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, R E 1 st 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. 1 st 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. 2 nd 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): Additional planet discoveries by PLANET, MOA & OGLE, also in preparation Data from OGLE, FUN, PLANET & MOA

11. 3 rd 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 M planet /M * 3 10 -7 –low-mass planet signals are brief and rare ~10% photometric variations –required photometric accuracy demonstrated M planet /M *, separation (w/ a factor of 2 accuracy) –M planet and M * measured separately in > 30% of cases –follow-up observations measure M planet, 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