GAIA: SOLAR SYSTEM ASTROMETRY IN DR2

P. Tanga 1, F. Spoto1,2

1Observatoire de la Côte d’Azur, Nice, France 2IMCCE, Observatoire de Paris, France P. Tanga – Gaia and the Solar System 2 Gaia is observing - Scanning the sky since July 2014 (operational phase) - Solar elongations 45° to 135° - Limiting magnitude V~20.5

- 100.000 observations (CCD level) / 45°

Sun

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 3 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 14 099 astrometry 1 997 702 CCD positions 287 904 transits (52% : photometry) Typ. accuracy <1 mas (along scan)

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 4 Gaia DR2 - Solar System statistics 1:0e5 1200

8e4 1000

6e4 800

600 4e4 400

2e4 200

0 0 10 11 12 13 14 15 16 17 18 19 20 0 100 200 300 400 500 600 Number of observations per object G mag

1000

100

10

1

1 2 5 10 20 50 a (AU)

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 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

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 6

Asteroid (386) Siegena - residuals from orbital fit

Combining archive data (2776 obs.) to GDR2

0:4 0:03 4 0:3 0:02 0:2 2 ) ) ) 0:01 s s s

a 0:1 a a ( ( ( l l l a a a u 0 u u 0 d d 0 d i i i s s s e e e r r r c c c ¡ :

e e 0 01 e ¡0:1 D D D ¡2 ¡0:2 ¡0:02 ¡4 ¡0:3 ¡0:03

¡0:4 ¡4 ¡2 0 2 4 ¡0:4 ¡0:3 ¡0:2 ¡0:1 0 0:1 0:2 0:3 0:4 ¡0:02 0 0:02 RA residual (as) RA residual (as) RA residual (as) factor 100 X improvement!

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 7 The situation, before Gaia

• 1.9 x 108 measurements in the archives of the Center • mostly CCD imaging • average accuracy: 0.4 as

• ~2000 radar ranging measurements • equivalent accuracy : 10-50 mag

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 8 Some challenges for asteroid astrometry

Discover asteroid satellites New / precise asteroid 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

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 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

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 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)

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 11 members in DR2 4676 family members in DR2 (from AstDys membership)

0:6

0:5

0:4 ) i (

n :

i 0 3 s

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)

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 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 0 0

-0.1 -0.1 O-C Dec (as) O-C Dec (as)

LT -0.2 -0.2

-0.4 -0.3 -0.2 -0.1 0 0.1 -0.4 -0.3 -0.2 -0.1 0 0.1 O-C RA (as) O-C RA (as)

(credits: Gaia GBOT team)

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 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)!!

1e 4 15 ¡ • Occultations: very accurate asteroid 14

) 13

u 1e 5 a position at the level of the star ¡ s ( 12 n a o ¾ 11 i 1e 6 t astrometry s a

n ¡

10 t l o i u

t 9 c a • 1e 7 c …Beyond the duration of Gaia! t l ¡ 8 o u c

7 N c 1e 8 O ¡ equal accuracy 6 5 1e 9 4 ¡1e 9 2e 9 5e 9 1e 8 2e 8 ¡ ¡ ¡ ¡ ¡ All observations ¾a(au)

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 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 OBSERVATIONS DATA

EXPLOITATION: YARKOVSKY, IMPROVED OCCULTATION DYNAMICAL MODEL PREDICTIONS AND ORBITS

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 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…

IAU 30 GA - Wien - 2018 P. Tanga – Gaia and the Solar System 16

Diffusion of asteroid alertsFrom 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)

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IAU 30 GA - Wien - 2018