GAIA: SOLAR SYSTEM ASTROMETRY IN DR2 P. Tanga 1, F. Spoto 1,2 1 Observatoire de la Côte d Azur, Nice, France 2 IMCCE, Observatoire de Paris, France
2 Gaia is observing asteroids - Scanning the sky since July 2014 (operational phase) - Solar elongations 45 to 135 - Limiting magnitude V~20.5-100.000 asteroid observations (CCD level) / day 45 Sun
P. Tanga Gaia and the Solar System Gaia DR2 - Solar System On the base of a pre-selected list of known objects > 10 FOV transits over the 22 months of DR2 August 5, 2014 - May 23, 2016 Objects Epoch astrometry Typ. accuracy 14 099 1 997 702 CCD positions 287 904 transits (52% : photometry) <1 mas (along scan) 3
4 Gaia DR2 - Solar System statistics 1:0e5 1200 8e4 1000 6e4 800 4e4 600 400 2e4 200 0 10 11 12 13 14 15 16 17 18 19 20 G mag 0 0 100 200 300 400 500 600 Number of observations per object 1000 100 10 1 1 2 5 10 20 50 a (AU)
5 Gaia DR2 - Solar System - single transit astrometric performance Residuals from the orbital fit of Gaia DR2 data only (along-scan direction) Spoto et al. 2018
6 Asteroid (386) Siegena - residuals from orbital fit Combining archive data (2776 obs.) to GDR2 0:4 4 0:3 0:03 Dec residual (as) 2 0 2 Dec residual (as) 0:2 0:1 0 0:1 Dec residual (as) 0:02 0:01 0 0:01 0:2 0:02 4 0:3 0:03 4 2 0 2 4 RA residual (as) 0:4 0:4 0:3 0:2 0:1 0 0:1 0:2 0:3 0:4 RA residual (as) 0:02 0 0:02 RA residual (as) factor 100 X improvement!
7 The situation, before Gaia 1.9 x 108 measurements in the archives of the Minor Planet Center mostly CCD imaging average accuracy: 0.4 as ~2000 radar ranging measurements equivalent accuracy : 10-50 mag
8 Some challenges for asteroid astrometry Discover asteroid satellites New / precise asteroid masses from their wobbling 0.5-10 mas 1000s encounters/year > 10 mas Improve predictions of stellar occultations ~a few mas Measure the orbital drift due to Yarkovsky force
9 Yarkovsky effect: why we care Yarkovsky applications: NEO and meteorite transport physical properties (spin, density ) family dispersion, ages Spoto et al. 2015 The challenge: measure a few, apply to many > connection to spectro- photometry by Gaia: low-res spectra: taxonomy in the visible mmag photometry: shape determinations Large sample by end of mission: ~ 350 000 Delbo, Galluccio, De Angeli, ESA/Gaia/DPAC
10 Yarkovsky and Gaia? ~ 3 x 10-9 AU Pre-Gaia orbit quality (MPC data) 1/D (1/km) In DR2: 3 NEOs with measured Yarkovsky 5 NEOs with marginal detection about 20 good MB candidates Orbit uncertainty (km)
11 Asteroid family members in DR2 4676 family members in DR2 (from AstDys membership) 0:6 0:5 0:4 sin(i) 0:3 0:2 0:1 0 1:6 1:8 2:0 2:2 2:4 2:6 2:8 3:0 3:2 3:4 3:6 3:8 4:0 a (AU)
12 Joint exploitation Gaia + pre-gaia astrometry: bias example Asteroid (1132) Hollandia Liverpool Telescope + VST (8 hours apart) & MPC ground-based data (~1900 positions) 0.2 0.2 PPMXL calibration Gaia calibration 0.1 VST 0.1 LT+VST O-C Dec (as) 0-0.1 O-C Dec (as) 0-0.1-0.2 LT -0.2-0.4-0.3-0.2-0.1 0 0.1 O-C RA (as) -0.4-0.3-0.2-0.1 0 0.1 O-C RA (as) (credits: Gaia GBOT team)
13 The occultation astrometry with Gaia DR2 already successfully exploited Triton (Octobre 3, 2017) 2014 MU69 (V=27.5, 50 km KBO, ~1.4 mas)!! Occultations: very accurate asteroid position at the level of the star astrometry Beyond the duration of Gaia! Occultations ¾a(au) 1e 4 1e 5 1e 6 1e 7 1e 8 equal accuracy 1e 9 1e 9 2e 9 5e 9 1e 8 2e 8 All observations ¾ a (au) 15 14 13 12 11 10 9 8 7 6 5 4 N occultations
14 Robotic observations of asteroid occultations Extension to faint magnitudes and small asteroids Only method providing ~Gaia accuracy 50 cm robotic telescope at OCA, France ASTROMETRIC DATA OBSERVATIONS EXPLOITATION: YARKOVSKY, IMPROVED DYNAMICAL MODEL AND ORBITS OCCULTATION PREDICTIONS
15 Conclusions Gaia DR2 asteroid data: the first sample, already useful to test subtle dynamical effects Yarkovsky determination general orbit improvement application to stellar occultations Combination with other observations: it starts to work but weighting of the data is critical accurate debiasing is required a new method successfully implemented and tested Yarkovsky detection in the Main Belt is getting closer
16 P. Tanga Gaia and the Solar System Diffusion of asteroid alerts From the Gaia-FUN-SSO web site (the red contours are computed by the SSO-ST pipeline and show the search area on the sky, on three different dates): Topocentric predictions http://gaiafunsso.imcce.fr B. Carry (OCA), W. Thuillot (IMCCE) d n a r e t s i Reg! e t u b i r t n co