Planetary Microlensing Events

Planetary microlensing events are a subset of multiple lens events where the mass ratio is quite small. Planetary events have light curves that appear quite similar to the single lens light curves shown in Fig. 3.2, but for a brief period of time they deviate from the single lens form and display the characteristics of a binary lens light curve. We will define a requirement on the mass ratio q = < 0.03 to separate planetary microlensing events from stellar binary events following Bond et al. (2004), because q « 0.03 is the approximate location of the "brown dwarf desert" that appears to separate stellar from planetary secondaries. We will also initially only consider events with only one detectable planet, as these represent the majority of planetary microlensing events and this will simplify the discussion.

The caustic structure of a binary lens system is determined by the mass ratio and the separation of the lenses (Schneider & Weiss, 1986). For a separation d ^ 1 (in units of RE ), there are three caustics, two triangular caustics with 3 cusps each and a caustic close to the center of mass which has 4 cusps. These merge into a single caustic with 5 cusps at d ~ 1, which splits into two caustic curves with 4 cusps each for d ^ 1. For small values of the mass ratio, q, the division between these regimes occurs near d « 1, so most events have multiple caustic curves. As shown in Fig. 3.3.

OB05-071: d=0.759, q=0.006 caustic topology

OB05-071: d=1.299, q=0.006 caustic topology

OB05-071: d=0.759, q=0.006 caustic topology

OB05-071: d=1.299, q=0.006 caustic topology

Fig. 3.3. Grey-scale magnification patterns for two models of the planetary microlensing event OGLE-2005-BLG-71. The darkness of the image is proportional to the logarithm of the magnification. The nearly vertical lines in each panel indicate the path of the source star for each model. The source passes close to the central, or stellar, caustic, and as discussed in Sect. 3.4, the magnification pattern in the vicinity of the central caustic is similar for planetary systems related by d o 1/d, The magnification patterns for the planetary caustics (on the outside of each panel) are clearly very different and easy to distinguish. (These images are provide courtesy of Daniel Kubas.)

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