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 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

Context: the star formation paradigm

Evolution: from primordial protoplanetary accretion disks

To planetary systems with debris disks

Fomalhaut debris disk, HST/ACSHD 141569 transition 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:

– dM/dt R-3/4

dM/dt provides a crucial constraint!

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

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

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

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

Hillenbrand 2003

NIR excess: R~0.1 AU

Gas evolution: mass accretion rates

viscous disk similarity solutions

70% 30% 5%accretor fraction:

Probing cooler dust - Spitzer

MIR excess (< 10 m) 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

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)

NGC 2068/2071

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

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

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

Lada et al. 2006

Carpenter et al. 2006

A0 G0 K0 M0

Mass-dependent disk dissipation

Upper Sco

brown dwarf transition disk

● M6.5, M~0.075 Msun

● not accreting?

● inner hole size ~0.5-1 AU

Muzerolle et al. (2006)

Inner disk clearing mechanisms

● photoevaporation

● dust grain growth

● giant planet formation

● binary dynamics??

Quillen et al. 2004

Najita, Strom, & Muzerolle 2007

giant planet formation?

photoevaporation?

demographics Taurus disk masses, accretion rates:

transition disks occupy unique loci

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

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

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)