Transcript wfirst

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
Exoplanet sensitivity down to sub-Earth masses at 0.5 AU - ∞
down to 0.1 Earth-masses over most of this range
free-floating planets down to 0.1 Earth-masses
• free-floating planet mass distribution is important for understanding planet
formation.
• Exoplanet Survey Question #2: How common are
potentially habitable worlds?
– η = fraction of planetary systems with an earth-like planet in the outer
habitable zone
– But what is earth-like?
– Kepler results imply a wide variety of planetary systems
– We need to answer question #1 to understand habitability
Microlensing Target Fields are in the
Galactic Bulge
Galactic center
8 kpc
Sun
1-7 kpc from Sun
Light curve
Source star
and images
Lens star
and planet
Telescope
10s of millions of stars in the Galactic bulge in order to detect planetary
companions to stars in the Galactic disk and bulge.
Space vs. Ground Sensitivity
ground
Expect ~190
free-floating
Earths
space
Habitable Earths
orbiting G & K stars
accessible only
from space
How Low Can We Go?
Limited by Source Size
angular Einstein radius
Mp 
 E   as 

 M °Ú 
1/2
R 
*  as  * 
 R°—
angular source star radius
(Bennett & Rhie 1996)
For E  * :
Mars-mass planets
low-mass planet signals are rare
detectable
and brief, but not weak
if solar-type sources can be
monitored!
Ground-based confusion, space-based resolution
CTIO
HST
• Space-based imaging needed for high precision photometry of
main sequence source stars (at low magnification) and lens star
detection
• High Resolution + large field + 24hr duty cycle => Space-based
Microlensing Survey
• Space observations needed for sensitivity at a range of
separations and mass determinations
High-magnification: Low-mass planets
OGLE-2005-BLG-169Lb
• Detection of a ~17 M
planet in a Amax= 800 event
• Caustic crossing signal is
obvious when light curve is
divided by a single lens
curve.
• Detection efficiency for ~10
M planets is << than for
Jupiter-mass planets
• Competing models with an
Earth-mass planet had a
signal of similar amplitude
• So, an Earth-mass planet
could have been detected
in this event, if it had a
separation ~ RE !
FUN, OGLE,
MOA & PLANET
Close Separation planets by Microlensing
s = 0.50 RE
s = 0.25 RE
• Faint main sequences sources needed to detecting low-mass planets
• At separations < RE, planetary signals occur at low stellar magnification
• Ground-based photometry seems to have systematic errors proportional to the
flux of blended stellar light.
• For close-in (or HZ) planets, higher angular resolution & longer exposures help
WFIRST vs. Kepler
WFIRST – w/ extended mission
Kepler ~12 yr mission
Figures from B. MacIntosh of the ExoPlanet Task Force
Extraction of Exoplanet Light Curve Signal
Twice Earth
Earth
Half Earth
No Planet
3
3.5
Magnification
Magnification
by stellar lens
3
Magnification
Time-series
photometry
is combined
to uncover
light curves
of
background
source stars
being
lensed by
foreground
stars in the
disk and
bulge.
2.5
Deviation Due
to Planet
2.5
2
1.5
-20
Offset from peak gives
projected separation
-10
0
Days
Planets are revealed as short-duration
deviations from the smooth, symmetric
magnification of the source due to the
primary star.
10
2
20
9.2
9.4
9.6
9.8
Days
Detailed fitting to the photometry
yields the parameters of the
detected planets.
Free Floating Planet Events
have tE < 2 days
RE (M,D)
tE 
~ M / M J day
vt
~２０ days for stars
M：lens mass
MJ: Jupiter mass
D：distance
vt: velocity

~Neptune mass
Sumi et al. 2011
As Many FFP as stars!
WFIRST can detect Earth-mass FFP
WFIRST SDT Final Report is Obsolete
There are issues with the exoplanet microlensing predictions
• Data Rate restrictions were ignored for DRM-2
– With planned hardware, we can only observe 6 DRM-2
fields per 15 minute cycle (not 7)
• Field locations and sampling was not optimized
• Obsolete microlensing optical depth/rate numbers used
Microlensing Optical Depth & Rate
WFIRST
Red Clump Stars Only
• MOA-II measures τ in vicinity of WFIRST field for 1st time (Sumi et al. 2012)
– using 474 events with well measured microlensing parameters (215 in central fields)
– luminosity function errors tend to cancel with all-star sample
• Selection of Red Clump Giant source stars is problematic
– due to blending and possible luminosity function errors
• WFIRST SDT Final report on obsolete (on this point)
GSFC NRO Design
“only” 18
H4RGs
These calculations use the GSFC design.
WFIRST Field Selection & Data Rate
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
Comparison of Earth-mass Detection Numbers
DRM1rep
DRM2rep
DRM-1
DRM-2
NRO
L2-limited
data rate
239
-
581
402
440
No data
rate limit
239
176
610
483
821
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
W
Ecliptic
Plane
Sun
Equatorial
Plane
W
MPF field
Vernal
Equinox
Orbit
23.5
28.7
Orbit Plane
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.
Detector Radiation Shield for
Outer van Allen Belt
STScI Design
MPF study indicates “sealed” multilayer
shield can remove trapped e- radiation
To get 100K focal plane may require
multiple radiators protected by louvers
Lens System Properties
• Einstein radius : E= *tE/t* and projected Einstein radius, r%
E
– * = the angular radius of the star
– r%
E from the microlensing parallax effect (due to Earth’s orbital motion).
2
r%
4GM
c
RE   E DL , so   E  2
. Hence M 
 E r%
E
DL c  E DL
4G
Finite Source Effects & Microlensing
Parallax Yield Lens System Mass
• If only E or rE is measured,
then we have a mass-distance
relation.
• Such a relation can be solved if
we detect the lens star and use
a mass-luminosity relation
– This requires HST or ground-based
adaptive optics
• With E, rE , and lens star
brightness, we have more
constraints than parameters
mass-distance relations:
c 2 2 DS DL
ML 
E
4G DS  DL
c 2 DS  DL
ML 
r%
E
4G
DS DL
2
c2
ML 
r%
E E
4G
HST Observations of
OGLE2003-BLG-235L/MOA-2003-BLG-53L
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= 3.30.4 mas/yr from light curve analysis
(rel= */t*) implies 0.6 mas separation in B - I
HST Observation Predictions for
OGLE-2003-BLG-235L/MOA-2003-BLG-53L
Fraction of total flux
due to lens star.
Centroid Shift
between HSTACS/HRC
passbands for
follow-up images.
(Units are 25 mas
pixels.)
Relative proper motion rel= 3.30.4 mas/yr
from light curve analysis (rel= */t*)
Lens Star Identification from Space
• Lens-source proper motion
gives E = reltE
• rel= 8.40.6 mas/yr for
OGLE-2005-BLG-169
• Simulated HST ACS/HRC
F814W (I-band) single orbit
image “stacks” taken 2.4
years after peak
magnification
Simulated HST images:
ML= 0.08 M
ML= 0.35 M
– 2 native resolution
– also detectable with HST
WFPC2/PC & NICMOS/NIC1
• Stable HST PSF allows clear
detection of PSF elongation
signal
• A main sequence lens of any
mass is easily detected (for
this event)
ML= 0.63 M
raw image
PSF subtracted
binned
Stacked HST I-band Image of OGLE2005-BLG-169 Source
HST images
taken 6.5 yrs
after event
Analysis by
Jay Anderson
Subtracted Neighbor…
PSF IS
GOOD!
Almost no
residuals
When we
Subtract a
PSF from a
(brighter)
neighbor
Subtracted F814W Stack
This means
that the
residuals of
the target-star
subtraction
are real.
Fit and Subtract Two Stars: Source & Lens
Very good
subtraction
residuals
when we fit
for two
sources
Two-source Solution:
• Offset consistent in the
F814W, F555W, and
F438W data:
– x = 1.25 pixels = 50 mas
– y = 0.25 pixel = 10 mas
– FLUX:
(left)
(right)
•
•
•
•
•
•
F814W
F555W
F438W
fI = 0.51
fV = 0.35
fB = 0.25
3392 e 3276 e2158 e 3985 e338 e 1029 e
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
High-magnification: Low-mass planets
OGLE-2005-BLG-169Lb
• Detection of a ~17 M
planet in a Amax= 800 event
• Caustic crossing signal is
obvious when light curve is
divided by a single lens
curve.
• Detection efficiency for ~10
M planets is << than for
Jupiter-mass planets
• Competing models with an
Earth-mass planet had a
signal of similar amplitude
• So, an Earth-mass planet
could have been detected
in this event, if it had a
separation ~ RE !
FUN, OGLE,
MOA & PLANET
HST vs. WFIRST Astrometry
• Our examples are all with HST data
– 2.4m diffraction limited optics
– Imaging at λ = 0.8 μm
– ~40 mas/pixel
– 6-8 images per passband
• WFIRST
– 1.1-2.4m diffraction limited optics
– Imaging at λ = 1-2 μm
– ~110-180 mas/pixel
– 500-50,000 images per passband
– Very wide filter 0.92-2.4 μm filter implies color dependent PSF
– rel uncertainty scales as ~ FWHM2 so ~4× better w/
NRO
Relative Proper Motion
Gaudi’s relative proper motion from White Paper for J = 23 source+lens
Lens Detection Provides Complete
Lens Solution
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
HST/WFPC2
CTIO
Crowded fields give
higher lensing rate, but
complicate mass
determination ->
redundancy needed
HST/WFC3/IR