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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