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Chemistry and Dynamics in Protoplanetary Disks

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Presentation on theme: "Chemistry and Dynamics in Protoplanetary Disks"— Presentation transcript:

1 Chemistry and Dynamics in Protoplanetary Disks
Thomas Henning and Dima Semenov Max-Planck-Institut für Astronomie, Heidelberg _______________ Chemistry and Dynamics in Protoplanetary Disks _______________ Courtesy of David E. Trilling

2 Motivation Initial conditions for planet formation
Chemical composition of primitive bodies in the solar system Gas depletion and dissipation in disks – Molecules as tracers of disk history Chemistry – Physical state of the disk (temperature, density, radiation, ionization, transport) General note Observational fatcs and data: solid bodies from early Solar System young disks around young stars Chemistry-Physics connections in disks

3 Any Hot Topics? Coupling between dynamics and chemistry
Complete evolutionary track from cold cores to disks (e.g. deuteration sequence) Coupling between solid-phase and gas-phase disk components (grain evolution and settling) Early stellar activity (winds, X-rays, UV, …) Coupling Chem with Dyn: timescales! (e.g., freeze-out ~10^3-4 yrs, mixing = H2/D ~ 10^4-5, but surface chemistry ~ 10^6) Imprint of core properties & composition in young disks Accretin/desorption, grain growth & settling, rocessing of ices, etc. X-ray, UV non-thermal rádiation fields (variability), stellar winds

4 Disk Structure ~500 AU 100 AU 1 AU ~1000 AU
Observable region with interferometers wind photon-dominated layer cold midplane warm mol. layer accretion (magneto- rotational instability) hν, UV, X-rays turbulent mixing IS UV, cosmic rays snowline Only region r> AU is accessible with (sub)mm-interferometers “Sandwich”-like 3-layered structure: cold midplane (freeze-out), warm inermediate layer (mild UV, X-ray, rich chemistry, lines!), surface (only simle species/atoms) Accretion is thought to be driven by the so-called magnetorotational instability (next slide)

5 Highly Dynamical Environment
Disk Physics __________ Highly Dynamical Environment Klahr, Henning, and Kley (1999) -7 ° - 7 ° (#20) ΩK = 12 Flux limited RT AU (#51) __________

6 MRI Overview __________ __________ Rotational axis
Magnetic field geometry faster rotation slower rotation centrifugal force & magnetic tension  loop generation (turbulence) How MRI works? Imagine a rotating disk with a radial velocity gradient (a natural consequence of angular momentum transport) and magnetic field oriented vertically. Simple 2D picture: if there is a tiny dispatch of the material in outward direction, then disbalanced centrifugal force coupled to magnetic tension will tend to increase the dispatch and create a loop configuration with disturbed velocities. Eventually, it leads to turbulence generation. The MRI is a robust mechanism that works even if weak plazmas with ionization fracion > 10^-10-10^-8. N. Dziourkevitch & H. Klahr (2006), ApJ, in prep. __________

7 Ionization Structure of a Disk: Effect of Grain Evolution
__________ Shown here are two identical models of a young disk around a Sun-like star. Upper panel depicts the final ionization degree in the disk at 5 Myr computed with extended chemistry and tiny dust grains (0.1 mum). A „dead“ zone for MRI in the midplane with very low ionization degree is clearly visible. Lower panel shows the same model but computed in assumption that dust grains are large, cm-sized, and setteld toward the midlane. In this case there is no low ionized region in the disk, so MRI operates everywhere and accretion is more efficient. Time evolution is presented on the next slide. __________

8 Ionization Structure of a Disk: Effect of Grain Evolution
__________ „Layered“ vertical structure Semenov, Wiebe, Henning (2004) __________

9 N2H+ in disks: CID Collaboration
(Bordeaux – Heidelberg – Jena – Grenoble - Paris) __________ N2H+/HCO+ ~ 0.03 HCO+ is dominant ion N2H+ is not a good tracer of ionization 1012 We observed N2H+ in two disk around Sun-like stars (DM Tau, LkCa15) and around a more massive star (MWC 480). Surprisingly, detected N2H+ column density is 100 times lower that was previously observed (Lkca 15, Qi et al. 2003?). We also obtained HCO+ column density and estimated N2H+/HCO+ ration ~ , similar to the value measured in cold cores. Using a realistic disk physical structure & extended gas-grain chemical model with surface reactions, we predicted HCO+ & N2H+ abundance distributions & column densities. Our model is fairly good in comparison with observational data (but radial dependence is not well constrained). Important conclusion is that due to much lower abundance and the fact that it is locally peaked at lower part of warm molecular layer, N2H+ is not a reliable tracer of the ionization degree. In this respect, HCO+, being the dominant ion, looks more promising ionization degree tracer in disks. 10 1010 Dutrey, Henning et al. (2006), A&A, submitted __________

10 CID I: N2H+ in three disks
Main results: HCO+ remains the more abundant molecular ion in disk [N2H+]/[HCO+] ~ , similar to dense cores A lower limit on the ionization fraction can be obtained from HCO+ column density (and 13CO abundance) [e] > 10-10 HCO+ and N2H+ Comparison observations / model (chemical model by Semenov et al., 2005) Good agreement (even if the slope p Is not yet properly reproduced) !

11 Dynamics and Chemistry
__________ Chemically reacting flow system „Well-mixed reactor system“ Flow along predominant direction including mixing __________

12 __________ Theoretical Milestones
Anomalous viscosity (von Weizsäcker, early 40s) α model of disk viscosity (Shakura & Sunyaev 1973) Magneto-rotational instability (Balbus & Hawley 1991) Observational Evidence Non-thermal line broadening (~100 m/s) Crystalline silicates in comets and disks (van Boekel et al. 2005, Crovisier et al. 1997, Wooden et al. 2005) Chondritic refractory inclusions in meteorites (MacPherson et al. 1988) Gas-phase CO at T<25K in DM Tau (Dartois et al. 2003) __________

13 __________ Observational Evidence
Non-thermal line broadening (~100 m/s) Crystalline silicates in comets and disks (van Boekel et al. 2005, Crovisier et al. 1997, Wooden et al. 2005) Chondritic refractory inclusions in meteorites (MacPherson et al. 1988) Gas-phase CO at T<25K in DM Tau (Dartois et al. 2003) __________

14 Steady Inner Disk Model
__________ no vertical mixing vertical mixing CS Ilgner, Henning et al. (2004) __________

15 Previous Studies __________ __________
Gail & Tscharnuter (>2000): 2D hydro + RT inner disk, gas-phase combustion chemistry, grain evolution  crystalline silicate distribution, carbon chains Ilgner et al. (2004; 2006a,b): 1+1D inner disk, 1D vertical mixing & radial transport, gas-grain chemistry  molecular abundances Lyons & Young (2005): inner solar nebula, 1D vertical mixing, photochemistry  16/18 oxygen isotopic anomalies Willacy et al. (2006): 1+1D outer disk, 1D vertical chemistry, gas-grain + surface chemistry  molecular abundances __________

16 Chemistry with Dynamics
__________ Evolution = Formation - Destruction Diffusion Advection [ Chemistry ] [ Dynamics ] Input: Physical conditions, diffusion coefficient & flow data Initial abundances of species A chemical network A numerical solver Benchmarking Chemical evolution of species can be described using a system of ordinary differential equations that are stiff due to vastly different magnitudes of reaction rate values. If one wants to model chemistry with dynamics, one has to include also two additional terms due to turbulent diffusion and advective transport. Both these terms are characterized by diffusion coefficient, D ~ alpha x sound speed x scale height, and flow properties (U). Thus, necessary input information for ´modeling chemistry with dynamics is T, rho, radiation field, etc., then diffusion coefficient and flow data. Also, one has to chose appropriate set of chemical reactions and efficient numerical solver. The last step is comparison with existing mixng chemical models. __________

17 Surface Chemistry __________ __________ 2-body reaction Desorption
Accretion Surface reaction (thermal hopping) 2-body reaction UV, CR, X-ray heating Mantle Grain Gas-phase reactions, accretion & desorption (UV, cosmic rays, collision?), surface chemistry (thermal hopping but unlikely quanum tunneling) __________

18 Chemical Network __________ __________ Updated UMIST’95:
Limited deuterium chemistry Photochemistry: Cosmic rays, UV, X-rays & radioactivity Accretion & desorption + surface reactions (0.1 μm grains) Molecular initial abundances t = a few million years Semenov et al. (2005) __________

19 t ~ N3 (amount of species in the model)
Chemistry with Mixing __________ 2D-implicit scheme for chemistry with mixing Fickian diffusion Full/reduced chemical networks 1D-benchmarking with K. Willacy & D. Wiebe t ~ N3 (amount of species in the model) 2D-implicit scheme for chemistry but not for dynamics (no operator splitting!). Fickian diffusion is approximation (dn/dt = d2/dr2(n) ). 2D code is time-consuming, time increases as 3rd power of number of species in the model -> robust reduction is needed! Semenov , Wiebe, & Henning (2006), ApJL, submitted __________

20 Disk Model __________ __________
1+1D flared disk (D‘Alessio et al. 1999) Mdisk= 0.05M, Mdot = 10-8M/yr, M = 0.65M, R >10 AU Mixing efficiency D ~ 0.01csH (Johansen & Klahr 2005) Radial D = 1.5 x vertical D ~ 1015 – 1018 cm2/g __________

21 Overview of Mixing Results
__________ 10 AU AU Deuterated chemistry is limited in our model, so the results for DCO+ and HDO should be taken with care! __________

22 Disk Ionization Degree
__________ Stationary Vertical mix. Radial mix D-Mixing 30x65 grid, 200 species in 1600 reactions 10 AU AU Comp. Time: 2h h h >200h Chemical equilibrium is reached within 10^3-10^4 yrs for ionization degree. On the other hand, diffusion timescale, which is ~ H2/D, is about 10^4-10^5 yrs for this disk. Thus, chemical evolution proceeds at timescales shorther than dynamics and resulting electron concentration is not sensetive to mixing. However, as I‘ll show later, it is not true the presence of steep abundance gradients anymore. Unaffected by diffusion since chemical equilibrium is reached quickly __________

23 Stationary Vertical mix. Radial mix. 2D-Mixing
Gas-phase CO at T<25K __________ Stationary Vertical mix. Radial mix D-Mixing 10 AU AU Dark blue color means nothing (10^-15 value for relative abundance). From left ot right: static (non-mixing) model, model with vertical mixing, model with radial mixing, and 2D-mixing model. In static model, in cold outer midplane CO frezes out quickly, but in fact a large reservoir of CO gas at T~13 K it has been detected in DM Tau by Dartois et al. (2003). In contrast, in 2D model some of CO is transported from inner midplane region where accretion heating keeps CO in the gas phase, as well as from warm molecular layer above (these 2 effects are clearly visible on 2 & 3 plots). Moreover, it is apparent that 2D mixing can be considered as a joint aaction of radial & vertical mixing. Abundant CO gas in cold midplane despite fast freeze-out (steep local abundance gradients) __________

24 Gas-phase CO at T<25K __________ __________
N(CO) ~ 1017 cm-2 (2D-model) optical depth is ~ 1 explains the observations of Dartois et al. (2003) To get observed tau <~ 1 in the cold midplane, column density of CO should be around 10^17 cm^-2. We computed CO column density in the midlane and the rest of the disk, using T=25K as a limiting value. Upper panel shows N(CO) for vertical disk extend but midplane, and lower panel corresponds to the CO column density in the midplane. As clearly seen, CO is only significantly enhanced in the case of 2D-model. __________

25 Diffusion-dependent H2CO enrichment due to slow surface processes
Gas-phase H2CO 100x lower diffusion Stationary Vertical Radial D-Mixing 10x lower __________ The same slide as before but for H2CO. It shows the resuls for 4 chemical models with & without mixing included, and 3 cases of the diffusion coefficient that correspond to alpha = 10^-2 (upper panel), 10^-3, and 10^-4 (bottom panel). The chemical evolution of H2CO is governed by surface reactions: CO + H -> HCO, HCO + H -> H2CO. The first channel involves a barrier of ~1000K, and thus is not that efficient at very low T. Moreover, H2CO easily freezes out at T<~30K. Thus, in static model it appears in he gas-phase only in the shielded but warm intermediate layer and inner midplane region heated by viscous accretion heating. In contrast, vertical mixng model and 2D-mixing model show significant enhancement of the H2Co gas-phase concentration. This is due to 2 factors. First, mixing brings instanteneously frozen H2CO from cold disk part such that it can thermally desorb back to the gas phase. Second, main effect is due to slow surface hydrogenation reactions that have timescales longer that typical mixing timescale of 10^4-10^5 yrs. Thus, the chemical evolution of H2CO becomes sensitive to details of mixing process. Since the mixing is able to transport grains from very cold regions to more warmer regions, a larger fraction of surface CO is converted to surface H2CO (remeber barrier for the first reaction H + CO -> HCO!). As we lower diffusion coefficient, the increase of the H2CO abundances goes down (compare upper panel with middle and bottom panels). Thus, his molecule is sensetive to the magnitude of diffusion coefficient, i.e., mixing efficiency. 10 AU AU Diffusion-dependent H2CO enrichment due to slow surface processes __________

26 Basic Results __________ __________
“Sandwich”-like disk structure is preserved Ionization degree is hardly affected Abundance of photo-controlled species are not affected Abundances of more complex (organic) species can be enhanced (grain mantle components, e.g. H2CO) __________

27 Disk Chemistry __________ __________
Large range of temperatures and densities Importance of radiation fields Strong coupling between chemistry and dynamics (ionization, temperature structure, …) Chem and Dyn are connected such that Dyn is related to ionization state of a disk, which in turn depends on Chem __________

28 Collaborators __________ __________
CID collaboration (A. Dutrey, S. Guilloteau, V. Pietu, A. Bacmann, R. Launhardt, Y. Pavlyuchenko, J. Pety, K. Schreyer, V. Wakelam) D. Wiebe (Moscow): Chemistry with mixing M. Ilgner (London): Chemistry with mixing H. Klahr, A. Johansen (MPIA): Disk dynamics K. Dullemond (MPIA): Grain evolution __________

29 The End __________ __________

30 (broad emission lines: Hα, etc)
NIR/Mid-IR thermal dust (VLTI: Midi, Amber) Scattered Light in Optical and NIR (opt. thick) Domain sampled in current images at 150 pc mm / submm thermal dust emission (opt. thin) Optically thick CO lines Accretion shock thin lines Accretion disk ~ Kuiper Belt Stellar magnetosphere Passive reprocessing disk 200 Flaring? Approximate H(r) at 500 AU (AU) Vertical scale near the star Acc. Rate ~10-8M⊙/yr Planetesimals? Wind Accretion columns (broad emission lines: Hα, etc) - 200 ~ 1 ~ 50 ~ 100 ~ 500 Approximate radius r (AU) Adapted from A. Dutrey Cumber03.ppt

31 Angular Momentum Machines and Chemical Factories
_______________ Cumber01.ppt

32 Previous Models (Non-Equilibrium Thermodynamics) Markwick et al., 2002
Aikawa et al., 2002 „Molecular distributions in the inner „Warm molecular layers in regions of protostellar disks“ protoplanetary disks“ Willacy et al., 2000 Aikawa et al., 1999 „The importance of photoprocessing „Evolution of molecular abundances in proto- planetary disks with accretion flow“ in protoplanetary disks“ Finocchi et al., 1997 Willacy et al., 1998 „Gas and grain chemistry in „Chemical reactions in protoplanetary disks“ protoplanetary disks“ Aikawa et al, 1996 Bauer et al., 1997 „Evolution of molecular abundance in „Simulation of chemical reactions and gaseous disks around young stars“ dust destruction in protoplanetary disks“

33 (gas phase and surface chemistry incl. vertical mixing)
Steady Disk Model (gas phase and surface chemistry incl. vertical mixing) accretion rate  -7 10 M / yr yr accretion rate  -8 510 M / yr yr

34 (gas phase and surface chemistry incl. vertical mixing)
Steady Disk Model (gas phase and surface chemistry incl. vertical mixing) opacity (Henning & Stognienko) yr opacity (Bell & Lin) yr

35 Chemistry Reduction Code “ART”
__________ “Automatic Reduction Technique” to reduce sizes of chemical networks: “Important” species Main destruction and formation reactions Several time steps and disk regions Remove “insignificant” reactions Final benchmarking Wiebe, Semenov, Henning (2003) __________

36 Chemical simulations t ~ N3!
Reduced yet reliable accuracy  5X time gain Surface chemistry  >10X loss Old: DVODE + dense matrix formalism  slow (1 disk point ~ 5-30 min) Now: DVODPK + sparse matrix formalism  fast >200X gain (1 disk point ~1-5 s) t ~ N3!

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