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2014 Hubble Fellows Symposium
Talk Abstracts

Listing of Talk Abstracts

Gas Dynamics in Protoplanetary Disks
Dr.  Xuening Bai (SAO)
The gas dynamics of protoplanetary disks is crucial for understanding many aspects of planet formation, yet its nature is still not well understood due to the complex microphysics of non-ideal magnetohydrodynamical (MHD) effects introduced by extremely weak level of ionization. The conventional picture of layered accretion considered only the effect of Ohmic resistivity under the framework of magnetorotational instability (MRI) driven accretion. Using numerical simulations that self-consistently take into account two other non-ideal MHD effects, namely the Hall effect and ambipolar diffusion, I will show that a new paradigm is emerging: In the inner disk up to around 10 AU, the MRI is suppressed entirely, leaving the region largely laminar with accretion driven by magnetocentrifugal wind. Beyond this radius, the disk becomes weakly turbulent, and angular momentum transport is likely to proceed through a combination of wind and the MRI, and eventually dominated by the MRI in the outer disk. Implications for disk evolution and planet formation will be briefly discussed.
Using Specific Star Formation Rates to Constrain Unobserved Galaxy Populations
Peter  Behroozi  (STScI )
We present a new method for inferring the evolution of galaxies in dark matter halos using their specific star formation rates. We use this method to predict stellar masses, star formation rates, and number densities of galaxies at 8
The Tiny Titans Survey: Cosmological Simulations and Multi-Wavelength Observations of Isolated Dwarf Groups
Dr.  Gurtina Besla (Columbia University)
Dwarf galaxies are the most numerous type of galaxy in our universe. These low mass, low metallicity galaxies encompass a wide range of morphologies, gas fractions and star formation properties. Yet it is unclear how or if these different subtypes are evolutionarily linked or how starbursts are triggered in such galaxies. Being low in mass, dwarf galaxies are more prone to environmental factors (tides, ram pressure stripping), which can facilitate morphological transformations. But is environment the only factor? What about dwarf galaxies in the most isolated environments? Do interactions between dwarf galaxies themselves play a role? I will outline preliminary results from our combined theoretical and multi-wavelength observational program (the Tiny Titans Survey) to constrain the merger sequence of dwarf galaxies in isolated environments.
The Stability of the Mass Fundamental Plane since z~2
Dr.  Rachel Bezanson (University of Arizona)
Once thought to be relics of a much earlier epoch, the most massive local galaxies are red and dead ellipticals, with little ongoing star formation or organized rotation. In the last decade, observations of their assumed progenitors have demonstrated that billions of years ago, massive galaxies were more compact and morphologically different, possibly with more disklike structures. The details of this observed evolution can place constraints on the physical processes that have driven massive galaxy evolution. I will discuss the structural and dynamical properties of massive galaxies through cosmic time. In particular, I will highlight the evolution of the mass Fundamental Plane, the empirical relation between the size, stellar mass surface density, and velocity dispersion. Despite the observed dramatic evolution in the sizes and morphologies of massive galaxies since z~3, I will show that quiescent galaxies lie on the mass Fundamental Plane out to z~2.
Dynamical modeling of tidal streams
Dr.  Jo Bovy (Institute for Advanced Study)
Tidal streams hold enormous promise as probes of both the large-scale structure of the Milky Way halo’s density distribution and its small-scale fluctuations. However, their practical use has been hampered by the absence of simple tools for modeling their structure in realistic galactic potentials. I will present a new framework for describing the dynamics of tidal streams that consists of simple models for the distribution of tidal debris in frequency--angle space which can be converted into observable coordinates through a novel method for calculating action-angle coordinates for any potential. I will discuss how to quickly calculate the average location of a stream, estimate its width, draw mock stream data, and how to evaluate the full probability distribution function as well as its moments. In addition to providing a computationally-efficient practical tool for modeling the dynamics of tidal streams, the action–angle nature of the framework helps elucidate in exactly what manner streams do not follow single orbits, how the observed width of the stream relates to the velocity dispersion or mass of the progenitor, and how the progenitors of “orphan” streams could be located.
Three-Dimensional Simulations of Core-Collapse Supernovae
Dr.  Sean Couch (University of Chicago)
Core-collapse supernovae are fundamentally three-dimensional. I will discuss the enormous differences between 2D and 3D CCSN simulations and how recent work has shown that 3D is a necessity for understanding the CCSN mechanism. In particular, the development of crucial instabilities and turbulence in the CCSN context is completely different between 2D and 3D. I will also highlight how the need for 3D extends all the way back to the CCSN progenitor evolution and what steps we are taking to better understand and describe the final minutes of a massive star’s life.
The Life and Death of Galaxy Groups
Dr.  Alis Deason (University of California, Santa Cruz)
Galaxy groups are the characteristic structures formed at the present epoch, and provide unique laboratories for the study of galaxy evolution and dark matter. I will describe the use of the "mass-gap" statistic -- defined as the logarithmic difference in halo or stellar mass between the two most massive members of a galaxy group -- as a probe of halo age and concentration. Using large samples of SDSS selected galaxy groups, I demonstrate how stellar mass-gap can be used as an observational inference of halo age, and discuss its relation with galaxy properties (e.g. star formation, compactness). Finally, I will discuss the implications of these recent results for 1) the nature of fossil groups, and 2) our own Milky Way galaxy.
Solar System History from the Asteroid Belt's Perspective
Dr.  Francesca DeMeo (Harvard College Observatory)
Asteroids and other small bodies are markers, like tiny beacons, relaying information about the initial temperature and composition conditions of our Solar System revealed by their surface compositions, as well as the Solar System’s evolution fossilized in the scattering record of these bodies. Today we are armed with major advancements from the past decade that have revolutionized the field of asteroids in areas such as discovery, physical characterization, meteorite links, and dynamical models. Based on tens of thousands of measurements from the Sloan Digital Sky Survey, in this talk I present a new compositional map of the asteroid belt that reveals a greater diversity of asteroids as a function of size and distance. This new map differs greatly from the maps created three decades ago that showed a smooth gradient with distance from the sun suggesting a "calm" Solar System history. I will review the state of current models in the context of this new distribution map that support a much more dynamic evolutional history of the Solar System.
Isolated Young Brown Dwarfs at the Exoplanet Mass Boundary
Dr.  Jacqueline Faherty (Carnegie Institution of Washington)
Young brown dwarfs and directly-imaged exoplanets have enticingly similar photometric and spectroscopic characteristics, indicating that their cool, low gravity atmospheres should be studied in concert. Similarities between the peculiar shaped H band, near and mid-IR photometry as well as location on color magnitude diagrams provide important clues about how to extract physical properties of planets from current brown dwarf observations. In this talk I will discuss systems newly assigned to 10-150 Myr nearby moving groups, highlight the diversity of this uniform age-calibrated brown dwarf (or planetary mass) sample, and reflect on their implication for understanding current and future planetary data.
Massive Galaxies in their Prime - Star Formation Quenching in Cosmological Simulations
Robert Feldmann (University of California, Berkeley)
Some galaxies at z~2-3 are puzzling characters. They are over-massive, under-sized, and mostly done with forming stars. I will discuss the lives of these galaxies from a numerical perspective focussing on the transformation processes that convert blue, star forming galaxies into red and largely quiescent ones.
Helium Atmosphere on Neptune-Sized Exoplanet GJ 436b Formed by Irradiation Driven Escape
Dr.  Renyu Hu (Jet Propulsion Laboratory)
Many Neptune-sized exoplanets in orbits smaller than Mercury’s are thought to have experienced extensive atmospheric evolution. The consequence of this evolution has not been observed. Recently, the atmosphere of a warm Neptune-sized exoplanet GJ 436 b has been found to be poor in methane and rich in carbon monoxide, contrary to theoretical predictions for a hydrogen-dominated atmosphere, while the constraints on the bulk density of the planet require an extensive low-density gas envelope. The conundrum may be resolved if most of the original hydrogen on GJ 436 b were lost via irradiation-driven escape, and the remnant atmosphere is mainly helium with trace amounts of hydrogen, carbon and oxygen. Using a recently established photochemistry-thermochemistry and radiative transfer model to simulate the thick atmosphere on GJ 436 b, we find that a helium atmosphere with a hydrogen elemental abundance of 10-4 would lead to spectral signatures consistent with observations. Such atmospheres, although not found on any Solar System planets, may well be common on Neptune-sized exoplanets around stars more than 7 billion years.
High-Redshift Quasars and Galaxies at the Epoch of Cosmic Reionization
Dr.  Linhua Jiang (Arizona State University)
In the last decade, we have witnessed great advances in detecting high-redshift (z>=6) galaxies and quasars using both space and ground-based telescopes. These distant objects provide powerful tools to probe the epoch of cosmic reionization. In the first part of my talk, I will present some physical properties of a large sample of spectroscopically confirmed galaxies at z>=6. These galaxies have extremely deep imaging data in the optical bands, HST near-IR bands, and the Spitzer mid-IR bands. The spectroscopic redshifts of the sample and the wealth of the multi-band data provide great advantages to measure their physical properties. I will then present our deep spectroscopic identification of z>7 galaxy candidates, and show what the results tell us about cosmic reionization. In the second part, I will talk about high-redshift quasars. I will mostly review our past and current searches of z>=6 quasars, their physical properties, and implications to reionization. Finally I will briefly mention our future work on this topic.
The Gold Rush
Mansi M Kasliwal  (Carnegie )
Advanced gravitational wave (GW) interferometers promise to routinely hear neutron star mergers later this decade. Seeing the electromagnetic (EM) counterpart would be a litmus test for whether these mergers are indeed the long sought site of r-process nucleosynthesis. However, the challenge is unambiguously identifying the predicted faint and fast EM counterpart in the coarse GW localizations. I present end-to-end simulations that leverage the sensitivity limit to the local universe. I present the rapidly growing inventory of transients in the local universe that are fainter, faster and rarer than supernovae. New classes of transients have bridged the luminosity gap between novae and supernovae and represent missing pieces in two fundamental pictures: the fate of massive stars and the evolution of compact binaries. The next frontier in gap transients is the discovery of an EM-GW merger. The surge of EM-GW excitement may literally be the 21st century gold rush!
Weighing the Milky Way using Tidal Streams of Globular Clusters
Dr.  Andreas Küpper (Columbia University)
The Milky Way hosts about 200 globular clusters which continuously lose mass into the Galactic tidal field. In addition, a largely unknown number of already dissolved globular clusters should have deposited their stars into the halo of the Milky Way. In surveys like SDSS these dissolution product show up as coherent structures tracing the clusters’ orbits around the Galaxy. We can use these observations to constrain the gravitational potential of the Galaxy and at the same time reconstruct the clusters' dissolution histories. I will demonstrate this using the most prominent of these dissolving clusters, Palomar 5. Joint efforts from NYU, Yale and Columbia are underway to collect more information on this exceptional tidal stream and improve its numerical modeling.
Discovery of a Thorne-Zytkow Object Candidate in the Small Magellanic Cloud
Emily Levesque (University of Colorado at Boulder)
Thorne-Zytkow objects (TZOs) are a theoretical class of star in which a compact neutron star is surrounded by a large, diffuse envelope. Supergiant TZOs are predicted to be almost identical in appearance to red supergiants (RSGs), with their very red colors and cool temperatures placing them at the Hayashi limit on the H-R diagram. The best features that can be used at present to distinguish TZOs from the general RSG population are the unusually strong heavy-element lines present in their spectra. These elements are the unique products of the star's fully convective envelope linking the photosphere with the extraordinarily hot burning region in the vicinity of the neutron star core. As part of a recent high-resolution spectroscopic search, my collaborators and I have discovered a TZO candidate in the Small Magellanic Cloud. It is the first star to display the distinctive chemical profile of anomalous element enhancements thought to be unique to TZOs. The positive detection of a TZO would provide the first direct evidence for a completely new model of stellar interiors, a theoretically predicted fate for massive binary systems, and never-before-seen nucleosynthesis processes that would offer a new channel for heavy-element production in our universe.
On the physical conditions of star-forming galaxies near and far
Dr.  Xin Liu (UCLA)
The emission-line ratios [O III]5007/H-beta and [N II]/H-alpha (BPT diagram) are routinely used for diagnosing the excitation mechanisms of ionized gas. Emission-line galaxies at z = 0 divide into two general sequences, depending on whether the gas ionization is dominated by young stars or by an active galactic nucleus(AGN). Recent observations show that star-forming (SF) galaxies at z > 1 are systematically offset from the z = 0 SF sequence on the BPT diagram. It remains controversial, however, what are driving the offset, and in particular what the precise roles of AGN excitation are. I will discuss a study combining near-infrared spectroscopy with multi-wavelength observations of a representative sample of SF galaxies at z = 1.0 -- 1.5. AGN excitation is negligible in most, if not all cases. Instead, the offset is primarily caused by differences in gas physical conditions including higher ionization parameters and higher electron densities, which root in an elevated specific star formation rate surface density at z > 1. The results establish a direct connection between the redshift evolution of the positions on the BPT diagram and that of the global properties for SF galaxies, which has only been suggested by models or by indirect evidence from local analogs of high-redshift galaxies. The results highlight the importance of accounting for the differences in physical conditions in modeling the cosmic evolution of the BPT diagram, and provide further evidence that typical high-redshift star formation is physically different from that occurring in low-redshift galaxies.
New Insights into Galaxy Cluster Evolution from the South Pole Telescope
Dr.  Michael McDonald (MIT)
In the past 4 years, the number of known galaxy clusters at z>0.5 has grown by a factor of >5, thanks primarily to Sunyaev Zel'dovich surveys such as Planck, the Atacama Cosmology Telescope, and the South Pole Telescope (SPT). Here, I present new results from an X-ray analysis of SPT-selected clusters in the SPT 2500 deg^2 survey, which has discovered hundreds of new galaxy clusters, the majority of which are at z>0.5. These new data allow us to address such outstanding issues as the cooling flow problem, the effects of AGN feedback and how these vary with time, and subhalo accretion at the virial radius. These studies are providing the most detailed constraints to date on the evolution of galaxy clusters on all physical scales, and will continue to improve with the next generation of surveys already upon us.
dispersion measure estimates to cosmological sources
Dr.  Matthew McQuinn (UC Berkeley)
Recently, Thornton and coworkers confirmed a class of millisecond radio bursts likely of extragalactic origin that is well-suited for estimating dispersion measures (DMs). In addition, there have been three papers in 2013 proposing schemes for measuring DM to time-steady incoherent sources, such as quasars. I will discuss why DM measurements to cosmological sources would be extremely interesting for studying the WHIM and galaxy formation. Unfortunately, DM cannot be measured towards time steady, incoherent sources, and rather a variable source is always required. I will discuss why.
Machine-Learning Enabled Stellar Classification and the Prediction of Fundamental Atmospheric Parameters From Photometric Light Curves
Dr.  Adam Miller (Jet Propulsion Laboratory/Caltech)
The falling costs of computing and CCD detectors has led to a great boon in wide-field time-domain surveys during the past ~decade, with several new surveys expected prior to the arrival of the Large Synoptic Survey Telescope (LSST). This observational boon, however, comes with a catch: the data rates from these surveys are so large that discovery techniques heavily dependent on human intervention are becoming unviable. In this talk I will detail new methods, which utilize semi-supervised machine-learning algorithms, to automatically classify the light curves of time-variable sources. Using these methods, we have produced a data-driven probabilistic catalog of variables found in the All Sky Automated Survey (ASAS). I will also present a new machine-learning-based framework for the prediction of the fundamental stellar parameters, Teff, log g, and [Fe/H], based on the photometric light curves of variable stellar sources. The method was developed following a systematic spectroscopic survey of stellar variability. I will demonstrate that, for variable sources, the machine-learning model can determine Teff, log g, and [Fe/H] with a typical scatter of ~130 K, 0.37 dex, and 0.25 dex, respectively, without obtaining a spectrum. Instead, the random-forest-regression model uses SDSS color information and light-curve features to infer stellar properties. The precision of this method is competitive with what can be achieved with low-resolution spectra. These results are an important step on the path to the efficient and optimal extraction of information from future time-domain experiments, such as LSST. We argue that this machine-learning framework, for which we outline future possible improvements, will enable the construction of the most detailed maps of the Milky Way ever created.
Short-Period Sub-Earths: Discovering the Smallest Extrasolar Planets with Accurate Stellar Classification
Dr.  Philip Muirhead (Boston University)
Short-Period sub-Earths are small extrasolar planets found to orbit low-mass stars. They may represent a link between the formation of planets in the solar system and the formation of satellites around gas-giant planets. The prototypical system is Kepler 42, consisting of three rocky exoplanets all orbiting a mid-M star, each with an orbital period of less than 2 days. I will present recent results from my spectroscopic survey of M-type stars observed by NASA's Kepler spacecraft, including the identification of two new candidate short-period sub-Earth systems orbiting mid-M dwarfs. I will also discuss a new survey of eclipsing binary stars which will provide accurate and precise stellar parameters for M dwarfs monitored by NASA's upcoming Transiting Exoplanet Survey Satellite (TESS) Mission, which will likely discover many more short-period sub-Earth systems.
Probing deaths of massive stars with hadronic emission from subphotospheres
Dr.  Kohta Murase (Institute for Advanced Study)
Gamma-ray bursts and supernovae are violent explosive phenomena in the Universe. Neutrinos can provide us with precious opportunities to probe these phenomena even deep under the photosphere, which cannot be seen by electromagnetic observations. In particular, nonthermal neutrinos from jets have been commonly considered assuming that cosmic rays are accelerated, but this is not guaranteed at subphotospheres. We derive general constraints for production of TeV-PeV neutrinos, and show that low-luminosity jets are more favored. The same argument is applied to supernovae, and relativistic supernovae and super-luminous supernovae are found to be promising sources. Finally, we discuss roles of neutron-loaded outflows and show the importance of quasithermal emission in the GeV-TeV range. The neutron-proton-converter acceleration mechanism further boosts particle energies, enhancing the detectability of GeV-TeV emission from gamma-ray bursts and supernovae.
Outflow Driven Turbulence in Molecular Clouds
Dr.  Stella Offner (Yale University )
The most fundamental property of molecular clouds is that they are supersonically turbulent. The origin of the turbulence is unknown, although it is suspected that kinematic feedback from stars may be partially responsible. In order to investigate the origin of the observed turbulence I present multi-physics, adaptive mesh refinement simulations modeling stellar feedback processes acting on molecular clouds. I focus especially on the influence of protostellar outflows, and I investigate the resulting turbulent energy injection, gas entrainment, and star formation efficiency.
The Diffuse Universe, The Object Problem, and Magnetized HI Fibers
Dr.  Joshua Peek (Columbia University)
Astronomy has always been a science of lists. From the ancient names for stars to modern databases, we understand our universe primarily through rows and columns. Our other modality is the study of the CMB -- a (mostly) Gaussian random field, described entirely by its power spectrum. I will discuss how new and upcoming data sets upset this interpretation paradigm, and propose some ways forward. As an example, I will present work on neutral features of the interstellar medium that we have been able to parse using image processing computer vision methods. For the first time we are able to use the pure shape information in the ISM to predict underpinning quantities: in this case, the interstellar magnetic field.
Observational Signatures of the Explosion Mechanism of Core-Collapse Supernovae
Dr.  Ondrej Pejcha (Princeton University)
The core of a massive star at the end of its life collapses and launches an outgoing shockwave. Simulations show that the shock wave evolves into a quasi-static accretion shock, which should eventually turn into supernova explosion when the neutrino luminosity from the collapsed core exceeds a critical value (the "neutrino mechanism"). However, realizing this scenario proves difficult in many simulations. We study the physics and parameter dependence of the neutrino mechanism. We quantify the connection between the steady-state isothermal accretion flows with bounding shocks and the neutrino mechanism and use it to derive new "antesonic" condition, which characterizes the transition to explosion over a broad range of parameters. The physics of the explosion mechanism and the progenitor structure are imprinted in the observed properties of the explosion and the compact remnants. We combine the formalism of the neutrino mechanism with time evolution of a range of progenitor stars. We construct a semi-analytic model of supernovae that ties together the mass function of the compact remnants with the rate of successful explosions and the distribution of explosion energies. All of these quantities can be constrained by observations. We use Bayesian analysis to compare the double neutron star mass distribution with supernova explosion models and find direct implications for the mechanism of supernova explosions.
Demographics of Gamma-Ray Burst Host Galaxies from z=0 to z>4
Dr.  Daniel Perley (Caltech)
Long-duration gamma-ray bursts are produced by the core-collapse of massive stars; their rate, redshift distribution, and host-galaxy demographics directly reflect the evolution of the cosmic star-formation rate and of the star-forming galaxy population over the course of cosmic history. I will summarize the results of a recently-completed Spitzer Large Program (supplemented by a large, ongoing ground-based effort) to survey the host galaxies of over 130 GRBs spanning from z=0 to z=4 and beyond. As a population, the sites of GRBs and the sites of most star-formation (as inferred from traditional galaxy surveys) show similarities but also important differences, confirming that an additional factor beyond the SFR alone - most likely metallicity - strongly influences the GRB rate out to at least z~2. Once this dependency is quantitatively understood, GRB host studies will provide unique constraints on the chemical evolution as well as the star-formation rate of the early universe.
As the Dust Settles: Using Time-Domain Observations to Reveal Cloud Structure in Substellar Atmospheres
Jacqueline  Radigan  (STScI )
The combination of condensate clouds and rapid rotation has long motivated searches for weather phenomena in ultracool (late-M, L and T) dwarf (UCD) atmospheres. Pioneering work in this field dating back as early as 1999 suggested that variability is quite common for UCDs. Yet these early studies were ambiguous: detections were often low-amplitude and/or lacking periodicity, and the mechanisms responsible remained unclear. Observations made in the past 5 years, utilizing continuous monitoring strategies, better instruments, and larger telescopes have demonstrated conclusive and surprisingly large near-infrared variability for a subset of brown dwarfs at the transition between L and T spectral types, suggesting a patchy distribution of silicate clouds in their atmospheres. Brightness variations as large as 25% on readily observable rotational timescales allow light curves of exquisite precision, worthy of detailed analysis, to be obtained from both ground and space based facilities. While the L/T transition is the realm of spectacular variability, recent space-based efforts have confirmed lower levels of variability for a substantial fraction of UCDs at all spectral types. I will describe how such multi-wavelength, multi-epoch observations are contributing to an emerging picture of cloud structure in brown dwarf and exoplanet atmospheres.
Supernovae in the Early Universe with HST
Dr.  Steven Rodney (Johns Hopkins University)
The explosion of a white dwarf in a binary system produces a Type Ia Supernova (SN Ia). These are powerful tools for measuring cosmic distances, but we still lack a complete understanding of how these systems explode. With the CLASH and CANDELS programs on the Hubble Space Telescope we have been pushing back the frontier of SN discovery, to an era when the universe was only ~3 Gyr old. With this unique high-z sample we are testing SN Ia progenitor models by measuring how many SN Ia explode promptly after formation. With the new HST Frontier Fields survey, we are also using gravitationally lensed SN Ia as a tool for examining the dark matter in massive galaxy clusters.
Glimpsing the Composition Distribution of Sub-Neptune-Size Exoplanets
Dr.  Leslie Rogers (California Institute of Technology)
The Kepler Mission, combined with ground based RV follow-up and TTV dynamical analyses, has revolutionized the observational constraints on sub-Neptune-size planet compositions. I present an updated planet mass-radius diagram, including new Keck radial velocity measured masses (or mass upper limits) for 42 small Kepler transiting planet candidates. From the observed planet mass-radius distribution, I theorize about why parts of the distribution are unpopulated and about whether this could be a signature of planet formation and evolution. I focus on the intriguing transition between rocky exoplanets (comprised of iron and silicates) and planets with voluminous layers of volatiles (H/He or astrophysical ices). Applying a hierarchical Bayesian model to the current sample of sub-Neptune-size Kepler planet candidates with RV follow-up, I constrain how the fraction of planets that are sufficiently dense to be rocky varies with planet radius. I find that the majority 1.6 Earth-radii planets are not rocky.
Feeding and Feedback in nearby Low-luminosity AGNs
Dr.  Roman Shcherbakov (University of Maryland)
Most galaxies in the Universe host low-luminosity AGNs. These systems exhibit a vast range of dynamical and radiative effects, which require high sensitivity and high angular resolution to study. The closest objects with the largest central supermassive black holes are revealing their secrets with the improvement of instrumentation. I will review recent progress on observing with Chandra and modeling of the accretion flow onset regions in Sgr A* and NGC3115. The hot accretion flow in these sources is shaped by the combined effects of (1) radius-dependent mass injection by stellar winds, (2) galactic gravitational potential, (3) small-scale feedback such as conduction, and (4) supernova feedback. The natural outcomes of modeling are the virial/supervirial gas temperature and inhibited accretion with shallow density profile.
Searching for Sub-Parsec Binary Supermassive Black Holes with Time-Domain Spectroscopy
Dr.  Yue Shen (Carnegie Observatories)
Sub-parsec binary supermassive black holes (BBHs) are predicted products of hierarchical galaxy mergers, and are of great interest to both galaxy formation studies and gravitational physics. Finding these systems in the EM domain, however, is challenging given the stringent spatial resolution requirement. Peculiar spectroscopic features of the broad emission lines of quasars have long been used to identify spatially-unresolved sub-pc BBH candidates. But followup spectroscopic monitoring of these candidates is required to confirm their binary nature. I will present our recent results in searching for sub-pc BBHs in quasars with this spectroscopic technique using repeated optical spectroscopy from SDSS.
Accretion Lags in Stellar-Mass Black-Hole Binaries
Dr.  James Steiner (Smithsonian Astrophysical Observatory )
Accreting black hole binaries show a strong correlation between X-ray and optical bands, but with a time delay of several weeks. In two systems, we employ these lags and develop a model for the accreting systems involving viscous-dissipation in the disk, X-ray heating of the distorted companion star, and emission from compact jets. By removing optical variability linked to the X-rays, our model can be used to recover dynamical information from outbursting black holes, a feature of particular importance for those exceptionally X-ray bright systems which balk traditional observational techniques.
Toward Understanding Proto-Planetary disk and Binary Star Formation
Dr.  John Tobin (NRAO)
The formation of disks and binary systems is generally thought to begin early in the star formation process. However, there have not been sufficient numbers of young protostars (Class 0 and I phases) observed with high enough resolution to determine when and where most binaries form and whether or not large disks are common at early times. I will present several case studies examining the forming circumstellar disks and binary systems using a combination of ALMA, CARMA, SMA, VLA, and Gemini data. Finally, I will present initial results from a 264 hour Jansky VLA large program that has observed all protostars in the Perseus molecular cloud (N ~ 80) with an ultimate spatial resolution of 15 AU. The results on the protostellar multiplicity frequency and separation distribution will significantly improve our understanding of the binary formation mechanisms and prevalence of close binaries and disks during the early stages of star formation.
Faint Dwarf Galaxies Beyond the Local Group Satellites
Dr.  Erik Tollerud (Yale University)
Satellite galaxies of the Milky Way and M31 are key elements of Near-Field Cosmology. That is, they provide a number of unique constraints on galaxy formation and LCDM. However, inferring general results from just two (possibly interacting) systems will always be concerning. I will describe efforts to understand faint dwarf galaxies beyond the satellites of the Local Group, how they might improve our understanding of the nearby satellites, and describe some challenges and surprises they may hold.
3D Model Atmospheres of White Dwarfs
Dr.  Pier-Emmanuel Tremblay (STScI)
Most stars become white dwarfs at the end of the nuclear combustion cycle. The study of these old remnants in clusters and the halo provides essential information about the first stellar populations in our galaxy. We have recently computed the first grid of 3D radiation-hydrodynamics simulations for hydrogen-atmosphere white dwarfs. The simulations have been employed to compute 3D spectra which were then used in the spectroscopic analysis of the white dwarfs in the Sloan Digital Sky Survey and the White Dwarf Catalog. The white dwarfs with a radiative and a convective atmosphere have derived mean masses that are now the same, in much better agreement with our understanding of stellar evolution. Our improved model atmospheres can now be connected with structure models to predict improved ages and pulsation properties for these degenerate stars.
The High Mass Stellar Initial Mass Function
Dr.  Dan Weisz (University of California, Santa Cruz)
The high mass stellar initial mass function (IMF) is fundamental to a wide variety of Galactic and extra-galactic astrophysics. Yet, despite its widespread importance, the high mass IMF slope remains surprisingly unconstrained. I will present preliminary measurements of the IMF slope above ~2 solar masses from resolved star counts in hundreds of young star clusters in M31. I will also discuss ongoing efforts to quantify the environmental sensitivity of the high mass IMF.
The Structure and Origin of Circumgalactic Gas
Dr.  Jessica Werk (University of California, Santa Cuz)
Judging by their luminous matter alone, galaxies appear to be severely baryon-deficient compared to the cosmological baryon fraction (f_b = 0.17, by mass). Specifically, over two thirds of the baryons expected to be in a Milky-Way-size galaxy halo are not found in the gas and stars in the disk of the galaxy. Based on observations with the HST Cosmic Origins Spectrograph, I have found that a significant fraction of the purportedly 'missing baryons' in luminous galaxies may lie in gas at roughly 10^4 K, at distances between 10 and 150 kpc from the galaxy -- in the so-called Circumgalactic Medium (CGM). Now, I am working to characterize the physical conditions of the circumgalactic gas, its two-dimensional distribution, and its relation to outflows from the halo host galaxy. Most of the work I will present is ongoing, and raises new questions about the origin and fate of the CGM.
Transitional Disks: Young Planets in Protoplanetary Disks?
Dr.  zhaohuan zhu (Princeton University)
Transitional disks are protoplanetary disks with gaps and holes. I will discuss if young forming planets are responsible for these gaps and holes. Hydrodynamic and magnetohydrodynamic simulations have been carried out to understand the gap opening process by young planets. Furthermore, dust particles have been implemented into these simulations to understand dust dynamics separately from the gas dynamics. Dust dynamics is important since most observations will only tell us the dust distributions in disks. Related processes, such as particle drift and settling in disks and particle trapping by vortices, will also be discussed.
Strong and Weak Lensing with HST: Review and Recent Advancements
Dr.  Adi Zitrin (California Institute of Technology (Caltech))
The science of both strong and weak lensing has advanced significantly since (and much thanks to) the launch of the Hubble Space Telescope (HST) ~two decades ago, with acceleratingly interesting results in recent years such as the detection and study of high-redshift galaxies at z~10 and beyond, due to the magnification power of cosmic lenses such as galaxy clusters. In my talk I will review the strong and weak cluster lens modeling and related science, particularly with HST, with an emphasis on recent advancements . I will talk about our contribution to the field and present results from the Cluster Lensing And Supernova with Hubble (CLASH) and Hubble Frontier Fields programs, as well as other independent results, with some expectations for the near future.