Transcript Transition disks

Probing the Conditions for Planet Formation in
Inner Protoplanetary Disks
James Muzerolle
Motivation: diversity of planetary systems
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wide range of system
architectures: periods, masses,
eccentricities
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unexpected “hot Jupiters”, multiple
planets in resonances
wide range of parent star
properties
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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
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flat disk in keplerian rotation
gas accretes inward, angular
momentum transferred
outward
disk structure for “alpha” disk
model:
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S ~ dM/dt R-3/4
 dM/dt provides
a crucial constraint!
Magnetospheric accretion
Vinfall
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ballistic motion along magnetic field lines
– Vinfall ~ (GM*/R*)1/2
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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
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Standard method: UV excess from the
accretion shock
LUV ~ Lacc ~ GM*/R* dM/dt
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limited to low extinction,
low mass stars
Alternate method: emission line profiles
from magnetospheric accretion flows
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depends on radiative transfer modeling
Radiation from circumstellar disks
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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
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most disks are flared
 more flux at mid- to far-IR,
a > -4/3
Flared vs. settling
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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
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Radiative transfer modeling
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Gas emission line profiles from accretion flows
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SED models of disk structure
Optical/infrared observation
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Optical photometry & spectroscopy – ages, masses, accretion activity
of young stars
Infrared imaging & spectroscopy – dust emission from circumstellar
disks
Protoplanetary disk evolution
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What mechanism(s) drive disk evolution and dissipation?
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Is the dust and gas dissipation coupled?
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Is disk clearing radially dependent?
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Are there dependences on stellar mass, age, environment?
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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?
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Spectral slope probing
dust at r < 0.5 AU
decrease in mean value
at older ages
– precursor to dissipation?
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large dispersion at any
given age!
Hernandez et al. 2007
Disks in embedded clusters: NGC 2068/2071
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t~1-2 Myr
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~75% disk fraction
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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
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Understanding how protoplanetary disks dissipate:
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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?
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Do disks clear from the inside-out?
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Is there a dependence on mass or age?
Transition disks: where the clearing process has begun
NASA/JPL-Caltech/T. Pyle (SSC)
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dust holes ~2-24 AU
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2/3 still accreting gas
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inner optically thin disk in GM Aur
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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
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Transition disks identified via spectral slopes
Muzerolle et al. 2008
Spitzer statistics
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Transition phase appears even at
t <~ 1Myr
~1% of stars
 fast? 104 – 105 yrs
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fraction increases with age
~5-15% at 3-10 Myr
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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)
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M6.5, M~0.075 Msun
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not accreting?
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inner hole size ~0.5-1 AU
Inner disk clearing mechanisms
Quillen et al. 2004
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photoevaporation
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dust grain growth
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giant planet formation
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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
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Disks are not perfect axisymmetric structures!
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Accretion is known to be non-steady….
New time-series Spitzer
observations show common
mid-IR varability in YSOs
6 months
3 years
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> 30% of objects
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Daily – yearly timescales
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Amplitudes up to 1 mag
Variable transition disks
Surprising wavelength
dependence, timescales as
short as 1 week!
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warp or corotating
dynamical structure?
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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
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detailed follow-up of transition disks and other evolved systems
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systematic study of accretion via line profiles, veiling
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mm measurements of disk masses
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high spatial resolution imaging  binarity (WFC3, NICMOS)
multi-wavelength follow-up of mid-IR variables
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optical/NIR photometry – occultation events?
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variations of accretion signatures
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spectropolarimetry, high resolution polarimetric imaging (NICMOS)
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NIR veiling
expand age and environment baselines
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mass accretion rates of young protostars (COS, NIRSPEC)
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disk properties as a function of external UV environment
Further in the Future: JWST and beyond
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Detect optically thin dust around T Tauri stars
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early debris disks?
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Expand environmental samples
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Simultaneous measures of accretion, disk gas tracers
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Follow-up of dust structures implied by Spitzer SEDs
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high-resolution IR imaging of scattered light from evolved disks to
look for further evidence of dust sedimentation
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eventually resolve inner holes and the massive
planets that may create them? (ALMA, TMT/GMT)