Chemistry and line emission of outer protoplanetary disks

Chemistry and line emission of outer protoplanetary disks Inga Kamp • Introduction to protoplanetary disks and their modeling • Chemistry in the outer disks: - the influence of the central star, PAHs, and X-rays on the disk - Deuterium Chemistry • Pushing the limits of future observing facilities Collaborators: Kees Dullemond, Jesus Emilio Enriquez, Bastiaan Jonkheid, Ewine van Dishoeck, Michiel Hogerheijde, et al.

Why are the outer disks important?

Models of Protoplanetary Disks A quiet protoplanetary disk: - stationary 2 D disk models - irradiation by the star (+ accretion) determines the disk structure [Chiang & Goldreich 1997, Willacy & Langer 2000, Aikawa et al 2002, Jonkheid et al. 2004, Kamp & Dullemond 2004] A more dynamical picture of a protoplanetary accretion disk: - matter is mixed and transportet by turbulence - matter accretes onto the central star d. M/dt~10 -7 M Sun/yr Infalling gas and dust Protoplanet IR radiation Protostar Chemically active zone Visible and UV radiation Acc - matter continuously V~100 km/s falls in from the envelope causing an accretion shock at the disk surface [Aikawa et al. 1999, Gail 2001, Ilgner et al. 2004] Transport of matter and angular momentum reti on sho ck V~10 km/s Posters: Semenov et al. I. 63 Willacy et al. III. 73

Free parameters: • Stellar properties, L*, R*, M* • Dust properties, opacities, sizes • Elemental abundances • Disk dimensions, Ri, Ro • Surface density (disk mass) • turbulence/diffusion constants

The Chemical Network Example: CO formation and destruction C + OH CO + H CO + n C + O kijk~ 10 -10 … 10 -9 s-1 cm-3 ij ~ 10 -10 … 10 -8 s-1 - S(T)pa 2 ngvini + nini e (-E(ads)/k. T) • stationary solution with modified Newton-Raphson algorithm • time dependent solution using the Backward Differentiation Formula (BDF) e. g. VODE [ Hindmarsh 1980 ] • artificial neural networks [Asensio Ramos et al. 2005] [ Wedemeyer-Böhm, Kamp, Freytag, Bruls 2004]

What do we know about disk chemistry? The disks are layered: [Aikawa & Herbst 1999, Willacy & Langer 2000, van Zadelhoff et al. 2003, Semenov et al. 2004] surface layer --> photochemistry intermediate layer --> neutral & ion molecule gas chemistry disk midplane --> gas-grain chemistry

• The surface layers can get very hot (UV irradiation) • Gas and dust temperatures are not coupled in the surface layers • Photoelectric heating on PAHs set the gas temperature in the surface layer [Jonkheid et al. 2004, Kamp & Dullemond 2004, Nomura & Millar 2005] no PAHs Poster: Geers et al. I. 27

• Chemical destruction of H 2: H 2 + O --> H + OH • C/CO transition at lower/same optical depth as H/H 2 transition • Higher UV fluxes lead to lower molecule abundances in the disk atmosphere • Very confined OH layer in all T Tauri and Herbig models log n(H 2)/n(H) log n(CO)/ntot log n(OH)/ntot log n(HCO+)/ntot [Kamp et al. 2004, Nomura & Millar 2005] H/H 2

X-rays affect the chemistry and the disk temperature: R=700 AU, Z=220 AU • X-rays enhance the ionization fraction of the disk surface • Some molecules have higher abundances due to efficient ion-molecule chemistry (HCN) • X-rays can efficiently heat the disk in the absence of strong UV irradiation no X-rays [Aikawa & Herbst 1999, Kamp et al. 2005] Poster: Aikawa & Nomura III 02 Tgas in a 0. 01 M disk around an M star AU Mic no X-rays

Deuterium chemistry: • H 3+ is formed by cosmic rays throughout the disk H 3+ + HD --> H 2 D+ + HD --> HD 2+ + H 2 HD 2+ + HD --> D 3+ + H 2 (UV and X-rays do not penetrate that deep) • D/H in molecules is higher than the elemental D/H ratio in the ISM • Destruction via grain surface recombination and reactions with CO, N 2 [Aikawa & Herbst 1999, 2001, Ceccarelli & Dominik 2005] Posters: Ceccarelli et al. III. 13 Ceccarelli & Dominik III. 14

T Tauri star 0. 01 M OH layer above the disk photosphere: chromosphere log n(OH)/ntot hot [OI] 6300 Å OH + n O* + H O* is in the 1 D excited level; it decays to the ground state by emitting a 6300 Å photon gas + collisional excitation for Tgas > 3000 K OI 6300 Å emission in Orions proplyds is restricted to the skin of the disk d. M/dt = 10 -9 M /yr log n(OH)/ntot hot gas [Bally et al. 2000, Störzer & Hollenbach 1998; Orion proplyds] [Acke et al. 2005 (Herbig stars)]

Pushing the limits of future observations The mass of small dust grains decreases with stellar age (ISO, Spitzer) [Habing et al. (1999), Meyer et al. (2000), Habing et al. (2001), Spangler et al. 2001] Optically thin models (late stages of Herbig Ae star) 1. 5 x 10 -4 - 1. 5 x 10 -7 M How and when does the gas disappear from the disks? • Boundary conditions for planet formation • How many failed planetary systems are out there ? J=4 -3 J=3 -2 J=2 -1 ALMA detection limit J=1 -0 CO rotational lines

Conclusions: • Need for self-consistent disk models: disk structure + gas chemistry Posters: Semenov et al. I. 62, Jonkheid et al. III. 35, Nomura et al. III. 51 • Proper inclusion of ALL radiation sources: stellar UV, X-rays and external • Chemistry of the outer protoplanetary disks is driven by irradiation --> Importance of photochemistry • Future instrumentation will allow the detection of transition disks down to 0. 5 MEarth of gas Outlook: • Photochemistry, X-ray chemistry, three-body reactions • Gas-grain chemistry: desorption processes, molecule reactions on grains • next generation models: 2 D hydrodynamical disks with a realistic energy equation (gas temperature), radiative transfer, and full chemistry (gas, gas-grain and grain surface)