Physical State of the Deep Interior of CoRoT-7b

1. F. W. Wagner T. Rückriemen F. Sohl German Aerospace Center (DLR) IAU Symposium 276 - 13 October 2010 Physical State of the Deep Interior of CoRoT-7b

2. Introduction - Method - Results - Conclusions What we know • Mass and radius only known for two out of ~ 30 exoplanets below < 15 M • Radius (1.58±0.10) R  • (Bruntt, et al. 2010) • The mass challenge • 1-4 M Pont, et al. 2010 • (4.8±0.8) M Queloz, et al. 2009 • (5.2±0.8) M Bruntt, et al. 2010 • (5.7±2.5) M Boisse, et al. 2010 • (6.9±1.4) M Hatzes, et al. 2010 • Mean density (7.2±1.8) Mg m-3 (Bruntt, et al. 2010) • rocky planet? M-R Relations The CoRoT Family GJ 1214b CoRoT-7b CoRoT-7b 

3. Introduction - Method - Results - Conclusions Interior Structure Model • Spherical and fully differentiated • Mechanical equilibrium and thermal steady state Input: Mp, composition, Psurf, Tsurf, e Mechanical Thermal ᵋ T(r) conv. conv. Output: Rp, m(r), g(r), p(r), r(r), q(r), T(r)

4. Introduction - Method - Results - Conclusions Mixing Length Formulation • Heat flux T < Tref • Effective thermal conductivity due to • thermal convection l • Dynamic viscosity • Local Nusselt number T > Tref

5. Introduction - Method - Results - Conclusions Internal Structure of CoRoT-7b Density Bulk composition Radius, R/R Earth-like Mass, M/M Iron-depleted Core massfraction, wt.% • Density suggests rocky bulk composition

6. Introduction - Method - Results - Conclusions Present Thermal State of CoRoT-7b Temperature Pressure 1940GPa 7560K 6710K 1440GPa 5210K 5320K PCM (Valencia, et al. 2006) 656GPa 727GPa • Pressure-induced sluggish convective regime in the lower mantle • Substantial higher CMB temperatures in comparison to parameterized models • Mantle pressures within stability field of post-perovskite (125 –1000 GPa)

7. Introduction - Method - Results - Conclusions Radiogenic Heating Temperature CMB Specific heat production Age: 1.2 – 2.3 Gyr (Leger, et al. 2009) • Deep interior stays relatively hot despite decreasing radiogenic heat production • What is the role of accretional and tidal heating?

8. Introduction - Method - Results - Conclusions Physical State of the Core Activation volume, mantle Sulfur content, core 32.6 wt.% cmf ~ 15 wt.% S Melting point reduction ~3000K • Temperature strongly depending on rheology • Relatively high activation volume needed to initiate core melting • Solid state of lower mantle and iron core due to high pressure

9. Introduction - Method - Results - Conclusions Conclusions • The mean density of (7.2±1.8) Mg m-3 and high surface temperatures imply that CoRoT-7b is a dry and rocky planet. • Post-perovskite is expected to be the predominant mantle mineralogical phase. • Pressure-induced sluggish convection prevalent in the lower mantle. • Due to the large effect of pressure on melting, a pure iron core is expected to be solid. • But: A liquid core cannot completely be ruled out, depending strongly on mantle rheology and actual core composition.

10. Thank you for your attention!

11. Introduction - Method - Results - Conclusions Comparison with 2D Convection Model Upper mantle • Convection pattern strongly influenced by varying surface temperature 5M Deep interior • High pressure Highly sluggish layer • No lateral temperature variation from day-side to night-side  70 5,300K L. Noack

12. Introduction - Method - Results - Conclusions On the Existence of a Magma Ocean 1810K • Temperature variation within the lithosphere less distinct • Depth of a possible magma ocean depending on the predominant minerals • and actual surface temperatures

13. Introduction - Method - Results - Conclusions Equation of State • Equation of State (EoS) relates • pressure, temperature, and • density • Generalized Rydberg EoS • (Stacey, 2005): Fit to high- • pressure experiments • Reciprocal K-primed EoS • (Stacey, 2000): Fit to PREM • Problem: Extrapolation exoplanets Mao H., Hemley R.J., 2007: PNAS, 104, 9114-9115