planetary microlensing for dummies nick cowan april 2006
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Planetary Microlensingfor dummies
Nick Cowan
April 2006
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Outline
• Microlensing: it’s hard
• Results: it works
• Prospects: it’s useful, too
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Deflecting Light
• Einstein predicted that massive objects bend light.
• He was right.QuickTime™ and a
TIFF (Uncompressed) decompressorare needed to see this picture.
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QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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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.
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Caustic Crossing High Magnification
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High Amplification Event
Caustic Crossing Event
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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)
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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
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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).
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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
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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.
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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
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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.
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References
• Rattenbury, astro-ph/0604062
• Beaulieu et al., Nature (Jan 2006)
• Queloz, Nature (Jan 2006)