probing the conditions for planet formation in inner protoplanetary disks james muzerolle
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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 - PowerPoint PPT PresentationTRANSCRIPT
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Probing the Conditions for Planet Formation in Inner Protoplanetary Disks
James Muzerolle
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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?
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Disks: planetary birthplacesHow 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
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Context: the star formation paradigm
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Evolution: from primordial protoplanetary accretion disks
To planetary systems with debris disks
Fomalhaut debris disk, HST/ACSHD 141569 transition disk, HST/ACS
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Disk accretion in a nutshell
● flat disk in keplerian rotation
● gas accretes inward, angular momentum transferred outward
● disk structure for “alpha” disk model:
– dM/dt R-3/4
dM/dt provides a crucial constraint!
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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
Vinfall
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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
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Radiation from circumstellar disks● geometrically thin, optically thick
flat disk● heating from irradiation, viscous
dissipation
F = F* + Fvisc
~T*4 R*
3 ~dM/dt
T ~ R-3/4
=> F ~ -4/3
● most disks are flared
more flux at mid- to far-IR, > -4/3
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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
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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
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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?
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First evidence for dust disk evolution
Hillenbrand 2003
NIR excess: R~0.1 AU
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Gas evolution: mass accretion rates
viscous disk similarity solutions
70% 30% 5%accretor fraction:
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Probing cooler dust - Spitzer
MIR excess (< 10 m) R<~0.5 AU
Muzerolle et al. 2008
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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
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Flaherty & Muzerolle 2008
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)
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NGC 2068/2071
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NASA/JPL-Caltech/T. Pyle (SSC)
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
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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)
Calvet et al. 2005
CoKu Tau/4
D’Alessio et al. 2005
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Spitzer cluster survey● Transition disks identified via spectral slopes
Muzerolle et al. 2008
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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
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Lada et al. 2006
Carpenter et al. 2006
A0 G0 K0 M0
Mass-dependent disk dissipation
Upper Sco
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brown dwarf transition disk
● M6.5, M~0.075 Msun
● not accreting?
● inner hole size ~0.5-1 AU
Muzerolle et al. (2006)
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Inner disk clearing mechanisms
● photoevaporation
● dust grain growth
● giant planet formation
● binary dynamics??
Quillen et al. 2004
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Najita, Strom, & Muzerolle 2007
giant planet formation?
photoevaporation?
demographics Taurus disk masses, accretion rates:
transition disks occupy unique loci
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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
6 months 3 years
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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?
10/1/07
9/24/07
3/15/05
Artymowicz simulationVinkovic et al. 2006
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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
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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)