From Dust to Dust: Protoplanetary Disk Accretion, Hot Jupiter Climates, and the Evaporation of Rocky Planets

UC Berkeley UC Berkeley Electronic Theses and Dissertations Title From Dust to Dust: Protoplanetary Disk Accretion, Hot Jupiter Climates, and the Evaporation of Rocky Planets Permalink https://escholarship.org/uc/item/9jq3136f Author Perez-Becker, Daniel Alonso Publication Date 2013 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California From Dust to Dust: Protoplanetary Disk Accretion, Hot Jupiter Climates, and the Evaporation of Rocky Planets By Daniel Alonso Perez-Becker A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Physics in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Eugene Chiang, Co-chair Professor Christopher McKee, Co-chair Professor Eliot Quataert Professor Geoffrey Marcy Fall 2013 From Dust to Dust: Protoplanetary Disk Accretion, Hot Jupiter Climates, and the Evaporation of Rocky Planets Copyright 2013 by Daniel Alonso Perez-Becker 1 Abstract From Dust to Dust: Protoplanetary Disk Accretion, Hot Jupiter Climates, and the Evaporation of Rocky Planets by Daniel Alonso Perez-Becker Doctor of Philosophy in Physics University of California, Berkeley Professor Eugene Chiang, Co-chair Professor Christopher McKee, Co-chair This dissertation is composed of three independent projects in astrophysics concerning phenomena that are concurrent with the birth, life, and death of planets. In Chapters 1 & 2, we study surface layer accretion in protoplanetary disks driven stellar X-ray and far-ultraviolet (FUV) radiation. In Chapter 3, we identify the dynamical mechanisms that control atmospheric heat redistribution on hot Jupiters. Finally, in Chapter 4, we characterize the death of low-mass, short-period rocky planets by their evaporation into a dusty wind. Chapters 1 & 2: Whether protoplanetary disks accrete at observationally significant rates by the magnetorotational instability (MRI) depends on how well ionized they are. We find that disk surface layers ionized by stellar X-rays are susceptible to charge neutralization by condensates—ranging from µm-sized dust to angstrom-sized polycyclic aromatic hydrocarbons (PAHs). Ion densities in X-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI. In contrast, ionization by stellar FUV radiation enables full-blown MRI turbulence in disk surface layers. Far-UV ionization of atomic carbon and sulfur produces a plasma so dense that it is immune to ion recombination on grains and PAHs. Even though the FUV-ionized layer is 10–100 times more turbulent than the X-ray-ionized layer, mass accretion rates of both∼ layers are comparable because FUV photons penetrate to lower surface densities than do X-rays. We conclude that surface layer accretion occurs at observationally significant rates at radii & 1–10 AU. At smaller radii, both X-ray- and FUV-ionized surface layers cannot sustain the accretion rates generated at larger distance and an additional means of transport is needed. In the case of transitional disks, it could be provided by planets. Chapter 3: Infrared light curves of transiting hot Jupiters present a trend in which the atmospheres of the hottest planets are less efficient at redistributing the stellar energy absorbed on their daysides than colder planets. Here we present a shallow water model of the atmospheric dynamics on synchronously rotating planets that explains why heat 2 redistribution efficiency drops as stellar insolation rises. To interpret the model, we develop a scaling theory which shows that the timescale for gravity waves to propagate horizontally over planetary scales, τwave, plays a dominant role in controlling the transition from small to large temperature contrasts. This implies that heat redistribution is governed by a wave-like process, similar to the one responsible for the weak temperature gradients in the Earth’s tropics. When atmospheric drag can be neglected, the transition from small to large day-night p temperature contrasts occurs when τwave τrad=Ω, where τrad is the radiative relaxation time of the atmosphere and Ω is the planetary∼ rotation frequency. Our results subsume the more widely used timescale comparison for estimating heat redistribution efficiency between τrad and the horizontal day-night advection timescale, τadv. Chapter 4: Short-period exoplanets can have dayside surface temperatures surpassing 2000 K, hot enough to vaporize rock and drive a thermal wind. Small enough planets evaporate completely. Here we construct a radiative-hydrodynamic model of atmospheric escape from strongly irradiated, low-mass rocky planets, accounting for dust-gas energy exchange in the wind. Rocky planets with masses . 0:1M⊕ (less than twice the mass of Mercury) and surface temperatures & 2000 K are found to disintegrate entirely in . 10 Gyr. When our model is applied to Kepler planet candidate KIC 12557548b—which is believed to be a rocky _ body evaporating at a rate of M & 0:1 M⊕/Gyr—our model yields a present-day planet mass of . 0:02 M⊕ or less than about twice the mass of the Moon. Mass loss rates depend so strongly on planet mass that bodies can reside on close-in orbits for Gyrs with initial masses comparable to or less than that of Mercury, before entering a final short-lived phase of catastrophic mass loss (which KIC 12557548b has entered). We estimate that for every object like KIC 12557548b, there should be 10–100 close-in quiescent progenitors with sub-day periods whose hard-surface transits may be detectable by Kepler—if the progenitors are as large as their maximal, Mercury-like sizes. KIC 12557548b may have lost 70% of its formation mass; today we may be observing its naked iron core. ∼ i A mis padres ii Contents List of Figuresv List of Tables viii Acknowledgments ix 1 Surface Layer Accretion in Transitional and Conventional Disks: from Polycyclic Aromatic Hydrocarbons to Planets1 1.1 Introduction....................................2 1.1.1 The Need for Companions........................2 1.1.2 Companions are Not Enough: The Case for the Magnetorotational Instability for the Origin of Disk Viscosity...............5 1.1.3 The Threat Posed by Polycyclic Aromatic Hydrocarbons to the MRI6 1.2 Model for Disk Ionization............................8 1.2.1 X-ray and Cosmic-ray Ionization Rates and Gas Densities......8 1.2.2 Gas Temperature............................. 11 1.2.3 Chemical Network............................ 12 1.2.4 Properties and Abundances of Trace Species.............. 14 1.2.5 Collisional Charging of PAHs and Grains................ 17 1.2.6 Numerical Method of Solution...................... 19 1.3 Results....................................... 20 1.3.1 Charge Distributions on PAHs and Grains............... 21 1.3.2 Ionization Fraction vs. PAH Abundance................ 21 1.3.3 Degree of Magnetic Coupling: Re and Am ............... 31 1.3.4 Comparison with Previous Work: Ionization Fractions......... 37 1.4 Summary and Discussion............................. 41 1.4.1 Future Directions............................. 42 2 Surface Layer Accretion in Conventional and Transitional Disks Driven by Far-Ultraviolet Ionization 45 2.1 Introduction.................................... 46 2.2 Model for FUV ionization............................ 50 Contents iii 2.2.1 FUV Luminosity............................. 50 2.2.2 Total Gas Columns and Densities.................... 51 2.2.3 Gas Temperature............................. 51 2.2.4 Very Small Grains (VSGs)........................ 52 2.2.5 Polycyclic Aromatic Hydrocarbons (PAHs)............... 53 2.2.6 H2: Photodissociation and Re-Formation on Grains.......... 54 2.2.7 Carbon: Abundance and Ionization Equilibrium............ 55 2.2.8 Sulfur: Abundance and Ionization Equilibrium............. 56 2.2.9 Numerical Method of Solution...................... 57 2.3 Results for MRI-Active Surface Densities.................... 57 2.3.1 Photodissociation and Ionization Fronts................ 57 2.3.2 MRI-Active Surface Density Σ∗ ..................... 59 2.3.3 Parameter Survey, Including Sensitivity to PAHs........... 62 2.3.4 Hall Diffusion............................... 65 2.3.5 The Possibility of Turbulent Mixing: Chemical Equilibration Timescale vs. Dynamical Timescale......................... 66 2.4 Summary and Implications for Disk Accretion................. 66 2.4.1 Transitional Disks............................. 71 3 Atmospheric Heat Redistribution on Hot Jupiters 74 3.1 Introduction.................................... 75 3.2 The Model..................................... 80 3.3 Numerical Solutions................................ 87 3.3.1 Basic Behavior of the Solutions..................... 87 3.3.2 Physical Explanation for Forcing Amplitude Dependence....... 88 3.3.3 Metric for the Day-Night Contrast................... 89 3.4 Analytic Theory for Day-Night Differences................... 91 3.4.1 Drag-dominated: Valid for Both Equatorial and Non-equatorial Regions 96 3.4.2 Coriolis-dominated: Valid for Non-equatorial Regions Only...... 96 3.4.3 Advection- or Vertical-transport-dominated: Valid for Ro & 1 .... 97 3.5 Interpretation of Theory............................. 98 3.5.1 Timescale Comparison.......................... 98 3.5.2 Vertical Advection............................ 101 3.5.3 Wave Adjustment Mechanism...................... 102 3.6 Application to Hot Jupiter Observations.................... 104 3.7 Conclusions.................................... 105 4 Catastrophic Evaporation of Rocky Planets 108 4.1 Introduction...................................

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