Probing the Conditions for Planet Formation in Inner Protoplanetary Disks James Muzerolle

1. Probing the Conditions for Planet Formation in Inner Protoplanetary Disks • James Muzerolle

2. Motivation: diversity of planetary systems • wide range of system architectures: periods, masses, eccentricities • unexpected “hot Jupiters”, multiple planets in resonances • wide range of parent star properties • all masses yet surveyed, some metallictiy dependence Is the solar system atypical?

3. Disks: planetary birthplaces • How do planets form from circumstellar disks? • how do the gas and dust components of disks evolve? • what is the range of disk lifetimes? • is disk dissipation directly related to planet formation? • focus on the inner ~5 AU of protoplanetary disks: • accretion indicators to probe gas content at star-disk interface • infrared continuum excess at <24 micron to probe warm dust in the planet formation region of disks • identify and characterize disks in the process of being cleared out

4. Context: the star formation paradigm

5. Evolution: from primordial protoplanetary accretion disks To planetary systems with debris disks HD 141569 transition disk, HST/ACS Fomalhaut debris disk, HST/ACS

6. Disk accretion in a nutshell • flat disk in keplerian rotation • gas accretes inward, angular momentum transferred outward • disk structure for “alpha” disk model: • S ~ dM/dt R-3/4  dM/dt provides a crucial constraint!

7. Vinfall Magnetospheric accretion • ballistic motion along magnetic field lines • Vinfall ~ (GM*/R*)1/2 • most disk material accreted onto star, ~10% lost in wind • emission produced in the flow can be used to trace disk mass accretion rate

8. determine dM/dt as a function of mass & age to trace the evolution of gas in accretion disks • Standard method: UV excess from the accretion shock LUV ~ Lacc ~ GM*/R* dM/dt • limited to low extinction, low mass stars • Alternate method: emission line profiles from magnetospheric accretion flows • depends on radiative transfer modeling

9. Radiation from circumstellar disks • geometrically thin, optically thick flat disk • heating from irradiation, viscous dissipation Fn = F* + Fvisc ~T*4 R*3 ~dM/dt T ~ R-3/4 => nF ~ na , a = -4/3 • most disks are flared more flux at mid- to far-IR, a > -4/3

10. Flared vs. settling • Dust & gas well-mixed, vertical hydrostatic equilibrium  T ~ R-3/4, H ~ R9/8  flared surface • Grain growth – settling of large grains to midplane, reduced opactiy in irradiation surface – decrease MIR flux

11. Tools • Radiative transfer modeling • Gas emission line profiles from accretion flows • SED models of disk structure • Optical/infrared observation • Optical photometry & spectroscopy – ages, masses, accretion activity of young stars • Infrared imaging & spectroscopy – dust emission from circumstellar disks

12. Protoplanetary disk evolution • What mechanism(s) drive disk evolution and dissipation? • Is the dust and gas dissipation coupled? • Is disk clearing radially dependent? • Are there dependences on stellar mass, age, environment? • Can we see indirect evidence of planet formation?

13. First evidence for dust disk evolution NIR excess: R~0.1 AU Hillenbrand 2003

14. Gas evolution: mass accretion rates viscous disk similarity solutions accretor fraction: 70% 30% 5%

15. Probing cooler dust - Spitzer MIR excess (< 10 mm)R<~0.5 AU Muzerolle et al. 2008

16. Dust evolution via grain growth & settling? • Spectral slope probing dust at r < 0.5 AU • decrease in mean value at older ages – precursor to dissipation? • large dispersion at any given age! Hernandez et al. 2007

17. Disks in embedded clusters: NGC 2068/2071 • t~1-2 Myr • ~75% disk fraction • some disks with smaller excess at 3.6-8 and 8-24 microns • correlation of accretion activity with SED shape? • two “transition” disks (2% of total disk population) Flaherty & Muzerolle 2008

18. NGC 2068/2071

19. disk dissipation: transition disks • Understanding how protoplanetary disks dissipate: • What are the mechanisms for primordial disk dissipation? • What are the time scales? Does the gas go away at the same time as the dust? • Do disks clear from the inside-out? • Is there a dependence on mass or age? • Transition disks: where the clearing process has begun NASA/JPL-Caltech/T. Pyle (SSC)

20. Taurus • dust holes ~2-24 AU • 2/3 still accreting gas • inner optically thin disk in GM Aur • CoKu Tau/4 is a circumbinary disk!(Ireland & Kraus 2008) CoKu Tau/4 D’Alessio et al. 2005 Calvet et al. 2005

21. Spitzer cluster survey • Transition disks identified via spectral slopes Muzerolle et al. 2008

22. Spitzer statistics • Transition phase appears even at t <~ 1Myr ~1% of stars  fast? 104 – 105 yrs • fraction increases with age ~5-15% at 3-10 Myr • span full range of stellar spectral types, but less common in M stars? • mix of accretors & non-accretors Muzerolle et al. 2008

23. A0G0K0M0 Mass-dependent disk dissipation Lada et al. 2006 Upper Sco Carpenter et al. 2006

24. brown dwarf transition disk • M6.5, M~0.075 Msun • not accreting? • inner hole size ~0.5-1 AU Muzerolle et al. (2006)

25. Inner disk clearing mechanisms • photoevaporation • dust grain growth • giant planet formation • binary dynamics?? Quillen et al. 2004

26. giant planet formation? photoevaporation? demographics Taurus disk masses, accretion rates: transition disks occupy unique loci Najita, Strom, & Muzerolle 2007

27. 6 months 3 years A new wrinkle: variability • Disks are not perfect axisymmetric structures! • Accretion is known to be non-steady…. New time-series Spitzer observations show common mid-IR varability in YSOs • > 30% of objects • Daily – yearly timescales • Amplitudes up to 1 mag

28. 10/1/07 9/24/07 3/15/05 Vinkovic et al. 2006 Artymowicz simulation Variable transition disks Surprising wavelength dependence, timescales as short as 1 week! • warp or corotating dynamical structure? • may betray the presence of a giant planet or brown dwarf companion • variable accretion/dusty winds?

29. Next Steps • detailed follow-up of transition disks and other evolved systems • systematic study of accretion via line profiles, veiling • mm measurements of disk masses • high spatial resolution imaging  binarity (WFC3, NICMOS) • multi-wavelength follow-up of mid-IR variables • optical/NIR photometry – occultation events? • variations of accretion signatures • spectropolarimetry, high resolution polarimetric imaging (NICMOS) • NIR veiling • expand age and environment baselines • mass accretion rates of young protostars (COS, NIRSPEC) • disk properties as a function of external UV environment

30. Further in the Future: JWST and beyond • Detect optically thin dust around T Tauri stars • early debris disks? • Expand environmental samples • Simultaneous measures of accretion, disk gas tracers • Follow-up of dust structures implied by Spitzer SEDs • high-resolution IR imaging of scattered light from evolved disks to look for further evidence of dust sedimentation • eventually resolve inner holes and the massive planets that may create them? (ALMA, TMT/GMT)