Planetary Microlensingfor dummies

Nick Cowan

April 2006

Outline

• Microlensing: it’s hard

• Results: it works

• Prospects: it’s useful, too

Deflecting Light

• Einstein predicted that massive objects bend light.

• He was right.QuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Microlensing

• If the source is small and far away, we cannot resolve the multiple images.

• The extra images manifest themselves as a brightening of the source.

• If the lens consists of multiple point masses, there are regions of infinite magnification known as caustics.

Caustic Crossing High Magnification

High Amplification Event

Caustic Crossing Event

Step 1: The Detection

• July 11th 2005: The OGLE Early Warning System announces microlensing event.

• July 31st: Maximum magnification.• August 9th: Deviation from single lens.Source star:

RStar = 9.6 Rsun

G4 III (K giant)

Step 2: The Analysis

2 = 562.26

650 data points- 7 lens parameters- 12 flux normalization parameters

= 631 degrees of freedom

Reduced 2 = 0.89

More Analysis

• Use four different lens modelling codes just to be sure.

• Best single-lens model has ∆2 = 46 for one less model parameter.

• Microlensing only directly determines the planet-star mass ratio, q, and projected separation, d.

• Use Galactic models and Bayesian statistics to get the quantities of real interest (Mp and a).

Summary of Results• Five probable planetary microlensing events have

been observed so far:– MACHO-98-BLG-35– OGLE-2003-BLG-235/MOA-2003-BLG-53– OGLE-2005-BLG-071– OGLE-2005-BLG-390– OGLE-2006-BLG-169

How to detect an Earth (with current technology)

1. Stare at the Galactic Bulge: there are lots of stars in that direction.

2. Keep your fingers crossed: it’s pretty unlikely that any given star will get lensed.

3. Don’t blink: The planet-induced deviation from single-lens behavior takes place in mere hours, while the overall lensing event takes months.

Future Prospects

• MOA-2– 1.8 m telescope dedicated to microlensing– 2.2 sqr degree field of view– 23 fields, multiple times per night

• Earth-Hunter Network– Four 2 m class telescopes at different longitudes– 4 sqr degree f.o.v– A few fields per night

• Microlensing Planet Finder– Space telescope capable of continuous observations

Summary

• Microlensing is a viable way to detect terrestrial planets and icy giants with a > 1AU.

• The mass ratio and projected distance between the primary and secondary lens are readily determined.

• The actual mass and semi-major axis of the planet are inferred from models.

• An arbitrarily high signal-to-noise can be achieved, one just has to be lucky.

• Follow up studies are difficult.• Microlensing probes an area of M-a space which is

largely unexplored.

References

• Rattenbury, astro-ph/0604062

• Beaulieu et al., Nature (Jan 2006)

• Queloz, Nature (Jan 2006)