Planetary Microlensing for 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.

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 11 th 2005: The OGLE Early Warning System announces microlensing event. July 31 st : Maximum magnification. August 9 th : Deviation from single lens. Source star: R Star = 9.6 R sun G4 III (K giant)

Step 2: The Analysis  2 = 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 (M p 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/ Beaulieu et al., Nature (Jan 2006) Queloz, Nature (Jan 2006)