Protoplanetary Disks: The Initial Conditions of Planet Formation

Eric MamajekUniversity of Rochester, Dept. of Physics & Astronomy

Astrobio 2010 – Santiago de Chile – 15 January 2010

Special thanks to:

Michael Meyer (U. Arizona, ETH Zurich)

Dan Watson (U. Rochester)

Spitzer Early Release Observations

Why do circumstellar disks matter?

- initial conditions of planet formation.

- trace evolution of planetary systems.

- attempt to place our solar system in context.

Mayor & Udry (2008)

Motivation to understand disks:

The formation and evolution of planetary systems

Mayor & Udry (2008)

Motivation to understand disks:

The formation and evolution of planetary systems

Cloud collapse

104 yr

Planetary system+ debris disk

109 yr

105 yr

100 AU

107 yr

Tstar (K)

Lstar/LSun

Main sequence

8,000 5,000

10

1

2,000

Protostar+primordial disk

Planet building

Pre-main Sequence Evolution

Evolution of Circumstellar Disks

Primordial “Accretion Disks” Gas-rich, survive ~106-7 years.

Dusty “Debris Disks” Gas-poor, dusty disks seen around stars of all ages.

But dust lifetimes are ~103-106 yrs (blowout, PR drag). Hence planetesimal reservoirs needed!

What disk properties do we care about?*

Total disk mass: Mdisk, Mdisk/M*

Outer & inner radii: Rout, Rin

Surface density profile: Σ(r) = Σo r-p

Dust grain size distribution: n(a) ~ no a-q ; amin, amax

Dust grain opacity law: κν ~ νβ

Optical depth: τν = κν Σ(r)

Temperature profile: T(r) ~ To r-q

Scale height, Midplane density: H(r), ρo(r)

Viscosity: νv = α cs H ~ νvo rγ (MRI?)

Composition (gas, dust), Ionization, Azimuthal asymmetry,

etc.* While you are at it… we want to know the statistical moments of these parameters vary as a

function of stellar parameters, orbital radius, birth environment, and TIME!

Mass Time Disk Surface Density

Orbital Radius Primary Mass

An Analytical Estimate of Protoplanet Growth

Lodato et al.(2005)

“Recipe” for planet growth is sensitive to disk surface density, orbital distance, stellar mass, time

Ida & Lin (2004); Lodato et al. (2005); see also classic papers by Safronov (1969) & Pollack et al. (1996)

Star with

magnetospheric

accretion columns

Accretion disk

Disk driven

bipolar outflow

Infalling

envelope

Current Paradigm:

Infall Rate:

10-5 Msun

/yr

Accretion Rate:

10-8 Msun

/yr

Shu, Adams, & Lizano ARAA (1987)

Hartmann Cambridge Press (1998)

Mass Loss Rate:

10-9 Msun

/yr

Primordial accretion disk signatures for T Tauri stars

Spectroscopic:Emission lines from accreting gas (e.g. Hα)

Photometric:Infrared/mm excess from disk

(Mamajek+ 1999)

(Domminik+2003)

Kenyon & Hartmann (1995) Ann Rev Ast Astrophys.

FU Ori Outbursts

Time

M(a

ccr)

Protostellar Disks (105-106 yrs):

Initial Conditions of Planet Formation

• Standard model:

– Most of stellar mass passes through disk.

• Limits on disk masses:

– < 10-25 % of central mass or disk is gravitationally unstable (Adams et al. 1990).

• Size of disk grows with time with viscous evolution, and accretion rate falls

– Theory: R(disk) increases with specific angular momentum (Tereby et al. 1984).

– Observations: e.g. Kitamura et al. (2002), Isella et al. (2009)

• Cloud Infall Rate >> Disk Accretion Rate:

– Leads to disk instability and outburst (FU Ori stage).

• Outbursts decrease with time:

– The last one fixes initial conditions of remnant disk (=> planets)

Mm/Sub-mm constraints on disk parameters

Andrews & Williams (2005, 2007; SMA)

Also Kitamura et al. (2002; NMA), Isella et al. (2009; CARMA)

Lifetimes of “Primordial” DisksPlotted are the fraction of stars in clusters with primordial disks traced by Hα excess and/or Spitzer IRAC infrared excess

All stars: τ ~ 2.5 MyrHigh mass stars (>1.3 Msun)τ ~ 1 MyrBrown dwarfs (<0.08 Msun)τ ~ 3 Myr

See also Hernandez+2008,Haisch+2001

Mamajek (2009; arXiv:0906.5011; Subaru meeting on Exoplanets & Disks)

Lifetime of solar system’s protoplanetary disk?

Castillo-Rogez et al. 2007

Modeling thermalhistory of Iapetus(constraints on shape,heating by short-livedradionuclides)

Saturn formed fromgas-rich disk within2.5-5 Myr of CAIs

Factors Influencing Disk Evolution

• Stellar mass:

– Disk masses are proportional to stellar masses

– Lifetimes inversely related to mass (Carpenter et al. 2006,

Mamajek 2009)

• Close companions:

– dynamical clearing of gaps

(Jensen et al. 1995; 1997; Meyer et al. 1997b; Ghez et al. 1997;

Prato et al. 1999; White et al. 2001).

• Formation environment:

– cluster versus isolated star formation

(Hillenbrand et al. 1998; Kim et al. 2005; and Sicilia-Aguilar et al.

2004).

Transitional disk

R. Hurt, SSC/JPL/Caltech/NASA

Transitional disks

Transitional disks

• GM Aur (Calvet et al. 2005)• Model of IRS spectrum:

• 1.05 M classical T Tau star

• Wall of optically thick disk = outer edge of gap at 24 AU.

• Radial gap, 5-24 AU, with very little dust.

• Inner gas disk with radius 5 AU, and a minute amount of small dust grains.

• In agreement with submillimeter image of cold dust in the disk (Wilner et al. 2007).

Typical Disk ParametersParameter Median ~1σ Range

Log(M(disk)/M(star))[all ~1 Myr] [detected disks only]

-3.0 dex-2.3 dex

±1.3 dex±0.5 dex

Disk lifetime 2-3 Myr 1-6 Myr

Temperature power law [T(r) ~ r-q] 0.6 0.4-0.7

Taken from (or interpolated/extrapolated from):

Muzerolle et al. (2003), Andrews & Williams (2007), Hernandez et al. (2008), Isella et al. (2009)

Parameter Median ~1σ Range

R(inner) 0.1 AU ~0.08-0.4 AU

R(outer) 200 AU ~90-480 AU

Surface density power [Σ(r) ~ r-p] [Hayashi min. mass solar nebula][steady state viscous α disk]

0.61.51.0

0.2-1.0(predicted)(predicted)

Surface density norm. Σo (5AU) 14 g cm-2 ±1 dex

Chemistry

Differences in organic chemistry important as a function of stellar

mass? e.g. HCN/C2H2 (Pascucci+ 2009, Daniel Apai’s talk).

Ionization levels may vary significantly from protostar to protostar

(X-ray/UV fluences from central star & neighboring stars?

Cosmic rays?)

Water in young protoplanetary disks – Where? How much?

(Bill Dent’s talk is next)

Points to take away…Planet formation is relevant after M(disk)/M(star) < 10-1-10-2, and T Tauri disks

are observed to typically have M(disk)/M(star) ~ 10-3±1.

Protoplanetary disk lifetimes have big dispersion t ~ 106.4±0.4 years.

Disks survive longer around low-mass stars.

Evolution is not just age. There are “hidden variables” in disk evolution!

UV photoevaporation can disperse disks within 10 Myr;

A mechanism for short transition times and mass-dependence of disk lifetimes?

Transition disks: does planet formation help drive disk evolution?

Preliminary evidence of stellar mass-dependent disk chemistry.

Disk ionization controls MRI (viscosity mechanism) and disk chemistry, and so

control disk evolution and some aspects of planet formation

More observations (imaging and spectroscopy; especially

resolved observations) of disks in the IR/mm/radio are needed to

improve constraints on the properties of gas and dust in

protoplanetary disks, and thereby constrain the initial conditions

of planet formation!