Kojima-1Lb is a Mildly Cold Neptune around the Brightest Microlensing Host Star. (arXiv:1909.11802v1 [astro-ph.EP])

Kojima-1Lb is a Mildly Cold Neptune around the Brightest Microlensing Host Star. (arXiv:1909.11802v1 [astro-ph.EP])
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We report the analysis of additional multi-band photometry and spectroscopy,
and new adaptive optics (AO) imaging of the nearby planetary microlensing
event, TCP J05074264+2447555 (hereafter called Kojima-1), which was discovered
toward the Galactic anticenter in 2017 (Nucita et al.). We confirm the
planetary nature of the light-curve anomaly around the peak, while find no
additional planetary feature in this event. We also confirm the presence of
apparent blending flux and the absence of significant parallax signal reported
in the literature. The AO image reveals no contaminating sources, making it
most likely that the blending flux comes from the lens star. The measured
multi-band lens flux, combined with a constraint from the microlensing model,
allows us to narrow down the previously-unresolved mass and distance of the
lens system. We find that the primary lens is a dwarf on the K/M boundary
(0.581 pm 0.033 M_sun) located at 505 pm 47 pc and the companion (Kojima-1Lb)
is a Neptune-mass planet (20.0 pm 2.0 M_earth) with a semi-major axis of 1.08
^{+0.62}_{-0.18} au. This orbit is a few times smaller than those of typical
microlensing planets and is comparable to the snow line location at young ages.
We calculate that the a priori detection probability of Kojima-1Lb is only sim
35%, which may imply that Neptunes are common around the snow line as recently
suggested by the transit and radial-velocity techniques. The host star is the
brightest among the microlensing planetary systems (Ks = 13.7), offering a
great opportunity to spectroscopically characterize this system even with
current facilities.

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