Chemistry in evolving protoplanetary disks

1. Chemistry in evolving protoplanetary disks Ewine F. van Dishoeck Leiden Observatory + c2d team + many others, i.p. Fred Lahuis, Vincent Geers, Bastiaan Jonkheid Stars to Planets, Florida

2. Outline • Observational evidence for disk evolution • Chemistry in disks • Differences with molecular clouds • Parameter study for evolving Herbig Ae disks • Gas mass in the transitional disk around HD 141569

3. SEDs: different types of disks Flared Disk Geometrically flat disk (self shadowed disk) Models: e.g., Kenyon & Hartmann 1987, Chiang & Goldreich 1997, Dullemond et al. 2001, Calvet 2004, Dullemond & Dominik 2005, d’Alessio et al. 2005, …. Evolution from flared to flat geometry as grains grow and settle?

4. Evidence for grain growth: Silicate line profiles(continuum subtracted) 0.1mm 2.0 mm - Ratio of 11.3/9.7 mm fluxes is measure of flatness of profile Bouwman et al. 2001 Przygodda et al. 2003

5. Large fraction of T Tauri disks shows evidence for grain growth 10 mm band 20 mm band Data Obs Models Model Kessler-Silacci et al. 2006 Also: Furlan et al. 2006

6. Statistical analysis 10 mm band Small Large Kessler-Siliacci et al. 2006, 2007 • Large fraction of disks shows evidence for grain growth to a few mm • (first step in planet formation process) • Similar conclusion from 20 mm data • Includes several Brown Dwarfs Van Boekel et al. 2003, 2005 Przygodda et al. 2003 Spitzer GTO team

7. Evidence for grain growth inedge-on disks “Flying Saucer” Thermal emission Scattered light Pontoppidan et al. 2007 ApJL, in press Shape and depth of mid-IR “valley” very sensitive to grain size. For this source, grains at least ten mm in size are inferred.

8. But very small grains/PAHs can still be present • 4/ 38 T Tauri stars show PAH • Mostly G stars detected, one M star • PAHs results • Enhanced UV compared with stellar BB for some objects => accretion? chromosphere? • Absence in majority objects due to low PAH abundance: factor 10-100 low compared with ISM • 7.7 and 8.6 mm bands sometimes masked by silicate emission RR Tau PAH Optical, UV Geers et al. 2006 Visser et al. 2007 Dullemond et al. 2007

9. Separation of small and large grains 11.3 PAH 8.6 PAH 19.8 mm large grains IRS48 M0 star Oph PSF VLT VISIR image - Gap seen in large grains, but NOT in PAHs Geers et al. 2007, A&A, submitted

10. Cold/transitional disks around F & G stars Brown, Merin et al. 2007, ApJL Estimated outer radii of gap are >20 AU All 4 have PAH emission 3 out of 4 have gas inside gap (from CO 4.6 mm; Salyk et al. poster) Cycle 3 IRS Follow up of c2d candidates

11. Imaging gaps directly at submm Brown et al. poster SMA observations: Brown, Blake et al. 2007

12. [Ne II] in disks: tracer of X-ray/EUV radiation T Cha Cold disk • Detected in at least 20% of ~70 T Tauri stars • Fluxes consistent with recent models of X-ray irradiated disks Geers et al. 2006 Lahuis et al. 2007 Pascucci et al. 2007

13. [Ne II] [Ne III] [Fe I] Also hot H2 found in some disks Lahuis et al. 2007

14. Evolving disks • Grain growth and grain settling • Presence or absence of PAHs • Presence of intense UV and X-rays • High temperatures in inner disks for annealing/crystallization • Alternative: shocks • Development of gaps • Dust only? Is there gas inside the disk?

15. Three-layer chemical structure Tgas is larger • Most emission comes from warm • molecular layer B Photodissociation Freeze-out Aikawa et al. 2002 Van Zadelhoff et al. 2003 Semenov et al. 2006 Bergin et al. 2007 PPV midplane surface

16. PDRs in disks vs. clouds • Spectral shape radiation field: 30000=>4000 K • Affects H2, CO, C p.i., CN, N2, … • => rates as function of Teff • Resonance lines: Lyman a • => cross sections at 1216 Å • High intensity radiation fields up to 107xISRF • Grains grown to mm size • => photorates for larger grains • => H2 formation on grains suppressed • Gas/dust mass ratio not equal to 100 (> or <?) • Photodesorption/evaporation of ices Van Dishoeck et al. 2006, Far. Discussion

17. Importance of shape UV field A0 star Teff=10000 K Draine field • CH precursor of CO, but easily destroyed by stellar radiation • Limited CO and H2 photodissociation, C photoionization • => disk may consist largely of neutral atomic carbon

18. Importance of PAHs with grain growth • Formation of H2 • Formation of CH, CH+ => precursors of CO • Charge transfer • C+ + PAH/PAH- => C + PAH+/PAH • Absorbers of UV => shielding • Heating of gas

19. Disk evolution models Dullemond & Dominik 2004 • Overall mass loss: • 10-1 – 10-4 Msun, gas/small dust=100 • Dust evolution • growth and settling of dust grains • keep Mgas fixed at 10-1 Msun, increase gas/small dust from 102 to 106 • All models for Herbig stars • Main results • Gas temperature high in surface layers if small dust/PAHs present; very cold if small dust depleted • Intensity most molecules decreases with disk mass; atomic C becomes dominant at low disk mass (depending on shape radiation field) • Little change in H2,CO with changing gas/dust ratio; minor species affected Jonkheid et al. 2007

20. Transitional disk: HD 141569 IRAM PdB Massive gas-rich disk 12CO 2-1 Superposed on HST-STIS Debris disk Aim: constrain gas mass from CO observations Augereau, Dutrey et al. in prep

21. HD 141569 structure Trial gas Input Gas gas n ngas dust Dust T Deprojected HST-ACS scattered light image n ndust • Disk optically thin in UV • continuum • PAHs detected but at low • abundance • - Grains grown to a few mm Adopted dust and trial gas density distributions Jonkheid, Kamp et al. 2006

22. HD 141569 chemistry Radial distribution in midplane • Very low CO abundance if no gas in dust hole • Presence of PAHs affects H=>H2 and C+ => CO chemistry • (PAH abundance taken to fit observations) • CO abundance varies strongly with disk mass => tracer

23. Results CO lines • Best fit model has Mgas=75 Mearth; gas/dust close to 100 • Dust and gas disks evolve on similar timescales? • There is some gas in the inner hole • Not enough gas left to form a Jovian planet • Strong [C I] lines predicted (but not observed?) Zuckerman et al. 1995 Dent et al. 2005

24. Summary • Ample observational evidence for evolving disks • Grain growth + settling • Presence or not of PAHs • Gap formation • …. • Chemistry in disks different from PDRs • Shape UV field, X-rays • Grain growth • Importance of PAHs • Chemistry studies of evolving disks • Warm molecular layer shifts deeper with grain growth • Grain growth + settlingaffects mostly minor species, less H2 and CO • Mass loss affects all species • Gas may be left inside the dust gap; chemistry modeling essential to determine amount • Atomic [C I], [C II] as gas tracers in transitional disks?