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Astrophysics Colloquium


Fall 2009 - Coordinated by R. Humphreys
Held in Physics 210 at 3:35pm on Fridays
(unless otherwise noted)


September 18 Dr. Attila Kovacs, U Minnesota, Astronomy
Title: A Larger, Deeper Survey of Submillimeter Galaxies
Abstract: First results from a blind survey of galaxies at 870µm in the Chandra Deep Field South (CDFS) using the 12-m APEX telescope in Chile. The study offers both improved and new insight into the properties of distant submillimeter galaxies, which trace the major events of star formation throughout the volume of the visible universe.
Host: -
September 25 Dr. Giles Novak, Northwestern U
Title: Observing the Large-scale Magnetic Fields of Giant Molecular Clouds
Abstract: The role of interstellar magnetic fields in the process of stellar birth is at present very unclear. Observationally, one of the key goals is to determine the strength of the large-scale magnetic fields of giant molecular clouds. This is being addressed via Zeeman measurements of molecular lines as well as submillimeter polarimetric observations that probe magnetically aligned dust grains. For the latter technique, the main idea is to use the degree of order/disorder in the field as an indicator of the field strength, as was done for the diffuse Galactic field by Chandrasekhar and Fermi in 1953. I will review the current status of research on GMC fields and prospects for major advances with upcoming stratospheric observations.
Host: T.J. Jones
October 2 No Colloquium
Title: -
Abstract: -
Host: -
October 9 Dr. Terry Oswalt, Florida Inst. of Technology
Title: Fragile Binary Stars: Observational Leverage on Difficult Astrophysical Problems
Abstract: The early proper motion surveys conducted by Willem Luyten discovered thousands of so-called common proper motion binary stars. Such loosely bound pairs have separations ranging up to ~0.1 parsecs, implying extremely long orbital periods and no significant interaction between components. In many ways such "fragile binaries" are like open clusters with only two components of the same age. they provide a largely overlooked avenue for the investigation of many astrophysical questions. For example, the orbital distribution of fragile binaries with two long-lived main sequence components provides limits on the cumulative effects of the Galactic environment. In pairs where one component is evolved, the orbits have been amplified by post-main-sequence mass loss, potentially providing useful constraints on the initial-to-final mass relation for white dwarves. In addition, the cooling ages of white dwarf components provide useful limits on the ages of their main sequence companions, independent of other stellar age determination methods. This talk will summarize how fragile binaries provide useful leverage on these and other problems of astrophysical interest.
Host: R. Humphreys
October 16 Dr. Terry J. Jones, U Minnesota, Astronomy
Title: UM Astronomy, the LBT, and You
Abstract: -
Host: -
October 23 Dr. Robert Pepin, U Minnesota, Physics
Title: A Look at Stardust's Comet Cargo
Subtitle: Ed Ney wouldn't have been surprised, but many others were.
Abstract: -
Host: -
October 30 Dr. Robert Williams, STSci
Title: The Nova Outburst: Evidence for a New Paradigm?
Abstract: Spectroscopic observations of novae date back a century, and the fundamental nature of the outburst has been understood for 50 years. Yet, recent observations suggest a possible significant modification to the standard nova paradigm. A high-resolution spectroscopic survey of novae has revealed short-lived heavy element absorption systems near maximum light consisting of Fe-peak and s-process elements. The spectroscopic evolution of novae is interpreted in terms of two distinct interacting gas systems in which the bright continuum is produced by the outburst ejecta but absorption and emission lines originate in gas ejected by the secondary star in a way that may explain dust formation and X-ray emission from novae. The absorbing gas is circumbinary and it pre-exists the outburst. Its origin appears to be mass ejection from the accretion disk or secondary star, and it may be initiating the nova outburst.
Host: R. Gehrz
November 6 Dr. Jeffrey Larson, U.S. Naval Academy
Title: Mapping the Asymmetric Thick Disk: I. Field Star Distributions of the Hercules Thick Disk Cloud
Abstract: The Hercules Thick Disk Cloud (Larsen et al. 2009) was first initially discovered as an excess in the number of faint blue stars between quadrants I and IV of the Galaxy. The field stars responsible for the excess, are between 2 and 4 kiloparsecs from the Sun, 1.2 kpc above the Galactic plane, and the asymmetry feature or Cloud is kiloparsecs in length -- a major substructure in the Galaxy. The origin of the Cloud could be an interaction with the disk bar, a triaxial thick disk or a merger remnant or stream. To better map the spatial extent of the Cloud along the line of sight, we have obtained multi-color UBVR photometry for 1.2 million stars in 67 fields of approximately 1 square degree each. Our analysis of fields beyond the apparent boundaries of the excess rule out a triaxial thick disk as a possible explanation for the Cloud (Larsen et al., accepted). In this talk we present our results for the counts over all of our fields and characterize the size of the excess. Over the entire 500 square degrees of sky containing the Cloud, we estimate about a quarter of a million F/G type stars, bringing the estimated mass of the Cloud to over a million solar masses. Additionally, one of our quadrant IV fields contains a blue horizontal branch feature that implies that a large number of stars are clumped in a small range of distances. We have tentatively identified this clump with the Cetus Tidal Stream of Newberg et al. (2009).
Host: R. Humphreys
November 13 Dr. Timothy Beers, Michigan State U
Title: The Chemo-Dynamical History of the Milky Way as Revealed by SDSS/SEGUE
Abstract:

Although originally conceived as primarily an extragalactic survey, the Sloan Digital Sky Survey (SDSS-I), and its extensions SDSS-II and SDSS-III, continue to have a major impact on our understanding of the formation and evolution of our host galaxy, the Milky Way. The sub-survey SEGUE: Sloan Extension for Galactic Exploration and Understanding, excuted as part of SDSS-II, obtained some 3500 square degrees of additional broadband imaging, mostly at lower Galactic latitudes, in order to better sample the disk systems of the Galaxy. Most importantly, it obtained over 240,000 medium-resolution spectra for stars selected to sample Galactocentric distances from 0.5 to 100 kpc. In combination with stellar targets from SDSS-I, and the recently completed SEGUE-2 program, executed as part of SDSS-III, the total sample of SDSS spectroscopy for Galactic stars comprises some 500,000 objects.

The development of the SEGUE Stellar Parameter Pipeline has enabled the determination of accurate atmospheric parameter estimates for a large fraction of these stars. Many of the stars in this data set within 5 kpc of the Sun have sufficiently well-measured proper motions to determine their full space motions, permitting examination of the nature of much more distant populations represented by members that are presently passing through the solar neighborhood. Ongoing analyses of these data are being used to draw an increasingly clearer picture of the nature of our galaxy, and to supply targets for detailed high-resolution spectrscopic follow-up with the world’s largest telescopes. We discuss a few highlights of recently completed and ongoing investigations with these data.

Host: R. Humphreys
November 20 No Colloquium
Title: -
Abstract: -
Host: -
November 27 No Colloquium - Thanksgiving Holiday
Title: -
Abstract: -
Host: -
December 4 Dr. Judith Cohen, Caltech
Title: Going, Going, Gone - The Formation of the Galactic Halo
Abstract: A lot of new evidence has emerged in the past few years that provides clues to the formation of the Galactic halo. The ΛCDM model, so succesful in the cosmological context, failed spectacularly when initially applied to this problem, but recent work has largely resolved these discrepancies. I will discuss the newly emerging view of the relationship between the dwarf spheroidal satellites of the Milky Way, the Galactic globular cluster system, and the stellar halo of our galaxy.
Host: R. Humphreys
December 11 Dr. Farhad Zadeh, Northwestern U
Title: The Massive Black Hole at the Center of the Galaxy
Abstract: There has recently been a dramatic increase in our understanding of the supermassive black hole at the dynamical center of our galaxy, first identified as the nonthermal radio source Sgr A*. Stellar orbit measurements have shown a mass of four million solar mass coincident with Sgr A* and a luminosity which is several orders of magnitude lower than the Eddington luminosity. To study the underluminous nature of Sgr A*, we have been monitoring the variability of the emission from Sgr A* by making simultaneous multi-wavelength observations. I will present highlights of these measurements which are providing us with insights on the nature of the flow very near the event horizon as well as the emission mechanism in different wavelength bands. Time permitting, I will also discuss the origin of one or two discs of massive stars found within 0.5 pc of Sgr A* as well signatures of young massive star formation in the molecular ring orbiting Sgr A*.
Host: R. Humphreys