Exoplanet Microlensing Survey with the NEW Telescope (& WFIRST). David Bennett University of Notre Dame. WFIRST. Unique Science from Space-based Survey. Exoplanet Survey Question #1: How do planetary systems form and evolve? complementary to Kepler
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University of Notre Dame
1-7 kpc from Sun
10s of millions of stars in the Galactic bulge in order to detect planetary companions to stars in the Galactic disk and bulge.
Expect ~190 free-floating Earths
Habitable Earths orbiting G & K stars accessible only from space
Limited by Source Size
angular Einstein radius
angular source star radius
(Bennett & Rhie 1996)
For E * :
low-mass planet signals are rare and brief, but not weak
Mars-mass planets detectable
if solar-type sources can be monitored!
Ground-based confusion, space-based resolution
FUN, OGLE, MOA & PLANET
s = 0.50 RE
s = 0.25 RE
WFIRST – w/ extended mission
Kepler ~12 yr mission
Figures from B. MacIntosh of the ExoPlanet Task Force
Time-series photometry is combined to uncover light curves of background source stars being lensed by foreground stars in the disk and bulge.
Magnification by stellar lens
Deviation Due to Planet
Offset from peak gives
Detailed fitting to the photometry yields the parameters of the detected planets.
Planets are revealed as short-duration deviations from the smooth, symmetric magnification of the source due to the primary star.
MJ: Jupiter mass
~２０ days for stars
Sumi et al. 2011
As Many FFP as stars!
WFIRST can detect Earth-mass FFP
Red Clump Stars Only
These calculations use the GSFC design.
Preliminary optimization of DRM1 and DRM2 fields, with central MOA fields used for microlensing rate measurement. 9 and 6 fields, respectively, when optimum is 10.
Preliminary optimization of WFIRST-NRO fields, with central MOA fields used for microlensing rate measurement. L2 data rate limit is 5 fields. Optimum is 11 – assuming 15 minute sampling.
Assumes 40 sec slew & settle – probably conservative – we’d do more fields if we could
Current L2 data rate limit makes L2 version of WFIRST inefficient. More pixels per star => fewer planet discoveries.
Likely trade: L2-NRO will require more microlensing observing time and less dark energy time. But, a factor-of-2 increase in L2 downlink would remove this issue. Maybe a larger high-gain antenna would be enough?
Photometry improvement from NRO is modest – but it may be more significant near the habitable zone.
The results assume Cassan et al. (2012) exoplanet MF and a detection threshold of Δχ2 > 300 in mission simulations using an IR version of the Bennett & Rhie (2002) code.
MPF in Geosynchronous Orbit
MPF’s orbit allows continuous view of Galactic bulge planet search field and continuous data data downlink to a dedicated ground station in White Sands.
MPF study indicates “sealed” multilayer shield can remove trapped e- radiation
To get 100K focal plane may require multiple radiators protected by louvers
Lens and source perfectly aligned during event
Lens moves away form source after event, so centroid position of blended image is color dependent
Relative proper motion rel= 3.30.4 mas/yr from light curve analysis (rel= */t*) implies 0.6 mas separation in B - I
Fraction of total flux due to lens star.
Centroid Shift between HST-ACS/HRC passbands for follow-up images. (Units are 25 mas pixels.)
Relative proper motion rel= 3.30.4 mas/yr from light curve analysis (rel= */t*)
Simulated HST images:
ML= 0.08 M
ML= 0.35 M
ML= 0.63 M
HST images taken 6.5 yrs after event
PSF from a
Very good subtraction
residuals when we fit for two sources
HST BVI observations imply
M* = 0.63 M
Mp = 17 M
observed separation of 51 mas confirms planet model prediction of 54.3±3.7 mas
FUN, OGLE, MOA & PLANET
Gaudi’s relative proper motion from White Paper for J = 23 source+lens
Estimates of the parameter uncertainties from these methods for the proposed Microlensing Planet Finder mission concept, which was one of 3 WFIRST precursors (Bennett, Anderson & Gaudi 2007). WFIRST should do at least as well as this.
WFIRST-NRO should do much better.
However, the Central Bulge is More Crowded in the IR
Crowded fields give higher lensing rate, but complicate mass determination -> redundancy needed