1. Evolution of Gas in Disks Joan Najita National Optical Astronomy Observatory Steve Strom John Carr Al Glassgold

2. Evolution of Gas in Disks: Outline Why do we care? Observational tools to study gas in disks » In situ probes of gas in disks » Stellar accretion rates Using gas to probe the evolutionary status of » Transitional T Tauri stars » Weak T Tauri stars

3. Constrains planet formation processes and outcomes What are total (gas) disk masses? (>0.1 M*?) Lifetime of gas in the giant planet region? ( M J, 10Myr?) » Constrains mode of giant planet formation Role of Gas in Planet Formation Gravitational Instability (rapid, large masses) Core Accretion (slow, modest mass)

4. Role of Gas in Planet Formation Lifetime of gas in the terrestrial planet region? » Residual gas affects terrestrial planet M p, e, habitability via gas drag, resonances (Kominami & Ida; Agnor & Ward) ( 1 g/cm 2 at 1 AU? cf. 1500 g/cm 2 MMSN) Copyright Lynette Cook, used with permission

5. How do Gas Disks Evolve? Theoretical Expectation »Accretion (inner) + spreading »Photoevaporation (outer) »Planet formation 10 AU Hollenbach et al. 2000 Copyright Lynette Cook, used with permission

6. How do Gas Disks Evolve? Observationally »Dust excess and stellar accretion decline with age »Gas? Disk dissipation or grain growth? Debris production? How does scale with Mdot? Haisch et al. 2001 IR Excess Fraction Age (Myr) log (Accretion rate) log (Age/yr) Muzerolle et al. 2005

7. How do Gas Disks Evolve? Gas drag: rapid inspiral of dust in outer disk? »Grain growth to mm-sizes observed »Dissipates solids in 1Myr; long-lived gaseous disk? 10 6 Dust Gas Surface density Distance (AU) Takeuchi & Lin 2005

8. Tools: Probes of Gaseous Disks Terrestrial Planet Region: NIR CO, OH, H 2 O UV H 2 Kuiper Belt and Beyond: Millimeter transitions; NIR H 2 ro-vib; Optical atomic lines Copyright Lynette Cook, used with permission Giant Planet Region: Mid-IR atomic and molecular transitions

9. Kuiper Belt and Beyond: Millimeter Molecular Transitions Strengths CO: abundant, low n crit Disk sizes (> 100 AU) Disk rotation, M *, i Challenges Warm surface layer emission + midplane condensation; disk masses from dust Probes mainly large radii Simon et al. 2000 Aikawa et al. 2002

10. Millimeter Molecular Surveys CO Surveys Zuckerman et al. (1995): dissipation < 1 Myr » Depletion a concern More recent studies: WTTS, AeBe stars » 1 WTTS detected; limited sensitivity » Gas can survive (to 7Myr, >20 AU), mass uncertain (e.g., Duvert et al. 2000; Thi et al. 2001; Dent et al. 2005) Current Status/Future? Diagnostics e.g., HCO+ probe higher densities, smaller radii ALMA sensitivity + angular res. will probe < 30 AU Models needed to derive mass Greaves et al. 2004 HCO+ 4-3

11. Giant Planet Region: MIR Transitions Strengths Atomic and molecular lines (e.g. H 2 ) may be detectable Probes warm 100K) gas H 2 in gas phase, dominates mass Challenges Models needed to convert warm H 2 mass to total Depend on assumed disk structure High res spectra reduce ambiguity Gorti & Hollenbach 2004

12. Richter et al. 2005AB Aur TEXES/IRTF 17 m H 2 Giant Planet Region: MIR Transitions Surveys ISO: M J gas in 20 Myr systems » low n crit, extended emission? Unconfirmed by Spitzer or from ground » high angular + spectral res. (Thi et al.; Richter et al., Sheret et al., Sako et al.) Challenges Narrow width (r > 10 AU) Weaker emission from small r req. e.g., TEXES/Gemini, TMT Chen et al. (2004) Pic: 17 m Non-detection Spitzer M(warm H 2 ) < 11 M E

13. CO v=1-0, 2-1, 3-2, 13 CO lines detected. Najita et al., Brittain et al., Blake & Boogert Strengths Common 100% CTTS Probes R in to 1-2 AU 70 km/s FWHM Surprisingly warm gas 1000K gas cf. <400K dust Wide range of column density 10 -4 -- 1 g/cm 2 4.7 m CO Emission from CTTS Terrestrial Planet Region: CO v=1 Emission

14. Terrestrial Planet Region: Models TgTg TdTd X-rays Accretion =1 Gas-Dust =0.1 R = 1 AU H-H- 3-body Neutral reactions C/CO H/H 2 H2OH2O CO emission from warm, mid-z region ( > 0.001 g cm -2 ) Heated by accretion and X-rays Glassgold et al. (2004)

15. TW Hya UX Tau A: H EW=4A Age ~ 8 Myr V836 Tau: H EW=9A Mdot=4x10 -10 Age ~ 3 Myr TW Hya: Mdot=4x10 -10 -2x10 -9 Age ~ 8 Myr Gas in optically thin inner disks Najita et al; Rettig et al., Blake et al. CO Emission from Weak/Transitional TTS

16. Terrestrial Planet Region: CO v=1 Emission Challenges Emission strength correlates with accretion Structure in stellar photosphere Models needed to infer total column densities K-L Mass Accretion Rate Classical TTS Carr, Najita 2005 TW Hya 4.6 m CO Model stellar atmosphere * * * * *

17. Indirect Tool: Stellar Accretion For steady accretion: ~ Mdot / Strengths Given Mdot &, infer Independent measure Challenges Is relation valid? What is ? Measuring low Mdot One approach Use measured to determine Is a constant (with r and from source to source)? Muzerolle et al. 2005

18. Stellar Accretion Rates ~ Mdot / If = constant… Wide range in at any age » ~ 100 g/cm 2 at 1 AU for Mdot = 10 -8, =0.01 Long-lived gaseous disks WTTS V836 Tau: 3 Myr, Mdot=4x10 -10 or 4 g/cm 2 at 1AU TTS St34: 25 Myr, Mdot=2x10 -10 or 2 g/cm 2 at 1AU Dynamically significant e.g., producing Earth-like M p, e Muzerolle et al. 2005

19. Evolutionary Status: Transitional TTS Definition Photospheric at short, excess at long 10% of TTS Nature? Formed giant planets? Formed planetary cores? Photoevaporation + viscous dissipation? Constrains timescales either for forming planetary cores, accreting gaseous envelopes, or dissipating disks Median Taurus SED TW Hya Calvet et al. 2002 Quillen et al. 2004 log F Clarke et al. 2001 Surface density Distance

20. Case Study: TW Hya = 32g/cm 2 at 20 AU (SED) Outer disk is too massive for photoevap to create inner hole Mdot=5x10 -10 -5x10 -9 Evolutionary Status: Transitional TTS 0.1 g/cm 2 at 20AU,13Myr Cores or Planets? If =0.01, =5-50 g/cm 2 at 1AU giant planet formation? If =0.0003, =100-1000 g/cm 2 core formation? Measurements of disk gas content can resolve this

21. Evolutionary Status: Weak TTS Definition Weakly/non-accreting No IR excess 50% of TTS Ages 0.1--10 Myr Nature? Small initial M disk ? failed PF Large initial M disk ? successful PF Rapid inspiral of dust possible PF? Hartmann & Kenyon 1995 Surface density Distance (AU) Dust Gas Takeuchi et al. 2005 Mayer et al.

22. Evolutionary Status: Weak TTS Nature? Large vs. small initial M disk ? » Gravitational instability is quick; WTTS < 1 Myr old? » Search for massive, distant planetary companions Rapid inspiral of dust » Planet formation possible if cores have formed » Search for gas reservoir Measuring disk gas content + companion search can resolve this Dust Gas

23. Summary: Evolution of Gas in Disks An interesting problem! Disk masses and gas dissipation timescales: Constrain mode(s) of giant planet formation Outcome of terrestrial planet formation Giant planet migration, etc.

24. Summary: Evolution of Gas in Disks Interesting but difficult! Past decade: development of many probes of gas in disks. -UV, optical, IR, millimeter In situ diagnostics require -High sensitivity, high resolution observations (e.g., Spitzer, TEXES/Gemini, ALMA, TMT) -Reliable thermal-chemical models of disks Stellar accretion rates: dynamically significant reservoirs survive 10 Myr in some systems -How much gas and how frequently requires calibration of Mdot with gas measurements

25. Summary: Evolution of Gas in Disks Gas content probes evolutionary status Transitional TTS: constrains timescales for »Forming planetary cores »Accreting gaseous envelopes »Photoevaporating disks Weak TTS: »Failed »Successful »Possibly ongoing planet formation? Constrain planet formation processes and outcomes