Download Stephen E. Strom

GILLETT SYMPOSIUM SUMMARY
11- 13 April, 2002
Stephen E. Strom
Disk Accretion Phase
Disk Accretion Phase: Ubiquity
• 100% for stars with M < 1 MO (observed)
• 100% for higher mass stars (inferred)
– Collimated jets
– Evidence for ‘funnel flows’
– Rotational properties
• J/M vs M is continuous
– Accretion through disks enables assembly
• Radiation pressure would otherwise reverse accretion
Disk Accretion Phase: Lifetimes
• 0.1 < t < 10 Myr for solar-type stars
– Upper limit not well established
– Environment may affect lifetimes
• Photoevaporation; tidal interactions
• 0.1 <t < ? Myr for higher mass stars
– Disk lifetimes are shorter than for 1 MO stars
• Stellar rotation provides additional clues
– N(vsini) for a Per requires disk lifetimes up to ~5 Myr
– Rotation rates higher in high density regions
• Shorter disk lifetimes? Higher accretion rates?
• Caveat: PMS ages are highly uncertain for t < 3 Myr
– Tracks
– Birthline
– Uncertainties in L, Teff
Inner Accretion Disk Lifetimes (Meyer)
Disk Accretion Phase: Masses
• Total mass estimates based on dust mass
– mm continuum emission
• Range: Mdust ~ 10-3 to 10-5 MO
• These are lower limits
– Mass in t > 1 regions underestimated
– Large bodies not detected
• Mass estimates depend on (uncertain) grain properties
• No reliable estimates of gas content
• The total mass available for planet building may well
exceed the “minimum mass nebula”
– Mass comprising star passes through a disk
– Instantaneous disk mass is a lower limit
Disk Accretion Phase: Sizes
• Largest centrifugally supported disks: ~ 300 AU
– Larger, structures found, but no evidence of rotational support
• Orion silhouette disks provide direct measurement
– 20 < r < 200 AU (~ 50 disks)
• Photo-evaporation and tidal encounters may truncate disks
in rich, dense environments
– Correlation between disk size and proximity to q Ori
– Kuiper belt cutoff may reflect photo-evaporation (Hollenbach)
Disk Accretion Phase: Accretion Rates
• Estimated from ultraviolet excess emission
– Measures inner disk accretion rate
• dM/dt increases with increasing mass
– dM/dt ~ 10-7.5 for 0.5 MO; may increase ~ linearly with M
• Instantaneous rates are lower than time-averaged rates
– Accretion rates ~103 higher during FU Ori phase
– Recall that total mass passing through the disk is large
• Wide dispersion (> 10x) in dM/dt at fixed age and mass
– Q: might this dispersion reflect the effects of planets?
• Binarity and tidal encounters may affect dM/dt
Disk Accretion Phase: Larger Bodies
• Growth of grains (to 1 cm) estimated from SEDs
• Presence of larger bodies inferred from
– FEBs (but cf Grady: now suggests accretion origin in HAeS)
– Cyrstalline silicate emission in cold disk regions (?)
• Evidence favors planet formation during accretion phase
– N(a) for extrasolar planets suggests accretion-driven migration
– Best (only?) time to form gas giants
• Large O/IR telescopes and later, ALMA, should enable
detection of ‘gaps’ diagnostic of forming giant planets
– Establish the fraction of stars that form giant planets initially
– Establish the N(a) during the accretion phase
• Is there an (interesting) upper limit to a ?
Diagnosing Forming Planets
GSMT
AURA-NIO Point Design
ALMA
Star at 10pc
Debris Disk Phase
Debris Disk Phase
• Sparsely-sampled photometry and SEDs
– Statistics: <t>, s (t) vs age
– Radial distribution t (r) , (if SED is well sampled)
– mineralogical probes
• Statistical studies suggest
– Decrease of t with age (Ldust/ L* ~ t -1.75)
– Possible rapid decline of t for stars with ages t > 400 Myr
• Radial distributions: low t inner zones
• Mineralogy
– Solid state features can in principle be matched to source bodies
Debris Disk Phase
• Caveat regarding ages
– Transformation to L,Teff a problem for t < 200 Myr
• Large starspots + photosphere = composite spectral type
– Cluster ages (t < 200 Myr) are uncertain at 0.3 dex level
– Li-depletion ages
• Systematic difference with upper MS turnoff
• Rotation affects surface Li
• Efficacious for selected spectral type ranges only
– Activity-age estimates (e.g. Ca II)
• Large dispersion
– Conclude (Stauffer):
• Relative age uncertainties for field stars up to 0.6 dex
Debris Disk Phase
• Imaging: scattered light; thermal IR; mm
– Radial distribution of dust
– Selected cases reveal (cf Alycia Weinberger)
•
•
•
•
Low density inner zones
Warps
Rings
Non-axisymetric features
– Q: do these provide evidence of planets?
• Modeling suggests yes
• Analysis of solar system zodi strongly supportive
Beta Pictoris
HR4796a
Epsilon Eridani
Debris Disk Phase
• Spectroscopic searches for disk gas (steady state)
– CO emission (mm-wave measurements)
– H2 infrared features (ISO; ground)
– H2; CO ultraviolet absorption features
• Results
– Significant differences between (uv; ground) and ISO
– Concensus: [gas/dust] << ISM value
• Not enough to build giant planets
• Could mitigate dust migration; quantitative study needed
– Suggestion that gas is ‘secondary’: evaporated volatiles
• High priority for future work
– SIRTF observations of H2 spanning wide age range
• Knowledge of gas content key to modeling dust evolution
– Key Focus: young (5 – 20 Myr) debris disk stars
• Do post-accretion phase disks have gas sufficient to build giant planets?
Debris Disk Phase
• Spectroscopic monitoring (time variable)
– Episodic red-shifted absorption features from metals
– Suggest presence of ‘falling, evaporating bodies’
• Limited results suggest origin in refractory bodies
Debris Disk Phase: The Future
• SEDs from SIRTF
– Well sampled from 3m – 160m
– Diagnose t ( r ); infer presence of gaps
– Mineralogical features: probe parent body composition
• observe transition from ISM to debris-dominated disks
• High angular resolution imaging from the ground
– Map solid and gaseous components at sub-AU scales
Debris Disk Phase: Solar System Clues
• Sources of dust: collisions
– Outer solar system: Kuiper Belt objects
– Inner solar system: Asteroid Belt objects
• In both cases, orbits are ‘stirred’ by planets
– Observed disk properties; evolution depend on
• Planetesimal/cometesimal distribution; initial density
• Planetary architecture
• Effects of gravitational and drag forces
• Warps; rings; offsets in zodi dust linked to planets
• Density enhancements caused by resonant trapping
Debris Disk Phase: Solar System Clues
• Dust injection into zodiacal cloud stochastic
– Expect large variations in t
• Large collisions may give rise to ‘dust waves’
– Timescales ~ 104.5 to 106 yrs
– Possible significant climatic consequences
•
•
•
•
Kuiper belt primordial mass: > 10-4 MO
KB extends S ( r ) from inner solar system to > 100? AU
Slow Collisions produce 100-1000km bodies in ~ 100Myr
Fast Collisions erode initial KBOs
– Produce dust (observed by Pioneer?); comets
– Total KB mass has likely decreased 1000-fold over 5 Gyr
• KB probably similar to outer regions of extrasolar disks
Extrasolar
Planetary
Systems
Extrasolar Planetary Systems
• (Jovian) planet detection rate ~ 10%
• Of these, multiple planet systems common (> 50%)
• Unexpected distribution of a for giant planets
– N(a) distribution suggests migration
• What stops migration (cf Artymowicz)?
– Favors formation during accretion phase
• Stars with inner Jupiters may have higher Z
• Caveats
– N(a) suffers from strong observational bias
– High M sini favored by Doppler techniques
– Consistent analysis of metallicities is critical (underway)
• Focus on F stars (thinner convection zones)
GSMT: www.aura-nio.noao.edu
AURA-NIO Concept
Direct detection with ExAO
GSMT enables direct detection and analysis of planets
Debris Disks and the Formation of Planets
A true celebration of Fred Gillett whose
Insight
Imagination
Persistence
Care
Integrity
Decency
Generosity of spirit
Sense of community
are a continuing inspiration to us all