Kojima-1Lb Is a Mildly Cold Neptune around the Brightest Microlensing Host Star

Abstract

We report the analysis of additional multiband photometry and spectroscopy and new adaptive optics (AO) imaging of the nearby planetary microlensing event TCPJ05074264+2447555 (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 finding 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 multiband 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 ± 0.033 M e) located at 505±47 pc, and the companion (Kojima-1Lb) is a Neptune-mass planet (20.0 ± 2.0 M ⊕) with a semimajor axis of-+ 1.08 0.18 0.62 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

Microlensing Optical Depth and Event Rate in the OGLE-IV Galactic Plane Fields

“…Finding microlensing events requires frequent monitoring of a large Table 3 Microlensing Optical Depth and Average Timescales in EROS Fields (Rahal et al 2009) Region area along the Galactic plane. This is why the first Galactic plane events were detected mostly serendipitously (e.g., Fukui et al 2007Fukui et al , 2019Gaudi et al 2008;Nucita et al 2018;Zang et al 2019). Only the recent few years have brought about more detections of microlensing events in the Galactic plane, mostly thanks to efforts by the Gaia and ZTF groups.…”

“…Fukui et al (2007) and Gaudi et al (2008) presented the discovery of a bright microlensing event in the constellation Cassiopeia (l=117°). Nucita et al (2018), Fukui et al (2019), and Zang et al (2019) reported the characterization of a planetary-mass companion in microlensing event TCP J05074264+2447555 that was located toward the Galactic anticenter (l=179°). This was the first event, in which the two images generated by microlensing were resolved thanks to the use of the Very Large Telescope Interferometer GRAVITY (Dong et al 2019).…”

Spitzer + VLTI-GRAVITY Measure the Lens Mass of a Nearby Microlensing Event

“…Thus, the blended light basically all comes from the lens. Fukui et al (2019) detected no contamination at the position of the microlens on their Keck AO images, and using priors of an upper limit of p E and q E measured by Dong et al (2019), they found that the majority of the blend is most likely from the lens star rather than an ambient star or a companion to the source or the lens star. Our analysis directly measures the lens mass and distance using measured p E and q E , thus it is a direct demonstration that the blend mostly comes from the lens.…”

“…To estimate the lens apparent brightness, we use the angular Einstein radius q =  1.891 0.014 E mas and p =  0.469 0.060 E of the "luminous lens" model, and follow the procedure in Section 4.1. For extinction, we adopt the extinction law of Cardelli et al (1989) and estimate the extinction of the lens using the function of Fukui et al (2019). The predicted lens apparent magnitude is shown in Table 3.…”

“…Nucita et al (2018) discovered short-lived planetary features due to a super-Earth planet in this event. Fukui et al (2019) followed it up with Keck AO imaging, and combining it with an upper limit of p E and q E measurements from Dong et al (2019), they found that the blend was most likely due to the lens star with mass M L =0.581±0.033 M e .…”