Asteroid models for PACS and SPIRE calibration Thomas M¨uller,MPE, Jan. 19, 2012 1. Asteroid calibration observations 2. Sample for flux calibration PACS & SPIRE results 3. Asteroids as prime calibrators 4. Cross-calibration aspects 5. Next steps and future plans 1 Why asteroid calibration observations? • absolute flux calibration better than 5% in the entire flux regime • characterisation of point-spread-function (PSF) • important sources for filter leak tests and colour-corrections • spatial calibration aspects via asteroid-planet-star encounters • sources for detector linearity tests (connecting stars & planets) • establishment of relative spectral response function (RSRF) • technical telescope aspects: tracking & pointing system • cross-calibration between instruments, observing modes, with other missions, over time, across different wavelength & flux regimes, ... • anything where bright, point-like sources of known fluxes and with featureless SEDs are needed 2 Overview PACS/SPIRE asteroid calibration PacsCal PacsRangeSpec PV 16.2 h PacsCal Phot PSF FieldDistort 3.4 h PacsCal Spec Chopped Raster 1.2 h PacsCal WaveCalChop 1.7 h PacsPhoto 33.7 h PacsRangeSpec 41.4 h SpirePacsParallel 2.5 h SpirePhoto 67.9 h SpireSpectrometer 46.7 h • Until Dec. 2011, Herschel has spent about 215 h on asteroids for calibration purposes (PACS & SPIRE) • In total, more than 900 individual calibration observations (OBSIDs) • Thermophysical model calculations have been provided on request, typ- ically per OBSID, SEDs with pre-defined resolution or monochromatic flux densities at given reference wavelengths • All models have the same version, i.e., based on a fixed set of model parameters per object (for consistency reasons) 3 Asteroid sample for absolute flux calibration 4 Asteroid sample for absolute flux calibration • About 25 main-belt asteroids have been observed for flux calibration purposes by PACS and SPIRE • The original list of 55 asteroids has been down-selected to 12 high-quality objects (May 2011): 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, 6 Hebe, 8 Flora, 10 Hygiea, 20 Massalia, 21 Lutetia, 52 Europa, 88 Thisbe, 704 Interamnia • based on Herschel (PACS/SPIRE) observations and including checks against other data from ground (mid-IR/submm/mm) and space (IRAS, ISO, MSX, Spitzer, Akari) • For these 12 targets the derived Herschel (PACS/SPIRE) monochromatic flux densities are typically within 5% of the model predictions (full thermo- physical models, including shape models, rotation properties, thermal surface properties, true size and albedo values, ....). • Some are still outside the 5% corridor, but have the potential to reach that zone via external information (new shape and/or size information) 5 SPIRE: asteroid flux calibration (1) Timeline of SPIRE asteroid observations 6 SPIRE: asteroid flux calibration (2) 7 SPIRE: asteroid flux calibration (3) 8 PACS: asteroid flux calibration (1) • On average, the mini scan-map observations of the 18 asteroids agree within 2% in all 3 bands with the corresponding model predictions (giving each asteroid the same weight; after colour-correction) • blue band, 53 independent high-quality observations: stdev = 0.036 • green band, 53 independent high-quality observations: stdev = 0.036 • red band, 119 independent high-quality observations: stdev = 0.042 • Numbers do change slightly (a few percent), depending on settings for specific reduction steps: deglitching, masking/HPFW, aperture size, ... 9 10 11 12 Individual asteroids as prime calibrators? 21 Lutetia (Rosetta) 4 Vesta (DAWN) 13 21 Lutetia Rosetta flyby target (shape model by Carry et al. 2010) 14 15 16 21 Lutetia O'Rourke, M¨ulleret al. 2012 17 18 SPIRE FTS and photometer data on Pallas and Vesta 19 Individual asteroids as prime calibrators (2) • Best candidates: 1 Ceres, 2 Pallas, 4 Vesta, 21 Lutetia • Ceres and Vesta have more than 10 independent observations with each instrument, Pallas just below 10, Lutetia only a few • Model predictions are based on pre-Herschel model parameters • Derived fluxes depend slightly on data reduction details • Completely different calibration schemes for PACS & SPIRE Ast. 70 100 160 250 350 500 µm 1 1.05±0.02 1.03±0.02 1.07±0.02 1.00±0.03 1.01±0.02 1.00±0.03 2 1.06±0.02 1.05±0.03 1.10±0.03 1.09±0.03 1.10±0.03 1.10±0.03 4 1.02±0.03 1.08±0.03 1.14±0.04 1.07±0.02 1.11±0.02 1.09±0.02 21 1.07±0.07 1.07±0.05 1.11±0.07 1.00±0.06 1.01±0.04 0.94±0.08 Excellent cross-calibration results! 20 Individual asteroids as prime calibrators (3) • Several asteroids are on a path to become truly prime flux calibrators: Absolute flux predictions <5% in FIR/submm/mm, at any given time • Availability of highly accurate shape models, spin-axis characterisation, surface properties for 1 Ceres, (2 Pallas), 4 Vesta, 21 Lutetia • Large database of thermal observations available, for fine-tuning relevant pa- rameters, like surface roughness and thermal inertia • The PACS and SPIRE fluxes will play a key role when converting them to prime calibrators (in Herschel post-mission phase) • 21 Lutetia: the Rosetta shape model is available and publications are submit- ted (O'Rourke et al.; Carry et al. 2012), but only ∼60% of the surface was seen, the rest is re-constructed via other techniques • 4 Vesta: DAWN Mission is still at Vesta (until July 2012), shape model will be available soon • 1 Ceres & 2 Pallas: new shape models by Carry et al. 2009/2010 • 1 Ceres: DAWN Mission will arrive at Ceres in Feb. 2015 21 Asteroid cross-calibration aspects • SOFIA/APEX/ALMA: use the same list of 12 asteroids based on the same models • All 3 projects have meanwhile observed a few of the asteroids • coordinated activities with Planck on many asteroids (and planets), but no feeback on Planck fluxes so far • current main cross-calibration tasks - PACS photometer vs. spectrometer - SPIRE photometer vs. spectrometer - PACS vs. SPIRE - Connecting a stellar based calibration scheme (PACS, gb-MIR) to a planet based calibration scheme (SPIRE, gb-submm/mm) 22 Outlook & Next Steps • continuation of asteroid model calculations on request • collaborations with Benoit Carry (ESAC) and Josef Durech (Prague Univer- sity) to establish better shape/spin-vector models for key asteroids • collection and identification of key thermal data per object • ISSI-project with a focus on thermal surface effects has started Oct. 2011 (funded for 2 years) • aspect for the near future and post-operations phase: - collect all Herschel observations of calibrators (planets, stars, asteroids) - combine with auxiliary data from ground (lightcurves, occultations) - combine with space data (Planck, Rosetta, Dawn, ...) - tune calibrators for future projects: SOFIA, ALMA, JWST, SPICA, .... - updated calibrator models, including new primary standards! ) An important legacy of Herschel! 23.