitor	Host
May 7   Tech F210   4:00pm		Jay Strader     Michigan State University     Black Holes in Globular Clusters Hundreds of stellar-mass black holes are expected to form in the early lifetime of a typical globular star cluster, with many predicted to be ejected through close interactions in the cluster core following mass segregation. However, the efficiency of this ejection process in real globular clusters is poorly understood. The existence of black holes in globular clusters would have broad implications for the demography of black holes, gravitational wave sources, and the evolution of globular clusters. Our group has initiated a survey using deep radio continuum, X-ray, and optical data to search for stellar and intermediate-mass black holes in Milky Way globular clusters. I will discuss the current status of black hole candidates in several clusters, planned follow-up observations, and empirical constraints on the relative abundance of neutron stars and black holes in globular clusters.	Fred Rasio and Sourav Chatterjee
June 3   Tech F160   11:00am		Pablo Marchant    University of Bonn    The Interplay of Internal Mixing, Tides and Mass Transfer in Massive Binaries Recent observations have shown that the majority of massive stars reside in close pairs, which will strongly interact through mass transfer. After interaction, a significant amount of these objects will appear as single stars, hiding their former nature to surveys that attempt to exclude binaries using radial velocity variations. Understanding observed populations of massive stars is then intrinsically related to an understanding of the integrated properties of binary systems. In this talk I will present results from simulations of grids of binaries appropriate for comparison to the LMC, including the effects of stellar rotation and tides. I focus on the occurrence of Nitrogen enrichment due to accretion of CNO processed material, and tidal spinup, both tracers of binary interaction, and demonstrate that rotational mixing plays only a minor role in the final post-interaction products. I will also show early results on the modelling of contact systems, allowing for a better assesment of the initial conditions for mergers.	Francesca Valsecchi

Winter Quarter 2015

Date/Time		Visitor	Host
Jan. 16   Tech F210   2:00pm		Maria Cunningham     University of New South Wales     The probability density function in molecular gas in the G333 and Vela C     molecular clouds The probability density function (PDF) is a simple analytical tool for determining the hierarchical spatial structure of molecular clouds. It has been used frequently in recent years with dust continuum emission, such as that from the Herschel space telescope and ALMA. These dust column density PDFs universally show a log-normal distribution in low column density gas, characteristic of unbound turbulent gas, and a power-law tail at high column densities, indicating the presence of gravitationally bound gas. We have recently conducted a PDF analysis of the molecular gas in the G333 and Vela C giant molecular cloud complexes, using transitions of CO, HCN, HNC, HCO+ and N2H+. The results show that CO and its isotopologues trace mostly the log-normal part of the PDF, while HCN and HCO+ trace both a log-normal part and a power law part to the distribution. On the other hand, HNC and N2H+ mostly trace only the power law tail. The difference between the PDFs of HCN and HNC is surprising, as is the similarity between HNC and the N2H+ PDFs. The most likely explanation for the similar distributions of HNC and N2H+ is that N2H+ is known to be enhanced in cool gas below 20K, where CO is depleted, while the reaction that forms HNC or HCN favours the former at similar low temperatures. The lack of evidence for a power law tail in 13CO and C18O, in conjunction for the results for the N2H+ PDF suggest that depletion of CO in the dense cores of these molecular clouds is significant. In conclusion, the PDF has proved to be a surprisingly useful tool for investigating not only the spatial distribution of molecular gas, but also the wide scale chemistry	Laura Fissel
Jan. 22   Tech F160   10:30am		Frederick Davies     UCLA     The Fluctuating Intergalactic Ionizing Background Across Cosmic Time Understanding the ionizing background of the universe is crucial to interpreting observations of intergalactic gas in the context of large-scale structure. The epochs of H and He reionization, currently under intense observational and theoretical investigation, set the boundary conditions for the propagation of ionizing photons in the universe. Using novel 1D and semi-numerical 3D calculations, we find that fluctuations in the ionizing background due to rare or clustered sources can be very important, in contrast to common assumptions in previous work. We show that fluctuations in the radiation field cause the mean free path of ionizing photons to vary, leading to large-scale correlations that may explain recent observations of the H and He Lyman-alpha forests following their respective reionization epochs.	Claude-André Faucher-Giguère
Jan. 28   Tech F160   12:00pm		Nicholas McConnell     University of Hawaii     Puzzles in Massive Galaxy Assembly Giant elliptical galaxies are the most massive stellar systems in the present-day universe. The full story of their origins has eluded astronomers, even while high-redshift surveys have unearthed statistical trends in star formation and galaxy growth across cosmic time. Are giant ellipticals simply the most massive participants in a common sequence of growth and quenching, or are they the relics of exceptional objects that formed violently in the early universe? I will describe ongoing deep spectroscopic surveys of nearby ellipticals, designed to reveal two of their most extreme components: their low-mass stellar populations and their supermassive black holes. (1) The Black Hole Safari will measure stellar kinematics and central black hole masses (MBH) in over 30 giant elliptical galaxies, spanning a range of cosmic environments. The resulting census of black holes in the local universe will assess whether the linear relation between MBH and stellar spheroid mass arises naturally from hierarchical merging, and it may shed light upon the origins of recently-discovered overmassive black holes. (2) The most massive elliptical galaxies exhibit an extremely bottom-heavy stellar initial mass function (IMF), at odds with galaxy formation models where they are assembled from mergers of lower-mass systems. One possible means of reconciliation would be the detection of radial gradients in the IMF. I will present new measurements of IMF-sensitive spectral features with unprecedented radial coverage, suggesting that massive ellipticals' extreme stellar populations are indeed confined to small radii.	Daniel Angles-Alcazar
March 4   Tech F227 A   11:00am		James Guillochon     Harvard     A Dark Year for Tidal Disruption Events The disruption of a main-sequence star by a supermassive black hole results in the initial production of an extended debris stream that winds repeatedly around the black hole, producing a complex three-dimensional figure. When this stream self-intersects, accretion onto the black hole begins; this process slows down some flares and naturally leads to a "dark period" in which the flare is not observable for months to years. In my talk I'll discuss how this affects which flares are the most likely to be found by present-day surveys, and how they suggest a large fraction of TDEs, perhaps as large as 90%, are not identified. This implies that the true rate of tidal disruptions may be significantly higher than what is suggested by the number in the observed sample.	Fred Rasio