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
• 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
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– 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
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
• 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
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
Best fit light curve simulated on an OGLE image
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
2nd Exoplanet Discovery by lensingOGLE 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):
Additional planet discoveries by PLANET, MOA & OGLE, also in preparation
Data from OGLE, FUN,PLANET & MOA
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
• 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
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