Microlensing Surveys for Finding Planets

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Microlensing Surveys for Finding Planets

• Kem Cook
• LLNL/NOAO

With thanks to Dave Bennett for most of these
slides
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Microlensing Surveys Ushered in the Current Era
of time-domain surveys

• 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
• Huge databases of light curves over 1000s of days
  for millions of stars
• Anomalous microlensing detected--binary lensing
• Extreme binary system is star and planet
• Follow-up collaborations formed to detect planets
  in 1995
• PLANET collaboration
• Probing Lensing Anomalies NETwork
• MPS collaboration
• Microlensing Planet Survey
• Current follow-up
• PLANET
• MicroFUN
• Current Galactic Surveys
• OGLE
• MOA

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PLANET Telescope System
Collaboration member telescopes
MOU in place with RoboNet
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The Physics of Microlensing

• Foreground lens star planet bend light of
  source star
• Multiple distorted images
• 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 Ground-based observations
  announced already

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Lensed images at ?arcsec resolution
A planet can be discovered when one of the lensed
images approaches its projected
position. Animation from Scott Gaudi
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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.

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Microlensing surveys need VOEvents

• Alert to new microlensing events
• Currently done via email and web post
• Multiple surveys mean possible confusion
• Analysis of ongoing events suggests anomaly
• Email anomaly alerts (2nd level alerts)
• Analysis may suggest optimum sampling time
• Photometry follow-up for planets
• Spectroscopic follow-up
• Spatial resolution of source star (eg limb
  darkening)
• Multiplication of source star flux
• Current follow-up networks use email, telephone
  and web pages to relay information

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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 gt 0.03.
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MOA/OGLE Planetary Event
Best fit light curve simulated on an OGLE image
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2nd Exoplanet Discovery by ?lensing
OGLE 2005-BLG-71 (Udalski, Jaroszynski, et al -
OGLE ?FUN. Addl data from MOA PLANET).
Central caustic light curve perturbation (d
1.3 or 1/1.3)
Data from OGLE, ?FUN, PLANET MOA
Additional planet discoveries by PLANET, MOA
OGLE, also in preparation
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3rd Exoplanet Discovery by ?lensing
Short duration deviation suggests planetary mass
ratio binary--details in Nature, January 2006
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Exoplanets via Gravitational Microlensing

• Planetary signal strength independent of mass
• if Mplanet/M ? 3?10-7
• 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 gt 30 of
  cases
• follow-up observations measure Mplanet , M,
  separation for most G, K, and some M star lenses
• finds free-floating planets, too

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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

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Comparison of Planet Detection Techniques

• Transit detection planetary systems are blue
  boxes
• Microlensing from ground or space quite
  competitive
• MPF is a proposed satellite microlensing mission
• Microlensing discoveries are purple dots

Updated from Bennett Rhie (2002) ApJ 574, 985
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VOEvent and Microlensing

• VOEvent will simplify communication
• Between surveys and follow-up
• Within a follow-up team
• Among follow-up teams
• VOEvent content needed for
• Anomaly type
• Prediction of behavior
• Prioritization of follow-up
• Other potential needs
• Verification of follow-up
• Optimum resource allocation