Transcript Document
Microlensing Surveys for Finding
Planets
Kem Cook
LLNL/NOAO
With thanks to Dave Bennett for most of these slides
Microlensing Surveys Ushered in the
Current Era of time-domain surveys
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MACHO, OGLE, EROS started in the early 1990s
Microlensing search needed repeated observations of millions of stars
Simple point-source point-lens detected and proved the principle
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Anomalous microlensing detected--binary lensing
Extreme binary system is star and planet
Follow-up collaborations formed to detect planets in 1995
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PLANET collaboration
• Probing Lensing Anomalies NETwork
MPS collaboration
• Microlensing Planet Survey
Current follow-up
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Huge databases of light curves over 1000s of days for millions of stars
PLANET
MicroFUN
Current Galactic Surveys
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OGLE
MOA
PLANET Telescope System
Collaboration member telescopes
MOU in place with RoboNet
The Physics of Microlensing
• Foreground “lens” star +
planet bend light of “source”
star
• Multiple distorted images
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Total brightness change is
observable
• Sensitive to planetary mass
• Low mass planet signals
are rare – not weak
• Peak sensitivity is at 2-3
AU: the Einstein ring radius,
RE
• 1st Discovery from Groundbased observations
announced already
Lensed images at arcsec resolution
A planet can be discovered when one of the lensed
images approaches its projected position.
Animation from Scott Gaudi
Simulated Planetary Light Curves
• Planetary signals can be
very strong
• There are a variety of
light curve features to
indicate the planetary
mass ratio and
separation
• Exposures every 10-15
minutes
• The small deviation at
day –42.75 is due to a
moon of 1.6 lunar
masses.
Microlensing surveys need VOEvents
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Alert to new microlensing events
– Currently done via email and web post
– Multiple surveys mean possible confusion
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Analysis of ongoing events suggests ‘anomaly’
– Email anomaly alerts (2nd level alerts)
– Analysis may suggest optimum sampling time
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Photometry follow-up for planets
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Spectroscopic follow-up
– Spatial resolution of source star (eg limb darkening)
– Multiplication of source star flux
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Current follow-up networks use email, telephone and web pages to relay
information
1st Exoplanet Discovery by lensing
The OGLE 2003-BLG-235/MOA 2003-BLG-53 light curve (Bond et al, 2004). The
right hand panel shows a close-up of the region of the planetary caustic. The
theoretical light curves shown are the best fit planetary microlensing light curve
(solid black curve indicating a mass ratio of q = 0.0039), another planetary mass
binary lens light curve (green curve with q = 0.0069), and the best fit non-planetary
binary lens light curve (magenta dashed curve), which has q > 0.03.
MOA/OGLE Planetary Event
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Best fit light curve simulated on an OGLE image
2nd Exoplanet Discovery by lensing
OGLE 2005-BLG-71
(Udalski, Jaroszynski, et al OGLE & FUN. Addl’ data
from MOA & PLANET).
Data from OGLE, FUN,
PLANET & MOA
Central caustic light curve
perturbation (d = 1.3 or
1/1.3):
Additional planet discoveries by PLANET,
MOA & OGLE, also in preparation
3rd Exoplanet Discovery by lensing
Short duration deviation suggests planetary mass
ratio binary--details in Nature, January 2006
Exoplanets via Gravitational
Microlensing
• Planetary signal strength independent of mass
– if Mplanet/M* 310-7
– low-mass planet signals are brief and rare
• ~10% photometric variations
– required photometric accuracy demonstrated
• Mplanet/M*, separation (w/ a factor of 2 accuracy)
– Mplanet and M* measured separately in > 30% of cases
– follow-up observations measure Mplanet , M*, separation for most
G, K, and some M star lenses
• finds free-floating planets, too
Planetary Parameters from Microlensing
• Mass ratio & planetary separation in Einstein radius units
– Radial velocity planets only give mass ratio sin(I)
– But the properties of the source star are well known for radial velocities!
• High resolution observations can reveal source star
– Light curve fit gives source star brightness
– HST observations may reveal a source apparently brighter than required
by the fit - due to light from the lens
• Pending HST DD proposal by Gould, Bennett & Udalski
– Favorable case due to long timescale event and indications of
blending in ground-based photometry - could be K dwarf at 2 kpc
• 30-50% of events have detectable sources
– Future JWST or AO observations will confirm the lens star ID and
determine the lens-source proper motion (~10 years later)
• Measurement of microlensing parallax plus finite source effect gives
planetary mass directly
– Weak parallax detection for OGLE-235/MOA-53 gives mass between
~0.06 and ~0.7 M (Bennett & Gould, in preparation)
– MOA upgrade from 0.6m to 1.8m telescope and increased OGLE sampling
rate should improve data for future events
Comparison of Planet Detection
Techniques
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Solar System planets
are blue boxes
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Microlensing from
ground or space quite
competitive
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MPF is a proposed
satellite microlensing
mission
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Microlensing discoveries
are purple dots
Updated from Bennett & Rhie (2002) ApJ 574, 985
VOEvent and Microlensing
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VOEvent will simplify communication
– Between surveys and follow-up
– Within a follow-up team
– Among follow-up teams
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VOEvent content needed for
– Anomaly type
– Prediction of behavior
– Prioritization of follow-up
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Other potential needs
– Verification of follow-up
– Optimum resource allocation