A Framework for Atmospheric Escape from Low-Mass Planets. Ruth Murray-Clay Harvard-Smithsonian Center for Astrophysics

Transcription

1 A Framework for Atmospheric Escape from Low-Mass Planets Ruth Murray-Clay Harvard-Smithsonian Center for Astrophysics 1

2 hot Jupiter Mercury 0.39 AU Earth 1 AU ~0.05 AU ~ 10 R * Star Sun: LUV ~ 10-6 Lbol x10 3 during T Tauri phase planets occupy a large phase space M p,r p,a,l,l UV,,e,Ṁ w,b p initial atmosphere 2

3 Low mass gas giants and the atmospheres of solid planets can be significantly depleted Fraction of Mass Lost Over 10 Gyr planet is 0.05AU from a solar mass star ice giants gas giants Neptune 0.02 HD b super Earth Planet Mass (M J ) 3

4 Atmospheric escape is crucial to characterization of planetary atmospheres Charbonneau et al

5 Atmospheric escape is crucial to Super-Earths or mini-neptunes? characterization of planetary atmospheres Charbonneau et al

6 Atmospheric escape is crucial to characterization of planetary atmospheres Super-Earths or vs. mini-neptunes? mini-neptunes: primordial atmosphere vs. ongoing outgassing and loss Charbonneau et al

7 Two classes of escape mechanisms: Each can be thermal or non-thermal kinetic loss to space of individual atoms hydrodynamic bulk outflow of a collisional fluid exobase 5

8 Two classes of escape mechanisms: Each can be thermal or non-thermal kinetic loss to space of individual atoms hydrodynamic bulk outflow of a collisional fluid exobase Jeans escape non-thermal processes, often mediated by B-fields limits of thermal escape hydrodynamic escape Roche lobe overflow ram pressure stripping 5

9 UV photons heat the upper atmosphere by photoionization Before: After: UV photon e - collisions distribute energy from ejected electron H p + solar system planets have thermospheres heated this way 6

10 Three fates for deposited energy 1. radiated away in place 2. conducted lower in the atmosphere then radiated away 3. drives an outflow, which can be energy limited : several possible structures Difficulty: temperature and escape rate are coupled 7

11 No local energy loss Earth 10 5 Parker wind T conduction Jeans dm/dt 8

12 What generates a Parker wind? > 0: bad! fluid, isothermal hydrostatic 9

13 What generates a Parker wind? > 0: bad! fluid, isothermal accelerates the gas outward v energy for PdV work in outward flow comes from this assumption 9

14 Parker winds flow through a critical point T : sonic point: cs ~ vesc x r s = GM p /(2c 2 s) De Laval Nozzle Ṁ =4πr 2 ρv rs exponential dropoff Von Braun with the Saturn V rocket 10

15 Drop isothermal assumption still assume fluid (collisional) heating from photoionization sets lower boundary condition only photoionization heating and pdv work deposited primarily at ~ 1 : n 0 1 σh P ~ nanobars, altitude set by lower atmosphere 11

16 Jeans escape If FUV low many scale heights to sonic point; no longer collisional & model isn t self-consistent (modified) Jeans escape fluid outflow v < cs 12

17 No local energy loss Earth 10 5 Parker wind T conduction Jeans dm/dt 13

18 Conduction low UV flux regime T r Watson et al

19 No local energy loss Earth 10 5 Parker wind T conduction Jeans dm/dt 15

20 No local energy loss Earth: CO Parker wind T 10 5 Jeans conduction dm/dt 16

21 No local energy loss hot Jupiter 10 5 Parker wind T 10 4 conduction Jeans dm/dt 17

22 Radiative cooling T ~ 10^4 K EUV Ly α ~ 1 X-ray H3 + T ~ 10^3 K can kill flow altogether if too high 18

23 Mass-Loss rates remain roughly energy limited at late times for hydrogen-dominated super-earths Lyα cooling :;66!,<66*1;=0*/2367!"!$!"!#!"!!!"!"!" 9 > ">9 :*!*+ ()?@*#"9%&8A B-C01!D;1=E > ">' :*!*+ ()!" 8!" #!" $!" %!" &!" ' Energy-limited ()*+,-.*/ #

24 Three fates for deposited energy 1. conducted lower in the atmosphere then radiated away Earth and Venus 2. radiated away in place hot Jupiters orbiting T Tauri stars (in place radiation + outflow) 3. drives an outflow, which can be energy limited : several possible structures hot Jupiters; early Earth and Venus (inflated by conduction) 20

25 Summary Given UV fluxes typical of hot Jupiters orbiting Sun-like stars, atmospheric escape is ~ hydrodynamic and energy limited with r ~ Rp for observed exoplanets if they have hydrogen-dominated atmospheres, but... this need not be so for smaller planets, lower UV fluxes, higher UV fluxes, much higher X-ray fluxes, and different chemistries. Considerations: Coupling between upper and lower regions of outflow, radiative cooling, conduction, high energy spectrum, outer boundary conditions 21