Chemistry and line emission of Chemistry and line emission of outer protoplanetary disks outer protoplanetary disks

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Chemistry and line emission of Chemistry and line emission of

outer protoplanetary disks outer protoplanetary disks

Inga Kamp Inga Kamp

• Introduction to protoplanetary disks and their modelingIntroduction to protoplanetary disks and their modeling

• Chemistry in the outer disks: - the influence of the central star, Chemistry in the outer disks: - the influence of the central star,

PAHs, and X-rays on the diskPAHs, and X-rays on the disk

- Deuterium Chemistry- Deuterium Chemistry

• Pushing the limits of future observing facilitiesPushing the limits of future observing facilities

Collaborators: Kees Dullemond, Jesus Emilio Enriquez, Bastiaan Jonkheid, Ewine van Dishoeck, Collaborators: Kees Dullemond, Jesus Emilio Enriquez, Bastiaan Jonkheid, Ewine van Dishoeck,

Michiel Hogerheijde, et al.Michiel Hogerheijde, et al.

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Why are the outer disks important?

Why are the outer disks important?

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Models of Protoplanetary Disks Models of Protoplanetary Disks

A quiet protoplanetary disk:

- stationary 2D disk models

- irradiation by the star (+ accretion) determines the disk structure

A more dynamical picture of a protoplanetary accretion disk:

- matter is mixed and transportet by

turbulence

- matter accretes

onto the central star dM/dt~10-7 M Sun/yr - matter continuously falls in from the

envelope causing an accretion shock at the disk surface

Infalling gas and dust

Protostar Protoplanet

Chemically active zone

Transport of matter and angular

momentum

V~100 km/s

V~10 km/s Visible and

UV radiation

IR radiation Accretion s

hock

[Aikawa et al. 1999, Gail 2001, Ilgner et al. 2004]

[Chiang & Goldreich 1997, Willacy & Langer 2000, Aikawa et al 2002, Jonkheid et al. 2004, Kamp & Dullemond 2004]

Posters: Semenov et al. I.63 Willacy et al. III.73

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Free parameters:

• Stellar properties, L*, R*, M*

• Dust properties, opacities, sizes

• Elemental abundances

• Disk dimensions, Ri, Ro

• Surface density (disk mass)

• turbulence/diffusion constants

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The Chemical Network The Chemical Network

dn

i

dt

=

k

ijk

jk

n

j

n

k

k

jik

jk

n

i

n

k +

Γ

ij

j

n

j

Γ

ji

n

i

Example: CO formation and destruction

C + OH  CO + H

k

ijk~ 10-10 … 10-9 s-1 cm-3 CO +   C + O

ij ~ 10-10 … 10-8 s-1

[ Wedemeyer-Böhm, Kamp, Freytag, Bruls 2004]

• 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]

- S(T)a

2

n

g

v

i

n

i +

n

i

i

e

(-E(ads)/kT)

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What do we know about disk chemistry?

What do we know about disk chemistry?

The disks are layered: surface layer --> photochemistry

intermediate layer --> neutral & ion molecule gas chemistry

disk midplane --> gas-grain chemistry

[Aikawa & Herbst 1999, Willacy & Langer 2000, van Zadelhoff et al. 2003, Semenov et al. 2004]

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no PAHs

no PAHs

• 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]

Poster: Geers et al. I.27

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• Chemical destruction of H2: H2 + O --> H + OH

• C/CO transition at lower/same optical depth as H/H2 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(H2)/n(H) log n(CO)/ntot log n(OH)/ntot log n(HCO+)/ntot

[Kamp et al. 2004, Nomura & Millar 2005]

H/H2 H/H2

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X-rays affect the chemistry and the disk temperature:

• 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

[Aikawa & Herbst 1999, Kamp et al. 2005]

AU Mic no X-rays

Tgas in a 0.01 M

disk around an M star

R=700 AU, Z=220 AU

no X-rays

Poster: Aikawa & Nomura III02

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Deuterium chemistry:

• H3+ is formed by cosmic rays throughout the disk (UV and X-rays do not H3+ + HD --> H2D+ + H2 penetrate that deep)

H2D+ + HD --> HD2+ + H2 HD2+ + HD --> D3+ + H2

Posters:

Ceccarelli et al. III.13 Ceccarelli & Dominik III.14

• D/H in molecules is higher than the elemental D/H ratio in the ISM

• Destruction via grain surface

recombination and reactions with CO,N2

[Aikawa & Herbst 1999, 2001, Ceccarelli & Dominik 2005]

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[OI] 6300 Å [OI] 6300 Å

OH layer above the disk photosphere:

OH +   O* + H

O* is in the 1D excited level; it decays to the ground state by emitting a 6300 Å photon + collisional excitation for Tgas > 3000 K OI 6300 Å emission in Orions proplyds is restricted to the skin of the disk

[Bally et al. 2000, Störzer & Hollenbach 1998; Orion proplyds]

[Acke et al. 2005 (Herbig stars)]

T Tauri star 0.01 M

dM/dt = 10-9 M/yr hot gas

hot gas log n(OH)/ntot

log n(OH)/ntot chromosphere

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Pushing the limits of future observations 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]

J=2-1

J=1-0

J=3-2 J=4-3

ALMA detection limit

CO rotational lines How and when does the gas disappear

from the disks?

• Boundary conditions for planet formation

• How many failed planetary systems are out there ?

Optically thin models (late stages of

Herbig Ae star) 1.5 x 10-4 - 1.5 x 10-7 M

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Conclusions:

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:

Outlook:

• Photochemistry, X-ray chemistry, three-body reactions

• Gas-grain chemistry: desorption processes, molecule reactions on grains

• next generation models: 2D hydrodynamical disks with a realistic energy equation (gas temperature), radiative transfer, and full chemistry (gas, gas-grain and grain surface)

Figure

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References

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