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Looking at the Kuiper Belt from the Thermal Side

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Looking at the Kuiper Belt from the Thermal Side Looking at the Kuiper Belt from the thermal side Arielle Moullet, NRAO 1930 Slipher et al. 1988 1992 1000 KBOs (classical, Plutinos) 200 Centaurs 200 Scattered objects Faint sources: 22<V<28 Courtesy of Minor Planet Center Nasa Images Center ~20 large objects ( diameter>300 km ) Large fraction of multiple systems Icy surfaces (H2O, N2, CH4), possibly atmospheres Why study Kuiper Belt objects ? (and related populations) ● Physical and chemical evolution of cold/distant surfaces and atmospheres ●Pristine / unaltered objects : information on the conditions in the primitive Solar nebula ● Analog of planetesimals in stellar debris disk Orbital structure Remarkable features : ● Sharp outer edge ● Mass depleted ● Excited populations → indication on giants migration, approaching stars, initial disk structure Nice model in the outer Solar System, Morbidelli et al., 2008 Size distribution Classical Objects Identification of slope and breaks → direct comparison to formation models : merging, accretion, collisions Scattered Objects Challenging size measurements Donnison et al., 2006 Composition of early solar nebula Great variety of bulk densities (rock/ice ratio) : → inhomogeneous ice/rock ratio in the outer disk ? → collisions on differentiated bodies? Very little data Brown et al., 2012 : Densities and diameters Surface altering Wide variety of surface composition (volatiles/organics), reflect Jewitt et al. : Space-weathering surface alteration processes : Collisional excavation Space weathering Volatile loss Thermal alteration Cryovolcanism Radiogenic heating... Brown et al., 2010 : Model of volatile retention on surfaces Sublimation-sustained atmospheres Detected on Pluto, plausible on large KBOs : sublimation of volatiles CO, CH4, N2 → Pressure exp. dependent on surface temperature : high variations (diurnal/seasonal) → Condensation cycles periodically recycling surfacess Schaller and Brown, 2008 Observing KBOs Individual characterization of the large bodies Large-scale studies : taxonomy, families, correlations Correlations between spectral/orbital properties Doressoundiram et al, 2005 : Inclination,semi-major axis, size, spectral index Techniques Optical – NIR photometry (>100 obj) → magnitude, spectral index Optical – NIR spectrosocopy (>40 obj): → icy/mineral bands Dumas et al., 2007 : Eris spectra with water/tholin/methane/ nitrogen ice model Techniques Optical – NIR photometry (>100 obj) → magnitude, spectral index Optical – NIR spectrosocopy (>40 obj): Noll et al., 2008 → icy/mineral bands HST Imaging (~10 objects) : → multiple system imaging, sizes 1'' Techniques Optical – NIR photometry (>100 obj) → magnitude, spectral index Optical – NIR spectrosocopy (>40 obj): → icy/mineral bands HST Imaging (~10 objects) : → multiple system imaging, sizes Occultations (~10 objects) : → sizes, atmospheric height Sicardy et al., 2012 : Eris occultation Techniques Optical – NIR photometry (>100 obj) → magnitude, spectral index Optical – NIR spectrosocopy (>40 obj): → icy/mineral bands HST Imaging (~10 objects) : → multiple system imaging, sizes Occultations (~10 objects) : → sizes, atmospheric height IR/mm/cm continuum : → thermal emission Moullet et al., 2008 : detection of 1999 TZ1 at IRAM-30m KBOs' thermal emission ← Eris 30 K 55 K 40 K 70 K Temperature ~1/√Dh Brightness 2 ~1/√Dh/Dg KBOs' thermal emission Rayleigh-Jeans regime 30-100 μm peak ← Solar reflected Frequency (GHz) Brightness temperature Tb = ε Tsurface ε : emissivity = departure from black-body Radiative effects Snell-Fresnel laws at surface/air interface : - reflection - non isotropic refraction - polarisation Emissivity depends on refraction index, surface roughness n=1 Thermal emission n>1 Radiative effects Surfaces not transparent at thermal wavelenghts : effectively sounding subsurfaces down to ~10λ Emissivity depends on absorption coefficient, vertical thermal profile Moullet et al., 2008b : Variation of Io's Tb with wavelength The total emission combines contributions from different depths Temperature distribution Temperature depends on geometric properties : shape, rotation rate orbital properties :: hel. Distance, pole direction surface properties : albedo, thermal inertia Mueller et al., 2008 : temperature distribution model for Haumea The radiometric method Morrison et al., 1977 Optical magnitude Thermal emission ~ albedo . D2 ~ B(ν,T((1-a)0.25)) . D2 Assuming thermal model Independant estimate of albedo and effective size If mass known (binaries) : density estimate The radiometric method Thermals models, defined through beaming parameter η Low inertia High inertia η=1 Varying η η=2 Slow Rotator model Quick Rotator model η constrained by multi-wavelengths thermal photometry Average value for KBOs : 1.2 Lightcurve interpretation Brightness variation during rotation (average 8h period) Time-resolved radiometric method can distinguish albedo distribution/ shape (apparent size variation) Lellouch et al, 2010 : Haumea's optical and thermal Lacerda et al, 2006 lightcurves with Herschel Results obtained Herschel Spitzer IRAM ALMA JVLA Frequency (GHz) Before 2010... ~4 sizes with ISO 90 μm ~45 sizes with Spitzer-MIPS (Centaurs) – 24 and 70 μm ~8 sizes with IRAM-30m MAMBO bolometer – 1.2 mm Sensitivity very limiting ! The Herschel Large Program : ''TNOs are cool'' 370 hours awarded (PIs Mueller and Lellouch) - 140 (40) targets at 60, 100 and 160 μm (PACS), 17 targets at 250 μm (SPIRE) : sizes (>200 km, error. 25%) / albedos - 25 (10) binaries : densities - 25 (1) lightcurves : shapes Vilenius et al., 2012 : KBO's albedo and inclination relation Jansky Very Large Array Best sensitivity with band Ka (1cm ) Detection of Quaoar and MakeMake (very cold : Tb~27K) Imaging of Pluto/Charon : Tb~40 / 55 K (different albedo) Makemake detection at EVLA (B. Butler) ALMA : sensitivity AND imaging - Mm-interferometer : less sky-confusion in galactic plane than IR - In bands 7/6, sensitivity better than Herschel - Possibility to investigate atmosphere through CO lines - Spatial resolution down to ~0.01'' (high frequency) Pluto : 0.1'', large KBO : 0.05'', most KBOs <0.015'' Thermal detection : Cycle 1 Diameter threshold for 5 σ detection Typical KBO thermal model assumed Albedo assumed (if necessary) : 0.08 1 hour on source, 16 GHz Band 6 : 18 μJy Band 7 : 31 μJy Thermal detection : Full science Diameter threshold for 5 σ detection Typical KBO thermal model assumed Albedo assumed (if necessary) : 0.08 Moullet et al., 2011 1 hour on source, 16 GHz Band 6 : 9 μJy Band 7 : 15 μJy Radiometric measurements Can be applied to ~ 500 bodies (>35% of total) for 1 h. obs each ~ 600 bodies (>40%) for 2 h. obs each >60 km diameter @ 20 AU >110 km @ 30 AU >160 km @ 40 AU >210 km @ 50 AU Errors on diameter 15-25% (dominated by model uncertainty) Mueller et al., 2008 Sizes and albedos surveys : science output - Significant increase of the size/albedo database for establishing correlations → retrieving physical, dynamical surface processes - Albedo necessary to interpret optical/IR spectra → surface chemical composition - Filling of the size distribution in the 100-200 km range → constrain formation and collisional history - Density measurements (binaries) → primitive disk composition/structure Size and shape : direct determination - Direct analysis of visibilities (~ imaging) - SNR/beam >20 - spatial resolution 0.6-1.2 x source size - Possible on ~ 30 bodies, Accuracy <15%, non-model dependant - Possible to identify ellipticity in the plane of sky on few bodies - Thermal lightcurves on ~ 30 objects Moullet et al., 2011 : simulated Charon visibilities @345 GHz Size and shape : science output - Independent size measurements → refinement of thermal models, albedo - Precise size determination for large sources : → compare to atmospheric/volatiles models - Shape determination on pole-on geometries - 3D shape combining lightcurves / imaging : → constraints on internal strength, density, formation Surface mapping - First KBOs thermal mapping possible with very extended configurations, resolution ~15mas - 10% temperature variations on 6 large bodies (4 h integration in bands 7,9 or 10) - Horizontal variations of albedo/ thermal inertia reflect surface collisions / resurfacing processes Surface mapping Pluto : variegated surface in albedo/composition. Expected Tsurf variations Young et al., 1998 Pluto, Band 7, very extended Pluto, Band 9, very extended configuration configuration Multiple system mapping - Large fraction of multiple systems: ~10%. Many ~equally-sized - Separation 2'' → contact binaries - Orbit determines mass - First resolved thermal imaging * → individual size/albedo → constraint on system formation (capture, disruption,...) Grundy et al., 2011 : improved - Better resolution than Hubble : orbits of large KBO binary systems binary searches, astrometry * except for Pluto/Charon system Multiple system mapping - Cycle 1 : handful of very separated systems - Full science : large-scale binary search, contact binaries imaging Hi'iaka' Vanth Beam ~ 0.5'' Beam ~ 0.2'' Haumea system, Band 7, Cycle1-3 Orcus system, Band 7, Cycle1-6 Atmospheres : Pluto - N2-based atmosphere, ~10-40 μbar pressure. - CO-ice detected : expected to maintain 1-10.e-4 abundance - Contradictory detections : q=0.5e-3 q~1e-1, optically thick Lellouch et al., 2011 : detection of the Greaves et al., 2011 : detection of the CO atmospheric lines near 2.3 μm CO(2-1) line at JCMT Atmospheric detectiondetection : Pluto - CO detection expected in ~1 hour Cycle 1. - Constraints on sublimation mechanism : horizontal/vertical ice segregation
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