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)
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
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