chemistry in circumstellar disks: overview · 2004. 4. 29. · thermochemistry models with...
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April 15, 2004 Ringberg workshop
Chemistry in circumstellardisks: overview
Ewine F. van Dishoeck
Leiden Observatory
OutlineOutline
� Inner disk chemistry (<10 AU)� Thermochemistry� Models with transport and mixing
� Outer disk chemistry (10-400 AU)� Radial transport models� 1+1D, 2D models of static flaring disks� Tenuous disks
� Summary
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I. Inner disk chemistry (<10 AU)I. Inner disk chemistry (<10 AU)
� Large literature on chemical models of the solar nebula since 1970’s� E.g., Lewis, Prinn, Fegley, Lunine, ….
� Models applied to large range of solar system observations� E.g., CO/CH4 planetary atmospheres, comet abundances,
meteorites
� Chemistry thought to be dominated by thermal equilibrium (3-body) rather than kinetic (2-body) processes� 1 AU: n~1014 cm-3, T=800 K� 15 AU: n~1011 cm-3, T=80 K
ThermochemistryThermochemistry
� Principle chemistryCO + 3H2 ↔↔↔↔ CH4 + H2ON2 + 3H2 ↔↔↔↔ 2NH3
CO + H2O ↔↔↔↔ CO2 + H2
� Reactions proceed more to the right at lower T
� Fischer-Tropsch reactions on metallic iron can also convert CO to CH4
Lewis & Prinn 1980
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Quench surfaceQuench surface
� Equilibrium region: tchem < tmix
� Disequilibrium region: tchem>tmix� Chemical reactions too slow to be equilibrated; composition
similar to that at quench surface� E.g., CO and N2 dominant in solar nebula, since conversion to
CH4 and NH3 in outer region kinetically inhibited
Other processesOther processes� Ice condensation and evaporation: 4-20 AU (species
and time dependent)� Dust evaporation (<1 AU)� Weak (accretion) shocks
� Q: at what radius does most of the gas and dust enter disk?� Lightning� UV and X-rays from Sun/star� Radial and vertical mixing � Ionization by extinct radionuclides (26Al, 60Fe, …)� …..
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Modern modelsModern models� 1D models including radial mass transport
� E.g. Bauer et al. 1997, Finocchi & Gail 1997, Gail 2001-2004, Willacy et al. 1998, Aikawa et al. 1997, 1999
� 1+1D and 2D vertical models including UV, X-rays, but dynamics uncoupled� Markwick et al. 2002: no stellar radiation => cold surface� Nomura & Milar 2004: with stellar radiation => hot surface
Markwick et al. 2002
Chemical structuredominated by freeze-outand thermal desorptionfrom grains => T-structure crucial
Three-body reactions do not play a role (?)
Freeze-outCS
� Include mass transport processes in steady 1+1D model, both radial advection and vertical mixing � Vertical mixing much shorter timescale => static
atmosphere� No stellar irradiation in heating => cold upper layers� Move parcel of gas from 10 to 1 AU in 8.5x104 yr� Henning & Ilgner 2003, Ilgner et al. 2004, Gail et al. 2004
1 105 AU0
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Temperature distribution
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Models with/without vertical mixingModels with/without vertical mixing
CS
CS
Without
With
- CS more abundant at greater z at 8-10 AU due to vertical mixing- CS destroyed at <5 AU by reactions with O
(O higher because more H2O in gas phase?)
Ilgner et al. 2004=> Large effects on chemistry, but difficult to disentangle various effects
SO2
SO
No observational tests (yet)
II. Outer disk chemistryII. Outer disk chemistry
� Consider chemical evolution of parcel of gas as it moves radially from >100 AU to few AU
� Include large gas-phase chemistry network (few hundred species, few thousand reactions) and gas-grain adsorption/desorption processes
� Chemistry dominated by temperature profile: virtually no gas-phase molecules >10 AU, active gas-phase chemistry <10 AU� E.g., Bauer et al. 1997, Finocchi & Gail 1997, Gail 2001-2004, Willacy
et al. 1998, Aikawa et al. 1997, 1999
1D Radial transport models
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ExampleExample
Aikawa et al. 1999
Abundances after 3x106 yr
=> Everything frozen out at >10 AU
But this is NOT what is observed!
Intermezzo: observations
MWC 480OVRO
LkCa 15
MWC 480
CN 3-2 HCN 4-3
13CO 3-2 HCO+ 4-3
JCMT
Dutrey et al. 1997Kastner et al. 1997Qi 2001Thi et al. 2004
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Some observational findingsSome observational findings
� Simple gas-phase molecules observed� Ion-molecule reactions (HCO+)� Photon processes (high CN/HCN)� High deuterium fractionation (DCO+) � Low abundances complex species (H2CO, CH3OH)
� Data only sensitive to >50 AU� Lines comes from warm 20-40 K layer with n=106-108 cm-3
� Disk-averaged abundances are “depleted” by factor of 5-100 (using mass from dust continuum and assuming gas/dust=100)
� Solid CO and H2O detected in edge-on disks
Detection of DCO+ in a circumstellar disk
TW Hya face-on disk1.1 µµµµm 1.6 µµµµm
HCO+/2
H13CO+
DCO+Scattered light => radius 200 AU
Weinberger et al. 2002
van Dishoeck et al. 2003
DCO+/HCO+=0.035 => gas in disks is cold with heavy depletions
JCMT
Dutrey et al. 1997Kastner et al. 1997
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Temperature structureTemperature structure
40 K50
602010
12CO 3-2
13CO
- 12CO ττττ=1 surface near top of disk- 13CO emission from entire disk Van Zadelhoff et al. 2001
Dartois et al. 2003
Starting to image the chemistry in disks
Kessler, Qi, Blake et al. 2003
OVRO mm array
Aikawa et al. 2003
CO HDO
HCO+ CN
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Flaring disk models (>50 AU)Flaring disk models (>50 AU)� Calculate chemistry in vertical and radial
directions in 1+1D static flaring models� Aikawa et al. 1999, 2001:
� Kyoto minimum solar mass disk model � Low temperatures => needed artifically low sticking coefficient
S=0.03 to match observations� Willacy & Langer 2000
� Two-layer Chiang & Goldreich model� All molecules photodissociated in warm layer� All molecules frozen on grains in cold layers => needed high
photodesorption rate to match observations� Aikawa et al. 2002, van Zadelhoff et al. 2003
� D’Alessio et al. models with continuous T,n gradient� Warm molecular layer where molecules stay off the grains even
with S=1
� No radial or vertical mixing included, but models allow important processes and parameters to be identified
Chemical structure of disks- Surface layer: molecules dissociated by UV photons- Warm intermediate layer: molecules not much depleted, rich chemistry- Cold midplane: molecules heavily frozen out
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Model flowchartModel flowchart
Vertical structure (R=200 AU)Vertical structure (R=200 AU)
midplane surface
PhotodissociationFreeze-out
Tgas is larger
Ionization fraction:- Surface: 10-4 (C+)- Intermediate: 10-9 (HCO+)- Midplane: ~10-11 (H3
+,H2D+)
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A few words about chemistryA few words about chemistry
� Chemistry in upper layers resembles that of a PDR � Photodissociation H2 and CO only at 912-1100 Å� Important to treat H →→→→H2 and C+ →→→→C→→→→CO transitions accurately
(Leiden PDR benchmark workshop), e.g., sensitivities to self-shielding, mutual shielding, PAH abundance, …
� Photorates are rapid => chemical equilbrium in ~103 yr� High abundances of radicals (CN, C2H, …) in PDR zone
Gas-phase chemistry
0 2 4 6AV
A few words about chemistryA few words about chemistry
� Elemental abundances (e.g. high vs. low metals) and cosmic ray ionization rate important input parameters
� Gas-phase chemistry not very sensitive to temperature in 10-200 K regime
� Many different chemical networks containing a few hundred to a few thousand reactions => reduction?� Wiebe, Semenov et al. 2003, 2004
� Best agreement with well-studied PDRs and dark clouds is a factor of a few – ten => better agreement for disks would be a miracle!
Gas-phase chemistry
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A few words about chemistryA few words about chemistry
� Freeze-out/thermal desorption depend sensitively on dust temperature profile� Species dependent: CO T>20 K, H2O T>100 K; binding
energies not well known for all species and depend on type of ice or surface
� Fundamental issues with formulation grain surface reactions (diffusion-limited vs. accretion-limited)
� Timescales: tads~2x109/nH yr => strongly dependent on density
Gas-grain interactions
Recent efforts: importance of shape Recent efforts: importance of shape and treatment UV radiation fieldand treatment UV radiation field
Van Zadelhoff et al. 2003
Normal ISRF: ~1x108 ph s-1 cm-2
�Enhanced by factor > 1000 in upper layers
Full 2D calculationincluding absorptionand scattering
Penetration depthdepends on opticalproperties grains
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Full 2D vs. 1+1DFull 2D vs. 1+1D
1+1D
Full 2D:CO and H2not p.d. by star
Full 2D:CO and H2p.d. by star
Van Zadelhoff et al. 2003
Effect of stellar UVEffect of stellar UV
Green
Red
Blue
=> Molecules extended to greater height if no UVVan Zadelhoff et al. 2003
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Observational diagnostics stellar UVObservational diagnostics stellar UV
- Difficult to disentangle models with single-dish data- Need ALMA resolution to probe variations- HCN dissociated by Ly αααα, CN not
Importance of Lyman Importance of Lyman αααααααα
-Photodissociation rate kpd =���� σ(λ)σ(λ)σ(λ)σ(λ)I(λλλλ)dλλλλ-Some molecules are dissociated by Lyαααα (e.g., H2O, HCN), others
are not (e.g. CN, CO, H2)Bergin et al. 2003
BP TauTW Hya
Observed FUV spectra of T Tauri stars
ISRFx540
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Tenuous disks: Tenuous disks: ββββββββ PictorisPictoris
Kamp & Bertoldi 2000, Kamp & Samnar 2004
- Disks are optically thin to UV and IR radiation => analytic solution for Tdust
- Use stellar atmosphere model for stellar radiation- Recent work: extension to G-stars including chromosphere
CO CO vsvs HH22Stellar UV Stellar + interstellar UV
=> Absence of CO does not mean absence of H2!Kamp & Bertoldi 2000
CO p.d.
H2 self-shielding
Note: both CO and H2 are only dissociated at 912-1100 Å
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CO gas versus iceCO gas versus iceStellar UV Stellar + interstellar UV
CO gas
CO ice
C and CC and C++ as gas tracers of tenuous disksas gas tracers of tenuous disks
0.2 MEarth
2 MEarth
C
C+
ββββPic
2 MEarth 0.2 MEarth
Kamp et al. 2003
Vdrift=max Vdrift=0
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Summary chemistrySummary chemistry� Inner disks: highly sophisticated models including radial
transport and vertical mixing being developed, but no observational tests yet (need full ALMA with 64x12m)
� Outer disks: 1+1D static flaring disk models reproduce submillimeter lines of simple species within factors of few� Emission comes from intermediate warm layer� Chemistry in warm layer dominated by photodissociation and
thermal desorption => important to know Tdust and UV field accurately
� Molecules strongly depleted in cold midplane => high deuteration fraction
� CO is not a good gas mass tracer (alternatives H2, C, C+ to be determined)
� Importance of vertical mixing to be determined: can chemistry put limits?
Summary chemistrySummary chemistry
� Does chemistry affect disk structure?� Gas temperature in upper disk layers can be
significantly higher than that of dust (see talks tomorrow) => affects vertical structure and line emission; chemistry determines abundances coolants O, C+, C and CO
� Freeze-out and dust destruction modify grain opacities
� Ionization fraction => affects magnetohydrodynamics (but can probably be estimated by small network)
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