OGLE-2015-BLG-1670Lb: A Cold Neptune beyond the Snow Line in the Provisional WFIRST Microlensing Survey Field
2019
We present the analysis of the microlensing event OGLE-2015-BLG-1670, detected in a high-extinction field, very close to the
Galactic plane. Due to the dust extinction along the line of sight, this event was too faint to be detected before it reached the peak of magnification. The microlensing
light-curvemodels indicate a high-magnification event with a maximum of $A_\mathrm{max}\gtrsim200$, very sensitive to planetary deviations. An anomaly in the
light curvehas been densely observed by the microlensing surveys MOA, KMTNet, and OGLE. From the
light-curvemodeling, we find a planetary anomaly characterized by a
planet-to-host mass ratio, $q=\left(1.00^{+0.18}_{-0.16}\right)\times 10^{-4}$, at the peak recently identified in the mass-ratio function of microlensing
planets. Thus, this event is interesting to include in future statistical studies about
planetdemography. We have explored the possible degeneracies and find two competing planetary models resulting from the $s\leftrightarrow1/s$ degeneracy. However, because the projected separation is very close to $s=1$, the physical implications for the
planetfor the two solutions are quite similar, except for the value of $s$. By combining the
light-curveparameters with a Galactic model, we have estimated the
planetmass $M_2=17.9^{+9.6}_{-8.8}\,\mathrm{M}_\oplus$ and the lens distance $D_\mathrm{L}=6.7^{+1.0}_{-1.3}\,\mathrm{kpc}$, corresponding to a
Neptune-mass
planetclose to the Galactic bulge. Such events with a low absolute latitude ($|b|\approx 1.1\,\mathrm{deg}$) are subject to both high extinction and more uncertain source distances, two factors that may affect the mass measurements in the provisional
Wide Field Infrared Survey Telescopefields. More events are needed to investigate the potential trade-off between the higher lensing rate and the difficulty in measuring masses in these low-latitude fields.
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