Transcript Document

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 2D 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
dM/dt~10-7 M Sun/yr
Infalling gas and dust
Protoplanet
IR radiation
Protostar
Chemically active zone
Visible and
UV radiation
- 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
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
kijk~ 10-10 … 10-9 s-1 cm-3
ij
CO + n  C + O
~ 10-10 … 10-8 s-1
dn  k n n  k n n   n   n
- S(T)pa2ngvini + nini e (-E(ads)/kT)
dt
i
ijk
jk
j
k
jik
jk
i
k
ij
j
ji
i
j
• 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
no PAHs
Poster: Geers et al. I.27
• 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
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 III02
Tgas in a
0.01 M
disk around
an M star
AU Mic
no X-rays
Deuterium chemistry:
• H3+ is formed by cosmic rays throughout the disk
H3+ + HD --> H2D+ + H2
H2D+ + HD --> HD2+ + H2
HD2+ + HD --> D3+ + H2
(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,N2
[Aikawa & Herbst 1999, 2001,
Ceccarelli & Dominik 2005]
Posters:
Ceccarelli et al. III.13
Ceccarelli & Dominik
III.14
T Tauri star 0.01 M
chromosphere
log n(OH)/ntot
[OI] 6300 Å
OH layer above the disk photosphere:
OH + n  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
dM/dt = 10-9 M/yr
log n(OH)/ntot
[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: 2D hydrodynamical disks with a realistic energy
equation (gas temperature), radiative transfer, and full chemistry (gas,
gas-grain and grain surface)