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Chemical evolution from cores to disks

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

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Chemical evolution from cores to disks

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

  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 transferRADMC: Dullemond & Dominik 2004 • Physics compare well with hydrodynamical modelsYorke & Bodenheimer 1999, Brinch et al. 2008a,b • Density profiles compare well with observationsJørgensen et al. 2009 Visser et al. (2009), Visser & Dullemond (subm.), Visser et al. (in prep.)

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

  8. Where does material go to? Inside-out collapse gives a layered disk Start of collapse End of collapse intostar/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 intostar/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 CO HCOOH HNC HCN H2CO CH3OH grain-surface chemistry Visser et al. (in prep.) • Dotted line: hypothetical one-to-one relationship • Many model abundances differ from cometary abundances • Suggestive of mixing or grain-surface chemistry?

  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. 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 Kand 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 • 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 • 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

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