Gas in protoplanetary disks. Dmitry Semenov Max Planck Institute for Astronomy Heidelberg, Germany

Transcription

1 Gas in protoplanetary disks Dmitry Semenov Max Planck Institute for Astronomy Heidelberg, Germany

2 Suggested literature A. G.G.M. Tielens, "The Physics and Chemistry of the ISM" (2007), CUP "Protoplanetary Dust" (2010), eds. D. Apai & D. Lauretta, CUP "Protostars & Planets V" (2005), Part VI, eds. B. Reipurt et al., Univ. Arizona P. D. Semenov, "Chemistry in protoplanetary disks", Encyclopedia of Astrobiology, Springer Ver. ISBN:

3 Outline Formation of molecular lines Molecules as disk probes Disk chemical structure Observations of molecules in disks Modeling disk chemical evolution with dynamics Predictions for ALMA Accretion (advective transport) Turbulent mixing Stellar UV IV. Inner zone ( ~100 yr) Puffed-up inner rim Interstellar UV, cosmic rays -Super-resolutio!dv = 0.1 km/s! for 0.5 Msun star

4 Advantages of millimeter observations Optically thin dust emission (outer disk) Rotational transitions of many molecules High frequency resolution: ~10 6 (~0.05 km/s) Sensitive to cold regions: T~10K Interferometers: sub-arsec resolution Many spectral lines within a bandwidth Day-time observations Plateau de Bure interferometer, Submillimeter Array, Very Large Array, CARMA, ATCA IRAM 30-m, Apex 12-m, Effelsberg 100-m, Aresibo 100-m, JCMT 15m, Nobeyama 45-m

5 I. Basics of line excitation and line analysis

6 Courtesy: Ya. Pavlyuchenkov Analysis of emission line data n, T + chemistry + excitation + kinematics + radiative transfer: line Excitation: radiation & collisions Excitation & RT: non-local problem 6D: 3D n,t + 1D ν + 2D sky plane Incomplete coverage of (u,v) plane Optically thick lines: Intensity ~ Texc ( 12 CO, H2O) Optically thin lines: Intensity ~ τ*texc

7 Excitation temperatures: HCO + (1-0) T bg = 2.73 K T bg = 50 K T kin T kin Tex<0 n(h 2 ) radiatively dominated? thermalized zone T exc = Tbg T exc = Tkin Non-LTE LTE Courtesy: Ya. Pavlyuchenkov

8 Excitation conditions in PPDs Rotational transition: thermally, sub-thermally, or super-thermally excited Line is thermalized Line formation depends on chemistry Molecules in disks populate dense regions: nh > cm -3 Thermalized: low-lying transitions of observed molecules Asymmetric molecules have perplex level structure: H2O High-lying transitions: LTE or non-lte? Pavlyuchenkov et al. (2007)

9 Analysis of emission line data Physical model Chemical model Line radiative transfer Iterative fitting Observations LTE assumption Tkin is often fixed Chemistry is often ignored: fixed abundances Optical thin approx./lvg/escape probability

10 LRT tools & databases: 1/2/3D Line Radiative Transfer codes: RADEX/RATRAN (F. van der Tak, M. Hogerheijde) URANIA (Ya. Pavlyuchenkov) RADLITE (K. Pontoppidan) RADMC-3D (C. Dullemond) LIME (C. Brich & M. Hogerheijde) Photon-Dominated Region (PDR) code (F. Le Petit) Collisional rates: Leiden Atomic & Molecular Database: Line frequencies: Cologne Database for Molecular Spectroscopy: NIST, JPL,...

11 I. LRT basics: Summary Formation of emission in molecular lines is a tricky problem LTE/non-LTE Observed molecules: Texc~Tkin High-lying lines may reach τ>1 Complex molecules: Texc=? LRT codes & databases (limited) Full modeling cycle to fit interferometric data

12 II. Molecules as probes

13 Molecules in space (~170) Detected in disks: CO, HCO +, DCO +, CN, HCN, DCN, HNC, N 2 H +, H 2 CO, CS, HDO, C2H2, CO2, OH, H2O, Ne, Fe, Si, H2

14 Molecules as probes of T and nh

15 Other molecular tracers Tracer Quantity Zone of Zone of Zone of Inner Large-dipole ices molecules moment molecules: radicals density zone 12 CO, 13 CO Temperature mm Optically-thick mm lines: temperature mm IR H 2 IR Ions: ionization NH 3 cm cm CS, H 2 CO Density Radicals: mm FUV/X-ray radiation IR CCH, HCN, CN Photochemistry mm IR HCO + Ionization mm Complex mm molecules: surface chemistry/ N 2 H +,H 2 D + mm transport processes C + mm IR IR Metal ions Isotopes: fractionation & thermal history IR Complex organics Surface IR IR,mm IR,mm processes DCO +,DCN, Deuterium mm mm H 2 D + fractionation mm/cm and IR meanradio-interferometricandinfraredobservations,respectively. Semenov et al. (2010)

16 Dartois et al. (2003) CO isotopologues in disks: Tkin (z)

17 Reactions in disks Radiative association: Surface reactions: Neutral-neutral: H + C CH + hν H + O OH CH + NO HCN + O Ion-molecule: H CO H 2 + HCO + Ionization: H + hν, X, CRP H + + e - Photodissociation: CH C + H Charge exchange: H + + O H + O + Dissociative recombination: H 3 O + + e - H 2 O + H ~600 species & ~6000 reactions (no isotopes) Only ~10-20% of accurate rates Uncertainty in abundances: ~0.5 dex

18 Gas-phase formation of complex C + + H 2 CH 2 + molecules CH H 2 CH H CH H 2 /O CH 5+ /HCO + + H 2 CH e- CH 3 + H 2 CH 3 + O H 2 CO CH H 2 O CH 3 OH 2 + CH 3 OH e- CH 3 OH + H

19 Gas-phase formation of complex C + + H 2 CH 2 + molecules CH H 2 CH H CH H 2 /O CH 5+ /HCO + + H 2 CH e- CH 3 + H 2 CH 3 + O H 2 CO CH H 2 O X CH 3 OH + 2 (too low rate, Luca et al. 2002) CH 3 OH e- CH 3 OH + H

20 Gas-phase formation of complex C + + H 2 CH 2 + molecules CH H 2 CH H CH H 2 /O CH 5+ /HCO + + H 2 CH e- CH 3 + H 2 CH 3 + O H 2 CO CH H 2 O X CH 3 OH + 2 (too low rate, Luca et al. 2002) CH 3 OH e- CH 3 OH + H (3 ± 2%, Geppert et al. 2006)

21 Surface formation of complex molecules Accretion Surface synthesis Photoprocessing of ices Desorption: T, UV, CRPs Surface chemistry: O OH H 2 O N NH NH 2 NH 3 C CH CH 2 CH 3 CH 4 CO HCO H 2 CO H 3 CO CH 3 OH C + C, CO + OH, etc. in warm regions

22 III. Observations of molecules in PPDs IR (space) spectroscopy: inner regions: <20 AU rotational/vibrational lines absorption/emission Boltzmann diagrams, LTE, Tkin (Sub-)millimeter observations: outer regions: > AU rotational/vibrational lines emission lines antennas: no spatial information, surveys interferometers: resolved structures, restricted disk sample

23 Observational findings: outer disks Gas: depletion of molecules Ices: H 2O, NH 3, CH 4, H 2 CO, CH 3 OH Vertical gradients of T Photo-dominated chemistry Cold CO, CCH, HCN "Dry" interiors: where is water? Keplerian rotation Non-thermal line broadening Bergin et al. (2007), Dutrey et al. (2007), Semenov et al. (2010)

24 IR revolution: molecules in planet-forming Keck zones Herschel ESA VLT Spitzer NeII, FeII, OI, H 2, OH, H 2 O, CO 2, HCN and C 2 H 2 Warm gas: T K No depletion Non-Keplerian profiles: disk wind? Herbig Ae disks appears to be deficient in H2O and organics (Lahuis ++ 06, Pascucci , Salyk , Pontoppidan , Carr & Najita 08, Kamp++11)

25 Simon et al. (2000) Kinematics: weighting stars CO(2-1) M* from high SNR measurements of ΩK: evolutionary track models

26 Chemistry in T Tau and Herbig Ae disks Large programs: "Chemistry in Disks" (CID), Europe CID strategy: observations + modeling "Disk Imaging Survey of Chemistry with SMA" (DISCS), USA DISCS strategy: observations Limited sample: large, face-on disks (~6) Lines are weak: ~0.3 3K (0.1 1Jy) 1 line: ~1 10 hours Herbig Ae: CO, HCO +, CN, HCN T Tau: CO, HCO+, HCN, N 2 H +, CCH, CS, H2CO, DCO +, DCN Dutrey et al. (2007), Schreyer et al. (2008), Henning et al.(2010), Öberg et al. ( )

27 Resolved surface density & T: DM Tau Parametric fitting DM 12 CO/ Tau 13 CO ~ 20 Cold CO, HCO + : ~13K Pietu et al. (2007)

28 Temperature in T Tau and Herbig Ae disks "Warm" Herbig Ae disks: Tkin > K "Cool" T Tauri disks: Tkin ~ 10 30K No gradient in LkCa 15: inner hole of ~ 45 AU Are transitional disks peculiar? Pietu et al. (2007)

29 Ionization: HCO + and N2H + LkCa15 (K5), DM Tau (M1) and MWC480 (A4) J=1-0, 2-1 Two 5σ detections: N2H + in LkCa15 & DM Tau Upper limit: MWC480 HCO + (1-0) Dutrey et al. (2007) N2H + (1-0)

30 Ionization: HCO + and N2H + HCO + is dominant ion: N2H + + CO HCO + + N2 [N 2 H + ]/[HCO + ] ~ 2 5% MWC 480 DM Tau LkCa 15 X(e) > ~ Radial profiles are poorly reproduced Absolute values are in qualitative agreement

31 10 X-ray-driven chemistry in disks DM Tau (M1), LkCa 15 (K5), MWC 480 (A4) CCH (1-0) & (2-1) Hard to photodissociate Chemistry is known 10 Henning et al. (2010)

32 X-ray-driven chemistry in disks Texc: ~6 K Large-scale mixing or sub-thermal excitation? Less CCH in MWC 480 Strong photodissociation by the Herbig Ae star? Low LX in MWC 480: less efficient ion-molecule chemistry?

33 Chemistry in T Tau and Herbig Ae disks AB Aur: less amount of complex molecules per CO Are Herbig Ae disks "deserts" for complex molecules? No CO freeze-out in Herbig Ae disks: no surface chemistry Lower LX: less efficient ion-molecule chemistry CO + He + C + + O + He + Schreyer et al. (2009), Öberg et al. ( )

34 Turbulence in disks Temperature from CO lines Keplerian velocity: M*, r Subsonic components: ~ km/s Comparable with MHD models Dutrey et al. (2007), Hughes et al. (2011)

35 III. Observations of molecules in PPDs: Summary Probes of disk structure Analysis techniques are available Vertical gradients of T CO freeze-out in "cold" T Tauri disks "Warm" Herbig Ae disks are less rich in molecules High-energy stellar radiation Models are in "qualitative" agreement Turbulent line broadening Rich inner disk chemistry

36 IV. Disk chemical structure from modeler's perspective

37 Zone of ions and radicals (atmosphere) Intense UV and X-rays Low densities High temperatures High ionization degree Limited gas-phase chemistry

38 Zone of molecules (intermediate layer) Partly shielded from UV and X-rays Moderate densities Moderate temperatures Oasis of rich chemistry: gas-surface cycling, photoprocessing of ices Most molecular lines are excited here

39 Zone of ices (midplane) Only cosmic rays can penetrate High densities Low temperatures Molecules are frozen out Rich chemistry on dust surfaces

40 High n, T Reactions with barriers 3-body collisions X-ray-driven processes No freeze-out Fast grain evolution Inner, planet-forming zone Courtesy of Calvin J. Hamilton

41 A scheme of disk structure Wide range of T & nh Semenov (2011) FUV, X-rays, cosmic rays Dynamical evolution Photoevaporation Grain evolution No equilibrium

42 IV. Disk chemical structure from modeler's perspective: Summary "Sandwich"-like chemical structure Cold midplane: freeze-out, thick ices, surface chemistry Hot atmosphere: dissociation/ionization, ions/radicals, gas-phase chemistry Warm molecular layer: oasis of molecules, UV-assisted gas-phase & gas-grain chemistry Dense planet-forming zone: endothermic neutral-neutral chemistry, X-rayassisted ion-molecule chemistry

43 V. Modeling disk chemistry

44 Chemical kinetics equations n i t = Σ j,k=i k jk n j n k n i Σ l k l n l + Dn H n i /n H Un i Evolution = Formation - Destruction + Diffusion + Advection [ Chemistry ] [ Dynamics ] Physical conditions Initial abundances of molecules Grain properties Reaction data Chemical code

45 5 1 Z/H r Timescales in disks: chemistry vs dynamics [yr] [yr] [yr] Vertical mixing R, AU 6 Mixing 3.4 (3) 3.4 (3) 1.3 (3) Gas-phase 1.4 (-5) 1.3 (-4) 1.0 (-2) IM UV chemistry >1.0 (7) Freeze-out 3.3 (4) Photochemistry 5.9 (2) Accretion 1.2 (-2) 1.1 (-1) 5.4 (0) Desorption 5.8 (-7) 6.0 (-7) <1.0 (-7) Surface >1.0 (7) >1.0 (7) <1.0 (-7) * Processes Midplane Warm layer Atmosphere * The warm layer and atmosphere are located at the z/h r = 0.8 and 1.75, respectively Table 4 Characteristic Timescales: Outer Disk (250 AU) Processes Midplane Warm layer Atmosphere [yr] [yr] [yr] Mixing 1.0 (6) 2.5 (5) 1.4 (5) Gas-phase 2.0 (-2) 1.8 (-1) 2.9 (0) UV >1.0 (7) 1.2 (6) 3.1 (1) Accretion 2.7 (1) 1.8 (2) 2.3 (3) Desorption 1.0 (6) 4.3 (0) <1.0 (-7) Surface * >1.0 (7) >1.0 (7) 1.4 (5) Semenov & Wiebe (2011) * The warm layer CO and atmosphere are located at the z/h r = 0.8 and 1.75, respectively Surf. chemistry The dis shown i 800 AU forming to the v shorter 1st pan timesca 1 The ty photodi order re reaction assess t in disks actions ally hav 2007; W tion is g reaction and diss ion-mol

46 Chemistry with dynamics Turbulence & accretion Isotopic homogeneity of the Solar Nebula Crystalline silicates in comets and outer disk regions Extended gas-grain chemistry 1D/2D turbulent mixing "ALCHEMIC" code "Qualification" fit to observations Reduced and oxidized ices in comets Semenov & Wiebe (2011)

47 Turbulence: Steadfast species Laminar Slow Mixing Fast Mixing Column Densities Fast gas-phase formation and destruction t: Gas-phase chemistry < Dynamics Example: CO, OH, H2O ice, CCH, C +, CN, HCN

48 Turbulence: Sensitive species Laminar Slow Mixing Fast Mixing Column Densities Slow surface formation & desorption t: Surface chemistry > Dynamics Hydrocarbons (C2H2), organics (HCOOH), SO, SO2,C2S, C3S

49 VI. The Brave New World: ALMA Atacama Large Millimeter Array (2013) 50 x 12m + 12x7m + 4x12m Spatial resolution: Spectral resolution: <0.05 km/s 8 GHz bandwidth for continuum GHz (250 µm 1 mm) x100 resolution x20 sensitivity Credit: ALMA (ESO)

50 ALMA imaging of gas in PPDs: "Hot core/corinos"-like complex molecules: >CH 3OH, CnHm Molecules with S, P, Si, Cl,... Anions: C 8H - Isotopologues: 15 N, 34 S, 17,18 O, D, 13 C Ionization structure Planet-forming inner regions Molecular layers Large- and small-scale dynamics Large surveys Unknown unknowns!

51 ALMA is working: TW Hya CO (3-2): Integrated intensity CO (3-2): Rotation HCO + (4-3): Integrated intensity HCO + (4-3): Rotation Science Verification observations of TW Hya at 345 GHz

52 ALMA is working: HD CO (3-2): Rotation ALMA commissioning

53 Disk models for ALMA: HCO + (4-3) 2D Monte-Carlo LRT calculations: Uniform abundances, radial T-gradient Uniform abundances, 2D T-gradient Chemical abundances, 2D T-gradient Semenov et al. (2008)

54 Channel maps of HCO + (4-3)@ 20 UNIFORM THERMAL CHEMICAL Face-on disks: no big difference

55 Channel maps of HCO + (4-3)@ 60 UNIFORM THERMAL CHEMICAL Edge-on disks: molecular layers & T-gradients become visible

56 Chemical vs. Temperature Gradients: 0.68 km/s channel of HCO+ 60 CHEMICAL THERMAL UNIFORM

57 Ideal image ALMA simulations Reconstructed images CHEMICAL 0.3" 0.6" I" THERMAL UNIFORM Semenov et al. (2008)

58 ALMA simulations (other transitions, disk sizes,and inclinations) Thermal gradients and chemical stratification in disks will become observable