1. Chemical evolution from cores to disks
Ruud Visser
Leiden Observatory
Ewine van Dishoeck, Steve Doty, Kees Dullemond , Jes Jørgensen, Christian Brinch, Michiel Hogerheijde, John Black
November 2, 2009

2. Low-mass star formation Evolution of gas and dust
Herbst & van Dishoeck (2009)

3. Main features of this study
One model from pre-stellar core to circumstellar disk
Two-dimensional, axisymmetric
Study chemical evolution
Goal in itself
Chemistry affects physics: temperature, MRI, ...
Diagnostic tool

4. Motivation How do size and mass of disk evolve?
When is the disk first formed?
How does matter flow from envelope to disk?
Explain observed cometary abundances
Existing models
treat only the envelope or only the disk, or both in 1D
often approximate temperature

5. Comets: a view of the past
Error bars indicate spread between sources
Dotted line: hypothetical one-to-one relationship
Comets in general similar to ISM, but individual differences exist CO
CH3OH
HCN
H2CO
HNC
HCOOH
Bockelée-Morvan et al. (2000, 2004), Schöier et al. (2002)

6. Analytical star formation model in 2D
Fast to run, high resolution, easy to change initial conditions
Cloud mass, rotation rate, sound speed, ...
Density & velocity: inside-out collapse Shu (1977), Terebey, Shu & Cassen (1984)
Dust temperature (important!) from full radiative transfer RADMC: Dullemond & Dominik 2004
Physics compare well with hydrodynamical models Yorke & Bodenheimer 1999, Brinch et al. 2008a,b
Density profiles compare well with observations Jørgensen et al. 2009
Visser et al. (2009), Visser & Dullemond (subm.), Visser et al. (in prep.)

7. From one to two dimensions
streamlines
disk surface
Previous collapse models treated disk as completely flat
Include vertical structure: accretion occurs further out
Visser & Dullemond (subm.)

8. Where does material go to?
Inside-out collapse gives a layered disk
Start of collapse End of collapse
into star/ outflow
Visser et al. (2009)

9. Infall trajectories
Need to solve chemistry dynamically: compute n, T along many trajectories
Many different trajectories
Jump in n, T upon entering disk
entering disk
Visser et al. (2009) , Visser et al. (in prep.)

10. Chemical evolution along one trajectory
A: volatiles evaporate (e.g. CO, N2)
B: intermediates evaporate (e.g. CH4, NO)
C: other ices evaporate (e.g. H2O, NH3, CH3OH)
photodissociation of many species
D: some species reformed
outflow wall
infall trajectory
disk surface
Visser et al. (in prep.)

11. Chemical zones: CO gas/ice
Start of collapse End of collapse
into star/ outflow
Red: CO remains adsorbed (pristine!)
Green: CO desorbs and re-adsorbs
Pink: CO desorbs and remains desorbed
Blue: multiple desorption/adsorption
Visser et al. (2009)

12. Chemical evolution of water
Before collapse: water frozen out onto cold dust
1. Always frozen
2. Evaporates, photodissociated, reformed, freezes out
3. Evaporates, photodissociated, reformed
4. Evaporates, photodissociated
5. Evaporates
6. Evaporates, freezes out, evaporates
7. Evaporates, freezes out
H2Oice
H2O
H2Oice O
H2Oice
H2O
Visser et al. (in prep.)

13. Chemical evolution of water
Before collapse: water frozen out onto cold dust
1. Always frozen
2. Evaporates, photodissociated, reformed, freezes out
3. Evaporates, photodissociated, reformed
4. Evaporates, photodissociated
5. Evaporates
6. Evaporates, freezes out, evaporates
7. Evaporates, freezes out
Visser et al. (in prep.)

14. Chemical evolution of water
Before collapse: water frozen out onto cold dust
1. Always frozen
2. Evaporates, photodissociated, reformed, freezes out
3. Evaporates, photodissociated, reformed
4. Evaporates, photodissociated
5. Evaporates
6. Evaporates, freezes out, evaporates
7. Evaporates, freezes out
Comet-forming zone
Visser et al. (in prep.)

15. Implications for comets
Dotted line: hypothetical one-to-one relationship
Many model abundances differ from cometary abundances
Suggestive of mixing or grain-surface chemistry? CO
HCOOH
HNC
HCN
H2CO
CH3OH
grain-surface chemistry
Visser et al. (in prep.)

16. Another application of our model: dust
Carbonaceous material
Graphite (small-scale limit: PAHs)
Hydrogenated amorphous carbon
Diamonds
Silicates
Amorphous (olivine, pyroxene, ...)
Crystalline (forsterite, enstatite, ...)
Ices
H2O, CO, CO2, CH3OH, NH3, ...

17. From crystalline to amorphous and back
up to 50% crystalline
0–2% crystalline
up to 50% crystalline
1–30% crystalline

18. How can we explain these cold crystalline silicates?
Dust processing
ISM: 1% crystalline; disks: 1–30% crystalline
Crystallization by thermal annealing requires 800 K
Works for material accreting close to the star
Crystalline silicates observed down to 150 K and in comets formed in 100–200 K regions
How can we explain these cold crystalline silicates?

19. Disk evolution model Inside-out collapse with rotation
Dust accreting in hot inner region is crystallized
Disk spreads out to conserve angular momentum
Crystalline material transported to colder areas
Dullemond, Apai & Walch (2006), Visser & Dullemond (subm.)

20. Crystalline fractions
Black: 2006 model
Red:
new model
time
obs.
0.3 Myr
1.0 Myr
3.2 Myr
Model results in good agreement with observed range!
Visser & Dullemond (subm.)

21. Implications for comets
Observations
Model
Silicate dust
Partially crystalline
Partially amorphous
Chemical composition
Generally similar to ISM
Individual differences
Silicate dust
Partially crystalline
Partially amorphous
Chemical composition
Outer disk unprocessed
Inner disk processed
Cometary material is of mixed origins

22. Caveats Gas temperature Shape of stellar spectrum
Gas-phase production of water
Increase inner-disk scale height
Shape of stellar spectrum
Allow photodissociation of H2, CO, N2
Trapping of volatiles in water ice
Increase ice abundances of CO, N2, O2, ...
Mixing
Chemical zones in disk more diffuse
Crystallinity paradox?

23. Future work Compute line profiles Add grain-surface chemistry
Compare with observations by SMA, JCMT, IRAM 30m, ...
Analyse water data from Herschel (WISH key program)
Make predictions for ALMA
Add grain-surface chemistry
Formation of complex organics
Add isotope-selective CO photodissociation
New model: Visser, van Dishoeck & Black (2009)

24. Conclusions
First model to go from pre-stellar cores to circumstellar disks in 2D
Important to do collapse and disk formation in 2D
Disk is divided into zones with different chemical histories
Outer part pristine, inner part processed
Cometary material is of mixed origins