Microlensing Surveys for Finding Planets

1. Microlensing Surveys for Finding Planets Kem Cook LLNL/NOAO With thanks to Dave Bennett for most of these slides

2. 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

3. PLANET Telescope System Collaboration member telescopes MOU in place with RoboNet

4. 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

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

6. 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.

7. 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

8. 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.

9. MOA/OGLE Planetary Event Best fit light curve simulated on an OGLE image

10. 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

11. 3rd Exoplanet Discovery by lensing Short duration deviation suggests planetary mass ratio binary--details in Nature, January 2006

12. 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 > 30% of cases • follow-up observations measure Mplanet , M*, separation for most G, K, and some M star lenses • finds free-floating planets, too

13. 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

14. 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

15. 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