September 29, 2010 | 3:30 PM | RI 480 | Astronomy Colloquium X-rays and Planet Formation Eric Feigelson, Penn State
High-resolution X-ray observations of star forming regions show that magnetic reconnection flares are powerful and frequent in pre-main sequence solar-type stars. Well-defined samples in the Orion Nebula Cluster and Taurus clouds exhibit flares with peak X-ray luminosities L_x ~ 1e29-1e32 erg/s, orders of magnitude stronger and more frequent than contemporary solar flares. X-rays are emitted in magnetic loops extending 0.1-10 stellar radii above the stellar surface and thus have a favorable geometry to irradiate the protoplanetary disk. The fluorescent FeK 6.4 keV emission line and X-ray absorption directly indicates X-ray irradiation of cold material in some young systems; this is supported by observations of infrared lines from ionized neon and excited molecules from the outer disk layers. A tail of penetrating hard X-rays with energies ~10-30 keV is sometimes present. There is thus considerable empirical evidence that X-rays from the host star irradiate protoplanetary disks, heating and ionizing their outer layers, and likely penetrating to the midplane in some disk regions. As ionization fractions need only reach ~ 1e-12 to induce the magnetorotational instability and associated turbulence, X-rays may be the principal determinant of the extent of the viscous `active zone' and laminar `dead zone' in the layered accretion disk. It may be important for the dissipation of gas in older disks via photoevaporation. X-ray irradiation may thus play a major role in planet formation processes: whether particle growth occurs by settling or in turbulent eddies; whether turbulence inhibits rapid inspiral from headwinds; whether protoplanets suffer secular migration or random walk interactions; whether transition disks are quickly dissipated. The violent magnetic flares from young stars may also explain shock melting of chondrules and a spallogenic origin of some anomalous short-lived radioisotopes found in ancient meteorites.
October 13, 2010 | 3:30 PM | RI 480 | Astronomy Colloquium Asymmetric Planetary Nebulae: Emerging Paradigms and Related Frontiers of Magnetohydrodynamics Eric Blackman, University of Rochester
Many, if not all, post-AGB stars rapidly transform from spherical to a powerful aspherical pre-planetary nebula (pPN) outflow phase before presumably fading into a less powerful planetary nebula (PN). The pPNe outflows require engine rotational energy and a mechanism to extract this energy into collimated outflows. Just radiation and rotation are insufficient, but an interplay between rotation, differential rotation and magnetic fields seems promising, not unlike the presumed symbiosis of these ingredients in other jetted sources. Present observational evidence for magnetic fields in evolved stars is suggestive of dynamically important magnetic fields, but both theory and observation are rife with opportunity. I will discuss why magnetohydrodynamic (MHD) shaping and launch might arise in pPNe and PNe. Scenarios involving binary driven dynamos and accretion engines cannot yet be ruled out. One noteworthy paradigm involves accretion onto the primary post-AGB white dwarf core from a low mass companion whose decaying accretion supply rate powers the asymmetric pPN. Strategies for distinguishing different engine mechanisms is a topic of active research. The related physics of dynamos and accretion disks underpins MHD launch and shaping scenarios throughout astrophysics and I will summarize some progress and challenges in our evolving understanding of the underlying principles.
October 20, 2010 | 3:30 PM | RI 480 | Wednesday colloquium The Generation and Evolution of Cosmic Magnetic Fields Ellen Zweibel, U Wisconsin-Madison
Despite spectacular recent progress in cosmology, the origin of magnetic fields in the Universe remains unknown. I will review the evidence, emphasizing recent detections of extremely weak intergalactic fields, discuss the evolution of these fields over time, and talk about their effects on astrophysical processes.
October 27, 2010 | 3:30 PM | RI 480 | Astronomy Colloquium Probing Star and Planet Formation: Gas and Dust Within 1 AU of Pre-Main-Sequence Stars Joshua Eisner, University of Arizona
Planetary systems form out of disks of dust and gas that are remnants of the star formation process. The structure of these protoplanetary disks within 1 AU of their central stars has important implications for terrestrial planet formation, giant planet migration, and disk accretion. I will present spatially and spectrally resolved observations of gas and dust within 1 AU of young stars, with a focus on hydrogen gas at stellocentric radii smaller than a tenth of an AU. I will describe the new instrumentation and techniques that enabled these measurements, and discuss the resultant constraints on star and planet formation processes.
November 3, 2010 | 3:30 PM | RI 480 | Wednesday colloquium Coherent Neutrino-Nucleus Scattering: From Supernovas to Reactors Todd Hossbach, Pacific Northwest National Laboratory
Coherent neutrino-nucleus scattering is a non-controversial prediction of the Standard Model of Particle Physics that has yet to be experimentally verified. In the first part of the talk I will discuss a new research effort to search for this elusive process at a spallation source using a small CsI(Na) detector. I will show how this detector can be easily scaled to a size relevant for studies of fundamental neutrino properties and discuss its potential as a supernova-neutrino detector. The second part of the talk will focus on exploiting the coherent scattering process and a new type of low-noise large-mass germanium detector to monitor the emission of antineutrinos from operating nuclear reactors. Details of this new germanium detector will be presented and I will discuss how this technology has allowed us to perform one of the most sensitive light-WIMP searches thus far.
November 10, 2010 | 3:30 PM | RI 480 | Astronomy Colloquium Climate of Gliese 581g -- The First Potentially Habitable Extrasolar World Raymond Pierrehumbert, The University of Chicago
On 2010 September 30, Vogt et al. reported the detection of two new planets orbiting the M-dwarf star Gliese 581. One of these two planets, 581g, is in what is loosely known as the "habitable zone," where a planet can support an Earth-like climate given a suitable atmosphere. I will discuss the factors governing whether this potentially habitable planet is indeed habitable, and the prospects for detecting whether the planet actually has an atmosphere which would render it habitable.
November 17, 2010 | 3:30 PM | RI 480 | Wednesday colloquium Understanding the Cosmic Recombination Epoch Christopher M Hirata, Caltech
December 1, 2010 | 3:30 PM | RI 480 | Wednesday colloquium Exploration of the "Circum-galactic" Medium of Galaxies at High Redshift Chuck Steidel, Caltech
During the peak epoch of galaxy formation, the intergalactic medium is both the source of gas fueling star formation in forming galaxies, as well as the waste dump for the products of star formation and black hole accretion that are not retained by galaxies. By studying the "Circum-Galactic Medium", the region within a few hundred physical kpc of forming galaxies, one can begin to constrain the flow of baryons into and out of galaxies. At present, there is a puzzling discrepancy between observations and theoretical expectations whose resolution may be the key to unraveling the aspects of galaxy formation that are least well-understood.
December 8, 2010 | 3:30 PM | RI 480 | Astronomy Colloquium Self-Regulation of Star Formation Rates in Disk Galaxies Eve Ostriker, University of Maryland
Star formation rates depend on both the total available interstellar gas mass and the physical state of that gas and the local galactic environment -- including the stellar and dark matter gravitational potentials. In a multiphase disk, the relative proportion of mass in gravitationally bound clouds vs. the diffuse ISM depends on energy injected by star formation. This energetic feedback both heats gas and drives turbulence, and it can lead to a self-regulated star-forming state. I will discuss recent numerical work showing that multiphase, turbulent ISM simulations are able to reproduce observed star formation rates, provided that the disks' vertical structure is resolved down to ~pc scales. These results are also consistent with empirical relationships that have been found between midplane pressure and the star formation rate (or molecular gas fraction). I will also introduce a new theoretical model that predicts the star formation rate as a function of the total gaseous surface density and the midplane density of stars + dark matter. This prediction derives from requirements for maintaining thermal and dynamical equilibrium in the diffuse gas. In HI-dominated outer-disk regions, star formation rates increase until the thermal pressure in the two-phase ISM matches the dynamic pressure. In the central regions of galaxies, the total surface density of HI is limited due to the high cooling rate of vertically confined, high-pressure gas. Cooling cannot exceed the heating provided by UV from young stars; this leads to a saturation of the HI surface density, consistent with observations. Application of this thermal/dynamical equilibrium theory to a set of spiral galaxies shows excellent agreement between predicted and observed star formation rates, especially for flocculent galaxies.
October 1, 2010 | 12:00 PM | LASR Conference Room | Friday noon seminar The Atacama Cosmology Telescope: Spectrum and parameters from the 2008 Renee Hlozek, University of Oxford
The Atacama Cosmology Telescope (ACT) has mapped the microwave sky to arcminute scales. We will present the spectrum at 148 and 218 GHz from the 2008 season of the Southern Survey of the Atacama Cosmology Telescope. Improved map-making and efficient spectrum estimation allow us to recover the second through seventh acoustic peaks, and provide independent confirmation of the LCDM paradigm. After discussing the telescope and tests of the spectra, we will describe constraints on both primary cosmological parameters and secondary parameters from the small-scale power spectrum, including diffuse SZ emission and foregrounds.
October 8, 2010 | 11:30 AM | RI 480 (Special Location) | Friday noon seminar How uncertain is our interpretation of cosmic ray data at high energy? Ralph R Engel, Karlsruhe Institute of Technology (KIT)
After reviewing methods for deriving the primary energy and estimating the mass composition of cosmic rays from observables of extensive air showers, the current standard model of cosmic rays is discussed and the status of some speculations about exotic scenarios is presented. Possible interpretations and apparent discrepancies regarding the composition of cosmic rays at very high energy are addressed in some detail.
October 15, 2010 | 12:00 PM | LASR Conference Room | Friday noon seminar Precision Cosmology from Optical Galaxy Clusters Hao-Yi Wu, KIPAC, Stanford University
Dark energy constraints from future optical galaxy cluster surveys will depend on how various systematic errors are controlled. I will first focus on the observable-mass distribution, discussing self-calibration and follow-ups for constraining cluster mass. I will then talk about theoretical uncertainties, discussing how precise calibrations of the dark matter halo mass function, bias function, and assembly history using N-body simulations will impact precision cosmology.
October 22, 2010 | 12:00 PM | LASR Conference Room | Friday noon seminar Results from the ANITA Search for Ultra-High Energy Neutrinos Abigail Vieregg, UCLA
The ANITA (ANtarctic Impulsive Transient Antenna) experiment is a balloon-borne radio telescope, designed to detect coherent Cherenkov emission from cosmogenic ultra-high energy neutrinos with energy greater than 10^18 eV. The second flight of the ANITA experiment launched on December 21st, 2008, and collected data for 30 days. This new data set allows for the most sensitive search to date for GZK neutrinos, which would reveal information about the source of the highest energy cosmic rays. I will present the results from the second flight of ANITA and discuss calibration techniques, analysis methods, and background rejection.
October 29, 2010 | 12:00 PM | LASR Conference Room | Friday noon seminar Constraining the dawn of cosmic structure and the epoch of reionization with the 21 cm line Jonathan Pritchard, Harvard-Smithsonian Center for Astrophysics
The first billion years of the Universe contains the formation of the first galaxies and reionization. This period lies beyond the current observational frontier presenting challenges to theory and observation. Low frequency observations of the redshifted 21 cm line of neutral hydrogen will be key in developing our understanding of this period. In this talk, I will describe two aspects of the 21 cm signal from the period of ''cosmic dawn'': the global 21 cm signal and 21 cm fluctuations. I will discuss what can be learnt about the first galaxies and reionization from this technique and explore some of the challenges and opportunities ahead for the first observations.
November 5, 2010 | 12:00 PM | LASR Conference Room | Friday noon seminar Low Threshold Dark Matter Search with CCDs Juan Estrada, Fermilab
I will report on the status of DAMIC, a low threshold dark matter search being performed with CCDs. The technical advantages of using CCDs for this type of search will be presented and the recent results obtained in a shallow underground site at Fermilab will be discussed. Developed for improving this search in the future will also be discussed.
November 12, 2010 | 12:00 PM | LASR Conference Room | Friday noon seminar The IMF, the UV bump, and AGB stars: New Insights From Stellar Population Synthesis Charlie Conroy, Harvard-Smithsonian Center for Astrophysics
Stellar population synthesis (SPS) combines stellar evolution and atmosphere calculations, an initial mass function, a dust model, and a star formation history in order to `predict' the emergent spectrum of a galaxy. SPS techniques have proliferated in the past decade, resulting in the routine estimation of stellar masses, star formation rates, and metallicities of large samples of galaxies. In this talk I will focus instead on the basic SPS ingredients, including the IMF, dust properties, and evolution of AGB stars, and demonstrate that these uncertain inputs can actually be constrained directly from the integrated light of galaxies. By confronting a flexible SPS model with a variety of data, I will show that the low-mass IMF is not universal, the 2175A dust feature seen in our Galaxy also exists in typical star-forming galaxies, and that AGB stars contribute substantially less light in the near-IR than state-of-the-art stellar evolution calculations predict. Implications of these results will also be highlighted.
November 19, 2010 | 12:00 PM | LASR Conference Room | Friday noon seminar Simulations of the Magellanic Stream in a First Infall Scenario Gurtina Besla, Harvard Smithsonian Center for Astrophysics
Recent high precision proper motions from the Hubble Space Telescope (HST) suggest that the Magellanic Clouds are either on their first passage or on an eccentric long period (>6 Gyr) orbit about the Milky Way (MW). This differs markedly from the canonical picture in which the Clouds travel on a quasi-periodic orbit about the MW (period of ~2 Gyr). Without a short period orbit about the MW, the origin of the Magellanic Stream, a young (1-2 Gyr old) coherent stream of HI gas that trails the Clouds ~150 degrees across the sky, can no longer be attributed to stripping by MW tides and/or ram pressure stripping by MW halo gas. We propose an alternative formation mechanism in which material is removed by LMC tides acting on the SMC before the system is accreted by the MW. Both Clouds are modeled as N-body systems using cosmologically motivated density profiles and infall masses. The orbit of the SMC about the LMC is chosen such that resonances maximize the efficiency of LMC tides. Contrary to previous models, the orbit of the LMC about the MW is not assumed to be a free parameter and is instead determined by the HST proper motions. The N-body simulations demonstrate that it is possible to explain the origin of the Stream under the assumption that the Clouds have not been long term satellites of the MW. More generally, they show that gas stripping is expected to occur between any gas-rich dwarf galaxy pair.
December 3, 2010 | 12:00 PM | LASR Conference Room | Friday noon seminar Overcoming astrophysical uncertainties to constrain fundamental physics with LSS measurements Beth Reid, Lawrence Berkeley National Laboratory
Large scale structure (LSS) measurements complement CMB observations in constraining fundamental physics parameters, such as neutrino properties and non-Gaussianity. However, our uncertainties in how galaxies trace the underlying matter density field complicate our interpretation of LSS observations in terms of these fundamental parameters. I will discuss how these uncertainties were (partially) overcome in our analysis of the SDSS Luminous Red Galaxy sample. In the second part of the talk, I will show that a dependence of galaxy properties on its host halo's assembly history can introduce significant uncertainties in the interpretation of large-scale clustering in terms of the non-Gaussianity parameter f_NL.
November 16, 2010 | 2:00 PM | LASR | Open Group seminar Reconstructing the Power Spectrum From Lyman-Alpha Simeon Bird, Institute of Astronomy, Cambridge
Current analyses of the Lyman-alpha forest assume the primordial power spectrum of density perturbations follows a simple power law form. We have used a large suite of numerical simulations to calibrate a minimally parametric framework for the power spectrum, dropping this assumption. I shall discuss our methods, including our use of the statistical technique of cross-validation to avoid over-fitting the data. Finally, I shall present our results, using current Lyman-alpha data from SDSS, and our forecasts for BOSS data.
December 3, 2010 | 1:30 PM | LASR Conference Room | Open Group seminar Testing modifications of general relativity with large-scale structures Lucas Lombriser, University of Zurich
Modifications of general relativity provide an alternative explanation for the cosmic acceleration without invoking dark energy. A wealth of astronomical observations can be utilized to test the different theories of gravity. Of particular interest in this talk are constraints on f(R) and DGP gravity models, as well as on phenomenological modifications of general relativity from probes of the large-scale structure.