ASTR 400/500: GRADUATE SEMINAR. TITLES AND ABSTRACTS (FALL 2016)

Aug 24: Dr. Patrick Hartigan
Title: Organizational Meeting
Abstract: The organizational meeting to set speaker schedules.


Aug 31: Dr. Charles Steinhardt
Title: Is Heirarchical Merging Broken?
Abstract: I will describe two sharp puzzles suggesting a possible mismatch between the current hierarchical merging paradigm and observations of evolving galaxies. Current models predict that the z ~ 4-8 universe should be a time in which the most massive galaxies are transitioning from their initial halo assembly to the later baryonic evolution seen in star-forming galaxies and quasars. Instead, massive galaxies appear to exist impossibly early, before their halos should even have been able to assemble. Further, small halos should have generally formed earlier than larger halos, yet the observed hierarchy in galaxy formation is inverted, with more massive galaxies completing their growth earlier. After describing these puzzles, we will consider hierarchical merging is truly inconsistent with observation or whether other explanations might be more likely.


Sep 7: Jason Ling
Title: Searching for Radio Emission from Stars and Exoplanets
Abstract: Radio astronomy provides us with a powerful tool to probe many astronomical phenomena. One such process is radio emission from stars and exoplanets that may provide a marker for magnetospheric interaction. Using results from the Very Large Array, I will discuss efforts to establish better constraints on detecting low-frequency radiation emanating from a variety of potential sources. Synthesizing multiple images of targets, stacked images are produced in order to lower the background noise and to possibly reveal an underlying common emission source. I will compare the results with theoretical models to propose upper limits on stellar and exoplanetary radio emission.


Sep 14: Joe Barchas
Title: An Exoplanet Around Proxima B, the Closest Star to the Sun
Abstract: In this journal talk, I will discuss the recent discovery of a potentially habitable exoplanet orbiting around our closest stellar neighbor. I'll discuss the observational techniques used by its discoverers, the history of observations that led to its discovery, and potential future missions to further study (or even visit) this promising exoplanet.


Sep 21: Andy Liao
Title: H-alpha Variability in PTFO8-8695 and the Possible Direct Detection of Emission from a 2 Million Year Old Evaporating Hot Jupiter
Abstract: The authors use high time cadence, high spectral resolution optical observations to detect excess H-alpha emission from the 2 - 3 Myr old weak lined T Tauri star PTFO8-8695. This excess emission appears to move in velocity as expected if it were produced by the suspected planetary companion to this young star. The excess emission is not always present, but when it is, the predicted velocity motion is often observed. The authors considered the possibility that the observed excess emission is produced by stellar activity (flares), accretion from a disk, or a planetary companion; we find the planetary companion to be the most likely explanation. If this is the case, the strength of the H-alpha line indicates that the emission comes from an extended volume around the planet, likely fed by mass loss from the planet which is expected to be overflowing its Roche lobe.


Sep 28: Adolfo Carvalho
Title: Evolution of Dust in the Orion Bar with Herschel
Abstract: The interstellar medium is populated by vast quantities of dust. Understanding this dust is an important part of understanding how galaxies, stars, and planets, form. How dust evolves in the interstellar medium due to excitation and local physical conditions can be studied using spectrophotometric observations of photo dissociation regions, such as the Orion Bar. These observations lend credence to coagulation as a large part of dust evolution. I will discuss the consequences of these observations, as well as the importance of coagulation as a part of dust evolution


SPEAKER 2: Mishaal Hassan
Title: The Chemical and Structural Beginnings of High Mass Stars
Abstract: While much is known regarding the mechanisms of low mass star formation the formation of high mass stars is less well understood. There are currently two prevailing theories regarding high mass star formation: monolithic collapse and competitive accretion. The best way to distinguish between these two models is to observe the internal structure of a cold, dense molecular cloud where high mass star formation is likely. This is done by identifying the molecular lines present in the cloud. The molecules present can provide important information regarding the kinematics and structure of the cloud. The clump that was observed, named G331.372-00.116, was identified as a cold, dense cloud through the MALT90 survey. ALMA was used to measure the internal structure due to its’ high sensitivity and angular resolution, meaning the data should reliably help distinguish between the monolithic collapse and competitive accretion models.


Oct 5: Dr. Nienke van der Marel
Title: Resolving gas and dust in transitional disks: the ALMA view on planet formation
Abstract: Protoplanetary disks of gas and dust around young stars are the birth cradles of planets. The study of these disks was for a long time based on unresolved observations, limiting our understanding of planet formation. Of particular interest are the so-called transitional disks with inner dust cavities, a sign of active evolution. The arrival of ALMA has revolutionized our view of the structure of these disks. ALMA observations in the last few years have revealed rings, asymmetries, dust/gas segregation, gas dynamics, evidence for dust trapping and vortices, and many more exciting phenomena that have been predicted for decades in disk models. Using new physical-chemical modeling tools it is now possible to constrain gas and dust densities and compare these with planet-disk interaction model predictions. In this talk I will discuss several recent ALMA discoveries and the next steps in planet formation studies.


Oct 12: Erik Weaver
Title: Measuring Temperatures in Protostellar Disks
Abstract: Accurate determination of temperature is key to understanding most features of disk evolution and planet formation, from dust aggregation to disk chemistry. Using a new ray-tracing code, two important problems have been identified in how gas temperature is derived from the observations of molecular line emission. The first involves inclusion of optically thin emission in deriving the temperature from integrated intensity maps (moment 0), while the second involves loss of flux during subtraction of continuum emission. These problems lead to a systematic underestimate of temperature, and cause unphysical features to appear in observations. We present an alternative method of deriving temperature which helps to mitigate these issues.
Speaker 2: Alison Farrish
Title: Mid-IR Luminosity and the Accretion Mode Dichotomy in Radio-Loud AGN
Abstract: I will provide an introduction to radio-loud active galactic nuclei (RL-AGN), and in particular the observed dichotomy between high-excitation and low-excitation radio galaxies (HERGs and LERGs, respectively). HERGs and LERGs are powered by different accretion modes, but can be difficult to classify based on observational signatures. I will detail a proposed method of separating HERGs and LERGs based solely on their 22-micron luminosities, and will describe tests that I carried out to determine whether this simple diagnostic is actually effective at separating RL-AGN into these two classes.


Oct 19: Dr. Marcelo Alvarez [CITA]
Title: Fundamental Physics with the Next Generation of Large Scale Structure Surveys
Abstract: The next generation of large scale structure (LSS) surveys will map out the universe in unprecedented detail, and are poised to answer long standing questions such as what were the initial conditions? and what is causing the mysterious accelerated cosmic expansion? Transformative techniques in computational astrophysics are now emerging as the optimal ways to extract information from these enormous surveys, with the possibility of realistic and routine simulations of the entire sky, for large ensembles of model universe realizations, finally within reach. I will describe work we have been doing to make this a reality, focusing on a new pipeline for efficiently generating high resolution simulated maps of the extragalactic sky from the radio to optical. I will then describe how these full sky simulations are being used to guide the design of coming LSS surveys. I will conclude with what questions are likely to be answered in the next decade and speculate about what sorts of surpising new discoveries may await.


Oct 26: Laura Flagg
Title: Newly Discovered Young, Low-Mass Spectroscopic Binaries
Abstract: Young spectroscopic binaries (SB) allow us to determine dynamical masses of young stars, which are needed to constrain evolutionary models and processes. During a search for new young moving group (YMG) members with ages between 10 and 300 Myr, we collected optical spectra of many nearby low-mass stars. These data allow us to measure key youth indicators and kinematics. The spectra may also reveal double-lined SBs. We have discovered three new, young, low-mass SBs. We calculated mass ratios and systemic velocities for these systems. Additionally, at least one of these SBs is not a kinematic match to any known YMG, yet has an age of less than 20 Myr. It may be the first system of a yet-to-be identified YMG.

Speaker 2: Yingchao Lu
Title: Magnetized jet creation using a hollow ring of laser beams
Abstract: The origin of magnetic fields starting from unmagnetized plasmas is a central question in astrophysics. Astrophysical objects such as accretion disks, stars, jets are greatly influenced by these fields. With recent advanced large laser facilities, we can scale the astrophysical system to laboratory. On the Omega laser, proton radiography was used to probe the magnetic field in the jet produced by 20 Omega beams to irradiate a flat plastic target in a hollow ring pattern. Proton images show ordered quasi-linear filaments whose divergence decreases with increasing ring radius. Proton density contrasts are consistent with filamentary field bundles with peak values in the tens of Teslas. These results demonstrate that magnetized jets created by a hollow ring of laser beams can become a versatile new platform for laboratory astrophysics.


Nov 2: Revati Mandage
Title: On the non-Kolmogorov nature of flare-productive solar active regions
Abstract: Understanding the differences between flare-productive active regions and flare-quiet active regions is of paramount importance in space weather forecasts. Since active regions are the manifestations of the magnetic fields, studying how the magnetic energy is distributed in an active region may help us see its likelihood of producing strong flares in its lifetime. The magnetic energy distribution is obtained using the magnetic power spectrum. I will talk about the results of the magnetic power spectral analysis performed on 53 HMI active regions patches that showed different levels of flare activity as they traveled across the solar disk.

Speaker 2: Nathan Tat
Title: Effective Temperature Flux Calculations for Limits of Life-Supporting Zones of Exoplanets
Abstract: Standard calculations of habitable zones around exoplanets assume that surface liquid water must be present for the planet to sustain life. However, other solvents could in principle serve the same function as water does for terrestrial biological systems. This paper generalizes the concept of a habitable zone to that of a `Life Supporting Zone' by including solvents other than water. The concept of LSZs adds additional perspectives to an exoplanet's ability to maintain life on its surface.


Nov 9: Dr. Luca Ricci (Rice)
Title: Investigating the Early Formation and Evolution of Planetary Systems with Sub-mm Interferometers
Abstract: Planets form through a huge growth of solids, starting from the tiny sub-micron sized grains found in the Interstellar Medium. Sub-mm observations of young circumstellar disks, the cradles of planets, can reveal key steps along this process. Pebbles as large as ~ 1-10 millimeter have been found in nearly all young disks observed so far, orbiting either young stars or brown dwarfs. The spatial distribution of these particles can be investigated in great detail using sub-mm and radio interferometers such as ALMA and the VLA. I will discuss how these observations inform models of the early evolution of solids toward the formation of planets. I will also present recent results from ALMA projects aimed at 1) detecting dust surrounding young protoplanets, and 2) studying the distribution of solids in a debris disk around a young Solar-like star, which gives an insight onto the dynamical interaction between a planetesimal belt and a planetary system embedded in the disk.


Nov 16: Dr. Yann Boehler
Title: Dust Trapping in Protoplanetary Disks
Abstract: The initial step to form planetesimals, the seed for planets, is the growth of micrometer-sized grains. Nevertheless, the theory predicts that When they reach mm-cm sizes, dust grains are subjected to a headwind from the gas, lose angular momentum and rapidly spiral onto the star. This catastrophic scenario is currently one of the main obstacle for the understanding of planets formation.

After quicky presenting theoretical hypothesis allowing to maintain dust grain in the disks, I will present ALMA observations at sub-millimerter wavelengths of two transition disks: HD 142527 and MWC 758, particular to present strong inhomogeneities in the dust and gas spatial distributions. I will discuss if these inhomogeneities can be 'dust traps', able to stop the dust radial migration.

Speaker 2: Dr. Jon Weisheit
Title: Research Opportunities at the National Laboratories
Abstract: The national labs are 17 entities under the auspices of the US Dept. of Energy. They operate most of America’s large research facilities, are responsible for all classified R&D relating our nuclear deterrent, and support major programs in the national interest. They conduct research across essentially all areas of physical science, engineering, applied math, and computer/computational science. They have FY16 budgets totaling about 30 billion and employ nearly 20,000 people, a large fraction of which are in technical positions.

These labs are excellent places to launch (postdoc) or conduct (staff member) a research career. After receiving my physics PhD from Rice in 1970, I spent about 40% of my own career in academic positions (Harvard, Princeton, Rice), and the rest in research and management at the Lawrence Livermore, Los Alamos, and Princeton Plasma Physics labs. In this informal presentation I will describe similarities and differences, as I experienced them, between research in the labs and in universities. Also, I will offer some tips for prospective applicants and suggest strategies to improve the likelihood of success as a young staff member.


Nov 23: Dr. T. Turkey
Title: Effects of Convective Mixing on the Internal Structure of Alpha Pavonis
Abstract: Convective mixing is a complex physical process that along with conducktive heating provides a means for energy transport in the presence of large temperature gradients. We show that the critical temperature for gastronomic stability in gallopavonic interiors like Alpha Pavonis is approximately 180 degrees. Effects of metallicity in the interior and hysteresis related to initial frozen equations of state are topics of active research and have not yet been fully modeled in 3D. The complex effects of water opacity in the optically thick cavity will be discussed.


Nov 30: Dr. Gijs Mulders [LPL, Arizona]
Title: The Kepler Exoplanet Population Around Different Types of Stars
Abstract: The Kepler spacecraft has monitored stars with a wide range of masses and metallicities for transiting planets. These stellar properties trace the conditions in the protoplanetary disk at the time of planet formation, and leave an imprint on the exoplanet population. We derive planet occurrence rates as a function of stellar mass and metallicity. In contrasts to giant planets, whose occurrence scales positively with both quantities, the occurrence of smaller planets is anti-correlated with stellar mass and does not depend on metallicity except at orbital periods less than 10 days.

These trends show that planet formation in protoplanetary disks does not occur in situ, and that the migration of planetary building blocks plays a prominent role in the planet formation process. How the planet formation process produces the observed trends with stellar mass and metallicity is not clear, but they can provide a crucial test to discriminate between different planet formation theories and provide a benchmark for planet population synthesis models.