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2015 Hubble Fellows Symposium
Talk Abstracts

Listing of Talk Abstracts

Magneto-photoevaporation of Protoplanetary Disks
Dr.  Xuening Bai (Smithsonian Astrophysical Observatory)
Mass loss from protoplanetary disks is believed to play a major role in disk evolution and dispersal, and hence planet formation. Conventionally, the disk mass loss is attributed to photoevaporation, where heating from external UV and X-ray photons to the disk surface drives a thermal wind. However, it has recently been realized that PPDs are very likely threaded by external magnetic flux, and inevitably launch a magnetocentrifugal wind that drives disk accretion. We develop a 1D wind model of magnetocentrifugal wind with simple treatment of thermodynamics as a proxy for external heating. Our results show that based on the wind launching conditions expected from the disk, mass loss from PPDs deviates significantly from the conventional photoevaporation picture, with mass loss rate likely a significant fraction of the mass accretion rate. This is mainly because external magnetic field threading the disk is expected to be weak (to be consistent with disk accretion rate). Implications on disk evolution and future prospects will also be discussed.
Observing Convection in Cool Stars through Light Flicker
Dr.  Fabienne Bastien (Pennsylvania State University)
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. 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 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 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 using our evolutionary diagram, permitting the use of discovery light curves to help prioritize follow-up observations of transiting exoplanets.
Using Galaxy Pairs to Probe Star Formation During Major Halo Mergers
Dr.  Peter Behroozi (STScI)
We present an observational method to select galaxies whose host haloes are preferentially undergoing major mergers. Applying the method to central L^* (10^10 Msun < M_* < 10^10.5 Msun) galaxies in the Sloan Digital Sky Survey (SDSS) at z<0.06, we find that major halo mergers can at most modestly reduce the star-forming fraction. Consistent with past research, however, mergers accompany enhanced specific star formation rates for star-forming L^* centrals: ~10% when a paired galaxy is within 200 kpc (approximately the host halo's virial radius), climbing to ~70% when a paired galaxy is within 30 kpc. These results are consistent with past suggestions that quenching in L^* galaxies is due to decoupling of the galaxy from existing cold and hot gas reservoirs, rather than a lack of available gas or gravitational heating from infalling clumps.
Tracing Massive Galaxy Growth Through Cosmic Time
Dr.  Rachel Bezanson (Steward Observatory, 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 the evolutionary histories of massive galaxies have been far from static. Instead, billions of years ago, massive galaxies were more compact and morphologically different, possibly with more disk-like structures and many were still forming stars. The details of this observed evolution can place constraints on the physical processes that have driven massive galaxy evolution through cosmic time. I will discuss recent observational studies of the structural and dynamical properties of massive high-redshift galaxies. Specifically, I will demonstrate that in spite of their dramatic structural evolution, the mass fundamental plane, or the empirical relation between dynamics, sizes, stellar mass surface density of massive galaxies, has been in place since z~2. This relation appears to hold for massive galaxies of all types, not just red, dead ellipticals. Therefore, this scaling relation is an ideal tool to follow the evolution of galaxy populations as it is minimally susceptible to progenitor biases due to the evolving stellar populations, structures, and dynamics of galaxies through cosmic time.
Exoplanet Atmospheres at High Spectral Resolution
Dr.  Matteo Brogi (University of Colorado at Boulder)
After only two decades from the first detection, we know today about 2,000 planets orbiting stars other than the Sun (exoplanets). The next step is to understand the nature and the properties of these planets and their atmospheres. Among the observing techniques for investigating the atmospheres of exoplanets, ground-based high-resolution (R>20,000) spectroscopy has recently excelled in delivering robust molecular detections and estimating their relative abundances. The key aspect of this technique is the ability to resolve molecular bands into the individual lines, and to detect their Doppler shift due to the planet orbital motion, complementing traditional measurements of stellar radial velocities. I will review the major breakthroughs achieved in recent years, among which are the first atmospheric detections for non-transiting planets, and the unprecedented measurements of their true masses and orbital inclinations. I will present current efforts focusing on constraining the atmospheric composition and on measuring the planet rotational rates. Finally, I will review the future prospects of ground-based, high-resolution spectroscopy, including the characterization of Earth-like planets and the identification of possible biomarkers.
The faintest galaxies as dark matter probes
Dr.  Michelle Collins (Yale)
The masses measured for the faintest galaxies in the Universe — the dwarf spheroidals — appear to be at odds with those expected from cosmological simulations. The tension is that the systems we observe around the Milky Way have lower masses than predicted by dark matter only models. This leads to what has been termed the ‘Too Big To Fail’ (TBTF) problem. In this talk, I will present spectroscopic observations of the dwarf spheroidal galaxies orbiting our nearest spiral neighbor, Andromeda, in order to test some of the proposed solutions to the TBTF problem. I will examine whether this tension can be resolved by assuming the Milky Way has a fairly low mass, and whether the tidal forces exerted on dwarf galaxies as they orbit their hosts are much stronger than assumed in dark matter only simulations, resulting in lower central densities for these faintest of galaxies. I will also show new results for several surprisingly low mass dwarf spheroidal galaxies in the Local Group, whose properties challenge our expectations, and discuss various processes that may have led them to their present state.
Observational Signatures of Planets in Protoplanetary Disks
Dr.  Robin Dong (UC Berkeley/LBNL)
It has been suggested that the spiral arms, gaps, and cavities recently discovered in transitional disks are produced by planets. To explore this scenario, we combine two-dimensional two fluid (gas + particle) hydrodynamical calculations with fully three-dimensional Monte Carlo Radiative Transfer simulations and study the observational signatures of features produced by one or several planets, making qualitative comparisons with observations. We find that a single planet as small as 0.2 MJ can produce a deep gap at millimeter (mm) wavelengths and almost no features at near-infrared (NIR) wavelengths, while multiple planets can open up a few times 10 AU wide common gap at both wavelengths. Both the contrast ratio of the gaps and the wavelength dependence of the gap sizes are broadly consistent with data. When viewed at a moderate inclination angle, a physically circular on-centered gap could appear to be off-centered from the star due to shadowing. This effect can be used to check the existence of an unseen inner disk. Planet-induced spiral density waves are more apparent at NIR than at mm wavelengths. However, they do not appear to resemble the morphology of the directly imaged spiral arms at NIR wavelengths recently. Overall, our results suggest that the planet-opening-gap scenario is a promising way to explain the origin of the transitional disks, while it is unclear whether planets are behind the spiral arm disk systems. Finally, inspired by the recent ALMA release of the image of the HL Tau disk, we show that multiple narrow gaps, well separated by bright rings, can be opened by 0.2MJ planets soon after their formation in a relatively massive disk.
Deciphering the Early Universe: Connecting Theory with Observations
Dr.  Cora Dvorkin (Harvard University)
Cosmological observations have provided us with answers to age-old questions, involving the age, geometry, and composition of the universe. However, there are profound questions that still remain unanswered. The origin of the small anisotropies that later grew into the stars and galaxies that we see today is still unknown. However, the nature of the anisotropies in the Cosmic Microwave Background (CMB) provides strong evidence that they were generated long before the CMB radiation had its last interaction with ordinary matter. In the first part of this talk, I will explain how we can use measurements of the CMB, which was last scattered when the universe was 380,000 years old, to reconstruct the detailed physics of much earlier epochs, when the universe was only a tiny fraction of a second old. In the last part of this talk, I will present the results of a joint analysis from BICEP2/Keck Array and Planck measurements of CMB polarization at different frequencies, and I will discuss the potential of upcoming high-sensitivity experiments to further constrain the physics underlying inflation.
Clouds in the Coldest Brown Dwarfs
Dr.  Jacqueline Faherty (Carnegie Institution of Washington)
The NASA WISE satellite has been extremely effective at discovering and characterizing the coldest brown dwarfs. Among the objects in the "300 K or below club" are our best analogs to Jupiter (~125 K). Since 2011 I have been using the Magellan FourStar infrared imager to measure parallaxes of a subset of these Y dwarfs. In this talk, I will report new parallax and photometric measurements for a subset of the population and examine atmospheric implications from model comparisons of color magnitude diagrams. While warmer T dwarfs are often regarded as cloudless, we find that clouds return as temperatures cool and sulfide clouds help explain the diversity in absolute magnitudes of Y dwarfs. In the case of the coldest brown dwarf known (W0855; Luhman 2014) there are indications that water and sulfide ice clouds are present in the atmosphere.
High-redshift galaxies in the Illustris Simulation
Dr.  Shy Genel (Columbia University)
I will present results from the Illustris simulation, which is a large cosmological hydrodynamical simulation that follows thousands of massive galaxies down to z=0 inside a (100Mpc)^3 volume, resolving <~kpc scales. It is run using the Arepo moving-mesh code, and models cooling, stellar population evolution, and various feedback processes. I will discuss a range of galaxy properties at z=0->5, including galaxy masses, morphologies, star-formation activity, sizes, and angular momentum. I will make comparisons to observables, where possibles, and discuss the evolution of several relations between these quantities with cosmic time.
Measuring Atmospheric Compositions of Giant Exoplanets and Distinguishing Water-World Exoplanets with Direct-Imaging Exoplanet Missions
Dr.  Renyu Hu (Jet Propulsion Laboratory)
Future direct-imaging exoplanet missions such as WFIRST/AFTA, Exo-C, and Exo-S will measure the reflectivity of exoplanets at visible wavelengths. Most of the exoplanets to be observed will be located further away from their parent stars than is Earth from the Sun. These “cold” exoplanets have atmospheric environments conducive for the formation of water and/or ammonia clouds, like Jupiter in the Solar System. I find the mixing ratio of methane and the pressure level of the uppermost cloud deck on these planets can be uniquely determined from their reflection spectra, with moderate spectral resolution, if the cloud deck is between 0.6 and 1.5 bars. The existence of this unique solution is useful for exoplanet direct imaging missions for several reasons. First, the weak bands and strong bands of methane enable the measurement of the methane mixing ratio and the cloud pressure, although an overlying haze layer can bias the estimate of the latter. Second, the cloud pressure, once derived, yields an important constraint on the internal heat flux from the planet, and thus indicating its thermal evolution. Third, water worlds having H2O-dominated atmospheres are likely to have water clouds located higher than the 10-3 bar pressure level, and muted spectral absorption features. These planets would occupy a confined phase space in the color-color diagrams, likely distinguishable from H2-rich giant exoplanets by broadband observations. Therefore, direct-imaging exoplanet missions may offer the capability to broadly distinguish H2-rich giant exoplanets versus H2O-rich super-Earth exoplanets, and to detect ammonia and/or water clouds and methane gas in their atmospheres.
Globular Cluster Streams as Galactic High-Precision Scales
Dr.  Andreas Küpper (Columbia University)
Compared to tidal streams from dwarf satellites like Sagittarius, globular cluster streams are kinematically cold and dense. This makes them ideal instruments for inferring masses, distances and velocities within our Galaxy through sophisticated modeling. Moreover, we can use cold streams to test the clumpiness of the Galactic dark matter halo by quantifying substructure in observed streams and comparing the signals with expectations from numerical simulations. To demonstrate the power of cold streams, I will present modeling results on the Galactic globular cluster Palomar 5 and its 23 degree long tidal stream. Furthermore, I will present results from the "Via Lactea Cauda" simulation, which uses the numerical resolution of the Via Lactea II simulation of a forming Milky-Way sized dark matter halo to study the effect of dark matter subhalos on globular cluster tidal streams.
Ultraviolet Spectroscopy and ISM Diagnostics of Star-Forming Galaxies
Dr.  Emily Levesque (University of Colorado at Boulder)
Rest-frame ultraviolet (UV) spectra of star-forming galaxies have a broad range of scientific applications. They are extremely valuable for testing stellar population synthesis and photoionization models, and also provide an observational framework for calibrating multi-wavelength interstellar medium (ISM) diagnostics. Finally, UV spectra are crucial when studying star-forming galaxies at z > 2; the Extremely Large Telescopes and JWST have been designed in part to probe the rest-frame UV properties of star-forming galaxies at high redshifts. We have recently observed a sample of 14 nearby (z ~ 0.03) star-forming galaxies in the rest-frame far-UV as part of a SNAP program using the Cosmic Origins Spectrograph on HST. I will discuss our results, comparing these spectra and their key diagnostic features to optical data and the predictions of stellar population synthesis models. These results also serve as a valuable proof-of-concept for a future HST Treasury survey aimed at assembling a large rest-frame UV library of spectra for star-forming galaxies.
Testing relativistic redshifts in AGN broad emission lines
Xin  Liu  (UCLA )
AGN broad emission lines are thought to arise from a broad line region close to the black hole where gas has been photoionized by the continuum emission. The proximity of the broad line region to the black hole should cause significant special- and general-relativistic effects on the broad emission lines. I will present the first statistical test of relativistic redshifts in AGN broad emission lines (based on spectra of over 20,000 quasars from the SDSS).
Observational Tests of Cosmic-ray Diffusion in the Magellanic Clouds
Dr.  Laura Lopez (Harvard-Smithsonian CfA)
Cosmic rays (CRs) play an important role in the interstellar medium: they ionize dense molecular gas, they are responsible for the light elements in the periodic table, and they account for 20% of the ISM energy budget. However, the means by which CRs are first accelerated and then transported through galaxies are not well understood. I will present results from a recent study of the Magellanic Clouds to constrain CR transport using Fermi gamma-ray observations. I will show how we have characterized the spatial distribution of gamma rays in the LMC and SMC and used the findings, in conjunction with available multiwavelength data, to constrain CR transport based on how the emission depends on physical parameters, such as gas density, massive star formation, magnetic field structure, and turbulence properties.
The Phoenix Galaxy Cluster: Cooling and Feedback in Action
Dr.  Michael McDonald (MIT)
We present new HST, Megacam, and Chandra data of the Phoenix cluster, one of the most massive galaxy clusters discovered to date. This system appears to be serendipitously caught at a pivotal evolutionary stage for galaxy clusters and their central, most-massive galaxies. These new data reveal thermally-unstable gas which appears to be fueling a massive starburst, as well as a young, recently-outbursting central AGN which appears poised to quench cooling in this system. We discuss how this unique system fits into our ever-improving understanding of how galaxy clusters and the most massive galaxies within them co-evolve.
A New Photometric Technique to Discover Extremely Metal Poor Stars
Dr.  Adam Miller (Jet Propulsion Laboratory)
I present a new non-parametric machine-learning method for predicting stellar metallicity ([Fe/H]) based on photometric colors from the Sloan Digital Sky Survey (SDSS). The method is trained using a large sample of ~120k stars with SDSS spectra and atmospheric parameter estimates (Teff, log g, and [Fe/H]) from the SEGUE Stellar Parameters Pipeline (SSPP). For bright stars (g < 18 mag) with 4500 K < Teff < 7000 K and log g > 2, corresponding to the stars for which the SSPP estimates are most reliable, the method is capable of predicting [Fe/H] with a typical scatter of ~0.27 dex. This scatter is similar to the typical uncertainty associated with [Fe/H] measurements from low-resolution spectra. Following minor adjustments to the model, the method is suitable for the discovery of extremely metal poor (EMP) stars ([Fe/H] < -3), as relatively pure (P > 25%), but inefficient (E ~ 5%), samples of EMP star candidates can be generated from the sources with the lowest predicted [Fe/H]. Once applied to wide-field, optical surveys, such as SDSS, Pan-STARRS, and LSST, the model will identify hundreds of previously unknown EMP stars.
A Census of Flares from the Galactic Center
Dr.  Joey Neilsen (MIT)
Sgr A*, the supermassive black hole at the center of our galaxy, is the poster child for profoundly quiescent accretion flows. Forty years after its discovery in the radio and fifteen years after its discovery in X-rays with Chandra, the extreme faintness of the closest supermassive black hole remains an important puzzle in black hole accretion. Why is Sgr A* so faint? Why is its deep sleep punctuated roughly once a day by brief flares? To study this remarkable source, Chandra (in concert with numerous ground- and space-based observatories) undertook a 3 Ms campaign on Sgr A* in 2012, providing an excellent opportunity to probe the physics of accretion in the Galactic Center. I will describe ongoing work on the variability of the supermassive black hole, focusing on efforts to understand the high-energy flares through statistical analysis and hinting at the exciting implications of this multiwavelength variability for the radiation physics of Sgr A*. Finally, I will discuss how variability studies of Sgr A* may help us understand our galaxy's place in the local universe.
Neutrino Emission from Core-Collapse Supernovae
Dr.  Evan O'Connor (North Carolina State University)
Core-collapse supernovae are one of the brightest phenomena in the modern universe. They mark the end stage of massive star evolution, they are the birth place of compact objects, and they spread the products of massive star nucleosynthesis throughout the galaxy. However, the central engine responsible for the explosion and that follows from the gravitational collapse of the iron core is shrouded from us by the overlying layers of the progenitor massive star. The best way to probe the central engine is through the observation of neutrinos from a galactic core-collapse supernovae. Neutrinos relay thermodynamic, dynamic, and structural information from the central engine to us virtually unimpeded by the overlying massive star mantle. In this talk I will discuss several aspects of neutrinos in core-collapse supernovae. I will briefly advertise a new open-source, spherically symmetric, general relativistic, neutrino transport code. I will also present neutrino signal predictions from both one and two dimensional simulations of core-collapse supernova and discuss how such an observations will help constrain the advanced stages of massive star evolution.
A Multiwavelength View of the HST Frontier Clusters
Georgiana Ogrean (Harvard-Smithsonian Center for Astrophysics)
Structure growth in the universe is one of the fundamental astrophysical problems. Merging galaxy clusters are the perfect laboratories to study the effects of structure growth on the thermal and the non-thermal particle populations in the ICM. I will present results from recent deep Chandra observations of the merging HST Frontier Clusters MACS J0416.1-2403, MACS J0717.5+3745, and MACS J1149.5+2223. I will talk about multiwavelength evidence for shocks and turbulence in the ICM, and discuss the constraints set on the merger geometries by optical, X-ray, and radio data.
Evaporation of close-in exoplanets
Dr.  James Owen (Institute for Advanced Study)
Given the numerous exoplanets discovered at close separations to their parents stars, where the stellar UV & X-ray radiation fields can heat the upper layers of the planets atmosphere to 10^4 K, evaporation is bound to occur. I will discuss evaporation in the hydrodynamic limit which can be driven by either the EUV or X-ray radiation, and the associated mass-loss rates. I will argue that evaporation of close-in exoplanets does not occur in `energy-limited' sense where PdV work dominates the energy loss, but that radiative cooling and recombinations are dominant energy sinks. Finally, I will present the results of multi-dimensional calculations and discuss the role planetary and stellar magnetic fields play in exoplanet evaporation, along with the long term impact of evaporation on exoplanets.
Burying a binary: explaining the candidate stellar merger V1309 Scorpii
Dr.  Ondrej Pejcha (Princeton University)
The "red nova" V1309 Sco was proposed to be a stellar merger based on the pre-outburst light curve of a contact eclipsing binary with a rapidly decaying orbital period. Using smoothed particle hydrodynamics simulations with realistic microphysics I show that V1309 Sco experienced dynamical mass loss through the outer Lagrange point. The resulting spiral stream is accelerated by the gravitational field of the binary and experiences shocks as the matter ejected during subsequent orbits comes into mutual contact. Part of the binary orbital energy is thus converted to radiation emanating from a flat disk. This simultaneously explains several features of V1309 Sco: dramatically accelerating period change, secular changes in the binary light curve and the final slow rise of luminosity to the optical maximum. I will discuss implications of our results for the physics of stellar mergers and provide observable predictions for the transient surveys.
Exploding Stars in Interesting Places
Dr.  Steve Rodney (Johns Hopkins University)
In recent years I have been extending the reach of HST by searching for supernovae behind strong-lensing galaxy clusters, which act as cosmic telescopes to amplify the flux of distant objects. This makes it possible to detect the explosions of stars that formed when the universe was less than a few billion years old. It also yields a small but special sample: highly magnified supernovae (SNe) that serve as sensitive probes of the lensing cluster's dark matter potential. I will describe two of the most exciting recent discoveries from this program: ``SN Spock,'' a peculiar transient source observed twice in a multiply-imaged galaxy; and ``SN Refsdal,'' a quadruply imaged supernova being lensed by both a galaxy-scale and a cluster-scale lens.
Origins and Demographics of Super-Earth and Sub-Neptune Sized Planets
Dr.  Leslie Rogers (California Institute of Technology)
Sub-Neptune, super-Earth-size exoplanets are a new planet class. Though absent from the Solar System, they are found by microlensing, radial velocity, and transit surveys to be common around distant stars. The nature of planets in this regime is not known; terrestrial super-Earths, mini-Neptunes with hydrogen-helium gas layers, and water-worlds with several tens of percent water by mass are all a-priori plausible compositions. Disentangling the contributions from each of these scenarios to the population of observed planets is a critical missing link in our understanding of planet formation, evolution, and interior structure. I will present statistical analyses constraining the nature and origins of short-period sub-Neptune-sized planets. With the suite of space-based exoplanet transit surveys on the horizon (K2, TESS, CHEOPS and PLATO) and the continuing development of ground-based spectrographs (e.g., Keck SHREK, EXPRES, SPIRou, Carmenes, HPF, ESPRESSO, G-CLEF), the pace of exoplanet discovery and characterization is poised to continue accelerating. I will conclude by describing pathways forward to identify bulk composition trends in the growing census of known exoplanets and to connect these composition trends back to distinct planet formation pathways.
The All-Sky Automated Survey for Supernovae (ASAS-SN)
Dr.  Benjamin Shappee (Carnegie Observatories)
Even in the modern era, only human eyes survey the entire optical sky for the violent, variable, and transient events that shape our universe. To change this, my collaborators and I have built and implemented the All-Sky Automated Survey for Supernovae (ASAS-SN). This is a long-term project designed to monitor the entire sky every 2 days using multiple telescopes in the northern and southern hemispheres. The primary goal of ASAS-SN is to find all the closest supernovae (SNe) with an unbiased search. This systematic all-sky technique will not only allow ASAS-SN to complete a census of local SNe, but it also leaves us with a movie of the sky through time which has allowed ASAS-SN to discover many other bright and interesting Galactic and extragalactic transients. During this talk I will give an overview on the status of ASAS-SN, describe our SN search results, give an example of our data release strategy, and high-light some of our most interesting non-SNe discoveries.
Imaging Cool Exoplanets with Cool Instruments
Andy Skemer (University of Arizona)
By directly imaging extrasolar planets, we can study the compositions, atmospheric properties and interiors of gas-giant planets, and eventually, the surfaces of rocky planets.  The current generation of directly imaged planets are abnormally warm, and future exoplanet studies will target cooler, "normal" planets, which emit the majority of their light in the mid-infrared (>3 microns).  At these wavelengths, the Large Binocular Telescope is unique in its capabilities as a high-contrast exoplanet imager.  I will describe a multi-faceted program at the LBT to study planets in the mid-infrared where we are (1) executing a large survey to search for new exoplanets, using instrumentation that is sensitive to lower-mass and smaller separation exoplanets than were visible to previous surveys, (2) studying the properties of the small number of massive, widely-separated exoplanets that are currently accessible to direct imaging, and (3) developing new optics and instrumentation to improve our ability to discover and characterize exoplanets.
Cosntraining Local Group Quenching
Dr.  Erik Tollerud (Yale University)
I will describe work focused on trying to understand the quenching timescales of Local Group dwarf galaxies. By combining observed star formation histories of Local Group galaxies with LCDM-motivated orbital histories, we can draw probabilistic conclusions about when, and possibly how, dwarf galaxies are quenched by the MW and M31. I will present initial results from this on-going effort and related work.
Evolution of White Dwarfs in 3D
Dr.  Pier-Emmanuel Tremblay (STScI)
The vast majority of stars will become white dwarfs at the end of the stellar life cycle. These remnants constrain the stellar formation history in the early developing phases of the Milky Way. I am in the process of computing a complete grid of 3D simulations for white dwarf atmospheres. I am also connecting these surface calculations to structure models, creating a robust framework to study all white dwarfs without resorting to free parameters for the treatment of convection. This will form the basis to analyse the breakthrough data set from Gaia from which we will extract a wealth of information about our Galaxy. In particular, I have developed techniques to constrain the stellar formation history and initial mass function in the Milky Way as well as the interior structure and progenitors of white dwarfs. My new theoretical tools will also be used in studies of former planetary systems that are currently being accreted in white dwarf convection zones with the aim of providing direct insight into the bulk composition of exo-terrestrial material.
The Active Galaxy Census: Does it Take a Special Galaxy to Fuel a Supermassive Black Hole?
Dr.  Jonathan Trump (Penn State)
Essentially every galaxy hosts a supermassive black hole in its center. But it is a puzzle why some of these black holes are passive, while others are rapidly accreting as active galactic nuclei (AGN). Does it take a special galaxy to efficiently fuel an AGN? Contrary to several popular models, no! I will use a variety of observations - with careful consideration of selection effects - to demonstrate that AGN host galaxies are not particularly special. Black hole growth is fueled by the same gas reservoirs which drive star formation, and requires neither massive hosts nor violent angular momentum transport (i.e. mergers, disk instabilities, or large-scale bars). Similarly, AGN feedback does not occur in special kinds of galaxies, and so is unlikely to play an important role in galaxy evolution. AGNs are largely passengers on the road of galaxy evolution, and the observed connection between black hole and galaxy masses is a result of coincidental growth rather than active feedback.
Stellar tidal disruption flares as new probes of black hole physics
Dr.  Sjoert van Velzen (JHU)
The tidal disruption of a star by a massive black hole leads to a most spectacular accretion event. In the first few months following the disruption, the fallback of stellar debris can exceed the Eddington limit by orders of magnitude. Yet about a decade later, the accretion rate will have dropped to a few percent of this limit. Stellar tidal disruption flares (TDFs) thus provide a new avenue to study black hole accretion physics. Furthermore, the frequency of stellar capture depends on how the orbits of stars evolve. A precise measurement of the TDF rate can thus be used to investigate the phase space disruption of stellar orbits close to the center of their host galaxy. The advent of large synoptic surveys has accelerated the search for optical TDFs. In this talk, I will present results from this ongoing revolution. I will focus on constraints on the emission mechanism and present the first measurement of the optical TDF rate.
Precision Measurements of the High-Mass Stellar IMF
Dr.  Daniel Weisz (University of Washington)
The stellar initial mass function (IMF) above a solar mass is central to a wide-variety of astrophysics ranging from the formation of stars from dense gas to interpreting the light of distant, star-forming galaxies. Despite decades of study, there remains remarkable uncertainty over the high-mass IMF and whether it is universal or varies with respect to environment. Here, I present two significant advances in our quest to understand the high-mass IMF. The first is the most precise measurement of the high-mass IMF slope to date, based on analyzing HST observations of young, resolved stars clusters in M31. The second is a new spectrophotometric analysis technique that enables direct measurement of the high-mass IMF in distant star-forming galaxies via their young star cluster populations.
The Large Reservoirs of Gas Around Galaxies
Dr.  Jessica Werk (University of California, Santa Cruz)
The Circumgalactic Medium (CGM) is where infalling gas that feeds star formation meets outflowing, feedback-enriched materials. It is where satellites are stripped and disrupted, and where gas ejected from galaxies may eventually be recycled. This medium is seen primarily in absorption, taking the form of diffuse, ionized gas bound to the dark matter halo of its host galaxy and extending to at least 300 kpc. In this talk, I will present two new results on my CGM work: (1) the first 2-D spatial map of the CGM in its multiple phases within 150 kpc of L* galaxies. To make this map, we combine data from Keck/NIRC2-LGS-AO and HST/SNAP imaging programs with quasar absorption-line data from HST/COS, and galaxy redshift data from Keck/LRIS; and (2) a dramatic absence of NV absorption in the halos of normal star-forming galaxies despite the presence of many other ionic transitions with similar ionization potential energies. Surprisingly, this absence of NV allows us to constrain the physical conditions in the highly-ionized CGM, and rule out several popular models. If I have time, I will discuss a new Gemini Large Program already underway that will allow us to obtain redshifts for thousands of new galaxies in the vicinity of well-studied quasars. With this program, our ultimate goal is to map the distances to which galaxies transport metals and the degree of gas and metal recycling in the CGM.
Transitional disks: Asymmetry and Young planets
Dr.  zhaohuan zhu (Princeton University)
Transitional disks are protoplanetary disks with gaps and holes. Recent near-IR polarization imaging (e.g. SEEDS, GPI, VLT) and submm (e.g. SMA, ALMA) observations have suggested that dust and gas start to decouple in these gaps and holes. The decoupling not only occurs in the inner disk but also in the outer disk, and moreover the decoupling can occur non-axisymmetrically in disks. Disk asymmetry will be discussed in detail. Global magnetohydrodynamic simulations including both dust particles and non-ideal MHD effects will also be presented. By comparing such realistic simulations with observations, we have constrained protoplanetary disk properties and revealed the early stage of planet formation. Finally, I will suggest the key to find young planets in transitional disks is looking for accreting circumplanetary disks, and best observational strategies will be presented.
Absorption Spectroscopy in the Big-data Era
Dr.  Guangtun Zhu (Johns Hopkins University)
Since the discovery of Fraunhofer lines in the solar spectrum over two centuries ago, absorption spectroscopy has been one of the most useful tools in astronomy, especially for studies of the diffuse gas in the Universe. Past works with absorption have been mostly focused on small samples of high-S/N spectra. I will introduce new statistical techniques to extract absorption information from large surveys. I will present the large-scale distribution of gas around galaxies, obtained using a cross-correlation technique, and show how galaxies interact with the circumgalactic medium, with some of the most recent data from the ongoing SDSS-IV survey. I will discuss the new constraints of these results bring on galaxy formation and show that these statistical techniques will greatly increase the potential of stage-IV dark energy surveys for galaxy sciences.
Fishing for Lines in the Lensed High-Z Universe
Dr.  Adi Zitrin (Caltech)
I will review recent lensing and high-redshift science results from the CLASH and Hubble Frontier Fields programs, reaching for the farthest galaxies. I will also discuss our new modest campaign to search for spectral lines in a subset of these lensed galaxies beyond z>~6.5, i.e. in the reionization era. Due to the attenuation of Lyman-alpha by neutral Hydrogen at z>6, other restframe-UV spectral lines may be the key for spectroscopically observing high-redshift galaxies. As these lines are substantially, intrinsically weaker than Lyman-alpha, lensing should play an important role in enhancing their fluxes enough to be targeted with current 10-m class telescopes, though even with lensing magnification these lines are quite challenging to observe in substantial numbers before next generation 30-m class telescopes or JWST. First few successful detections at z>6, however, have been recently made.