Directed Follow-Up of Gaia Photometry in Search of Transiting Planets

Shay ZuckerYifat Dzigan

Tel-Aviv University

• Dzigan & Zucker, MNRAS, 415, 2513 (2011)• Dzigan & Zucker, ApJL, 753, L1 (2012)• Dzigan & Zucker, MNRAS, accepted

Gaia in a Nutshell

• High precision astrometry (~5 μas).• Launch October 2013, five-year mission, L2 orbit.• + spectroscopy, + 1 mmag photometry.• 109 stellar targets, On average 70-80 samples per

target.• Hipparcos successor

Aposteriori Detection of HD209458b in Hipparcos Data

Söderhjelm 1999Robichon & Arenou 2000

Castellano et al. 2000

Why not a real detection?

• No one thought about it…• Photometry not precise enough• Very low cadence

Can we somehow use the data for detection?

Directed Follow-Up – General Idea

• Actually sampled transits provide some knowledge about P,T,w,d.

• MCMC (assuming a box-shaped transit) prvides a pdf over the parameters.

• Compute Instantaneous Transit Probability – ITP(t).• Follow-up observation when ITP is large.• Repeat the process (including the new follow-up

data).• Stopping criterion.

Could HD209458b have been detected in 2004?

Could HD209458b have been detected in 2004?

Could HD209458b have been detected in 2004?

Lessons from Hipparcos HD209458 Exercise

• With five sampled transits, low cadence data, augmented by DFU, allow detection.

• Data become useless after a few years.• For completeness – we should mention HD189733b.

Testing DFU on Gaia

• Simulated Gaia light curves inspired by known transiting planets (period and duration, coordinates)

• Gaia scanning law• Photon noise level – 1 mmag.• Phase depends on the simulated situation.• We defined three scenarios of detection.

First Scenario – Detection

• Gaia samples enough transits to allow detection of the transit using only Gaia data.

• 1-mmag precision makes this scenario possible.• Simulation inspired by CoRoT-1b

(P=1.51 d, w=0.1 d)• Assuming five sampled transits (Ntot=64)• d > 0.005 mag

First Scenario – Detection

Second Scenario – DFU-assisted detection

• Gaia samples enough transits to allow several possible solutions. One DFU observation is enough to constrain the period.

• Simulation inspired by CoRoT-4b (P=9.20 d, w=0.16 d)

• Assuming three sampled transits (Ntot=63)• d > 0.001 mag

Second Scenario – one DFU observation

Third Scenario – DFU Detection

• Gaia samples enough transits to allow several possible solutions. A few DFU observations are needed to constrain the period.

• Simulation inspired by WASP-4b (P=1.338 d, w=0.104 d)

• Assuming four sampled transits (Ntot=83)• d = 0.005 mag

Third Scenario – DFU Detection

Third Scenario – DFU DetectionOnly Gaia data

Gaia data + 1 DFU observation

Gaia data + 2 DFU observations

Rapid Response

• ITP degrades over time• Therefore – we should start observing even before the

end of the mission• Tres-1b (P=3.03 d, w=0.104 d)• Three sampled tranists (Ntot=48) (mid-life of mission)• Assume d=0.008 mag

Is it worth it?

Observational Window Function

70 Gaia Measurements 130 Gaia Measurements

Is it worth it?• Assuming 2 hr transit Simplified Galactic model (Bahcall & Soneira 1980) Transiting planet statistics (Gould et al. 2006)

• Down to 14th G magnitude:minimum 7 transits: ~70 transiting HJs and VHJsminimum 5 transits: ~200 minimum 3 transits: ~600

• Down to 16th G magnitude:minimum 7 transits: ~300 transiting HJs and VJHsminimum 5 transits: ~900 minimum 3 transits: ~2600

To do list:• Get organized

– Dedicated observatory network?– CU7 follow-up network?– Science Alert team?

• Prescreening (metallicity, brightness, activity etc.)• Improve on MCMC.• Objective criteria to triggering DFU

– Wald statistics– ITP values– ITP skewness

• Smaller planets (Neptunian and below?)• Other low-cadence surveys

Additional slides Contingencies for potential questions

HD189733 – Hipparcos and DFU observations

HD189733 – Hipparcos and DFU observations

HD189733 – Hipparcos and DFU observations

HD189733 – Hipparcos and DFU observations

Sanity checks

Gaia WASP-4b – degradation of ITP over 10 years