GESTIÓN DE RESIDUOS ÍNDICE
PERACIONES DE REUTILIZACIÓN, VALORACIÓN O ELIMINACIÓN A QUE RCD QUE SE GENERARÁN EN OBRA
400.01 — The Galilean Satellites Observed by Cassini: a testbed for icy terrestrial exoplanets
Laura Mayorga1; David Charbonneau1; Daniel
Thorngren2
1 Center for Astrophysics | Harvard & Smithsonian (Cambridge, Massachusetts, United States)
2 University of California, Santa Cruz (Santa Cruz, California, United States)
For terrestrial exoplanets with thin atmospheres or no atmospheres, the surface will dominate to the re- flected light signal of the planet. Direct observation of the disk-integrated brightness of bodies in the So- lar System, and the variation with illumination an- gle, wavelength, and planetary longitude, is essential for both planning imaging observations of exoplan- ets and interpreting the eventual datasets. We will present our analysis of approximately 5,000 Cassini observations of the Galilean satellites through an ex- oplanet lens and show their longitudinal and illumi- nation variations. The data span a range of wave- lengths from 400-950 nm and predominantly phase angles from 0-25 degrees with some constraining ob- servations near 120 degrees. Restricted to observa- tions at the same illumination angle, we show that we can clearly detect the spin period of each of the
four moons. We invert these light curves to recon- struct maps of the surfaces and we present compar- isons of these maps to direct images. In the case of Io, we detect a clear color variation that can be traced to geologic features with varying quantities of sul- fur compounds and silicates across the surface. De- spite the similarity in size and density between the moons, surface inhomogeneities result in significant changes in the disk-integrated reflectivity with plan- etocentric longitude and phase angle. This implies that future exoplanet observations could exploit this effect to deduce surface variations, determine rota- tion periods, and potentially infer surface composi- tion. Furthermore, the Galilean satellites are all dis- tinctly non-Lambertian with steep phase functions, implying that icy exoplanets will be fainter than ex- pected at quadrature and more demanding to char- acterize by direct imaging.
L.C.M. is supported by the Harvard Future Faculty Leaders Postdoctoral fellowship.
400.02 — Prospects for Using H-α Transits to Probe Escaping Atmospheres
Ruth Murray-Clay1; Mark Dijkstra1
1 Astronomy and Astrophysics, UC Santa Cruz (Santa Cruz, Cali- fornia, United States)
The recently-observed dearth of super-Earths with radii∼1.8 times that of Earth is a member of a rare class of discoveries: observational confirmation of a clear theoretical prediction. Two separate groups predicted this feature using models of photoevapo- rative atmospheric loss. Since the discovery of the super-Earth radius gap, these models have been used to constrain properties of this planetary population such as the core mass distribution, with exciting re- sults. As the quantitative results of photoevapora- tion models become more important (and as photoe- vaporation is compared to core-powered mass loss, a new competing theory for the source of energy driving escape), improved observational constraints on photoevaporation models are sorely needed. I will present new results showing that, though tran- sits in hydrogen’s H-α line have thus far provided limited information about escaping atmospheres, for thoughtfully-chosen planetary samples, this line has exciting potential. Most direct observations of pho- toevaporation in action have been conducted using transits in hydrogen’s Lyman-α line. Because this line’s center is obscured by ISM absorption, these ob- servations primarily provide information about out- flowing gas far from the planet, making mass loss rate calculations model dependent. For most planets currently observed to have escaping gas, the fraction
of escaping hydrogen in the n=2 state is too small for significant H-α absorption. I will present new the- oretical calculations showing that for planets expe- riencing a larger XUV flux, recombination cascades can populate the n=2 state at an observable level. I will comment on these results in the context of A- stars such as Kelt 9, flaring M-stars, and young stars, and attempt to convince conference attendees that additional observational campaigns are warranted in the conveniently accessible from the ground H-α line.
400.03 — A Sub-Neptune Exoplanet with a Low- Metallicity Methane-Depleted Atmosphere and Mie-Scattering Clouds
Björn Benneke1; Heather Knutson2; Joshua Lothringer3;
Ian Crossfield4; Julianne I. Moses5; Caroline Morley6;
Laura Kreidberg7; Benjamin Fulton8; Diana Dragomir4;
Andrew Howard2; Ian Wong4; Jean-Michel Desert9;
Peter McCullough10; Eliza Kempton10; Jonathan
Fortney11; Joshua Kammer12; Drake Deming10
1 University of Montreal (Montreal, Quebec, Canada)
2 University of Maryland (College Park, Maryland, United States) 3 Astronomy and Astrophysics, University of California, Santa Cruz (Santa Cruz, California, United States)
4 Southwest Research Institute (San Antonio, Texas, United States) 5 California Insitute of Technology (Pasadena, California, United States)
6 University of Arizona (Tucson, Arizona, United States) 7 Massachusetts Institute of Technology (Cambridge, Mas- sachusetts, United States)
8 Space Science Institute (Boulder, Colorado, United States) 9 Astronomy, University of Texas at Austin (Austin, Texas, United States)
10 Harvard University (Cambridge, Massachusetts, United States) 11 NASA Exoplanet Science Institute / Caltech-IPAC (Pasadena, California, United States)
12 Anton Pannekoek Institute for Astronomy (API), University of Amsterdam (UvA) (Amsterdam, Netherlands, Netherlands)
The discovery of thousands of exoplanets with masses and radii intermediate between Earth and Neptune was one of the biggest surprises of ex- oplanet science. These super-Earths and sub- Neptunes likely represent the most common out- come of planet formation. Mass and radius measure- ments indicate a diversity in bulk composition much wider than for gas giants; however, direct spectro- scopic detections of molecular absorption and con- straints on the gas mixing ratios have largely re- mained limited to planets more massive than Nep- tune. In this talk, we present the main results from an unprecedented HST/Spitzer data set (12 transits and 20 eclipses) of a sub-Neptune exoplanet, whose
mass of 12.6 Earth masses places it near the half- way point between previously studied exo-Neptunes (22-23 Earth masses) and exoplanets known to have rocky densities (7 Earth masses). Obtained over many years, our data set provides a robust detection of water absorption (> 5 σ) and a thermal emission detection from the lowest irradiated planet to date. We reveal a low-metallicity, hydrogen-dominated at- mosphere similar to a gas giant, but strongly de- pleted in methane gas. The low, near-solar metal- licity (O/H=0.2-18) sets important constraints on the potential planet formation processes at low masses as well as the subsequent accretion of solids. The low methane abundance indicates that methane is de- stroyed much more efficiently than previously pre- dicted, suggesting that the CH4/CO transition curve has to be revisited for close-in planets. Finally, we also find a sharp drop in the cloud opacity at 2-3 µm characteristic of Mie scattering, which enables nar- row constraints on the cloud particle size and makes the planet a keystone target for mid-IR characteriza- tion with JWST.
400.04 — Meteorite Outgassing Experiments to In- form Chemical Abundances of Super-Earth Atmo- spheres
Maggie Thompson1; Myriam Telus2; Jonathan Fortney1;
Toyanath Joshi3; David Lederman3
1 Department of Astronomy & Astrophysics, University of Califor- nia, Santa Cruz (Santa Cruz, California, United States)
2 Department of Earth & Planetary Sciences, University of Califor- nia, Santa Cruz (Santa Cruz, California, United States)
3 Department of Physics, University of California, Santa Cruz (Santa Cruz, California, United States)
At present, there is no first-principles understanding of how to connect a terrestrial planet’s bulk compo- sition to its atmospheric properties. Since terrestrial exoplanets likely form their atmospheres through degassing (Elkins-Tanton & Seager 2008), a logical first step to build such a theory for super-Earths is to assay meteorites, the left-over building blocks of planets, by heating them to measure the outgassed volatiles. Our Solar System presents a wide vari- ety of meteorite types, including chondrites which are primitive unaltered rocks believed to be repre- sentative of the material that formed the rocky plan- ets. We present the current results of our meteorite outgassing experiments in which we heated a vari- ety of chondritic meteorite samples, at carefully con- trolled rates to temperatures from 200 to 1200°C and
measured the partial pressures and relative abun- dances of the outgassed volatile species (e.g., CO2,
H2O, CH4, H2, O2, S, Na) as a function of temper-
ature and time. Our experimental set-up consisted of a residual gas analyzer connected to a furnace to heat samples at specified rates. We compare the re- sults of these experiments to Schaefer and Fegley’s prior theoretical chemical equilibrium and kinetics calculations which modeled thermal outgassing for a wide variety of chondrites to predict the composi- tion of terrestrial atmospheres formed via outgassing of specific types of meteorites (Schaefer & Fegley 2007, Schaefer & Fegley 2010). In addition to testing and validating Schaefer and Fegley’s models, the re- sults from our experiments inform the phase space of chemical abundances used in atmospheric models of super-Earth exoplanets.
400.06 — Helium-Enhanced Planets at the Edge of the Radius Gap
Leslie Rogers1; Isaac Malsky1
1 Astronomy & Astrophysics, University of Chicago (Chicago, Illinois, United States)
Primordial hydrogen-helium envelopes surround- ing sub-Neptune-sized planets are susceptible to mass loss driven by ionizing radiation from their host star. The effect of mass loss is imprinted on observed exoplanet populations in the form of a ”photo-evaporation desert” and a ”gap” at 1.6 Earth Radii in the planet radius distribution. To date, mod- els of the mass-loss evolution of exoplanets have assumed that the planetary envelope composition stays constant over time. However, after an initial
∼0.1 Gyr phase of rapid hydrodynamic mass loss, sub-Neptunes may experience a subsequent phase of thermal escape modulated by diffusive separation between hydrogen and helium wherein they gradu- ally become enhanced in helium and metals (relative to hydrogen) over billions of years. We predict that planets on the large radius edge of the ”radius gap” in planet occurrence rates could be significantly en- hanced in helium (or depleted in hydrogen) relative to solar composition. We have performed the first self-consistent calculations of the coupled thermal, mass-loss, and compositional evolution of hydrogen- helium envelopes surrounding sub-Neptune mass planets. Our simulations consistently produce plan- ets with envelope helium mass fractions in excess of Y=0.5 (at planet ages of 5 Gyr) near the upper edge of the radius gap. Our results have important impli- cations for the interpretation of atmospheric trans- mission and emission spectra of low-density sub- Neptune-size planets, which are prime targets for atmospheric characterization with HST and eventu- ally JWST. Enhancement in helium relative to hydro-
gen will affect both the scale height and equilibrium chemical abundances in the atmosphere (e.g., CO rel- ative to CH4). To date, most atmospheric retrieval
analyses have fixed the ratio of hydrogen and helium to solar abundances; this assumption must now be relaxed. Our prediction further provides a new ob- servational test for the extent to which the radius gap is caused by atmospheric mass loss versus an intrin- sically bimodal outcome of planet formation.