Radiation hydrodynamics simulations of brown dwarf atmospheres - - PowerPoint PPT Presentation

Radiation hydrodynamics simulations

• f brown dwarf atmospheres

Bernd Freytag & France Allard & Derek Homeier

CRAL ENS-Lyon

“Brown Dwarfs come of Age”, Fuerteventura (2013-05-23)

Bernd Freytag & France Allard & Derek Homeier (CRAL ENS-Lyon) Simulations of brown dwarf atmospheres 1 / 9

Introduction

Dynamical stellar atmospheres

Sun Betelgeuse Observations Simulations CO5BOLD code: (Magneto-)Hydrodynamics: 2D/3D, compressible, transsonic flow, shocks, ionization, gravity, advection

• f additional densities

Radiation transport:

• ptically thick and thin,

non-local, frequency-dependence via

• pacity binning

2 geometrical setups: local ’box-in-a-star’ & global ’star-in-a-box’ models

Bernd Freytag & France Allard & Derek Homeier (CRAL ENS-Lyon) Simulations of brown dwarf atmospheres 2 / 9

Introduction

General properties of solar-like granulation

1 2 3 4 5 x [Mm] 1 2 3 4 5 z [Mm]

1 2 3 4 5 x [Mm] 0.0 0.5 1.0 1.5 2.0 2.5 z [Mm]

Surface intensity snapshot and entropy slice of solar model; 5782 K, log g=4.44, 5.62×2.8 Mm3, 8002×400 grid points

Bright granules and dark intergranular lanes Overshoot Downdrafts: largest convective velocities, shear, turbulence, vorticity Waves travel into the photosphere and transform into shocks

Bernd Freytag & France Allard & Derek Homeier (CRAL ENS-Lyon) Simulations of brown dwarf atmospheres 3 / 9

From the Sun to brown dwarfs

Transition from the Sun to brown dwarfs

2 4 6 8 log(P/[dyn/cm2]) 0.8 1.0 1.2 1.4 1.6 1.8 2.0 s [109 erg/K/g] 2 4 6 8 log(P/[dyn/cm2])

• 5
• 4
• 3
• 2
• 1

log(vvert,rms/[km/s])

Teff=5782K, logg= 4.44 Teff=4510K, logg= 4.50 Teff=3275K, logg= 5.00 Teff=2617K, logg= 5.00 Teff=2250K, logg= 5.00 Teff=2056K, logg= 5.00 Teff=1862K, logg= 5.00 Teff=1532K, logg= 5.00

Entropy and rms vertical velocity for selected (sub)stellar models

Energy flux ↓, velocities, Mach numbers ↓ Radiative time scales ↑ Stabilizing buoyancy forces ↑, horizontal vs. vertical velocities ↑ Overshoot velocity scale height in BDs ↓ Acoustic waves ↓↓, gravity waves ↓

Bernd Freytag & France Allard & Derek Homeier (CRAL ENS-Lyon) Simulations of brown dwarf atmospheres 4 / 9

From the Sun to brown dwarfs

Consequences for RHD models

Solar-like stars

◮ Two state variables, e.g. ρ and ei ◮ EOS and κ from lookup tables

Dusty objects

◮ Dust properties depend on history ◮ Additional density arrays ρdust,i ◮ Nucleation, growth, grains sizes, composition, opacities ◮ Simple dust models: ⋆ 4-moment model for carbon-rich dust in AGB stars (Freytag & H¨

• fner 2008)

⋆ 2-bin model for forsterite in brown dwarfs (Freytag et al. 2010) ⋆ Multi-size-bin model for forsterite in brown dwarfs ◮ Mach numbers from ∼1 to small values ◮ Local and global scales Bernd Freytag & France Allard & Derek Homeier (CRAL ENS-Lyon) Simulations of brown dwarf atmospheres 5 / 9

Results

Dust cloud layers above the convection zone

Entropy fluctuations and dust concentration of 2D model with 1800 K/log g=5.

Dust clouds (affect spectra and cause variability) Mixing by waves, overshoot, and cloud convection (Freytag et al. 2010) Parametrized mixing put into Phoenix code (Allard et al. 2010, 2012)

Bernd Freytag & France Allard & Derek Homeier (CRAL ENS-Lyon) Simulations of brown dwarf atmospheres 6 / 9

Results

Small-scale cloud patterns in local 3D models

Surface intensity T sequence:

◮ 2600 K ◮ 2200 K ◮ 2000 K ◮ 1800 K ◮ at log g=5

Increasing thickness

• f the dark dust clouds

above the granules

Bernd Freytag & France Allard & Derek Homeier (CRAL ENS-Lyon) Simulations of brown dwarf atmospheres 7 / 9

Results

Global 3D toy models

st22g35n02 t= 82.0 h

• 10
• 5

5 10 x [Mm]

• 10
• 5

5 10

st22g35n04 t= 15.8 h

• 10
• 5

5 10 x [Mm]

• 10
• 5

5 10

st22g35n04 t= 15.8 h

• 10
• 5

5 10 x [Mm]

• 10
• 5

5 10

st22g35n04 t= 15.8 h

• 10
• 5

5 10 x [Mm]

• 10
• 5

5 10

BD/planet toy models: 2200 K/log g=3.5 Radius scaled 1:20 3753 points, no dust:

◮ Surface intensity

4153 points, dust (2-bin forsterite)

◮ Surface intensity ◮ Entropy slice ◮ Dust concentration

slice

Bernd Freytag & France Allard & Derek Homeier (CRAL ENS-Lyon) Simulations of brown dwarf atmospheres 8 / 9

Conclusions

Conclusions

Local mixing and transport of dust

◮ Overshoot, gravity waves, dust-cloud convection ◮ Fast small-scall intensity fluctuations

Feedback mechanisms

◮ Dust clouds → convection → mixing → dust clouds ◮ Dust clouds → no nucleation → no new grains

Future work

◮ Larger local and “global” models ◮ Improved dust microphysics ◮ Rotation ◮ Magnetic activity (M dwarfs) ◮ Postprocessing: spectrum synthesis with Phoenix Bernd Freytag & France Allard & Derek Homeier (CRAL ENS-Lyon) Simulations of brown dwarf atmospheres 9 / 9