University of Minnesota
University of Minnesota

Astrophysics Colloquia

University of Minnesota

Minnesota Institute for Astrophysics

Astrophysics Colloquium

Fall 2014 - Coordinated by Roberta Humphreys
Held in Physics 210 at 2:30pm on Fridays
(unless otherwise noted)

September 12 Dr. Jeff Larsen, USNA
Title: Reflections from a Glassy Sea: The Beauty, Hazard and Utility of The Near-Earth Asteroids as Examined through the Spacewatch Project
Host: R. Humphreys
September 19 Misel Lecture 9/16 SPA Colloquium 9/17
September 26 Dr. Marc Seigar, UMD
Title: New Approaches for Determining the Dark Matter Content in Spiral Galaxies

Our standard cosmological model, LCDM (Lambda Cold Dark Matter), reproduces the large scale structure of the Universe extremely well. However, on smaller, galaxy-sized scales, there are several issues. One of these is the so-called cusp-core problems, where LCDM predicts cuspy central density profiles on for dark matter halos of all mass scales. However, observational studies show that dwarf galaxies have lower than predicted densities and tend to have constant density cores. Another problem with LCDM is the Tully-Fisher zeropoint problem, which refers to the fact that standard models cannot reproduce the observed relation between galaxy luminosity and circular velocity (the Tully-Fisher relation) without over-producing the number density of galaxies at fixed luminosity. In this presentation, I will describe new methods we are pursuing to determine dark matter density profiles in galaxies, with a view to coming to a solution to the above issues.

Host: T.J. Jones
October 3 Dr. Nino Panagia, STSci
Title: Star formation as seen by low mass stars

We have undertaken a systematic study of pre-main-sequence (PMS) stars spanning a wide range of masses (0.5 - 4 Msun), metallicities (0.1 - 1 Zsun) and ages (0.5 - 30 Myr). We have used the Hubble Space Telescope to identify and characterise a large sample of PMS objects in several star forming regions in the Magellanic Clouds, namely 30 Dor, the SN1987A field and NGC1850 in the LMC and NGC 346 and NGC 602 in the SMC, and have compared them to PMS stars in similar regions in the Milky Way, such as NGC 3603 and Trumpler 14.
Thanks to a novel method that we have developed to combine broad-band (V,I) photometry with narrow-band Halpha imaging, we have determined the physical parameters (temperature, luminosity, age, mass and mass accretion rate) of more than 3000 bona-fide PMS stars still undergoing active mass accretion. This is presently the largest and most homogeneous sample of PMS objects with known physical properties, and includes not only very young objects but also PMS stars older than 10-20 Myr that are approaching the main sequence.
I will present the current results of our project, including the fact that the mass accretion rates are systematically higher in the Magellanic Clouds than in the Milky Way for stars of the same mass and age, scaling roughly with the inverse of the cube root of the metallicity. These results are bound to have important implications for, and constraints on our understanding of the star formation process in the Universe.

Host: C. Scarlata
October 10 Dr. Stephane Courteau, Queen's University
Title: Puzzles in the Structure of Disk Galaxies

Galaxies can be described in terms of their structure, dynamics, and stellar populations. Some very robust correlations between various galaxy structural properties, such as total luminosity, maximum circular velocity, and size show rather small scatter, hinting at well-regulated galaxy formation processes. A major challenge to understanding these scaling relations, and ultimately galaxy formation and evolution, is the elusive interplay between visible and dark matter. I will discuss the latest derivations of galaxy scaling relations and their link with modern cosmological models.

Host: C. Scarlata
October 17 Dr. Ed Guinan, Villanova University
Title: The Secret Lives of Cepheids: Surprising new Insights into Atmospheric X-UV Heating, Pulsation and Evolution of Classical Cepheids
Host: R. Humphreys
October 24 Dr. Enrique Rodriguez, U Texas—San Antonio
Title: Revealing the Core of Active Galactic Nuclei Through Infrared Polarimetry

Magnetohydrodynamical theories consider the torus of Active Galactic Nuclei (AGN) to be part of an outflow wind moving away from the central engine. In this framework, the torus is a particular region of the outflowing wind, where dusty and optically thick clouds are formed. The outflows are strongly related to the accretion rate and magnetic field strength, which play an important role in the creation, morphology and evolution of the torus. Through high-angular (~0.1"-0.6") infrared (IR, 1-13 μm) polarimetric observations, this talk presents (1) the role of the magnetic fields in the torus of AGN, and (2) the polarized synchrotron core of Cygnus A.

Although several models have been made to account for the outflowing dusty winds from the central engine, the magnetic field strength at the position of the torus remains poorly characterized. Through near-IR polarimetry using MMTPol at the 6.5-m MMT, the magnetic field strengths, accretion and outflow rates within the clumpy torus of NGC 1068 were estimated. These results provide information about the torus evolution within the AGN unified scheme. For the archetypical radio-loud AGN, Cygnus A, mid-IR imaging polarimetry using CanariCam (7.5-13 μm) on the 10.4-m Gran Telescopio CANARIAS revealed a highly polarized, 11+/-3% at 8.7 μm, unresolved core. Polarimetric modeling suggests that the MIR polarization arises from a synchrotron component from the pc-scale jet in the core of Cygnus A. This result represents the most compelling MIR polarization detection of synchrotron radiation from the pc-scale jet in Cygnus A.

Host: T.J. Jones
October 31 Dr. Ivan Linscott, Stanford
Title: The Radioscience Experiment (REX), on the New Horizons Mission to Pluto

New Horizons is the first, and likely the last spacecraft to explore Pluto in this century. Pluto is outbound on its eccentric, 248 year orbit and the New Horizons spacecraft is due to encounter the Pluto system in June 2015, while Pluto is still "warm" from its closest approach to the sun. In the suite of scientific instruments carried on the spacecraft is REX, the Radioscience experiment, an unusual pairing of an ultrastable oscillator with an on-board signal processor able to collect high fidelity recordings of X-band transmissions (uplinks) from earth refracted during occultation by Pluto and its largest moon Charon. Using Abel inversion, the temperature and pressure profiles of Pluto's (and possibly Charon's) tenuous atmosphere will be extracted from the refractivity. REX will additionally measure the radiometric temperatures of Pluto and its moons from their thermal emissions, and catch uplinks scattered from Pluto to find the surface roughness, and dielectric constant. Twice on the way to Pluto the spacecraft's view of earth was occulted by the earth's moon. In the spacecraft's nine-year voyage to Pluto no other object in the solar system provided any other occultation. While it is too soon to have any REX results form Pluto, the REX data from the earth's moon occultation has provided some of its own mysteries.

Host: C. Woodward
November 7 No colloquium this week.
November 14 Dr. Kevin Schawinski, ETH Zurich
Title: The complex ends of galaxy lives: star formation, black hole feedback and the fate of gas reservoirs.

Massive galaxies are broadly split into those forming stars on the main sequence, and those which are quiescent. The physical processes by which galaxies quench their star formation remain poorly understood. I analyze the properties of galaxies and track their evolutionary trajectories as they migrate from the blue cloud of star forming galaxies to the red sequence of quiescent galaxies via the 'green valley'. I show that there must be two fundamentally star formation quenching pathways associated with early- and late-type galaxies which are intricately linked to how hydrogen gas reservoirs are destroyed or shut off. In the quenching of late-type galaxies, environment (or halo mass) is a key parameter, while for early-types, an internal mechanism such as black hole feedback is more likely. I will present recent HI observations supporting this picture.

Host: L. Fortson
November 21 Dr. Steve Kawaler, Iowa State
Title: The Kepler Mission's Other Legacy: Space-based Asteroseismology

By awakening us to the amazing abundance of other planetary systems, the Kepler mission has been a landmark in advancing our understanding of the Universe. The basis for these discoveries is accurate photometry, with high duty cycle, of over 100,000 stars. Thousands of planetary candidates emerged through detection of rare transit events. The data corresponding to transit events, though, is far less than 0.1% of the total amount of data obtained. The remaining >99.9% of the data is an exquisite record of the time-domain behavior of an enormous variety of stars. These photometric time series enable us to probe the interiors of stars by revealing their normal modes of oscillation - a technique known as asteroseismology. In this talk I'll discuss the remarkable strides that asteroseismology has been able to make using the remarkable archive of Kepler photometry.

Host: R. Humphreys
November 28 No colloquium; U closed for Thanksgiving Holiday
December 5 Dr. Mark Giampapa, NSO/NOAO; Henry A. Erikson Lecture
NOTE: in Tate 170 3:30-4:30
Title: Perspectives on the Solar Cycle and Activity in Solar-Type Stars

The variability we observe in the Sun, as well as stars similar to the Sun is intimately related to the generation and emergence of magnetic fields. This process involves the interaction of convective motions, rotation and extant magnetic fields through a mechanism broadly referred to as the dynamo. Among the most visible manifestations of solar activity is the number of sunspots, which is modulated by the 11-year sunspot cycle. Similarly, solar-type stars exhibit periodic behavior analogous to the sunspot cycle on multi-year time scales along with the rotational modulation of localized regions of strong magnetic flux on their surfaces on multi-day time scales. Recently, the Sun has shown indications of a decline in activity relative to previous sunspot cycles in the modern era. Is the recent behavior of the solar cycle unusual and what do current observations indicate about the nature of the next solar cycle? Could the Sun be entering a "Grand Minimum" similar to the Maunder Minimum (1645 — 1715 AD) with its prolonged period of low sunspot activity? What is the nature of cycles and activity in solar-type stars and how do stellar cycles compare to the solar cycle? These are among the questions that will be addressed from the perspective of a solar-stellar astronomer.