Criteria in the Selection of Target Events for Planetary Microlensing Follow-up Observations

To provide criteria in the selection of target events preferable for planetary lensing follow-up observations, we investigate the variation of the probability of detecting planetary signals depending on the observables of the lensing magnification and source brightness. In estimating the probability, we consider the variation of the photometric precision by using a quantity defined as the ratio of the fractional deviation of the planetary perturbation to the photometric precision. From this investigation, we find results consistent with previous studies that the probability increases with the increase of the magnification. The increase rate is boosted at a certain magnification at which perturbations caused by central caustic begin to occur. We find this boost occurs at moderate magnifications of A ≲ 20, implying that probability can be high even for events with moderate magnifications. The probability increases as the source brightness increases. We find that the probability of events associated with stars brighter than clump giants is not negligible even at magnifications as low as A ~ 5. In the absence of the rare prime target of very high magnification events, we therefore recommend to observe events with the brightest source stars and highest magnifications among the alerted events. Because of the increase of the source size with the increase of the brightness, however, the probability rapidly drops off beyond a certain magnification, causing detections of low mass ratio planets (q ≲ 10^-4) that are difficult from the observations of events involved with giant stars with magnifications A ≳ 70.