Observations of Disks around Young Stellar Objects

Observations of Disks around Young Stellar Objects
G. Duchêne & F. Ménard (Obs. Grenoble)

G. Duchêne - Structure Formation in the Universe - May 2007
Goals of this talk Consider as wide a range of datasets as possible in 30 minutes! Will skip some very exciting aspects Discussion of selected physical aspects Leave out gas and chemistry Persuade you that we can now constrain some physical processes Yet many open questions remain… G. Duchêne - Structure Formation in the Universe - May 2007

Outline General motivation Observational methods Disks in the context of star formation Disks in the context of planet formation Debris disks: after planets formed Summary and perspectives G. Duchêne - Structure Formation in the Universe - May 2007

General Motivation G. Duchêne - Structure Formation in the Universe - May 2007

Why do we care about disks?
A natural outcome of star formation G. Duchêne - Structure Formation in the Universe - May 2007

Why do we care about disks?
Planetary system factories A natural outcome of star formation G. Duchêne - Structure Formation in the Universe - May 2007

Expected physical processes (I)
Influence of central star/environment Disk lifetime Total mass reservoir Overall structure Disk dispersal mechanism Viscous dissipation of angular momentum Photo-ionization Dynamical dispersal (companion) G. Duchêne - Structure Formation in the Universe - May 2007

Expected physical processes (II)
Substructure formation Spiral arms (instabilities, planets) Gap openings (planets) Dust evolution Grain growth Radial migration Vertical sedimentation Change in grain structure G. Duchêne - Structure Formation in the Universe - May 2007

Observations of disks G. Duchêne - Structure Formation in the Universe - May 2007

Unresolved datasets: SEDs
The simplest approach: gather the energy and try to invert to disk structure Flared disks in most cases Flat Flared Chiang & Goldreich (1997) Dullemond et al. (2007) G. Duchêne - Structure Formation in the Universe - May 2007

Unresolved datasets: SEDs
Useful approach for statistical purposes Can be dangerous on an object-to-object basis Need for resolved datasets! Burrows et al. (1996) All Taurus CTTS D’Alessio et al. (2001) G. Duchêne - Structure Formation in the Universe - May 2007

Resolved datasets A single image provides key parameters: Outer radius, position angle Inclination (sometimes) Optical depth (sometimes) Not a normal disk Guilloteau et al. (1999) Bertout et al. (1998) G. Duchêne - Structure Formation in the Universe - May 2007

Resolved datasets grain size VLT/VISIR composition structure Interf. Spitzer mass All probe different dust populations G. Duchêne - Structure Formation in the Universe - May 2007

Resolved datasets VLT/VISIR Interf. Spitzer Need for complementary complex RT models G. Duchêne - Structure Formation in the Universe - May 2007

Disks and Star Formation
G. Duchêne - Structure Formation in the Universe - May 2007

Disks and central object mass
How universal is star formation? Probe disk presence through IR excess Overall fraction up to 90% (in  Oph) Best studied population: the ONC disks at all masses ( M) Slight deficit at low mass end? Hillenbrand et al. (1998) G. Duchêne - Structure Formation in the Universe - May 2007

Detection is harder around VLMS/BD because of cooler Teff BDs: 40-75% up to ~5 Myr at least No substantial difference with stars Jayawardhana et al. (2003) Liu et al. (2003) G. Duchêne - Structure Formation in the Universe - May 2007

Not only is disk frequency independent of mass, their structure is, too! Hydrostatic (flared) passive disks ~0.1 M ~0.5 M ~2 M IRAS HK Tau PDS 144 Glauser et al. (2007) McCabe et al. (2007) Perrin et al. (2007) G. Duchêne - Structure Formation in the Universe - May 2007

The special case of high-mass stars: Aligned (rotating) methanol masers, but not so clear Norris et al. (1993), De Buizer et al. (2003) Wide-angle outflows A huge ‘silhouette disk’ Difficult to conclude yet Too far away Evolving too fast 20000 AU M17 Chini et al. (2004) G. Duchêne - Structure Formation in the Universe - May 2007

Disks and orientation of stars
Taurus molecular cloud = series of filaments orthogonal to B field So are individual pre-stellar cores CO map Prestellar cores Hartmann (2002) G. Duchêne - Structure Formation in the Universe - May 2007

Disks and orientation of stars
Disks around T Tauri stars indicate the system’s symmetry axis Systems are randomly oriented w.r.t. local magnetic field What happened? Non-magnetic collapse? Ménard & Duchêne (2004) G. Duchêne - Structure Formation in the Universe - May 2007

Disks and Planet Formation (overall disk properties)

Disks sizes and masses Typical disk size ~ 200 AU Compares well with Solar System Large scatter around median value! IRAS HV Tau ~ 1100 AU ~ 40 AU Stapelfeldt et al. (2003) Kitamura et al. (2002) Glauser et al. (2007) G. Duchêne - Structure Formation in the Universe - May 2007

Disks sizes and masses Disk masses can be derived from thermal radio fluxes/maps Uncertain dust opacities Uncertain gas/dust ratio Derived total masses: Consistent with MMSN Consistent with stability Natta et al. (2000) G. Duchêne - Structure Formation in the Universe - May 2007

Disks sizes and masses Radio interferometers (IRAM, OVRO) can resolve disks Typical surface density ~1 100AU Power law indices Temperature law Surface density ‘Flat’ MMSN-like disks Good for planets! But interpolation… Dutrey et al. (1996) G. Duchêne - Structure Formation in the Universe - May 2007

Disk asymmetries: large scales
Evidence for dynamical perturbation: Companion, planet, high-mass disk? What you see is NOT what you have… HD AB Aur Optically thin! Grady et al. (2001) Fukagawa et al. (2004) Piétu et al. (2005) G. Duchêne - Structure Formation in the Universe - May 2007

Disk asymmetries: gaps
Planets embedded in disks open ‘gaps’ Can these be observed? Gap size < 1AU High resolution + high contrast ALMA? New generation AO? Remember, however: Spatial resolution remains an issue Gaps may be partly filled in G. Duchêne - Structure Formation in the Universe - May 2007

Disk dissipation Using disk counts in independent SFRs provides survival time of inner disk Essentially nothing left after 10 Myr No environment effect OB vs T associations, clusters Large bodies may still be present and hidden Disk lifetime Meyer et al. (2000) G. Duchêne - Structure Formation in the Universe - May 2007

Disk dissipation Does disk dissipation depend on central object mass? Spitzer surveys of UpSco (~5Myr) G-B: 5 +/- 2 % K0-M5: 19 +/- 3% BDs: 37 +/- 9 % Disk lifetime is longer for lower mass objects Because of slower viscous timescale? } Carpenter et al. (2005) Scholtz et al. (2007) G. Duchêne - Structure Formation in the Universe - May 2007

Inner disk dispersal Disks disappear after inner hole clearing Evidence shows that disks dissipate inside-out in <105 yrs (viscous timescale) AU material Very few transition objects CoKu Tau 4 D’Alessio et al. (2005) McCabe et al. (2006) 0.5-2 AU material G. Duchêne - Structure Formation in the Universe - May 2007

Inner disk dispersal How long does the outer disk remain? Spitzer searches for disk with only outer disk material (>5-10 AU) Only a few percent of such objects Outer disk falls below detection threshold in <~ 105 yrs Too fast for viscosity? Padgett et al. (2006) G. Duchêne - Structure Formation in the Universe - May 2007

Disks and Planet Formation (dust properties)

Grain growth: mm view First approach: SED slope (mm regime) Typically, amax ~ few mm to few cm Large grains Observed distribution of spectral indices Small grains D’Alessio et al. (2001) Natta et al. (2007) G. Duchêne - Structure Formation in the Universe - May 2007

Grain growth: silicates view
Silicate feature is size-dependent Small (< 0.1 m) vs large grains (~1 m) Larger grains do not contribute Crystallinity produces sharp features Kessler-Silacci et al. (2006) G. Duchêne - Structure Formation in the Universe - May 2007

Grain growth: silicates view
Clear evolutionary sequence Larger grains come together with higher grain crystallinity (above a threshold) Higher crystallinity Smaller grains Kessler-Silacci et al. (2006) Van Boekel et al. (2005) G. Duchêne - Structure Formation in the Universe - May 2007

Grain growth: scattered light
Stellar photons can scatter off dust grains at the disk surface Phenomenon depends on /a Larger grains scatter preferentially forward, with a lower polarization rate Images and polarization maps can be used to infer grain sizes Up to amax ~ few m typically Advantage: longer  probes deeper!! G. Duchêne - Structure Formation in the Universe - May 2007

Grain growth: scattered light
Single power law size distribution Increasingly more isotropic scattering HK Tau images (increasingly ‘peakier’) reveal larger grains inside (up to 3-5 m) VLT/AO Keck/AO Keck/AO Keck 2.2 m 3.8 m 4.7 m 11.3 m McCabe et al. (in prep) McCabe et al. (2003) G. Duchêne - Structure Formation in the Universe - May 2007

Grain growth: the big picture
Each aspect probes A different region of disks Different grain sizes/populations In each case, analysis requires knowledge of additional information (radius, inclination, …) Ideally, comparison all datasets to a single (complex) radiative transfer model G. Duchêne - Structure Formation in the Universe - May 2007

Vertical sedimentation
If large grains disappear from the surface, thermal equilibrium is changed Change in disk SED Difficult to ascertain, however Sedimentation mimics a flat disk Dullemond & Dominik et al. (2004) G. Duchêne - Structure Formation in the Universe - May 2007

Confront mm regime and silicates Can be convincing (if composition is well distributed throughout the disk) IM Lup Small grains only Small and large grains Pinte et al. (in prep) G. Duchêne - Structure Formation in the Universe - May 2007

Confront mm regime and silicates Can be convincing (if composition is well distributed throughout the disk) IM Lup Small and large grains Small grains only Small and large grains sedimentation Pinte et al. (in prep) G. Duchêne - Structure Formation in the Universe - May 2007

Radial migration Interferometry + spectroscopy (MIDI) Silicate features a few AU from the star Higher crystallinity! Grain processing? RY Tau (K1) small van Boekel et al. (2004) crystalline Shegerer et al. (subm.) G. Duchêne - Structure Formation in the Universe - May 2007

Radial migration Difficult to quantify differentiation Many assumptions in analysis Nonetheless, there is evidence that grain properties depend on radial distance to the star However, we cannot prove that grains have migrated! Crystallinization may be a local processing G. Duchêne - Structure Formation in the Universe - May 2007

Further in time: debris disks

Debris disks: basics Debris disks are the final stage in planet formation before zodiacal disks Formed through collisions of solid bodies They are optically thin Easier to interpret Harder to observe SED is usually limited Rough constraints only Beichman et al. (2006) G. Duchêne - Structure Formation in the Universe - May 2007

Debris disks: porosity, aggregates
With many independent observables, finer models can be tested The AU Mic debris disk is made of (small) porous grains porous compact Fitzgerald et al. (2007) G. Duchêne - Structure Formation in the Universe - May 2007

Debris disks: porosity, aggregates
Another debris disk: HD All observables cannot be explained simultaneously with spherical grains Aggregates? SED vs ? Phase function Schneider et al. (2006) G. Duchêne - Structure Formation in the Universe - May 2007

Summary and Perspectives

Summary We have access to many types of complementary observations Several physical processes can be (somewhat) constrained Core collapse/fragmentation Disk dissipation and inner hole clearing Grain growth Dust settling Presence of planetesimals G. Duchêne - Structure Formation in the Universe - May 2007

Perspectives More observations will come with future instrumentation (e.g., ALMA) At this stage, we still need Complex modeling/analysis of datasets More multi-technique analysis Tests of the basic processes in models Wait for next talks, to get the theorists’ point of view! G. Duchêne - Structure Formation in the Universe - May 2007

Observations of Disks around Young Stellar Objects

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