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Probing the Conditions for Planet Formation in Inner Protoplanetary Disks James Muzerolle 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? 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 Context: the star formation paradigm Evolution: from primordial protoplanetary accretion disks To planetary systems with debris disks HD 141569 transition disk, HST/ACS Fomalhaut debris disk, HST/ACS 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! Magnetospheric accretion Vinfall ● 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 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 Radiation from circumstellar disks ● ● geometrically thin, optically thick flat disk heating from irradiation, viscous dissipation Fn = F* ~T*4 R*3 + Fvisc ~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 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 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 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? First evidence for dust disk evolution NIR excess: R~0.1 AU Hillenbrand 2003 Gas evolution: mass accretion rates viscous disk similarity solutions accretor fraction: 70% 30% 5% Probing cooler dust - Spitzer MIR excess (< 10 mm) R<~0.5 AU Muzerolle et al. 2008 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 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 NGC 2068/2071 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) ● dust holes ~2-24 AU ● 2/3 still accreting gas ● inner optically thin disk in GM Aur ● CoKu Tau/4 is a circumbinary disk! Taurus (Ireland & Kraus 2008) CoKu Tau/4 D’Alessio et al. 2005 Calvet et al. 2005 Spitzer cluster survey ● Transition disks identified via spectral slopes Muzerolle et al. 2008 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 Mass-dependent disk dissipation Lada et al. 2006 A0 Upper Sco Carpenter et al. 2006 G0 K0 M0 brown dwarf transition disk Muzerolle et al. (2006) ● M6.5, M~0.075 Msun ● not accreting? ● inner hole size ~0.5-1 AU Inner disk clearing mechanisms Quillen et al. 2004 ● photoevaporation ● dust grain growth ● giant planet formation ● binary dynamics?? Taurus disk masses, accretion rates: transition disks occupy unique loci demographics giant planet formation? photoevaporation? Najita, Strom, & Muzerolle 2007 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 6 months 3 years ● > 30% of objects ● Daily – yearly timescales ● Amplitudes up to 1 mag 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? Artymowicz Vinkovic et al.simulation 2006 10/1/07 9/24/07 3/15/05 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 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)