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 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 - Pluto is backing : atmosphere may freeze out soon ! Expected CO(3-2) disk-integrated - In full science : Haumea, lines on Pluto (from M. Gurwell) Makemake, Eris, Sedna Summary : ALMA has an important and unique role for KBOs studies - Radiometric measurements of sizes and albedos and hundreds of bodies - Direct size and shape estimation om large bodies - Detection and imaging of binaries down to 10mas close - First thermal maps of KBOs Possible wealth of physical information Invaluable tool complementing Herschel Observation Challenges - track moving sources (~1''/ hour) - handling of ephemeris objects (data reduction, array operation) - confusion (non-uniform background) - tight scheduling constraints (lightcurves, multiple system orbits) Future perspectives for KBO studies - detection : GAIA. LSST, Pan-Starrs - occultation search : TAOS - optical/near-IR spectroscopy : E-ELT, JWST - New Horizons: Pluto flyby in 2015. Comprehensive caracterization, no thermal instrument. Possible other KBO target