Asteroid models for PACS and SPIRE calibration

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1 Asteroid models for PACS and SPIRE calibration Thomas Müller, MPE, Jan. 19, 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

2 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

3 Overview PACS/SPIRE asteroid calibration PacsCal PacsRangeSpec PV PacsCal Phot PSF FieldDistort PacsCal Spec Chopped Raster PacsCal WaveCalChop PacsPhoto PacsRangeSpec SpirePacsParallel SpirePhoto SpireSpectrometer 16.2 h 3.4 h 1.2 h 1.7 h 33.7 h 41.4 h 2.5 h 67.9 h 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, typically 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)

4 Asteroid sample for absolute flux calibration

5 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 thermophysical 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)

6 SPIRE: asteroid flux calibration (1) Timeline of SPIRE asteroid observations

7 SPIRE: asteroid flux calibration (2)

8 SPIRE: asteroid flux calibration (3)

9 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 = green band, 53 independent high-quality observations: stdev = red band, 119 independent high-quality observations: stdev = Numbers do change slightly (a few percent), depending on settings for specific reduction steps: deglitching, masking/hpfw, aperture size,...

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13 Individual asteroids as prime calibrators? 21 Lutetia (Rosetta) 4 Vesta (DAWN)

14 21 Lutetia Rosetta flyby target (shape model by Carry et al. 2010)

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17 21 Lutetia O Rourke, Müller et al. 2012

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19 SPIRE FTS and photometer data on Pallas and Vesta

20 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 µm ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.08 Excellent cross-calibration results!

21 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 parameters, 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 submitted (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/ Ceres: DAWN Mission will arrive at Ceres in Feb. 2015

22 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)

23 Outlook & Next Steps continuation of asteroid model calculations on request collaborations with Benoit Carry (ESAC) and Josef Durech (Prague University) 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 (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!