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GRAIN PROCESSING IN PROTO-PLANETARY DISKS. OBSERVATIONS JEAN-CHARLES AUGEREAU LAOG (GRENOBLE, FRANCE) MAY 26, 2008 Spectral classification à la Adams, Lada & Shu 1987 dense core Time = 0 Protostar 104 yr 1000AU cTTs 106 yr cTTs EW Ha emission > 10 Å wTTs EW Ha emission < 10 Å M* ≤ 2 Msun 100AU PLANET FORMING wTTs ? Mature system PERIOD106- 4x107 yr 109 yr Diagram credit: Steven Beckwith 50AU Initial conditions of planet formation Weidenschilling (1977), Hayashi (1981): Minimum Mass Solar Nebulae total=3-6x104 r1.5 kg/m2 Surface density in solids 1% surface density in gas Total dust + gas mass : 0.01-0.1Msun Vertically flared structure 3 Initial conditions of planet formation Interstellar-like size distribution n(a)da a-3.5da, from ~0.005 to ~1m AMORPHOUS silicates, <1% of crystalline silicates Organic refractories (graphite and PAHs) 4 Grain growth, the very first step of planet formation Drag force on the grains. GRAINS HIGHLY COUPLED TO 1.9 µm SiO2 THE GAS LOW RELATIVE VELOCITIES BETWEEN THE GRAINS. Laboratory experiments : formation of FRACTAL AGGREGATES, m a Df, with Df typically between 1.4 and1.9 5 Settling time-scale issue z z r R 1 AU 10 AU 100 AU Grains tend to settle toward the disk midplane, with a typical settling time scale : tset ≈ 1/[a (d/) (2/cs)] tset with decreasing grain size (a) Micron-sized grains settle in about 105 years at 1 AU, although grains grow during their journey to the midplane => reduce the settling time-scale PREDICTION : DISK UPPER LAYERS DEVOID OF μM-SIZED GRAINS 7 Radial drift timescale issues Sub-keplerian gas disk (radial component of the gas pressure gradient) 1cm/s = 2.1AU/Myr AT R=100AU Radial drifts : ~cm-sized grains drift fast toward the star (dust well-coupled to the gas). ~meter-sized bodies also drift fast toward the star (see a headwind, vdust > vgas) PREDICTION : NO MM/CM-SIZED GRAINS IN DISKS 8 Predictions vs Observations Predictions ignore many aspects of the disk physics, including dust fragmentation and the turbulence. SEBASTIEN’S TALK FOR THE THEORETICAL ANGLE. THIS TALK : Observations of mm/cm-sized grains in disks Observations of micron-sized grains in disk upper layers Evidence for dust mixing and transportation in disks Evidence for settling from detailled modeling Compositional fitting of IR spectra Scattered light images Images + full SEDs Summary and prospect Just for your eyes : images of disks with holes 9 Millimeter observations mm/cm-sized grains in disks 10 Disk opacity Dust : most of the disk opacity The disk opacity decreases with increasing wavelength λ: visible 11 Disk opacity Short wavelengths probe the upper layers λ: near-IR 12 Disk opacity Short wavelengths probe the upper layers λ: mid-IR 13 Disk opacity Longer wavelengths probe deeper in the disk λ: mid/far-infrared 14 Disk opacity Longer wavelengths probe deeper in the disk λ: far-infrared 15 Disk opacity λ: sub-millimeter 16 Disk opacity The disk becomes optically thin at mm-wavelengths λ: millimeter 17 Disk opacity The flux is proportional to the mean dust opacity: F v 2 Measuring the millimeter emission slope <-> opacity slope βparameter: at millimeter wavelengths ISM grains ………………………….... : β= 2 Grains >> wavelength/2π …. : β= 0 REMEMBER THE PREDICTION : NO MM/CM-SIZED GRAINS IN DISKS WE EXPECTβ = 2 18 mm-sized grains in disks T Tauri stars DO Tau, Koerner et al. (1995): β=0.6±0.3 TW Hya, Calvet et al. (2002): β=0.7 DO Tau Intermediate mass stars (Herbig Ae/Be): CQ Tau, Testi et al. (2003) : ß=0.5-0.7 Natta et al. (2004): ß=0.4-1.5 GRAINS OF ~ THE SIZE OF THE OBSERVING MILLIMETER WAVELENGTH 19 mm-sized grains in disks Size distribution : n(a)daaqda (q=3.5 dans l’ISM) from amin<< 1mm and amax. Grains > a few millimeters are necessary Typically q~3-3.5 [the mass is contained in the large grains, while the cross section is dominated by the small grains] 20 mm-sized grains in disks No correlation with the stellar mass (luminosity) No correlation with star age THE MM/CM-SIZED GRAINS STAY IN DISKS FOR MILLIONS OF YEARS SOME MECHANISMS PREVENT THESE GRAINS TO DRIFT INWARD ON SHORT TIME-SCALES? REPLENISHMENT PROCESS? E.G. FRAGMENTATION? 21 Infrared spectroscopy Grains in disks upper layers 22 Mid-IR observations of disks 1 AU 10 AU 100 AU Mid-IR : thermal emission from warm dust grains, in the PLANET FORMING REGION (110AU), originating from the upper disk layers Imaging suffers from low spatial resolution, and poorly extended emission on the sky Spectroscopy : indications on the DUST PROPERTIES 23 Mid-IR spectroscopy of silicates Mid-IR : silicates features, which depend on: ABSOPTION/EMISSION EFFICIENCY COMPOSITION, LATTICE GRAIN SIZE STRUCTURE (CRYSTALLINE/AMORPHOUS), Si-O (stretching mode) MgSi O3 O-Si-O (bending mode) Mg2Si O4 WAVELENGTH IN MICRONS (SPITZER IRS SPECTRAL RANGE: 5>35MICRONS) 24 Pre-Spitzer understanding of circumstellar silicates Young solar-like (T Tauri) stars : ISO satellite: sensitivity limited Ground-based observations: silicates detected at 10m in a few cases, crystalline silicate seen in very few objects ? REMEMBER THE PREDICTION : NO MICRON-SIZED GRAINS IN DISKS UPPER LAYERS WE EXPECT POINTY (ISM-LIKE) SILICATE 10MICRON FEATURES 25 Large grains in disks upper layers ISM-like amorphous silicate feature, small grains < a few 0.1m) Herbig Ae/Be TTauri Brown Dwarfs Labo Features characteristic of micron-sized grains Evidence for grain growth in disks Large silicate grains in the disks upper layers This happens toward all kind of stars 26 Large grains in disks upper layers 10 micron feature: shape vs strength Strength: Fpeak Shape: F11.3/F9.8 27 Large grains in disks upper layers 10 micron feature: shape vs strength MOST objects have features consistent with micron-sized grains SOME GENERIC MECHANISMS PREVENT THE MICRON-SIZED GRAINS TO SETTLE FAST 28 Large grains in disks upper layers Grain size vs stellar age : no correlation Grain size vs accretion rate : no correlation 29 Grain size vs stellar luminosity Significant differences between A/B and M stars: SMALL LARGE GRAINS GRAINS LARGER GRAINS PROBED IN DISKS M STAR DISKS THAN IN A/B STAR 30 Grain size vs stellar luminosity Illumination effect? Observations …..… : grain size = f(L*) Disk model ………… : distance probed at 10m: R10m L*0.56 Grain size = f(R10m) Evidence for a radial dependence of size distribution? DIFFERENTIAL COAGULATION? DIFFERENTIAL SETTLING? DIFFERENTIAL VERTICAL MIXING? 31 Infrared spectroscopy Degree of crystallinity of silicates grains 32 Crystals in disks 1 AU 10 AU 100 AU The silicates grains incorporated in disks are amorphous (ISMlike grains), degree of crystallinity < 1% 2 processes of crystallization: Direct condensation from gas phase Thermal annealing (devitrification) of amorphous grains Both processes take place in hot disk regions (T>800K), very close to the star PREDICTION: LOW DEGREE OF CRYSTALLIZATION, EXCEPT CLOSE TO THE STAR 33 Crystals in disks Mg-rich crystalline olivine & pyroxene (forsterite & enstatite) 34 33 m complex 28 m complex 23 m complex Enstatite Forsterite Forsterite Amorphous silicates PERCENTAGE FEATURE PEAK POSITION (WAVELENGTH IN MICRONS) ~ 3/4 of the TTauris disks show at least one crystalline silicate feature Mg-rich crystalline silicates (forsterite, enstatite) Diopside (Ca-Mg rich silicate) Crystals in (103) TTauri disks 35 Outward transport of crystals 1 AU 10 AU 100 AU Largely uncorrelated 10 & 20micron emission zones Different wavelengths probe different regions, hence different dust populations Crystals are found in cold disk regions CRYSTALS HAVE BEEN TRANSPORTED OUTWARD 36 36 Modeling of individual objects Evidence for settling 37 AL Compositional fitting of Spitzer spectra OW MASS STAR SST-Lup3-1 : a very low mass star in Lupus with crystalline silicates M5.5-type star (amost a brown dwarf) L* = 0.08 Lsun M* = 0.1Msun Age ~ 1Myr 38 Mass opacities for 2 representative grain sizes 0.1 µm 1.5 µm Silica (Mie) Enstatite (DHS) Forsterite (DHS) Pyroxene (Mie) Olivine (Mie) 39 2 temperature compositional AL fitting OW MASS STAR Continuum-subtracted spectrum 40 AL Compositional fitting of Spitzer spectra OW MASS STAR High degree of crystallinity (> 15%) => low luminosity stars can crystallize significant amounts of amorphous grains Warm component : similar masses of small (0.1m) and big (1.5 m) grains => ONGOING VERTICAL MIXING? Cold component: mostly big grains (75-90%) => EVIDENCE FOR DUST SETTLING? 41 A Compositional fitting of Spitzer D spectra BROWN WARF A disk about a M7.25-type brown dwarf in Taurus 2 temperature compositional fitting: global degree of crystallinity: 20-30% Cold component (in blue, bottom panel) is 10-15 times more crystalline than the warm component (in red) => EVIDENCE FOR DUST TRANSPORT? 42 Crystallinity vs stellar mass 43 A BINARY TTAURI SYSTEM Scattered light images Pan-chromatic images of the GG Tau circumbinary disk Comparison with synthetic scattered light images EVIDENCE FOR SETTLING? Technique limited to external disk regions (the disk to star contrast being too high close to the star) 44 A T TAURI STAR IM Lup : a case study 45 A T TAURI STAR IM Lup : a case study Silicates features => micron-sized grains in the inner disk upper layers Blue dotted line: fit to the SED with wellmixed dust and gas, amax=3microns Silicates features repoduced, but not the mm observations 46 A T TAURI STAR IM Lup : a case study Millimiter slope => mm-sized grains. Green dashed line: fit to the SED with well-mixed dust and gas, amax=3mm Silicates features badly repoduced: the micrometer-sized grains do not dominate the midIR opacity MM GRAINS ARE PRESENT, BUT NOT IN THE UPPER DISK LAYERS 47 A T TAURI STAR IM Lup : a case study Parametrized stratification : H a-ξ, withξ=0.1 Red line: fit to the SED, amax=3mm Both the silicates features and the mm observations are more correctly adjusted SETTLING PROVIDES A SOLUTION TO THE FITTING OF THE SED 48 A T TAURI STAR IM Lup : a case study 421 200 models Simultaneous fit to: the SED Scattered light images, 2 wavelengths Millimeter maps Bayesian analysis SOLUTIONS WITH NO SETTLING AT ALL ARE LARGELY EXCLUDED 49 Summary Observations are consistent with grain growth in disks, at least up to cmsized particles + This seems to happen independent of the stellar luminosity There are evidence for vertical settling as well as vertical mixing There are evidence for radial dust transportation and differential grain growth - Different observations probe different regions: Disk opacity Temperature gradient => this complicates the interpretation Solid bodies larger than ~ cm are invisible ! No clear temporal dependence 50 Observations challenge the models Models have to explain the presence of mm/cm-sized grains in disks for millions of years Models have to explain the presence of micron-sized grains in disk upper layers Models have to explain the presence of large quantities of crystalline silicates in cold disk regions 51 Prospect ANR « Dusty Disks » (PI F. Ménard, Grenoble) « Routine» analysis of panchromatic observations of disks Massive modeling of Spitzer disk photometry Compositional fitting of dozens of Spitzer spectra (PhD student J. Olofsson, Grenoble) Herschel : GASPS key programme (Grenoble responsible for dust modeling) Settling : MHD models + Radiative Transfer 52 Disks with holes SMA very extended configuration 1´´ Acknowledgement and papers Johan Olofsson Christophe Pinte Francois Ménard The c2d Spitzer/IRS team: Jacqueline Kessler-Silacci, Kees Dullemond, Bruno Merin, Ewin van Dishoeck, Klaus Pontoppidan, Goeff Blake, Neal Evans, … Papers: Olofsson et al., 2008 in prep Pinte et al., 2008 submitted Merìn et al. 2007, ApJ 661 Bouy et al. 2008, accepted Kessler-Silacci et al. 2006, ApJ 639 Kessler-Silacci et al. 2007, ApL 659 54