The chemistry of protoplanetary disks
Inga Kamp (Sterrewacht Leiden)
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Literature
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A brief introduction to protoplanetary disks
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An introduction to chemistry
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Theory: How do chemical networks work?
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Observations: What do the models tell us?
PLANET network meeting 6-8.10.2003, Heidelberg
Literature
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Molecular Astrophysics II, 1998, eds.T.W. Hartquist,
D.A. Williams
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Dust and Chemistry in Astronomy, 1993, eds. T.J. Millar,
D.A. Williams
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Astrochemistry: From Molecular clouds to Planetary Systems,
2000, eds. Y.C. Minh, E.F. van Dishoeck
(IAU Symposium 197)
A brief introduction to
protoplanetary disks
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Protoplanetary disks form as a by-product of star formation
typical masses around 10-2 MSun
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They evolve from active accretion disks to more quiet
passive disks on timescales of 106 yr
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Protoplanetary disks provide the environment in which
planet formation takes place
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How do protoplanetary disks look like?
An observers view:
CO(2-1) observation of the
disk around the T Tauri star
LkCa15 (OVRO):
- Mdisk~10-2 M Sun
- tdisk = 6.2 arcsec (870 AU)
- gas in keplerian rotation
[Qi 2000]
A theoreticians view:
Dynamical picture of a protoplanetary
accretion disk:
- 3D radiation hydrodynamics
- convective cells, turbulent motions
[Klahr & Bodenheimer 2003]
A quiet protoplanetary disk:
- stationary 2D disk models
- irradiation by the star determines
the disk structure
[Chiang & Goldreich 1997]
A more dynamic picture of a protoplanetary accretion disk:
- matter is mixed and
Infalling gas and dust
transportet by
turbulence
Chemically active zone
Transport of
Protostar
Protoplanet
matter and angular
- matter accretes
momentum
onto the central star
dM/dt~10-7 M Sun/yr
Acc
reti
IR radiation Visible and
on
sh o
- matter continuously
ck
UV radiation
V~100 km/s
falls in from the
V~10 km/s
envelope causing an
accretion shock at the
disk surface
An introduction to chemistry
Gas phase processes
- formation processes:
radiative association
grain surface formation
X + Y --> XY + hν
X + Y:g --> XY:g --> XY + g
- destruction processes:
photodissociation
XY + hν --> X + Y
dissociative recombination XY+ + e --> X + Y
- rearrangement processes:
ion-molecule reaction
charge-transfer reaction
neutral-neutral reaction
X+ + YZ --> XY+ + Z
X+ + YZ --> X + YZ+
X + YZ --> XY + Z
The UMIST database
[Le Teuff, Millar & Markwick 2000, A&AS 146,157]
- contains 4113 gas-phase reactions involving
396 species composed of
12 elements (H, He, C, N, O, Na, Mg, Si, P, S, Cl, Fe)
- listed are three values α, β, γ to parameterize the reaction rates
Reaction rates
H2 S H T s
1
s
1
1
T 300
d
1
1 2 n H v H nd
3
T cm s
; k
1
AV s
s
exp
k
exp
- formation on dust grains
T 300
- cosmic ray reactions
k
- photoreactions
k
- Arrhenius form
k
1
T
1
chem
k ijk n H exp
Chemical timescales
10
8
for ion-molecule reactions
12
10
nH s
chem
9
10
k ijk
1000
10
10
9
10000 K
for neutral-neutral reactions
18
10
nH s
chem
chem
12
10
for photodissociation
8
10
10
10
k ij
10
k ijk
nH
s
How do chemical networks work?
jk
jik
jk
two-body formation
of species i from
species j and k
A i el n i
n el
ij
n j
n i n k
k
k ijk n j n k
dn i
dt
The rate equation
ji
j
destruction of
species i by
two-body reaction
with species k
formation of
photodestruction
species i by
of species i
photoreactions
particle conservation for the elements
i
dn i
dt
n i
0 stationary solution of the rate equation
---> Newton-Raphson algorithm
n i
t
n i
r
vz
n i
z
vr
stationary disk, time-dependent chemistry
n i
t
dn i
dt
dn i
dt
radial infall,
time-dependent chemistry
vr
vz
The system of equations is extremely stiff, because the timescales of
chemical reactions vary by orders of magnitude.
---> Backward Differentiation Formula (BDF)
e.g. GEAR [Hindmarsh 1974], LSODE [Hindmarsh 1980],
DAESOL [Bauer 1999]
What can the models tell us?
[Willacy et al. 1998]
timescale of
dynamical disk
evolution
quasi steady-state
R = 10 AU
The abundances remain stable over timescales of 1 Myr. After
that time e.g. CO is processed into more complicated species
and stays on the grains instead of going back to the gas phase.
[Aikawa et al. 1997]
(a)
R = 87 AU
(b)
R = 87 AU
In time dependent models, the results depend on the initial condition:
difference between (a) initial abundances ¼ of carbon in C, ¾ in CO,
nitrogen and oxygen in N, O and (b) all carbon in CO, nitrogen in N2,
oxygen in H2O
[Aikawa et al. 1999]
Material moving from ~400 to 10 AU in 3 106 yr.
--> H2O ice is interstellar, interstellar CO is transformed into CO2, H2CO
[Markwick et al. 2002]
H+ abundance in a disk model
with X-ray ionisation
... and ...
without X-ray ionisation
--> CN and C2H are good tracers
of the ionisation degree in
protoplanetary disks
[Millar et al. 2003]
The surface layers of
protoplanetary disks
look like high-density
photon dominated
regions.
--> abundances of
atoms, ions and
simple radicals
like C2H peak there
[van Zadelhoff et al. 2003]
R ~ 100 AU, t = 1 Myr
Models including UV radiative
transfer in 2D, time-dependent
chemistry in vertical slices of
the disk:
--> CO is photodissociated in
the upper layers of the disk.
Z
Stellar UV
R
[Kamp 2003]
C+
Stationary chemistry in an old
protoplanetary disk (few 10 Myr).
C
--> layered structure C/C+/CO
similar to photon dominated
regions (PDR's)
stellar UV
z
CO
r
[van Zadelhoff et al. 2001]
Different chemical
species probe
different layers of
the protoplanetary
disks!
A combination of
high spatial resolution
observations with
detailed 2D chemical
models can provide
further constraints on
disk models:
--> densities
temperatures
disk flaring
mixing etc....
gas temperature
CO 6-5
12
CO 3-2
12
CO 2-1
12
CO 3-2
13
CO 1-0
C18O 3-2
C18O 2-1
13
gas density
HCO+ 4-3
HCO+ 1-0
H13CO+ 4-3
H13CO+ 1-0
A chemist's view of a protoplanetary disk
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UV, X-rays
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PDR layer
Molecular layer
Ice layer
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