List of Past Astronomy Colloquia : 01-Jan-2013 to 01-Jun-2013

Date:   Wednesday 23-Jan-2013
Speaker:   Dr. Selma de Mink (STSci)
Title:  The Evolution of Massive Stars towards their Death: Rotation, Binarity and Mergers

Although they are rare and short-lived, massive stars play a major role in Universe. With their large luminosities, strong stellar winds and spectacular explosions they act as cosmic engines, heating and enriching their surroundings, where the next generation of stars and their planets are forming.

I will discuss recent developments in the massive star community triggered by new surveys and theoretical modeling, that are slowly changing our general picture of how massive stars live their lives. In particular, I will discuss examples of the effects of rotation and binarity can drastically change the properties of both stars (brightness, color, ionizing flux, chemical yields, X-rays etc.) as well as their final fate as core-collapse and pair-instability supernovae and gamma-ray bursts.

These developments call for a critical reconsideration of the implications for a wide variety of astrophysical problems where the classic stellar models are used, which do not account for these effects.

Date:   Wednesday 30-Jan-2013
Speaker:   Dr. Joseph Nuth (NASA)
Title:  Laboratory Astrophysics is Not an Oxymoron

There is no need to duplicate the exact conditions found in an astrophysical environment in order to perform experiments that yield interesting and very pertinent results. Laboratory astrophysics supplies the basic data required to model processes in astrophysical settings and occasionally reveals surprising behavior in physical systems that require more sophisticated explanations than initially expected. Three examples will be presented in order to demonstrate the utility of laboratory measurements in understanding astrophysical observations. First, laboratory studies designed to measure the vapor pressure of SiO molecules, the basic building block of silicate grains that condense in the outflows of oxygen rich stars will be presented. Second, experiments conducted to measure the efficiency of the growth of crystalline grains from atomic vapors will be described. Such grains might form at equilibrium in the atmospheres of AGB stars before a significant stellar wind has developed. These experiments were performed on NASA’s KC-135 Reduced Gravity Research aircraft to minimize the effects of thermally induced convection. Finally, a completely unexpected phenomenon observed during the condensation of Fe-Mg-SiO vapors will be discussed: condensate compositions controlled at metastable eutectic points in the MgO-FeO-SiO phase diagram. These experiments imply the formation of separate populations of iron silicate and magnesium silicate grains from a mixed vapor of iron, magnesium and SiO that would be found in typical stellar outflows and have interesting implications for observations of crystalline magnesium silicate minerals in high mass loss stars.

Date:   Wednesday 06-Feb-2013
Speaker:   Dr. Sally Oey (University of Michigan)
Title:  Massive stars in the field: Do they form there?

About one quarter of all OB stars exist in the field, rather than in star clusters. What is the nature of this field population of massive stars? A significant population of massive runaway stars is well documented to be ejected from clusters. But are all the field stars runaways? Our group has carried out a complete, spectroscopic survey of field OB stars in the SMC to better understand the nature of this ambiguous, but important population. I will present some results from our survey, including a comprehensive investigation of their IMF, and evidence that OB stars are indeed capable of forming in the field.

Date:   Wednesday 13-Feb-2013
Speaker:   Dr. Armin Rest (STSci)
Title:  An Astronomical Time Machine: Light Echoes from Historic Supernovae and Eruptions

Tycho Brahe's observations of a supernova in 1572 challenged the dogma that the celestial realm was unchanging. 438 years later we have once again seen the light that Tycho saw: some of the light from the 1572 supernova is reflected off dust and is only now reaching Earth. These light echoes, as well as ones detected from other historic events like Cas A and Eta Carinae's Great Eruption, give us a very rare opportunity in astronomy: direct observation of the cause (the explosion/eruption) and the effect (the remnant) of the same astronomical event. Furthermore, in some cases we can compare light echoes at different angles around a remnant, and thus investigate possible asymmetry in the explosion. I will discuss how the unprecedented view at these exciting events with light echoes allows us to unravel some of their secrets.

Date:   Wednesday 20-Feb-2013
Speaker:   Dr. Hal Levison (SWRI)
Title:  Tackling Some Issues in Planet Formation --- From Mars's Size to a Fast Formation of Neptune

The standard model of planet formation has difficulties explaining some of the features observed in our Solar System. Of particular note, it predicts that Mars should be as massive as the Earth. In addition, it has difficulty in building the cores of the giant planets before the nebula disappeared. Here, I will argue that current models of planet formation are missing two important processes - planetesimal-driven migration and collisional grinding. I will present new simulations that include these processes. Preliminary results suggest a heretofore unknown and radical mechanism for building the outer planets.

Date:   Wednesday 27-Feb-2013
Speaker:   Dr. Stella Offner (Yale University)
Title:  "Symbiotic Star Formation: Modeling the Complex Ecology of Molecular Clouds and Forming Stars"

The details of star formation are intimately related to the natal molecular cloud environment. This environment is in turn shaped by radiative and kinematic feedback from embedded forming stars. In this talk, I will present gravito-radiation-hydrodynamic simulations of clustered star formation. I will explore how protostellar radiation and mass outflows impact the stellar initial mass function, multiplicity, and molecular cloud evolution. I will discuss the importance of producing "synthetic observations", for example by modeling dust and CO line emission, in order to connect numerical results directly to observables.

Date:   Tuesday 05-Mar-2013
Speaker:   Dr. Claude-Andre Faucher-Giguere
Title:  The Physics of Galaxy Formation: Gas, Stars, & Black Holes

Galaxies most directly connect our origins in the Big Bang to the structure that we observe in the Universe today. Understanding the physics of galaxy formation and evolution is thus a fundamental problem, and a primary focus of current and upcoming astronomical observatories. I will first present predictions for how galaxies grow by accretion of gas from the intergalactic medium, with an emphasis on the predicted observational signatures. Recent spectroscopic observations have tentatively detected the predicted cold accretion streams, which will allow detailed statistical tests over the next few years. I will then discuss the physics of feedback from stars and black holes, which shapes the galaxies that we observe. In particular, I will present new physical models of feedback-regulated star formation in galaxies and of black hole-driven galactic winds. The models make predictions ideally suited to the capabilities of new observational facilities, including the Atacama Large Millimeter Array.

I will conclude by summarizing ongoing efforts to combine insights from analytic and numerical studies of feedback physics to significantly improve the predictive power of cosmological simulations, thus enabling a wide range of new galaxy formation and intergalactic medium science.

Date:   Tuesday 12-Mar-2013
Speaker:   Dr. Smadar Naoz (Harvard University)
Title:  The Origin of Retrograde Hot Jupiters

The search for extra-solar planets has led to the surprising discovery of many Jupiter-like planets in very close proximity to their host star, the so-called ``hot Jupiters'' (HJs). Even more surprising, many of these HJs have orbits that are eccentric or highly inclined with respect to the equator of the star, and some (about 25%) even orbiting counter to the spin direction of the star. This poses a unique challenge to all planet formation models. We show that secular interactions between Jupiter-like planet and another perturber in the system can easily produce retrograde HJ orbits. We show that in the frame of work of secular hierarchical triple system (the so-called Kozai mechanism) the inner orbit's angular momentum component parallel to the total angular momentum (i.e., the z-component of the inner orbit angular momentum) need not be constant. In fact, it can even change sign, leading to a retrograde orbit. A brief excursion to very high eccentricity during the chaotic evolution of the inner orbit allows planet- star tidal interactions to rapidly circularize that orbit, decoupling the planets and forming a retrograde hot Jupiter. We estimate the relative frequencies of retrograde orbits and counter to the stellar spin orbits using Monte Carlo simulations, and find that the they are consistent with the observations. The high observed incidence of planets orbiting counter to the stellar spin direction may suggest that three body secular interactions are an important part of their dynamical history.

Date:   Wednesday 13-Mar-2013
Speaker:   Dr. Shane Davis (University of Toronto)
Title:  Are Photons Movers and Shakers?

I will address the role radiation forces play in the dynamics of rapidly-star-forming galaxies. Can radiation accelerate high velocity outflows? Can radiation drive high-Mach-number turbulence? In particular, I will discuss efforts to assess whether instabilities limit how effectively the gas and radiation couple in these environments, focusing on results from numerical simulations. I will first describe some new tools that we have developed to evolve the coupled equations of radiation transfer and (magneto)hydrodynamics without having to rely on ad hoc prescriptions that are usually employed. I will then describe the Rayleigh-Taylor instability and discuss how it is modified by the presence of radiation. Our results confirm that this instability can limit the effectiveness of radiative acceleration in optically thick environments and suggest that it may reduce the efficiency of radiation feedback in star-forming galaxies.

Date:   Wednesday 27-Mar-2013
Speaker:   Dr. Michael Kuhlen (Berkeley)
Title:  Most Matter is Dark Matter, but that's not all that Matters

High resolution simulations of Galactic Cold Dark Matter halos reveal huge amounts of substructure, both in configuration space (subhalos) and in velocity space (tidal streams and debris flow). This substructure has important consequences for efforts aiming to detect non-gravitational signatures of dark matter in the sky and in Earth-bound underground laboratories. In this talk I will first briefly review predictions from ultra-high resolution dark matter-only simulations and the well documented small-scale challenges that plague such pure-CDM simulations when confronted with observational data. A crucial question is whether these discrepancies demand a departure from the cold and collisionless dark matter paradigm, or rather can be solved within the CDM framework through the inclusion of baryonic physics. To answer this question, it will be necessary to first robustly solve the galaxy formation problem, and in the bulk of my talk I will present some recent progress from my own efforts in this direction.

Date:   Wednesday 03-Apr-2013
Speaker:   Dr. Michael Boylan-Kolchin (UCI)
Title:  Near-Field Cosmology: Big Science With Small Galaxies

Dwarf galaxies are the most metal-poor and dark matter-dominated galactic systems known, and they therefore provide a unique window onto galaxy formation and the nature of dark matter. I will present predictions from N-body simulations based on the standard dark energy plus cold dark matter cosmological model for the abundance and structure of low-mass galaxies around the Local Group, highlighting potentially serious discrepancies between these predictions and observations. I will then discuss how baryonic processes or non-standard dark matter physics may modify this picture, and how the next generation of ground and space-based telescopes, coupled with more advanced numerical simulations, will vastly improve our knowledge. Finally, I will describe how studying the Local Group can guide our understanding of galaxy formation at cosmic dawn, and conversely, how observations of the high-redshift Universe can inform models of nearby galaxies.

Date:   Wednesday 10-Apr-2013
Speaker:   Dr. Kaitlin Kratter
Title:  Looking Forward: Binaries and their Planets:

Binary and multiple stellar systems are common outcomes of the star formation process. More importantly, they account for a disproportionately large fraction of our knowledge on a wide array of topics from stellar structure to cosmology. In this talk I will discuss the intersection of binary formation and planet formation. I will describe several formation mechanisms for multiple star systems, and then discuss the importance of finding planets that reside in binary systems. I will focus on circumbinary planets around main-sequence stars. Finally, I will examine the evolution of these and other hierarchical systems, showing how the interplay between dynamics and stellar evolution can produce odd-ball evolved binaries, and frequent stellar collisions.

Date:   Wednesday 17-Apr-2013
Speaker:   Dr. Will Grundy (Lowell)
Title:  Outer Solar System Ice Mineralogy and Remote Sensing

Visible and near-infrared spectral absorptions of frozen volatiles (H2O, CH4, N2, CO2, etc.) have been exploited for decades to identify and map the presence of these ices on solar system bodies. Many cosmochemically abundant ices are extremely colorful at these wavelengths, much more so than common silicates. Most have readily-identifiable spectral features, some with spectacular wavelength-dependent contrasts, ideal for detection using remote observations of spectral reflectance at wavelengths readily accessible to current technology. Yet remote sensing techniques for the details of silicate mineralogy are considerably more advanced at present, owing chiefly to the simplicity of studying them at room temperature. But mineralogy in volatile ices can also be complex and interesting, and the state of the art in remote sensing of cryogenic materials is rapidly advancing. A significant motivation for this progress is the impending flyby of the Pluto system by New Horizons in 2015. This talk will review the current state of remote sensing of cryogenic ices via reflectance spectroscopy supported by laboratory studies, and will also look ahead to what we can expect from New Horizons in 2015.

Date:   Wednesday 24-Apr-2013
Speaker:   Dr. Daniela Calzetti (University of Masschusetts)
Title:  May I have a Star Formation Rate with Dust on the side, please?

About 10 years after both Spitzer and GALEX were launched, and 4 years into the Herschel Space Telescope operations, we can finally link star formation rate indicators across the full UV-to-farInfrared wavelength range. The ultimate `enemy' of astronomers, dust, is now well along the way of being tamed and, actually, used. I will review progress in this area, both for global (galaxy-wide) and local (sub-galactic region) star formation rate indicators, with special attention to where challenges (i.e., dependencies on the stellar IMF, the star formation history, etc.) are still present.

Date:   Wednesday 01-May-2013
Speaker:   Dr. Eilat Glikman (Yale)
Title:  Dust Reddened Quasars: A Transitional Phase in Quasar/Galaxy Co-Evolution

Quasars are extremely luminous sources, powered by accretion of gas onto a supermassive black hole in the nucleus of some galaxies. Most of the >100,000 quasars identified in the literature have been selected using optical surveys through the "ultraviolet excess" (UVX) method. However, these samples are known to be incomplete and biased because of obscuration and anisotropic radiation. Furthermore, a population of dust-obscured, red quasars is predicted by merger-driven models of quasar/galaxy co-evolution. To test these models, I have conducted a survey for a large population of dust-reddened quasars identified by matching radio sources from the FIRST 1.4 GHz survey with the 2MASS near-infrared survey and selecting objects with red optical-to-near-infrared colors. Follow up spectroscopy of these candidates in the optical and/or near-infrared has identified 120 red quasars, defined as having at least one broad emission line and a reddening of E(B-V)>0.1. The sample spans a wide redshift range, 0.1 < z < 3 and reaches a reddening of E(B-V)<1.5. When corrected for extinction, red quasars are the most luminous objects at every redshift and the fraction of red quasars increases with luminosity. The properties of red quasars suggest that they are revealing a transitional phase where the heavily obscured quasar is emerging from its dusty environment prior to becoming a "normal" blue quasar. Based on the fraction of quasars that are in this red phase, I determine that its duration is 20% as long as the unobscured quasar’s phase: a few million years.

I will also present future and ongoing work to expand this sample to fainter flux limits and longer wavelengths. And I will describe additional observations focused on subsamples of these red quasars to better understand their role in the co-evolution of supermassive black holes and their host galaxies.

Date:   Wednesday 08-May-2013
Speaker:   Dr. John Tobin (NRAO)
Title:  From Filaments to Disks: Observing Solar System Formation

The formation of proto-planetary disks begins during the earliest phase of the star formation process, while the nascent protostar is still surrounded by a dense envelope of gas and dust. We are beginning to have views of the entire star formation process from 1 pc all the way down to ~10 AU scales using a variety of observational techniques. 8 micron shadow images from Spitzer to show that the dense envelopes around Class 0 protostars are generally morphologically complex, often being filamentary and asymmetric.The observed envelope structure indicates a likely origin in turbulent cloud structure rather than a quasi-static formation and increase the likelihood of fragmentation during collapse, forming close binaries. With sub/millimeter interferometers, CARMA in particular, we can zoom in on the envelope and resolve forming proto-planetary disks around Class 0 protostars for the first time in the Taurus and Perseus star forming regions. Moreover, we have used disk rotation to directly measure the mass of the protostar L1527 in Taurus. Probing deeper, we also find that binaries are forming during the protostellar phase with 100 AU separations, a strong indication that the disks themselves are fragmenting early-on. This lays the ground work for ALMA, which will be needed to make significant gains in the area of disk formation with vastly improved resolution and sensitivity. Most importantly, ALMA's ability to detect faint molecular lines will enable masses of a large number of Class 0 protostars to be measured for the first time.

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