Transcript The Pluto “Smackdown” and The “New” Solar System

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
105 yr
104 yr
Lstar/
LSun
Planet building
107 yr
109 yr
1
Planetary system
+ debris disk
100 AU
Cloud collapse
8,000
5,000
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)
Shu, Adams, & Lizano ARAA (1987)
Current Paradigm:
Hartmann Cambridge Press (1998)
Mass Loss Rate:
Infall Rate:
Star with
magnetospheric
10-5 Msun/yr
10-9 Msun/yr
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
FU Ori Outbursts
Time
Kenyon & Hartmann (1995) Ann Rev Ast 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; arXiv: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. 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 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
Disk lifetime
Temperature power law [T(r) ~ r-q]
2-3 Myr
0.6
±1.3 dex
±0.5 dex
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]
Surface density norm. Σo (5AU)
0.1 AU
200 AU
0.6
1.5
1.0
~0.08-0.4 AU
~90-480 AU
0.2-1.0
(predicted)
(predicted)
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/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!
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!