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Research
Colloquium Series

All talks are held on Wednesdays in the STScI John N. Bahcall Auditorium at 3:30 p.m. preceded by tea at 3:15 p.m.

Please direct questions or comments to the colloquium committee . The 2014-15 committee members are Rongmon Bordoloi, Susana Deustua, Peter McCullough and Eileen Meyer.

STScI presents live and archived webcasting of talks and Colloquium Series.

Date Speaker/Title
Jan. 15 Jason Kalirai (Space Telescope Science Institute)
Title: The Stellar Graveyard of the Milky Way
Abstract: In old stellar populations such as globular clusters and stellar halos, the bulk of the initial mass function beyond the current main-sequence turnoff resides on the white dwarf cooling sequence. This end state of stellar evolution is difficult to observe, given the intrinsic faintness of white dwarfs and their lack of nuclear energy sources. Over the past decade, I have led a Keck study to uncover populations of white dwarfs in a wide range of stellar systems with well-established properties. In these environments, the properties of the white dwarfs (e.g., mass, temperature) can be uniquely linked to the properties of their progenitors. This work has now to the first global mapping of the initial-final mass relation -- a fundamental input into our understanding of the stellar evolution process for low and intermediate-mass stars.
A robust initial-final mass relation enables many related astrophysical studies that were not previously possible. My team has used the relation to develop new and precise methods to date the Galactic halo and the oldest globular clusters, to calculate the lifetimes and luminosities of stars in evolutionary phases such as the thermally-pulsing AGB, and to measure the mass loss and chemical yields from giant stars. In this talk, I will describe the past and future development of the initial-final mass relation, and how it has provided answers to several outstanding astrophysical problems.
Host: TBD
Jan. 23 Suzanne Hawley (University of Washington); Caroline Herschel Speaker
Title: The Kepler View of Stellar Flares
Abstract: Stellar flares are explosive, energetic events that occur every few minutes on some very active stars. I will first review the standard model for flares based on the solar analogy, and highlight significant differences seen in flares on lower mass stars. I will then discuss data from the Kepler satellite, which has provided an unprecedented set of long timescale, uninterrupted light curves, enabling previously impossible statistical examination of flare properties. In particular, we now have observations of several thousand flares on one extremely active M dwarf, more than on any star other than the Sun. The Kepler data also allow us to examine the flaring rates of magnetically inactive stars, which may have important implications for planets in those systems.
Host: Neill Reid
Feb. 05 Jack Burns (University of Colorado)
Title: The Dark Ages Radio Explorer (DARE)
Abstract: In the New Worlds, New Horizons in Astronomy & Astrophysics Decadal Survey, Cosmic Dawn was singled out as one of the top astrophysics priorities for this decade. Specifically, the Decadal report asked “when and how did the first galaxies form out of cold clumps of hydrogen gas and start to shine—when was our cosmic dawn?” It proposed “astronomers must now search the sky for these infant galaxies and find out how they behaved and interacted with their surroundings.” This is the science objective of DARE – to search for the first stars, galaxies, and black holes via their impact on the intergalactic medium (IGM) as measured by the highly redshifted 21-cm hyperfine transition of neutral hydrogen (HI). DARE will probe redshifts of 11-35 (Dark Ages to Cosmic Dawn) with observed HI frequencies of 40-120 MHz. DARE will observe expected spectral features in the global signal of HI that correspond to stellar ignition (Lyman-alpha from the first stars coupling with the HI hyperfine transition), X-ray heating/ionization of the IGM from the first accreting black holes, and the beginning of reionization (signal dominated by IGM ionization fraction). These observations will complement those expected from JWST. We propose to observe these spectral features with a broad-beam bi-conical dipole antenna along with a receiver and digital spectrometer. We will place DARE in lunar orbit and take data only above the farside, a location known to be free of human-generated RFI and with a negligible ionosphere. In this talk, I will present the mission concept including initial results from an engineering prototype in Green Bank, WV and western Australia which is designed to perform end-to-end validation of the instrument and our calibration techniques. I will also describe our signal extraction tool, using a Markov Chain Monte Carlo technique, which measures the parameterized spectral features in the presence of substantial Galactic and solar system foregrounds.
Host: Bob Hanish
Feb. 12 (CoolSci Talk)
Feb. 19 Jason Tumlinson (Space Telescope Science Institute)
Title: The Circumgalactic Medium: A New Window on Galactic Fueling, Quenching, and Recycling
Abstract: Why do some galaxies quench while others continue to form stars? Where is all the normal matter that galaxies should have, but don't? What happens to all the heavy elements that stars produce? The gas flows that feed galaxies and return their enriched products back to their environments are arguably the most important and least understood processes driving galaxy evolution. I will survey our group's results from Hubble's Cosmic Origins Spectrograph on the diffuse "Circumgalactic Medium" surrounding galaxies that reveal it to be a massive and richly structured medium with important roles as the mediator of galaxy accretion and feedback and a potential answer to some of these open questions about galaxies.
Host: Dave Soderblom
Feb. 26 No Colloquium; Bahcall Lecture on Feb. 27th
Feb. 27 Brian Greene (Columbia University); Bahcall Lecture
Title: Where Do We Stand with String Theory?
Abstract: This year marks three decades since the “first string theory revolution,” and so is a natural time to assess where the theory currently stands. After a brief review of string theory’s essential features, I’ll discuss the theory’s progress toward the long-held goal of providing a predictive and testable unified theory of matter and forces.
Host: Bob Williams
Mar. 05 Mary Putman (Columbia University)
Title: Gas Flows in Galaxy Halos and Clusters
Abstract: The flow of gaseous baryons into galaxies and galaxy clusters continues at z=0. I will provide an overview of this process and discuss the link between a galaxy's halo and disk, and the halo to the IGM in different environments. The interplay of various gas flows may be key to understanding the prevalence or lack of star formation in a galaxy.
Host: Andrew Fox
Mar. 12 Richard Ellis (California Institute of Technology)
Title: Observations of Star Forming Galaxies in the Heart of the Reionization Era
Abstract: Deep exposures with the Hubble Space Telescope (HST) have provided the primary evidence that star-forming galaxies were present in the first billion years of cosmic history. Sometime during this early period the intergalactic medium transitioned from a neutral gas to one that is fully ionized. How did this `cosmic reionization' occur and were star-forming galaxies the primary agents? The electron scattering optical depth inferred from cosmic microwave background observations suggests that reionization occurred sometime in the redshift interval z=20 to z=6 so probing the abundance, luminosity distribution and spectral properties of galaxies during this period holds the key to addressing these fundamentalquestions. Recent imaging with HST's Wide Field Camera 3 in conjunction with Spitzer photometry and Keck spectroscopy has provided important new insight into understanding when reionization occurred and the role of early galaxies in the process. I will review this progress and discuss the remaining challenges ahead of plans for future facilities such as TMT and JWST.
Host: Jennifer Lotz
Apr. 02 Licia Verde (Institute of Cosmological Sciences)
Title: The Importance of "Local" Measurements for Cosmology
Abstract: Most cosmological constraints and statements are model-dependent. There is therefore an immense added value in measuring if possible, at least some of these parameters locally and/or in a way that is independent of the cosmological model. For instance, direct measurements of the Hubble constant have long been a workhorse to reduce CMB-parameters degeneracies. The comparison of low and high redshift measurements is not just useful to break parameters degeneracies (which, again, must be done within a cosmological model) but also to test the underlying model itself.
For example, local direct measurements of the Hubble constant seem to be at odds with the latest CMB data when interpreted within the context of the LambdaCDM model. This conclusion arises from a model-dependent, parameter-estimation analysis but it could be used in the context of model testing or model selection. To do so I introduce a statistic (called Tension) that stems from the Bayesian evidence ratio and is suited to assess whether two posterior distributions from two different experiments are in tension or not. I apply it to the latest cosmological data, present the findings of this analysis and discuss the implications for cosmology.
Host: Mario Livio
Apr. 09 (No Colloquium: Phase 1 Deadline)
Apr. 16 Andrew Howard (University of Hawaii-Manoa)
Title: Earth-sized Exoplanets
Abstract: The Kepler Mission has taught us that Earth-sized planets in the Habitable Zones of Sun-like stars are common and that most Earth-sized planets have rocky compositions. Three recent results highlight these remarkable properties of Earth-size planets. First, our team measured the mass of the planet Kepler-78b, the only Earth-sized planet with a measured mass and radius outside of the Solar System. The bulk density of 5 grams per cubic centimeter suggests a rocky composition with an insubstantial atmosphere, similar to Earth. In a separate project, we showed that high densities are common for small exoplanets. Based on three years of Doppler measurements of the masses of dozens of Kepler planets, we showed that planets smaller than about 1.5 times Earth size are mostly rocky, while thick gas atmospheres envelop larger planets. Finally, we re-analyzed the Kepler photometry to determine an independent, calibrated catalog of planets, including Earth-sized planets in the Habitable Zones of Sun-like stars (GK dwarfs). We calibrated the survey completeness using injection-and-recovery tests of transit signals into Kepler photometry using our custom-built TERRA pipeline. Combining our planet catalog and completeness estimate we find that 22 percent of Sun-like stars have a planet that is 1-2 times the size of Earth orbiting in the Habitable Zone. Warm, Earth-sized planets appear to be common.
Host: Dave Soderblom
Apr. 23 Joan Centrella (NASA/Goddard Space Flight Center)
Title: Listening to the Cosmos –Revealing the Hidden Universe with Gravitational Waves
Abstract: After many decades of effort, the gravitational wave window on the universe is beginning to open. This talk will present an overview of developments across the gravitational wave spectrum, highlighting challenges in detection and the anticipated science bonanza that can transform our understanding of the dark side of the universe.
Host: Andy Fruchter
Apr. 30 No Colloquium; Spring Symposium
May 07 Mark Marley (NASA/AMES)
Title: Understanding the Directly Imaged Young Jupiters: Where Has All the Methane Gone?
Abstract: We are now on the cusp of the next great wave of exoplanet detection: direct imaging. This year promises to witness the first planets to be discovered by the GPI and SPHERE coronagraphs; surveys employing these instruments should eventually find dozens of new, young giant planets. These worlds, still warm from their formation, are bright in the near-infrared and offer favorable contrast with their primary stars. The first directly imaged planets, HR 8799 b,c,d, and e, are prototypical examples. In addition NASA is now studying the possibility of adding a coronagraph to WFIRST as well as smaller, more focused, direct imaging space telescopes that would characterize exoplanets in reflected light. The directly imaged giant planets discovered by these surveys will address a multitude of questions related to the origin, evolution, and atmospheric chemistry and physics of giant planets. In my talk I will give an overview of this expected science yield and as an example I will discuss what we have learned from the HR 8799 planets. I will highlight their unexpected lack of atmospheric methane--a surprise for such cool gas giants--and examine possible explanations.
Host: John Stansberry
May 14 No Colloquium;
May 21 Shri Kulkarni (California Institute of Technology)
Title: There is More Room Sideways
Abstract: Astronomy has been and is continuing to enjoy a golden phase. The light gathering power of successive generations of both ground- and space-based telescopes has steadily increased and thereby made it possible for astronomers to probe the distant universe and study the faintest objects such as extra-solar planets. I call this progression as the traditional or linear path for progress. However, strong forces that lie beyond academia can have and have had equally big if not larger effect on academia. Exponential progress in semi-conductor industry has enabled astronomers to build huge detectors and transmit, analyze and archive torrents of data. Improvements in the quality of detectors and methodology (due to funding by space agencies and weapons industry) have sustained this exponential trajectory. The next frontier is defined by the decreasing cost in manufacturing (optics, mechanical structures, motors). These three developments offer a distinct new path for astronomical investigation (as opposed to the traditional approach). I term this approach as lateral or sideways path to exponential progress. The sideways approach is less capital intensive than the traditional approach and is thus very well suited for this decade where astronomers will have to learn do more with less. I will review the recent successes of this approach with two examples (SDSS, PTF) and speculate on projects over this decade.
Host: Dave Soderblom
May 28 Kelsey Johnson (University of Virginia)
Title: Probing the Physical Conditions of Massive Star Cluster Formation with ALMA
Abstract: Observationally constraining the physical conditions that give rise to massive star clusters has been a long-standing challenge. Now with the ALMA Observatory coming on-line, we can finally begin to probe the birth environments of massive clusters. In this talk I will give an overview of recent ALMA observations of three galaxies in which we have identified candidate proto-super star cluster molecular clouds. In particular, I will focus on our team's ALMA observations of the Antennae Galaxies reveal the molecular clouds in this prototypical starburst in unprecedented detail. We confirm and characterize a proto-super star cluster as a compact CO(3-2) cloud. I will give an overview of the ALMA observations, discuss the properties of this extraordinary source, and put these results in the context of what we know about the formation of massive star clusters.
Host: Hugues Sana and Paul Goudfrooij