CIPS SEMINARS - Fall 2016
|131 Campbell Hall|
|Wednesdays 3:10 - 4:00 pm|
|DATE||SPEAKER(S)||TITLE / TOPIC||Abstract||Host|
|24-Aug||Imke de Pater, |
|Probing Below the Visible Cloud Layers in Jupiter’s Atmosphere||Visible and near-infrared images of the giant planets reveal a multitude of clouds, ranging in size from tiny, hardly visible, features to giant storm systems, such as the Great Red Spot on Jupiter, and, during the Voyager era, the Great Dark Spot on Neptune.|
It is not clear how deep these systems are “anchored”, as we only see the top of these clouds at visible and infrared wavelengths. At radio wavelengths the clouds are relatively transparent; however, hitherto such data could not provide longitudinal structure as radio interferometric maps are integrated over many hours to meet the required sensitivity; this causes any potential structure in longitude to be smeared out. We recently used the upgraded Very Large Array, combined with an innovative technique  to synthesize together many hours of radio data to produce longitude-resolved maps of Jupiter
between ~1 and 6 cm.
In this talk we will present our first longitude-resolved maps of Jupiter at wavelengths between 1 and 6 cm. The shortest wavelength maps show many similarities to visible-light amateur images. We will present an analysis of several features based upon radiative transfer calculations. In addition, we will show progress on the data reduction of Uranus and Neptune, using similar techniques.
 Sault, R.J., C. Engel, and I. de Pater, 2004. Longitude-resolved Imaging of Jupiter at 2 cm. Icarus, 168, 336-343.
|31-Aug||Simon Lock, |
|A new exhibit in the planetary zoo: hot, rotating rocky planets||There is an incredible variation in the mass and radii of exoplanets. Generally, the properties of exoplanets are inferred from interior structure models that treat the bodies as cold, differentiated and non-rotating. However, exoplanets are not always in such states. Models of accretion predict that terrestrial bodies are formed with substantial angular momentum. Rocky bodies can be hot because of proximity to their host stars or from giant impacts during accretion. We present a new code (HERCULES) that solves for the equilibrium structure of rotating bodies as a series of concentric, constant-density layers. The HERCULES code is an efficient tool for calculating the structure of rotating exoplanets with realistic equations of state. Using HERCULES and a smoothed particle hydrodynamics (SPH) code, we show that hot, rotating bodies display diverse morphologies. In particular, for rotating bodies there is a thermal limit beyond which the rotational velocity at the equator intersects the Keplerian orbital velocity. Beyond this hot spin stability limit, and the body forms an extended, continuous structure with a corotating region and a disk-like region. By analyzing SPH calculations of giant impacts and N-body models of planet formation, we show that typical rocky planets reach substantially vaporized states multiple times during accretion. For the expected mean angular momentum of growing planets, most of these impact-generated states will exceed the hot spin stability limit. Hot, rotating structures can have a bulk density several times lower than an equivalent cold, non-rotating body. The density inferred from observations can also be inaccurate by a factor of a few, depending on the orientation of an oblate body. In addition, the range of structures for hot, rotating bodies has significant implications for the differentiation, cooling and internal dynamics of rocky bodies. Finally, post-hot spin stability limit structures lead to a new mode of satellite formation that can explain the unique chemical relationship between the Earth and Moon.||Sean Wahl|
|7-Sep||Trevor David, |
|Observational constraints on planet formation and migration timescales||Short-period planets have the power to unlock many of the mysteries of planet formation and, fortunately, they are abundant. There is growing evidence that high-eccentricity migration channels are not responsible for all short-period planets; Â this notion is supported by the recent discovery of K2-33b, a short-period, Neptune-sized exoplanet transiting a 5-10 Myr old star in the Upper Scorpius association. While in situ formation of K2-33b can not be ruled out, the planet is parked curiously close to the Keplerian co-rotation radius; Â this may be interpreted as tantalizing evidence of disk-driven migration. The planet is likely still contracting, and should eventually join the bountiful class of close-in sub-Neptunes. A number of other young members of Upper Sco also exhibit transit-like|
signatures in K2 photometry, some of which may be debris streams trapped at the co-rotation radius and others of which may be related to stellar astrophysics.
In addition to K2-33b, the Kepler/K2 mission has enabled the discovery of planets in the intermediate age Hyades and Praesepe clusters.Â Both of these close-in planets exhibit radii that are large given their semi-major axes and host star characteristics.Â It is possible that, even at ages of several hundred Myr, these planets have not finished contracting or are undergoing atmospheric mass loss.Â If this is the case, we are directly constraining the evolutionary timescales of short-period planets. Finally, I will discuss ongoing work on the evolution of debris disks using modern stellar ages.
|Rob de Rosa|
|Structure and Sources of Mars' Nightside Ionosphere from MAVEN Observations||Structure and Sources of Mars' Nightside Ionosphere from MAVEN ObservationsThe Mars Atmosphere and Volatile Evolution (MAVEN) mission arrived in September 2014 and is the first to make comprehensive particle and field measurements down to altitudes of ~150 km. This study focuses on the structure of Mars’ night-side ionosphere, as defined by the densities of the dominant thermal ion species (O2+ and O+, measured by STATIC and NGIMS) and thermal electron densities (measured by LPW). We map these densities as a function of solar zenith angle, local solar time, altitude, latitude and longitude, and record additional information, including solar wind dynamic pressure (from SWIA measurements), magnetic field strength (from MAG), superthermal electron flux (from SWEA) and extreme ultraviolet flux (from EUV). At altitudes above 150 km, the night-side ionosphere is thought to be supplied by transport from the sunlit hemisphere (via bulk flow and plasma pressure gradients) and created in situ by precipitating particles (ions and electrons). These maps will help to identify the primary sources in specific regions of solar zenith angle, local time and altitude. For example, electron impact ionization is an important source for the night-side ionosphere, but access to specific regions is controlled by the magnetic field configuration. Night-side thermal plasma densities will be correlated with measured hot electron fluxes to investigate this relationship. We will also focus on the variation of thermal plasma density across the terminator as a function of altitude to investigate the role of bulk transport and plasma pressure gradients in supplying the night-side ionosphere.|
|21-Sep||Andrew Vanderburg, |
|Searching for Planets with K2||After four years of successful planet hunting, the Kepler spacecraft suffered a mechanical failure which ended its original mission and severely limited its ability to point precisely. However, Kepler is still able to point somewhat precisely at fields along the ecliptic plane for up to 80 days at a time in its new K2 extended mission. In this talk, I will describe our search for transiting planets with K2. I will give an overview of the K2 mission, which is a shallower, but wider-field version of the original Kepler mission. I will describe data analysis challenges new in K2, and our solutions which have permitted the discovery of hundreds of planet candidates. Finally, I will highlight some of our science results, including the discovery of a disintegrating minor planet transiting a white dwarf.||Eve Lee|
|28-Sep||Nienke van der Marel, |
|Resolving gas and dust in transitional disks: the ALMA view on planet formation||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.
|5-Oct||Tiffany Meshkat, |
|How common are giant planets around stars with dusty debris disks?||Debris disks may be the signposts of recent planet formation. The dust, which is generated in collisional cascades of asteroids and comets, is enhanced by the gravitational stirring of gas giant planets. Thus bright debris disk systems are natural targets for imaging searches for planets, as it indicates that the host star likely possesses some kind of planetary system. In this work, we describe a joint high contrast imaging survey for planetary mass companions at Keck and VLT of the last significant sample of debris disks identified by the Spitzer Space Telescope. No new substellar companions were discovered in our survey of Spitzer targets. We combine these observations with from three published surveys, to put constraints on the frequency of planets around 127 unique debris disk stars, the largest sample to date. We convert the detection limits to probability maps for each target. We use Monte Carlo simulations to predict the number of giant planets (1-20 Jupiter masses) we expect to detect around each target, using the detection probability maps and assuming a fixed planet companion mass function extrapolated from 1 to 1000 AU. We find that the frequency of giant planets around stars with debris disks is 3.58(+24.17-1.27)%, compared to 0.6(+0.7-0.5)% from the Bowler 2016 meta control sample.||Jing Luan|
|12-Oct||Henry Ngo, |
|Constraining giant exoplanet formation and migration with direct imaging surveys||Over the past two decades, planet finding surveys have found gas giant exoplanets on orbital separations spanning more than four orders of magnitudes. The giant planets with the smallest semi-major axes were the first planets detected via radial velocity and transit surveys. Known as "hot Jupiters", these giant planets can have orbital semi-major axes as low as 0.01 AU. At the other extreme, recent direct imaging discoveries revealed giant planets beyond their system ice lines, with separations as large as hundreds of AU.|
However, it is not known whether these two groups of extreme giant planets formed in situ or if they instead formed at moderate distances and then migrated to their current locations. I will present two direct imaging surveys to explore the origins and to characterize the population of these extreme gas giant planets. The first survey focuses on the influence of stellar companions on hot Jupiter formation and migration. Our work shows that stellar companions are unlikely affect hot Jupiter migration but they may have an important role in giant planet formation. The second study searches for directly imaged giant planets near their system ice lines. We will compare our imaged giant population with the radial velocity detected giant planets to compare formation mechanisms. We are able to access these small separations for the first time using the new vortex coronagraph recently installed on Keck/NIRC2.
|19-Oct||Rebecca Jensen-Clem, |
|Emerging Science Capabilities of Modern Adaptive Optics Systems for Exoplanet and Stellar Astrophysics||In this two-part talk, I will describe new applications of modern adaptive optic systems to both exoplanet and stellar astrophysics. In Part I, I discuss an emerging science capability of the Gemini Planet Imager: the detection of polarized light from self-luminous exoplanets has the potential to provide key information about rotation, surface gravity, cloud grain size, and cloud coverage. While polarized emission from field brown dwarfs is well established, no exoplanet or substellar companion has yet been detected in polarized light. With the advent of high contrast imaging spectro-polarimeters such as GPI and SPHERE, such a detection may now be possible with careful treatment of instrumental polarization. I will discuss the role of polarimetry in brown dwarf and exoplanet science, test observations with GPI, as well as current and future polarimetric observing campaigns. Part II concerns the first dedicated adaptive optics observatory: Robo-AO is an autonomous laser guide star AO system operating every clear night through 2018 at the Kitt Peak 2.1-m telescope. With typical instrument overheads of less than one minute per target, Robo-AO is the most efficient LGS AO system to date. I will describe the instrument’s performance in the first year of operations, and highlight key science programs including the origins of stellar angular momentum.||Rob De Rosa|
|26-Oct||William Hubbard, |
University of Arizona
|Juno at Jupiter: initial science orbit||The solar-powered geophysical orbiter Juno was successfully injected into a low-periapse Jupiter orbit on 4 July. The first bound orbit completed with the first perijove (PJ1) on 27 August. Virtually all science instruments acquired data during an hour or two while the spacecraft approached to within ~5000 km of the cloudtops. The radio-science/gravity experiment of primary interest to interior studies was deliberately not operating at highest precision during this initial pass. There will now be a hiatus until after PJ2 (19 October) due to a further main-engine burn to reduce Juno's orbital period to 14 days for the main part of the mission. I expect to see the first high-precision gravity results after PJ3 on 2 November, although initial results are quite good and eliminate some models already. My talk will touch on some of the theoretical work on Jupiter's interior that we are testing with Juno, and I'll describe the most relevant experiments. During 2017 we'll harvest new data every two weeks, steadily driving down the noise floor and filling in a gravity and compositional mosaic.||Burkhard Militzer|
|2-Nov||Erez Michaely, |
Isreal Institute of Technology
|Planets outside the main-sequence: Warm/Hot Jupiter formation in the pre-main sequence phase||High-eccentricity migration scenario of Juvian planets in the Pre-MS (PMS) phase is presented. It is believed that the formation of Jovian planets requires the existence of a substantial reservoir of gas in the interplanetary disk where planets form. Therefore they must form far from the star (beyond the “snow-line”). Given the evidence for the short lifetime of interplanetary disks (typically <3 Myrs), such planets need to form rapidly at this time-frame. During their formation the host star is still in its PMS. Their large radii suggests tidal interaction with the orbiting planets on the PMS to form Warm/Hot Jupiters.|
|9-Nov||Peter Gao, Caltech||Clouds and Hazes in Exoplanet Atmospheres||Clouds and hazes have been shown to be ubiquitous in warm exoplanet atmospheres, particularly Super Earths and Mini Neptunes, where they block the spectral signatures of key atmospheric molecules in the transmission spectra of exoplanets. Not only does this prevent us from gaining knowledge about their atmospheric composition, but also require that we include physically motivated aerosol distributions in their atmospheric models in order to fully characterize them. In this talk, I will discuss two specific case studies of clouds and hazes in exoplanet atmospheres and how they affect our observations and our understanding of their atmospheric properties. |
In part one, I tackle the case of sulfur hazes in temperate giant exoplanets. Recent work has shown that elemental sulfur and its allotropes may arise in reducing atmospheres due to photolysis of H2S, and their condensation could result in the formation of sulfur hazes. I investigate the impact such a haze would have on a temperate giant exoplanet's geometric albedo spectrum using a suite of established radiative--convective, cloud, and albedo models in order to inform future direct imaging missions. Nominal haze masses are found to drastically alter a planet's geometric albedo spectrum: whereas a clear atmosphere is dark at wavelengths between 0.5 and 1 micron due to molecular absorption, the addition of a sulfur haze boosts the albedo there to ~0.7 due to its purely scattering nature. Strong absorption by the haze shortward of 0.4 microns results in albedos <0.1, contrasting the high albedos produced there by Rayleigh scattering in a clear atmosphere. Therefore, not only do sulfur hazes mask the spectral signatures of important atmospheric species, but they also change the colors of these planets from blue to orange. Detection of such a haze by future direct imaging missions like WFIRST is possible, though discriminating between a sulfur haze and any other reflective material, such as water ice, will require observations shortward of 0.4 microns, which is currently beyond WFIRST's grasp.
In part two, I investigate the impact of including cloud microphysics in describing exoplanet condensate clouds. Specifically, I apply a cloud microphysics model to simulate the potassium chloride (KCl) and zinc sulfide (ZnS) clouds of the Super Earth GJ 1214 b. I find that pure ZnS clouds do not form via homogeneous nucleation because of its high surface energy, and when it is allowed to heterogeneously nucleate onto KCl particles it remains a minor cloud component. Pure KCl cloud distributions are strongly influenced by the rates of homogeneous nucleation versus the rates of sedimentation/mixing and condensational growth. High eddy diffusivities promote high rates of nucleation due to increased upwelling of KCl vapor from depth and generate more massive, vertically extended clouds, while low eddy diffusivities lead to diminutive clouds that experience periodic bursts of nucleation. Higher metallicities drastically increase the cloud mass due to higher supersaturations leading to high nucleation rates, and as a result even moderately supersolar metallicities (0 < [Fe/H] < 1) may produce optically thick clouds at high altitude capable of producing flat transmission spectra.
|Imke de Pater|
|16-Nov||Meredith MacGregor||Millimeter Studies of Nearby Debris Disks||At least 20% of nearby main sequence stars are known to be surrounded by disks of dusty material resulting from the collisional erosion of planetesimals, larger bodies similar to asteroids and comets in our own Solar System. Since the dust-producing planetesimals are expected to persist in stable regions like belts and resonances, the locations, morphologies, and physical properties of dust in these ‘debris disks’ provide probes of planet formation and subsequent dynamical evolution. Observations at millimeter wavelengths are especially critical to our understanding of these systems, since the large grains that dominate emission at these long wavelengths do not travel far from their origin and therefore reliably trace the underlying planetesimal distribution. The newly upgraded capabilities of millimeter interferometers like ALMA are providing us with the opportunity to image these disks with unprecedented sensitivity and resolution. I will present some of my ongoing thesis work, which uses observations of the angularly resolved brightness distribution and the spectral dependence of the flux density to constrain both the structure and grain size distribution of a sample of nearby debris disks.||Paul Kalas|
|30-Nov||Megan Ansdell (IfA Hawaii)||Planet formation from the inside out||Recent exoplanet surveys such as NASA’s Kepler mission have opened the field of exoplanet statistics, revealing an unexpected diversity in exoplanet systems but also emerging trends with host star properties. Uncovering the origins of these exoplanet characteristics will be critical to understanding planet formation, but requires similar demographic surveys of the preceding protoplanetary disks, where planets are thought to assemble from orbiting dust and gas within 5-10 Myr. Until recently, large-scale surveys of dust and gas in protoplanetary disks were limited by the resolution and sensitivity of sub-mm arrays, which are our best tool for probing these cold and faint objects. My research utilizes the recently commissioned Atacama Large Millimeter Array (ALMA) to overcome these observational barriers and conduct some of the first protoplanetary disk surveys capable of placing statistical constraints on the evolution of fundamental disk properties. Additionally, I have been combining ALMA’s sub-mm data with high-precision optical light curves from NASA’s K2 mission as well as ground-based infrared spectroscopy to reveal unexpected inner disk dynamics. I will present results from these multi-wavelength observations to explore planet formation across both time (i.e., stellar age) and space (i.e., radially in the disk), while also discussing potential relations to the exoplanet population.||Paul Kalas|
|7-Dec||Chris Mankovic, |
UC Santa Cruz
|Dense matter, helium rain, and the evolution of Jupiter and Saturn||There are a number of reasons why the interiors and evolution of our local gas giants, Jupiter and Saturn, remain an unsolved problem. With contemporary space missions returning ever more precise constraints on their gravity and magnetic fields, our understanding of these planets' composition and structure is limited by uncertain input physics, especially the thermodynamics of matter at extreme pressures. |
Of fundamental importance is the limited solubility of neutral helium in pressure-ionized hydrogen, lending otherwise fully convective giant planets a mechanism for differentiating their two most abundant constituents. In the age of quantitative predictions for the hydrogen-helium phase diagram from first principles molecular dynamics simulations, evolutionary models can teach us how Jupiter and Saturn's helium distributions evolved with time. I'll discuss how this phase transition bears on these planets' thermal evolution and present structure, including the important feedback between thermodynamic equilibrium and convective stability. I'll also discuss the potential for giant planet seismology to answer outstanding questions that the gravity fields alone cannot.