OGLE-2017-BLG-0173Lb: Low-mass-ratio Planet in a "Hollywood" Microlensing Event

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2018
Authors
Hwang K-H
Udalski A
Shvartzvald Y
Ryu Y-H
Albrow MD
Chung S-J
Gould A
Han C
Jung YK
Shin I-G
Abstract

We present microlensing planet OGLE-2017-BLG-0173Lb, with planet-host mass ratio either q≃2.5×10−5 or q≃6.5×10−5, the lowest or among the lowest ever detected. The planetary perturbation is strongly detected, Δχ2∼10,000, because it arises from a bright (therefore, large) source passing over and enveloping the planetary caustic: a so-called "Hollywood" event. The factor ∼2.5 offset in q arises because of a previously unrecognized discrete degeneracy between Hollywood events in which the caustic is fully enveloped and those in which only one flank is enveloped, which we dub "Cannae" and "von Schlieffen", respectively. This degeneracy is "accidental" in that it arises from gaps in the data. Nevertheless, the fact that it appears in a Δχ2=10,000 planetary anomaly is striking. We present a simple formalism to estimate the sensitivity of other Hollywood events to planets and show that they can lead to detections close to, but perhaps not quite reaching, the Earth/Sun mass ratio of 3×10−6. This formalism also enables an analytic understanding of the factor ∼2.5 offset in q between the Cannae and von Schlieffen solutions. The Bayesian estimates for the host-mass, system distance, and planet-host projected separation are M=0.39−0.24+0.40M⊙, DL=4.8−1.8+1.5\kpc, and a⊥=3.8±1.6\au. The two estimates of the planet mass are mp=3.3−2.1+3.8M⊕ and mp=8−6+11M⊕. The measured lens-source relative proper motion μ=6\masyr will permit imaging of the lens in about 15 years or at first light on adaptive-optics imagers on next-generation telescopes. These will allow to measure the host mass but probably cannot resolve the planet-host mass-ratio degeneracy.

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gravitational lensing: micro, planetary systems
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Fields of Research::51 - Physical sciences::5101 - Astronomical sciences::510109 - Stellar astronomy and planetary systems
Field of Research::02 - Physical Sciences::0201 - Astronomical and Space Sciences::020102 - Astronomical and Space Instrumentation
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