arXiv subjects

Masayuki Nagakane

Publications and source records attributed to Masayuki Nagakane.

22 records · Page 2Linked to original sources

arXiv2018

A Planetary Microlensing Event with an Unusually Red Source Star: MOA-2011-BLG-291

We present the analysis of planetary microlensing event MOA-2011-BLG-291, which has a mass ratio of $q=(3.8\pm0.7)\times10^{-4}$ and a source star that is redder (or brighter) than the bulge main sequence. This event is located at a low Galactic latitude in the survey area that is currently planned for NASA's WFIRST exoplanet microlensing survey. This unusual color for a microlensed source star implies that we cannot assume that the source star is in the Galactic bulge. The favored interpretation is that the source star is a lower main sequence star at a distance of $D_S=4.9\pm1.3\,$kpc in the Galactic disk. However, the source could also be a turn-off star on the far side of the bulge or a sub-giant in the far side of the Galactic disk if it experiences significantly more reddening than the bulge red clump stars. However, these possibilities have only a small effect on our mass estimates for the host star and planet. We find host star and planet masses of $M_{\rm host} =0.15^{+0.27}_{-0.10}M_\odot$ and $m_p=18^{+34}_{-12}M_\oplus$ from a Bayesian analysis with a standard Galactic model under the assumption that the planet hosting probability does not depend on the host mass or distance. However, if we attempt to measure the host and planet masses with host star brightness measurements from high angular resolution follow-up imaging, the implied masses will be sensitive to the host star distance. The WFIRST exoplanet microlensing survey is expected to use this method to determine the masses for many of the planetary systems that it discovers, so this issue has important design implications for the WFIRST exoplanet microlensing survey.

astro-ph.EP↗

arXiv2018

OGLE-2014-BLG-0962 and the First Statistical Validation of Bayesian Priors for Galactic Microlensing

OGLE-2014-BLG-0962 (OB140962) is a stellar binary microlensing event that was well-covered by observations from the Spitzer satellite as well as ground-based surveys. Modelling yields a unique physical solution: a mid-M+M-dwarf binary with $M_{\rm prim} = 0.20 \pm 0.01 M_\odot$ and $M_{\rm sec} = 0.16 \pm 0.01 M_\odot$, with projected separation of $2.0 \pm 0.3$ AU. The lens is only $D_{\rm LS} = 0.41 \pm 0.06$ kpc in front of the source, making OB140962 a bulge lens and the most distant Spitzer binary lens to date. In contrast, because the Einstein radius ($θ_{\rm E} = 0.143 \pm 0.007$ mas) is unusually small, a standard Bayesian analysis, conducted in the absence of parallax information, would predict a brown dwarf binary. We test the accuracy of Bayesian analysis over a set of Spitzer lenses, finding overall good agreement throughout the sample. We also illustrate the methodology for probing the Galactic distribution of planets by comparing the cumulative distance distribution of the Spitzer 2-body lenses to that of the Spitzer single lenses.

astro-ph.SR↗

arXiv2017

MOA-2012-BLG-505Lb: A super-Earth mass planet probably in the Galactic bulge

We report the discovery of a super-Earth mass planet in the microlensing event MOA-2012-BLG-505. This event has the second shortest event timescale of $t_{\rm E}=10 \pm 1$ days where the observed data show evidence of planetary companion. Our 15 minute high cadence survey observation schedule revealed the short subtle planetary signature. The system shows the well known close/wide degeneracy. The planet/host-star mass ratio is $q =2.1 \times 10^{-4}$ and the projected separation normalized by the Einstein radius is s = 1.1 or 0.9 for the wide and close solutions, respectively. We estimate the physical parameters of the system by using a Bayesian analysis and find that the lens consists of a super-Earth with a mass of $6.7^{+10.7}_{-3.6}M_{\oplus}$ orbiting around a brown-dwarf or late M-dwarf host with a mass of $0.10^{+0.16}_{-0.05}M_{\odot}$ with a projected star-planet separation of $0.9^{+0.3}_{-0.2}$AU. The system is at a distance of $7.2 \pm 1.1$ kpc, i.e., it is likely to be in the Galactic bulge. The small angular Einstein radius ($θ_{\rm E}=0.12 \pm 0.02$ mas) and short event timescale are typical for a low-mass lens in the Galactic bulge. Such low-mass planetary systems in the Bulge are rare because the detection efficiency of planets in short microlensing events is relatively low. This discovery may suggest that such low mass planetary systems are abundant in the Bulge and currently on-going high cadence survey programs will detect more such events and may reveal an abundance of such planetary systems.

astro-ph.EP↗

arXiv2017

MOA-2016-BLG-227Lb: A Massive Planet Characterized by Combining Lightcurve Analysis and Keck AO Imaging

We report the discovery of a microlensing planet --- MOA-2016-BLG-227Lb --- with a large planet/host mass ratio of $q \simeq 9 \times 10^{-3}$. This event was located near the $K2$ Campaign 9 field that was observed by a large number of telescopes. As a result, the event was in the microlensing survey area of a number of these telescopes, and this enabled good coverage of the planetary light curve signal. High angular resolution adaptive optics images from the Keck telescope reveal excess flux at the position of the source above the flux of the source star, as indicated by the light curve model. This excess flux could be due to the lens star, but it could also be due to a companion to the source or lens star, or even an unrelated star. We consider all these possibilities in a Bayesian analysis in the context of a standard Galactic model. Our analysis indicates that it is unlikely that a large fraction of the excess flux comes from the lens, unless solar type stars are much more likely to host planets of this mass ratio than lower mass stars. We recommend that a method similar to the one developed in this paper be used for other events with high angular resolution follow-up observations when the follow-up observations are insufficient to measure the lens-source relative proper motion.

astro-ph.EP↗