List of Past Astronomy Colloquia : 01-Sep-2015 to 31-Dec-2015

Date:   Wednesday 25-Mar-2015

Date:   Wednesday 09-Sep-2015
Speaker:   Dr. Adam Burrows (Princeton)
Title:  Theoretical Models of Exoplanet Spectra, Atmospheres, and Evolution

To understand the photometric and spectral data being, and soon to be, obtained concerning exoplanet atmospheres and their character a variety of theoretical tools and techniques need to be developed. These include, but are not limited to, planetary atmospheres codes, thermochemical databases for equilibrium and non-equilibrium composition studies, evolutionary and structural modeling capabilities, and multi-dimensional circulation models. I will review primary and secondary transit theories, wide-separation giant planet evolution in the context of high-contrast imaging, and super-Earth and mini-Neptune evolution in an effort to find a unifying theme and approach to the exploration of not only the exoplanets investigated to date, but also those to be discovered.

Date:   Wednesday 16-Sep-2015
Speaker:   Dr. Rachel Ivie (AIP)
Title:  Differences between Women’s and Men’s Careers in Astronomy

The Longitudinal Study of Astronomy Graduate Students (LSAGS), a joint project of the American Astronomical Society (AAS) and the American Institute of Physics (AIP), emerged from the Women in Astronomy II conference held in Pasadena, CA, USA in 2003. At the conference, concern about possible differential attrition for women arose from the relatively high percentage of female junior (student) AAS members compared to the lower representation of women among astronomy faculty members.

The Committee for the Status of Women in Astronomy (CSWA) and the AAS Council concluded that a longitudinal study was needed to collect data about which variables affect career choices in astronomy and to determine whether any of these variables exert a disproportionate force on either sex. In 2007-08, the project team, under the direction of Rachel Ivie, Director of AIP’s Statistical Research Center (SRC), asked all graduate students in astronomy and astrophysics in the US (~1500) to complete the first survey. The LSAGS follows this same group of students as they leave graduate school and enter their careers. In 2012-13, more than 800 responded to the second survey, and most had completed PhDs. We plan a third round of data collection in 2015.

Results from the study show that there are statistically significant differences between women and men in dissertation research methods, PhD subfield, and ratings of graduate school advisors. We found that after graduate school, women were more likely than men to take a break from work of six months or longer and to have made accommodations for a partner’s career opportunities (the “two-body problem”). In turn, women were more likely to work outside the field of astronomy because they were more likely to have experienced the “two-body problem.” In addition, we found women were more likely than men to have experienced harassment or discrimination at school or work. Finally, even while controlling for employment sector and years since PhD, we found that on average, women have lower salaries than men. These findings have implications for programs seeking to increase the representation of women in astronomy and physics. Funded by National Science Foundation AST-1347723.

Date:   Wednesday 23-Sep-2015
Speaker:   Dr. Erin Kara (UMD-College Park)
Title:  X-ray reverberation mapping the inner accretion flow around supermassive black holes

The energy released from accretion on to a supermassive black hole has significant implications for the evolution of its host galaxy. Much of this energy is released in the form of radiative feedback that is concentrated within a few tens of gravitational radii from the central black hole. Therefore studying the inner accretion flow—at the intersection of infall and outflow—is essential for understanding how the feedback mechanism works and the effect it will have on the surrounding environment.

The aim of my research is to understand these extreme, relativistic environments through observations of X-ray reverberation mapping. Similar to Optical reverberation mapping, where time delays of days or weeks between the continuum and the emission lines from scattered light in Broad Line Region clouds map out kiloparsec scales, X-ray reverberation reveals time delays of tens of seconds, which map out submicroparsec scales in the accretion flow—well beyond the spatial resolution power of any instrument. This technique has just been discovered in the past 6 years, so in this talk I will give an overview of how the measurements are taken, and the discoveries and advancements in this quickly developing field. I will show how reverberation is breaking degeneracies in our physical models and how it is helping us understand the geometry and kinematics of the inner accretion flow with unprecedented sensitivity.

Date:   Wednesday 30-Sep-2015
Speaker:   Dr. Kerry Ann O'Meara (University of Maryland)
Title:  Advancing Graduate Student Agency: The Role of Departments, Students, and Faculty

In this presentation, Dr. O'Meara shares the results of several studies on conditions within academic departments, and the nature of advising relationships that matter to graduate student agency and success. She also shares conditions that constrain graduate student agency in career advancement, and are especially important for supporting women and under-represented minority graduate students.

Date:   Wednesday 07-Oct-2015
Speaker:   Dr. Johanna Teske (DTM)
Title:  The Bane and Boon of Binary Exoplanet Host Stars

The Kepler mission has revealed that most stars host at least one planet. We also know that almost half of stellar systems in the solar neighborhood belong to multiple-star systems. Thus, binarity must be considered if we are to characterize and understand as an ensemble the known exoplanets and exoplanet candidates. First, I will discuss why companions to exoplanet host stars can be a “bane” to exoplanet astronomers. Kepler transiting planet candidates rely on spectroscopic and imaging follow-up observations to rule out false positives and detect blended stars; such observations can change measured planet radii, and even rule out planetary status. Traditionally the two techniques have probed different host star companion parameters spaces, but how well, and under what conditions, do the planet host companion parameters derived from the two techniques agree? A new study by my colleagues and I tries to address whether we really need both types of observations to validate Kepler planets. Second, I will explain why companions to exoplanet host stars can be a "boon" to exoplanet astronomers. While initially suggested as a sign of accretion of H-depleted material onto the star, the giant planet-metallicity correlation is now established as a mostly primordial effect -- stellar composition affects planet formation. But is it still possible that planet formation may also alter host star composition? Previous studies hinted at a few cases of compositional differences between stars in binary systems, and now high-precision abundance analyses are exploring this possibility in systems known to host planets. I will discuss the important role binary host stars have to play in extending correlations between stellar composition and the presence/type of planets that form, including brand new (not yet published!) results.

Date:   Wednesday 14-Oct-2015
Speaker:   Dr. Marta Volonteri (IAP-Paris)
Title:  The first black holes in the first galaxies

Massive black holes, weighing millions to billions of solar masses, inhabit the centers of today's galaxies. The progenitors of these black holes powered luminous quasars within the first billion years of the Universe. The first massive black holes must therefore have formed around the time the first stars and galaxies appeared, and then evolved along with their hosts for the past thirteen billion years. I will discuss some aspects of the cosmic evolution of massive black holes, from their formation to their growth and the interplay between black holes and galaxies.

Date:   Wednesday 21-Oct-2015
Speaker:   Dr. Michael A'Hearn (University of Maryland)
Title:  Cometary Science from Rosetta

The Rosetta mission has returned more data from comet Churyumov-Gerasimenko than we have ever had before about a single comet. These data have spawned a very large number of papers, many of them still available only on-line and others not yet publicly available. A comparable volume of data is still to come. With the myriad of results to sort through, it takes time to sort out what we have really learned about comets in general and their role in understanding the early solar system. This will be a very preliminary attempt to highlight some interesting things about C-G and to sort out the big picture

Date:   Wednesday 28-Oct-2015
Speaker:   Dr. Rodolfo Montez (Vanderbilt)
Title:  Insights into Binary Stars, Stellar Winds, and Astrophysical Plasmas from X-ray Observations of Planetary Nebulae

Planetary nebulae (PNe) provide textbook examples of astrophysical plasma and shock processes and provide essential constraints for theories of stellar evolution and the chemical enrichment of the universe. The varied shapes of PNe reveal the actions of interacting stellar winds from the late stages in the life of intermediate-mass stars, and growing evidence suggests that many PNe are the products of interacting binary star systems. As a result, studies of PNe can yield insight into other astrophysical objects governed by binary processes, such as, low mass X-ray binaries and Type Ia supernovae. Best known for their ten thousand degree optical line emission, the Chandra X-ray Observatory has established that a fraction of PNe display extended X-ray emission from shock-heated plasmas of a few million degrees and that the central stars harbor hotter than expected point-like emission from plasmas that reach tens of millions of degrees. I describe the discoveries, insights, and questions raised by Chandra observations of PNe with emphasis on those results gleaned from the Chandra Planetary Nebulae Survey (ChanPlaNS), which is the first systematic X-ray survey of PNe in the solar neighborhood.

Representative image of my research:

Head shot of me:

Date:   Wednesday 04-Nov-2015
Speaker:   Dr. Adam Kowalski (NASA/GSFC)
Title:  The Optical and Near-Ultraviolet Continuum Emission in Stellar Flares

Flares are thought to result from the reconnection and relaxation of magnetic fields in the upper layers (coronae) of stellar atmospheres. The highly dynamic atmospheric response produces radiation across the electromagnetic spectrum, from the radio to X-rays, on a range of timescales, from seconds to days. In this talk, I will focus on the observed continuum and emission line characteristics of flares in M dwarf stars in the optical and near-ultraviolet wavelength regimes. The hydrogen line and continua emission at these wavelengths contain a large fraction of the radiated energy during flares and critically constrain the heating mechanism(s) in the lower, dense stellar atmosphere (the chromosphere and photosphere). I will discuss new radiative-hydrodynamic flare models that reproduce the observed spectral properties around the Balmer jump and in the optical wavelength regime. I will also compare to new models of solar flares motivated by recent far- and near-ultraviolet spectroscopic observations from the Interface Region Imaging Spectrograph (IRIS).

Date:   Wednesday 11-Nov-2015
Speaker:   Dr. Lou Strolger (STSci)
Title:  High Redshift Supernovae: Beyond The Epoch of Dark Energy

For nearly two decades the Hubble Space Telescope has been heavily used to locate supernovae in high redshift environments, with the primary goal of improving constraints on the nature of dark energy. Along the way we have made surprising observations on the nature of supernovae themselves, and clues to their elusive progenitor mechanisms, some of which are difficult to reconcile with observations at much lower redshift. From complete volumetric supernova rate histories, that for the first time extend to z > 2, we find type Ia supernova delay-time distributions are consistent with a power law of index -1, but with the fraction of prompt (t_d < 500 Myr) much less than expected from various ground-based surveys. Core collapse supernova rates trace the cosmic star formation rate history, but require stellar progenitors more massive than has been seen in deep studies of nearby events (M > 20 M_sol). I will also detail our current campaigns on clusters of galaxies (RELICS and the Frontier Fields), where gravitational lens magnification provides a real potential for locating the first, primordial supernovae, while also providing useful constraints on the mass models of the foreground gravitational lenses.

Date:   Wednesday 18-Nov-2015
Speaker:   Dr. Fabienne Bastien (Penn State)
Title:  Convection in Cool Stars, as Revealed through Stellar Brightness Variations

As a result of the high precision and cadence of surveys like MOST, CoRoT, and Kepler, we may now directly observe the very low-level light variations arising from stellar granulation in cool stars. Here, we discuss how this enables us to more accurately determine the physical properties of Sun-like stars, to understand the nature of surface convection and its connection to magnetic activity, and to better determine the properties of planets around cool stars. Indeed, such sensitive photometric "flicker" variations are now within reach for thousands of stars, and we estimate that upcoming missions like TESS will enable such measurements for ~100 000 stars. We present recent results that tie “flicker” to granulation and enable a simple measurement of stellar surface gravity with a precision of ~0.1 dex. We use this, together and solely with two other simple ways of characterizing the stellar photometric variations in a high quality light curve, to construct an evolutionary diagram for Sun-like stars from the Main Sequence on towards the red giant branch. We discuss further work that correlates “flicker” with stellar density, allowing the application of astrodensity profiling techniques used in exoplanet characterization to many more stars. We also present results suggesting that the granulation of F stars must be magnetically suppressed in order to fit observations. Finally, we show that we may quantitatively predict a star's radial velocity jitter from its brightness variations, permitting the use of discovery light curves to help prioritize follow-up observations of transiting exoplanets.

Date:   Wednesday 02-Dec-2015
Speaker:   Dr. Leslie Young (SWRI)
Title:  Pluto and its five moons as revealed by NASA's New Horizons spacecraft

From decades of observations, astronomers have known that Pluto is a dynamic and motley world in the third, icy zone of our solar system, with one large moon, Charon, and a puzzling collection of four smaller moons. To study these, New Horizons flies remote sensing instruments that operate at ultraviolet, visible, near-infrared, and radio wavelengths, and three in-situ instruments to measure the dust and plasma environment. A complex observing sequence was executed to study the atmospheres, surface composition, and geology of these bodies. Since the flyby of the Pluto system in July 2015, we know Pluto and its moons are even more fascinating than anticipated. Pluto's surface ranges from dark, cratered terrains to tall mountain ranges to geologically young ice flows. Charon sports a completely unexpected dark, reddish area near its north pole. Pluto's atmosphere is hazy out to hundreds of km -- particularly astonishing since Pluto itself is only about 1200 km in radius. I will discuss these and other revelations from the New Horizons spacecraft.

Date:   Wednesday 09-Dec-2015
Speaker:   Dr. Janice Lee (STSci)
Title:  "Star Formation in Dwarf Galaxies."

How might the process and measurement of star formation in "dwarf" galaxies, similar in metallicity, mass, and size to the Magellanic Clouds, be different when compared with more commonly studied massive galaxies like the Milky Way? Dwarf galaxies are metal-poor, typically have low gas and stellar densities, and do not have bulges or spiral structure. Thus, the dependence of star formation on gravitational stability, density, pressure and metallicity can be studied. I will review results based on the Local Volume Legacy (LVL) survey, which has provided UV-IR imaging of a complete sample within 11 Mpc of 258 galaxies of which >80% are dwarf galaxies, and provide a preview of work from the HST Legacy ExtraGalactic Ultraviolet Survey (LEGUS), which has recently obtained complete five band imaging in NUV, U, B, V and I, for 50 a representative sample of nearby galaxies.

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