2020/08/29 – ESOP XXXIX (Online) João Ferreira (Observatoire de la Côte d’Azur & IA-Lisboa) Paolo Tanga (Observatoire de la Côte d’Azur, Nice and Gaia Mission) Pedro Machado (IA-Lisboa) Census of Occultations
Over 4 700 stellar occultations performed to asteroids (planets and satellites excluded): 1961-2020 (August 14th); Over 1 300 asteroids involved; Good historical average astrometry (85 mas in 2015) compared to other ground-based methods (~400 mas for CCD imaging);
Big growth in last few years, thanks to more reliable catalogues and larger community.
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Census of Occultations
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Asteroid Orbit: Weighting Astrometric Data Not all observations are equal, because they don’t all present the same precision;
MPC does not publish each observation’s position uncertainties;
It’s necessary to assess which observations are more reliable;
Solution: each observation has its own weight in the fit, inversely proportional to its uncertainty.
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Occultations in MPC
Identified with Observatory Code “244”;
Observed RA/Dec coordinates filled by D. Herald’s astrometric solution;
More chords in event => Better solution;
Many occultations scattered in a long arc can lead to better orbital improvements.
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Sources of Uncertainty – Across- track
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Sources of Uncertainty – Across- track
Single chord occultations are the most common (~60%);
Ambiguity in asteroid’s position introduced;
More chords => Better position constraints => More precise results (sharp decrease typically from 3 chords forward).
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Sources of Uncertainty – Timing
Dashed lines correspond to timing uncertainty of chords
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Sources of Uncertainty – Timing Essential to have the most precise timing possible;
Larger uncertainties mean it’s easier to fit, but also that the result is less robust;
Optimal Precision vs Signal balance is required;
Final solution’s timing uncertainty depends on: Individual chords’ timing precision; Model of chord combination on the Bessel fundamental plane; Star uncertainty at moment of event.
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Formula Used by Occult in “Poor” Events
Chord > 80% of Max. Duration: 5% precision; 60% < Chord <= 80%: 10% precision; Chord <= 60%: 20% precision.
Some particular values for elongated objects, like (216) Kleopatra, (433) Eros and (25 143) Itokawa: 20% of maximum duration always.
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Debiasing Old Observations Different catalogues are used as time goes by, to calibrate images and place moving objects in the sky. Were the old ones biased either globally or locally?
Existence of very accurate position/proper motion recent catalogues allows assessment of old catalogue bias;
Most prolific star catalogues in asteroid observations (UCAC, USNO, 2MASS, GSC, Tycho, etc.) have been compared to PPMXL/2MASS (old model) and Gaia DR2 (new model) and corrected accordingly;
Old observations can be improved and made more reliable through this method. Performance of Occultation Astrometry with Gaia DR2, João Ferreira Debiasing Example (Farnocchia et al., 2014)
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Observation Weights – What Matters? Besides observation method, other parameters can be used to weight a position:
Observation date (observations tend to get better with time); Observatory (some observers have access to more advanced telescope/camera systems); Catalogue of star(s) used as reference; Asteroid brightness and rate of motion.
Weighting scheme should depend on all of these parameters, and how they combine.
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Two Weighting Methods for Observations “Old”: Farnocchia et al., 2014 (fcct14). Hierarchy of methods. Uses: 1. Observer; 2. Star catalogue; 3. Epoch; 4. Observation method.
“New”: developed by Spoto et al. (to be published). Multi- dimensional analysis. Uses: Same as previous, but with different weight distributions; Object brightness.
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Current Weighting Scheme – Farnocchia et al (2014)
UCAC4 Tycho
Occultations Average uncertainties per date, per catalogue used and per observation method, for both RA and Dec. Performance of Occultation Astrometry with Gaia DR2, João Ferreira OrbFit
Used to fit an asteroid orbit from all observations available at MPC;
Can use both weighting methods. Comparison possible;
Used to check occultation’s astrometric solution vs asteroid’s orbital solution with and without Gaia;
Possible to make mass review of how well occultation fits to available information at the time with each weighting scheme.
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Events With 20+ Chords Number Name Date Old (mas) New (mas) 2 Pallas 29-05-1983 95,5 53,6 9 Metis 12-09-2008 101,9 59,0 9 Metis 07-03-2014 8,5 17,1 51 Nemausa 11-09-1983 295,3 33,4 Anomalous (472) Roma event 87 Sylvia 29-10-2019 34,7 24,1 excluded from calculations. 90 S/2000 19-07-2011 64,1 44,5 130 Elekra 21-04-2018 51,3 45,4 Distances (average ± std): 135 Hertha 11-12-2008 41,7 56,5 212 Medea 08-01-2011 72,9 9,6 - Old method: (86 ± 103) mas; 216 Kleopatra 12-03-2015 27,1 28,2 - New method: (46 ± 40) mas. 234 Barbara 21-11-2009 65,7 33,1 345 Tercidina 17-09-2002 55,9 24,0 372 Palma 26-01-2007 54,9 45,7 381 Myrrha 13-01-1991 444,9 200,4 Better Method: 420 Betholda 26-08-2003 68,9 32,9 - Old: 3 events; 451 Patientia 05-10-2016 57,8 40,4 - New: 15 events; 472 Roma 08-07-2010 821,9 795,9 - Similar: 3 events. 498 Tokio 17-02-2004 35,5 31,4 704 Interamnia 23-03-2003 98,6 92,3 Bold: Improvement above 50%. 925 Alphonsina 22-12-2003 54,5 17,4 1263 Varsavia 18-07-2003 0,3 34,7 Performance of Occultation Astrometry with Gaia DR2, João Ferreira Events With 20+ Chords
(51) Nemausa (1 263) Varsavia
(212) Medea
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Gaia’s Influence on Orbital Fit
Chords of occultations by (118) Peitho on 2019/10/12 using fit with MPC observations and MPC+Gaia. (0;0) is centre of Astrometric solution given by D. Herald. Performance of Occultation Astrometry with Gaia DR2, João Ferreira Occultation Predictions with Gaia
44%
Prediction of occultation of (850) Altona using only MPC data for the orbital fit. Performance of Occultation Astrometry with Gaia DR2, João Ferreira Occultation Predictions with Gaia
98%
Gaia data added to the fit. Cross marks positive report (Rui Gonçalves, Tomar, Portugal) Performance of Occultation Astrometry with Gaia DR2, João Ferreira Conclusions? For the events with most chords, new error model outperforms the old one in most cases. Full run on all occultations to be made soon;
Gaia DR2 can improve orbits by a large factor, though combining it with other MPC data still requires more work;
Gaia DR3 will bring data on ~150 000 asteroids, an order of magnitude up from DR2, and during a longer arc. Results to be seen next year!
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Thank you!
Performance of Occultation Astrometry with Gaia DR2, João Ferreira Anomaly: (472) Roma occultation
Performance of Occultation Astrometry with Gaia DR2, João Ferreira