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