Protoplanetary Disks The Initial Conditions of Planet Formation

Protoplanetary Disks: The Initial Conditions of Planet Formation Special thanks to: Michael Meyer (U. Arizona, ETH Zurich) Dan Watson (U. Rochester) Eric Mamajek University of Rochester, Dept. of Physics & Astronomy Astrobio 2010 – Santiago de Chile – 15 January 2010

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.

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 Mayor & Udry (2008)

Pre-main Sequence Evolution 10 Protostar+ primordial disk Lstar/ LSun 105 yr 104 yr Planet building 107 yr 109 yr 1 Planetary system + debris disk 8, 000 100 AU Main sequ ence 5, 000 Cloud collapse 2, 000 Tstar (K)

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!

An Analytical Estimate of Protoplanet Growth Mass Time Disk Surface Density Lodato et al. (2005) Orbital Radius Primary Mass “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)

Current Paradigm: Shu, Adams, & Lizano ARAA (1987) Hartmann Cambridge Press (1998) Mass Loss Rate: Infall Rate: Star with 10 -5 Msun/yr 10 -9 Msun/yr magnetospheric accretion columns Accretion disk Accretion Rate: 10 -8 Msun/yr Disk driven bipolar outflow Infalling envelope

Primordial accretion disk signatures for T Tauri stars Spectroscopic: Emission lines from accreting gas (e. g. Hα) (Mamajek+ 1999) (Domminik+2003) Photometric: Infrared/mm excess from disk

M(accr) FU Ori Outbursts Time Kenyon & Hartmann (1995) Ann Rev Astrophys.

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” Disks Plotted 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 Myr High mass stars (>1. 3 Msun) τ ~ 1 Myr Brown dwarfs (<0. 08 Msun) τ ~ 3 Myr See also Hernandez+2008, Haisch+2001 Mamajek (2009; ar. Xiv: 0906. 5011; Subaru meeting on Exoplanets & Disks)

Lifetime of solar system’s protoplanetary disk? Modeling thermal history of Iapetus (constraints on shape, heating by short-lived radionuclides) Saturn formed from gas-rich disk within 2. 5 -5 Myr of CAIs Castillo-Rogez et al. 2007

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. 1997 b; 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 disks Transitional disk R. Hurt, SSC/JPL/Caltech/NASA

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 Parameters Parameter 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 Temperature power law [T(r) ~ r-q] 2 -3 Myr 0. 6 1 -6 Myr 0. 4 -0. 7 Parameter Median ~1σ Range R(inner) R(outer) Surface density power [Σ(r) ~ r-p] [Hayashi min. mass solar nebula] [steady state viscous α disk] 0. 1 AU 200 AU 0. 6 1. 5 1. 0 ~0. 08 -0. 4 AU ~90 -480 AU 0. 2 -1. 0 (predicted) Surface density norm. Σo (5 AU) 14 g cm-2 ± 1 dex Taken from (or interpolated/extrapolated from): Muzerolle et al. (2003), Andrews & Williams (2007), Hernandez et al. (2008), Isella et al. (2009)

Chemistry Differences in organic chemistry important as a function of stellar mass? e. g. HCN/C 2 H 2 (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! Transition disks: does planet formation help drive disk evolution? UV photoevaporation can disperse disks within 10 Myr; A mechanism for short transition times and mass-dependence of disk lifetimes? 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!