arXiv subjects

H. Shibai

Publications and source records attributed to H. Shibai.

At least 19 recordsLinked to original sources

arXiv2019

OGLE-2013-BLG-0911Lb: A Secondary on the Brown-Dwarf Planet Boundary around an M-dwarf

We present the analysis of the binary-lens microlensing event OGLE-2013-BLG-0911. The best-fit solutions indicate the binary mass ratio of q~0.03 which differs from that reported in Shvartzvald+2016. The event suffers from the well-known close/wide degeneracy, resulting in two groups of solutions for the projected separation normalized by the Einstein radius of s~0.15 or s~7. The finite source and the parallax observations allow us to measure the lens physical parameters. The lens system is an M-dwarf orbited by a massive Jupiter companion at very close (M_{host}=0.30^{+0.08}_{-0.06} M_{Sun}, M_{comp}=10.1^{+2.9}_{-2.2} M_{Jup}, a_{exp}=0.40^{+0.05}_{-0.04} au) or wide (M_{host}=0.28^{+0.10}_{-0.08} M_{Sun}, M_{comp}=9.9^{+3.8}_{-3.5}M_{Jup}, a_{exp}=18.0^{+3.2}_{-3.2} au) separation. Although the mass ratio is slightly above the planet-brown dwarf (BD) mass-ratio boundary of q=0.03 which is generally used, the median physical mass of the companion is slightly below the planet-BD mass boundary of 13M_{Jup}. It is likely that the formation mechanisms for BDs and planets are different and the objects near the boundaries could have been formed by either mechanism. It is important to probe the distribution of such companions with masses of ~13M_{Jup} in order to statistically constrain the formation theories for both BDs and massive planets. In particular, the microlensing method is able to probe the distribution around low-mass M-dwarfs and even BDs which is challenging for other exoplanet detection methods.

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arXiv2019

An analysis of binary microlensing event OGLE-2015-BLG-0060

We present the analysis of stellar binary microlensing event OGLE-2015-BLG-0060 based on observations obtained from 13 different telescopes. Intensive coverage of the anomalous parts of the light curve was achieved by automated follow-up observations from the robotic telescopes of the Las Cumbres Observatory. We show that, for the first time, all main features of an anomalous microlensing event are well covered by follow-up data, allowing us to estimate the physical parameters of the lens. The strong detection of second-order effects in the event light curve necessitates the inclusion of longer-baseline survey data in order to constrain the parallax vector. We find that the event was most likely caused by a stellar binary-lens with masses $M_{\star1} = 0.87 \pm 0.12 M_{\odot}$ and $M_{\star2} = 0.77 \pm 0.11 M_{\odot}$. The distance to the lensing system is 6.41 $\pm 0.14$ kpc and the projected separation between the two components is 13.85 $\pm 0.16$ AU. Alternative interpretations are also considered.

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arXiv2018

MOA-2015-BLG-337: A Planetary System with a Low-mass Brown Dwarf/Planetary Boundary Host, or a Brown Dwarf Binary

We report the discovery and the analysis of the short timescale binary-lens microlensing event, MOA-2015-BLG-337. The lens system could be a planetary system with a very low mass host, around the brown dwarf/planetary mass boundary, or a brown dwarf binary. We found two competing models that explain the observed light curves with companion/host mass ratios of q~0.01 and ~0.17, respectively. From the measurement of finite source effects in the best-fit planetary model, we find a relatively small angular Einstein radius of theta_E ~ 0.03 mas which favors a low mass lens. We conduct a Bayesian analysis to obtain the probability distribution of the lens properties. The results for the planetary models strongly depend on the minimum mass, M_min, in the assumed mass function. In summary, there are two solutions of the lens system: (1) a brown dwarf/planetary mass boundary object orbited by a super-Neptune (the planetary model with M_min=0.001 M_sun) and (2) a brown dwarf binary (the binary model). If the planetary models is correct, this system can be one of a new class of planetary system, having a low host mass and also a planetary mass ratio (q <0.03) between the host and its companion. The discovery of the event is important for the study of planetary formation in very low mass objects. In addition, it is important to consider all viable solutions in these kinds of ambiguous events in order for the future comprehensive statistical analyses of planetary/binary microlensing events.

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arXiv2018

SPICA - a large cryogenic infrared space telescope Unveiling the obscured Universe

Measurements in the infrared wavelength domain allow us to assess directly the physical state and energy balance of cool matter in space, thus enabling the detailed study of the various processes that govern the formation and early evolution of stars and planetary systems in galaxies over cosmic time. Previous infrared missions, from IRAS to Herschel, have revealed a great deal about the obscured Universe, but sensitivity has been limited because up to now it has not been possible to fly a telescope that is both large and cold. SPICA is a mission concept aimed at taking the next step in mid- and far-infrared observational capability by combining a large and cold telescope with instruments employing state-of-the-art ultra-sensitive detectors. The mission concept foresees a 2.5-meter diameter telescope cooled to below 8 K. With cooling provided by mechanical coolers instead of depending on a limited cryogen supply, the mission lifetime can extend significantly beyond the required three years. SPICA offers instrumentation with spectral resolving powers ranging from R ~50 through 11000 in the 17-230 $μ$m domain as well as R~28.000 spectroscopy between 12 and 18 $μ$m. Additionally SPICA will provide efficient 30-37 $μ$m broad band mapping, and polarimetric imaging in the 100-350 $μ$m range. SPICA will provide unprecedented spectroscopic sensitivity of ~5 x $10^{-20}$ W/m$^2$ (5$σ$/1hr) - at least two orders of magnitude improvement over what has been attained to date. With this exceptional leap in performance, new domains in infrared astronomy will become accessible, allowing us, for example, to unravel definitively galaxy evolution and metal production over cosmic time, to study dust formation and evolution from very early epochs onwards, and to trace the formation history of planetary systems.

astro-ph.IM↗

arXiv2017

Galaxy evolution studies with the SPace IR telescope for Cosmology and Astrophysics (SPICA): the power of IR spectroscopy

IR spectroscopy in the range 12-230 micron with the SPace IR telescope for Cosmology and Astrophysics (SPICA) will reveal the physical processes that govern the formation and evolution of galaxies and black holes through cosmic time, bridging the gap between the James Webb Space Telescope (JWST) and the new generation of Extremely Large Telescopes (ELTs) at shorter wavelengths and the Atacama Large Millimeter Array (ALMA) at longer wavelengths. SPICA, with its 2.5-m telescope actively-cooled to below 8K, will obtain the first spectroscopic determination, in the mid-IR rest-frame, of both the star-formation rate and black hole accretion rate histories of galaxies, reaching lookback times of 12 Gyr, for large statistically significant samples. Densities, temperatures, radiation fields and gas-phase metallicities will be measured in dust-obscured galaxies and active galactic nuclei (AGN), sampling a large range in mass and luminosity, from faint local dwarf galaxies to luminous quasars in the distant Universe. AGN and starburst feedback and feeding mechanisms in distant galaxies will be uncovered through detailed measurements of molecular and atomic line profiles. SPICA's large-area deep spectrophotometric surveys will provide mid-IR spectra and continuum fluxes for unbiased samples of tens of thousands of galaxies, out to redshifts of z~6. Furthermore, SPICA spectroscopy will uncover the most luminous galaxies in the first few hundred million years of the Universe, through their characteristic dust and molecular hydrogen features.

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arXiv2017

Ground-based parallax confirmed by Spitzer: binary microlensing event MOA-2015-BLG-020

We present the analysis of the binary gravitational microlensing event MOA-2015-BLG-020. The event has a fairly long timescale (about 63 days) and thus the light curve deviates significantly from the lensing model that is based on the rectilinear lens-source relative motion. This enables us to measure the microlensing parallax through the annual parallax effect. The microlensing parallax parameters constrained by the ground-based data are confirmed by the Spitzer observations through the satellite parallax method. By additionally measuring the angular Einstein radius from the analysis of the resolved caustic crossing, the physical parameters of the lens are determined. It is found that the binary lens is composed of two dwarf stars with masses $M_1 = 0.606 \pm 0.028M_\odot$ and $M_2 = 0.125 \pm 0.006M_\odot$ in the Galactic disk. Assuming the source star is at the same distance as the bulge red clump stars, we find the lens is at a distance $D_L = 2.44 \pm 0.10 kpc$. In the end, we provide a summary and short discussion of all published microlensing events in which the annual parallax effect is confirmed by other independent observations.

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arXiv2017

OGLE-2013-BLG-1761Lb: A Massive Planet Around an M/K Dwarf

We report the discovery and the analysis of the planetary microlensing event, OGLE-2013-BLG-1761. There are some degenerate solutions in this event because the planetary anomaly is only sparsely sampled. But the detailed light curve analysis ruled out all stellar binary models and shows that the lens to be a planetary system. There is the so-called close/wide degeneracy in the solutions with the planet/host mass ratio of $q \sim (7.5 \pm 1.5) \times 10^{-3}$ and $q \sim (9.3 \pm 2.9) \times 10^{-3}$ with the projected separation in Einstein radius units of $s = 0.95$ (close) and $s = 1.19$ (wide), respectively. The microlens parallax effect is not detected but the finite source effect is detected. Our Bayesian analysis indicates that the lens system is located at $D_{\rm L}=6.9_{-1.2}^{+1.0} \ {\rm kpc}$ away from us and the host star is an M/K-dwarf with the mass of $M_{\rm L}=0.33_{-0.18}^{+0.32} \ M_{\odot}$ orbited by a super-Jupiter mass planet with the mass of $m_{\rm P}=2.8_{-1.5}^{+2.5} \ M_{\rm Jup}$ at the projected separation of $a_{\perp}=1.8_{-0.5}^{+0.5} \ {\rm AU}$. The preference of the large lens distance in the Bayesian analysis is due to the relatively large observed source star radius. The distance and other physical parameters can be constrained by the future high resolution imaging by ground large telescopes or HST. If the estimated lens distance is correct, this planet provides another sample for testing the claimed deficit of planets in the Galactic bulge.

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arXiv2016

The Exoplanet Mass-Ratio Function from the MOA-II Survey: Discovery of a Break and Likely Peak at a Neptune Mass

We report the results of the statistical analysis of planetary signals discovered in MOA-II microlensing survey alert system events from 2007 to 2012. We determine the survey sensitivity as a function of planet-star mass ratio, $q$, and projected planet-star separation, $s$, in Einstein radius units. We find that the mass ratio function is not a single power-law, but has a change in slope at $q \sim 10^{-4}$, corresponding to $\sim 20 M_{\oplus}$ for the median host star mass of $\sim 0.6 M_{\odot}$. We find significant planetary signals in 23 of the 1474 alert events that are well characterized by the MOA-II survey data alone. Data from other groups are used only to characterize planetary signals that have been identified in the MOA data alone. The distribution of mass ratios and separations of the planets found in our sample are well fit by a broken power-law model of the form $dN_{\rm pl}/(d{\rm log} q\ d{\rm log} s) = A (q/q_{\rm br})^n s^m \, {\rm dex}^{-2}$ for $q > q_{\rm br}$ and $dN_{\rm pl}/(d{\rm log} q\ d{\rm log} s) = A (q/q_{\rm br})^p s^m \, {\rm dex}^{-2}$ for $q < q_{\rm br}$, where $q_{\rm br}$ is the mass ratio of the break. We also combine this analysis with the previous analyses of Gould et al. and Cassan et al., bringing the total sample to 30 planets. This combined analysis yields $A = 0.61^{+0.21}_{-0.16}$, $n =-0.93\pm 0.13$, $m = 0.49_{-0.49}^{+0.47}$ and $p = 0.6^{+0.5}_{-0.4}$ for $q_{\rm br}\equiv 1.7\times 10^{-4}$. The unbroken power law model is disfavored with a $p$-value of 0.0022, which corresponds to a Bayes factor of 27 favoring the broken power-law model. These results imply that cold Neptunes are likely to be the most common type of planets beyond the snow line.

astro-ph.EP↗

arXiv2016

OGLE-2012-BLG-0950Lb: The First Planet Mass Measurement from Only Microlens Parallax and Lens Flux

We report the discovery of a microlensing planet OGLE-2012-BLG-0950Lb with the planet/host mass ratio of $q \sim 2 \times 10^{-4}$. A long term distortion detected in both MOA and OGLE light curve can be explained by the microlens parallax due to the Earth's orbital motion around the Sun. Although the finite source effect is not detected, we obtain the lens flux by the high resolution Keck AO observation. Combining the microlens parallax and the lens flux reveal the nature of the lens: a planet with mass of $M_{\rm p} = 35^{+17}_{-9} M_{\oplus}$ is orbiting around a M-dwarf with mass of $M_{\rm host} = 0.56^{+0.12}_{-0.16} M_{\odot}$ with a planet-host projected separation of $r_{\perp} =2.7^{+0.6}_{-0.7}$ AU located at $D_{\rm L} = 3.0^{+0.8}_{-1.1}$ kpc from us. This is the first mass measurement from only microlens parallax and the lens flux without the finite source effect. In the coming space observation-era with $Spitzer$, $K2$, $Euclid$, and $WFIRST$, we expect many such events for which we will not be able to measure any finite source effect. This work demonstrates an ability of mass measurements in such events.

astro-ph.EP↗

arXiv2016

OGLE-2012-BLG-0724Lb: A Saturn-mass Planet around an M-dwarf

We report the discovery of a planet by the microlensing method, OGLE-2012-BLG-0724Lb. Although the duration of the planetary signal for this event was one of the shortest seen for a planetary event, the anomaly was well covered thanks to high cadence observations taken by the survey groups OGLE and MOA. By analyzing the light curve, this planetary system is found to have a mass ratio $q=(1.58\pm0.15)\times10^{-3}$. By conducting a Bayesian analysis, we estimate that the host star is an M-dwarf star with a mass of $M_{\rm L}=0.29_{-0.16}^{+0.33} \ M_{\odot}$ located at $D_{\rm L}=6.7_{-1.2}^{+1.1} \ {\rm kpc}$ away from the Earth and the companion's mass is $m_{\rm P}=0.47_{-0.26}^{+0.54} \ M_{\rm Jup}$. The projected planet-host separation is $a_{\perp}=1.6_{-0.3}^{+0.4} \ {\rm AU}$. Because the lens-source relative proper motion is relatively high, future high resolution images would detect the lens host star and determine the lens properties uniquely. This system is likely a Saturn-mass exoplanet around an M-dwarf and such systems are commonly detected by gravitational microlensing. This adds an another example of a possible pileup of sub-Jupiters $(0.2 < m_{\rm P}/M_{\rm Jup} < 1)$ in contrast to a lack of Jupiters ($\sim 1 - 2 \ M_{\rm Jup}$) around M-dwarfs, supporting the prediction by core accretion models that Jupiter-mass or more massive planets are unlikely to form around M-dwarfs.

astro-ph.EP↗

arXiv2014

OGLE-2008-BLG-355Lb: A Massive Planet around A Late type Star

We report the discovery of a massive planet OGLE-2008-BLG-355Lb. The light curve analysis indicates a planet:host mass ratio of q = 0.0118 +/- 0.0006 at a separation of 0.877 +/- 0.010 Einstein radii. We do not measure a significant microlensing parallax signal and do not have high angular resolution images that could detect the planetary host star. Therefore, we do not have a direct measurement of the host star mass. A Bayesian analysis, assuming that all host stars have equal probability to host a planet with the measured mass ratio implies a host star mass of M_h = 0.37_{-0.17}^{+0.30} M_Sun and a companion of mass M_P = 4.6^{+3.7}_{-2.2} M_Jup, at a projected separation of r_proj = 1.70^{+0.29}_{-0.30} AU. The implied distance to the planetary system is D_L = 6.8 +/- 1.1 kpc. A planetary system with the properties preferred by the Bayesian analysis would be a challenge to the core-accretion model of planet formation, as the core-accretion model predicts that massive planets are far more likely to form around more massive host stars. This core accretion model prediction is not consistent with our Bayesian prior of an equal probability of host stars of all masses to host a planet with the measured mass ratio. So, if the core accretion model prediction is right, we should expect that follow-up high angular resolution observations will detect a host star with a mass in the upper part of the range allowed by the Bayesian analysis. That is, the host would probably be a K or G dwarf.

astro-ph.EP↗

arXiv2013

MOA-2008-BLG-379Lb: A Massive Planet from a High Magnification Event with a Faint Source

We report analysis of high microlensing event MOA-2008-BLG-379, which has a strong microlensing anomaly at its peak, due to a massive planet with a mass ratio of q = 6.9 x 10^{-3}. Because the faint source star crosses the large resonant caustic, the planetary signal dominates the light curve. This is unusual for planetary microlensing events, and as a result, the planetary nature of this light curve was not immediately noticed. The planetary nature of the event was found when the MOA Collaboration conducted a systematic study of binary microlensing events previously identified by the MOA alert system. We have conducted a Bayesian analysis based on a standard Galactic model to estimate the physical parameters of the lens system. This yields a host star mass of M_L = 0.66_{-0.33}^{+0.29} M_Sun orbited by a planet of mass m_P = 4.8_{-2.4}^{+2.1} M_Jup at an orbital separation of a = 4.1_{-1.5}^{+1.9} AU at a distance of D_L = 3.6 +/- 1.3 kpc. The faint source magnitude of I_S = 21.30 and relatively high lens-source relative proper motion of mu_rel = 7.6 +/- 1.6 mas/yr implies that high angular resolution adaptive optics or Hubble Space Telescope observations are likely to be able to detect the source star, which would determine the masses and distance of the planet and its host star.

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arXiv2011

The SPICA coronagraphic instrument (SCI) for the study of exoplanets

We present the SPICA Coronagraphic Instrument (SCI), which has been designed for a concentrated study of extra-solar planets (exoplanets). SPICA mission provides us with a unique opportunity to make high contrast observations because of its large telescope aperture, the simple pupil shape, and the capability for making infrared observations from space. The primary objectives for the SCI are the direct coronagraphic detection and spectroscopy of Jovian exoplanets in infrared, while the monitoring of transiting planets is another important target. The specification and an overview of the design of the instrument are shown. In the SCI, coronagraphic and non-coronagraphic modes are applicable for both an imaging and a spectroscopy. The core wavelength range and the goal contrast of the coronagraphic mode are 3.5--27$μ$m, and 10$^{-6}$, respectively. Two complemental designs of binary shaped pupil mask coronagraph are presented. The SCI has capability of simultaneous observations of one target using two channels, a short channel with an InSb detector and a long wavelength channel with a Si:As detector. We also give a report on the current progress in the development of key technologies for the SCI.

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arXiv2010

Imaging of a Transitional Disk Gap in Reflected Light: Indications of Planet Formation Around the Young Solar Analog LkCa 15

We present H- and Ks-band imaging data resolving the gap in the transitional disk around LkCa 15, revealing the surrounding nebulosity. We detect sharp elliptical contours delimiting the nebulosity on the inside as well as the outside, consistent with the shape, size, ellipticity, and orientation of starlight reflected from the far-side disk wall, whereas the near-side wall is shielded from view by the disk's optically thick bulk. We note that forward-scattering of starlight on the near-side disk surface could provide an alternate interpretation of the nebulosity. In either case, this discovery provides confirmation of the disk geometry that has been proposed to explain the spectral energy distributions (SED) of such systems, comprising an optically thick outer disk with an inner truncation radius of ~46 AU enclosing a largely evacuated gap. Our data show an offset of the nebulosity contours along the major axis, likely corresponding to a physical pericenter offset of the disk gap. This reinforces the leading theory that dynamical clearing by at least one orbiting body is the cause of the gap. Based on evolutionary models, our high-contrast imagery imposes an upper limit of 21 Jupiter masses on companions at separations outside of 0.1" and of 13 Jupiter masses outside of 0.2". Thus, we find that a planetary system around LkCa 15 is the most likely explanation for the disk architecture.

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arXiv2010

Kiloparsec-scale star formation law in M81 and M101 based on AKARI far-infrared observations

We assess the relationships between the surface densities of the gas and star formation rate (SFR) within spiral arms of the nearby late-type spiral galaxies M81 and M101. By analyzing these relationships locally, we derive empirically a kiloparsec scale Kennicutt-Schmidt Law. Both M81 and M101 were observed with the Far-Infrared Surveyor (FIS) aboard AKARI in four far-infrared bands at 65, 90, 140, and 160 um. The spectral energy distributions of the whole galaxies show the presence of the cold dust component (Tc~20 K) in addition to the warm dust component (Tw~60 K). We deconvolved the cold and warm dust emission components spatially by making the best use of the multi-band photometric capability of the FIS. The cold and warm dust components show power-law correlations in various regions, which can be converted into the gas mass and the SFR, respectively. We find a power-law correlation between the gas and SFR surface densities with significant differences in the power law index N between giant HII regions (N=1.0) and spiral arms (N=2.2) in M101. The power-law index for spiral arms in M81 is similar (N=1.9) to that of spiral arms in M101. Conclusions: The power-law index is not always constant within a galaxy. The difference in the power-law index can be attributed to the difference in the star formation processes on a kiloparsec scale. N~2 seen in the spiral arms in M81 and M101 supports the scenario of star formation triggered by cloud-cloud collisions enhanced by spiral density wave, while N~1 derived in giant HII regions in M101 suggests the star formation induced by the Parker instability triggered by high velocity HI gas infall. The present method can be applied to a large galaxy sample for which the AKARI All Sky Survey provides the same 4 far-infrared band data.

astro-ph.CO↗

arXiv2009

AKARI Far-Infrared All Sky Survey

We demonstrate the capability of AKARI for mapping diffuse far-infrared emission and achieved reliability of all-sky diffuse map. We have conducted an all-sky survey for more than 94 % of the whole sky during cold phase of AKARI observation in 2006 Feb. -- 2007 Aug. The survey in far-infrared waveband covers 50 um -- 180 um with four bands centered at 65 um, 90 um, 140 um, and 160 um and spatial resolution of 3000 -- 4000 (FWHM).This survey has allowed us to make a revolutionary improvement compared to the IRAS survey that was conducted in 1983 in both spatial resolution and sensitivity after more than a quarter of a century. Additionally, it will provide us the first all-sky survey data with high-spatial resolution beyond 100 um. Considering its extreme importance of the AKARI far-infrared diffuse emission map, we are now investigating carefully the quality of the data for possible release of the archival data. Critical subjects in making image of diffuse emission from detected signal are the transient response and long-term stability of the far-infrared detectors. Quantitative evaluation of these characteristics is the key to achieve sensitivity comparable to or better than that for point sources (< 20 -- 95 [MJy/sr]). We describe current activities and progress that are focused on making high quality all-sky survey images of the diffuse far-infrared emission.

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arXiv2009

Exoplanet Characterization and the Search for Life

Over 300 extrasolar planets (exoplanets) have been detected orbiting nearby stars. We now hope to conduct a census of all planets around nearby stars and to characterize their atmospheres and surfaces with spectroscopy. Rocky planets within their star's habitable zones have the highest priority, as these have the potential to harbor life. Our science goal is to find and characterize all nearby exoplanets; this requires that we measure the mass, orbit, and spectroscopic signature of each one at visible and infrared wavelengths. The techniques for doing this are at hand today. Within the decade we could answer long-standing questions about the evolution and nature of other planetary systems, and we could search for clues as to whether life exists elsewhere in our galactic neighborhood.

astro-ph.EP↗

arXiv2009

Far-infrared all sky diffuse mapping with AKARI

We discuss the capability of AKARI in recovering diffuse far-infrared emission, and examine the achieved reliability. Critical issues in making images of diffuse emission are the transient response and long-term stability of the far-infrared detectors. Quantitative evaluation of these characteristics are the key to achieving sensitivity comparable to or better than that for point sources (< 20 -- 95 MJy sr-1). We describe current activity and progress toward the production of high quality images of the diffuse far-infrared emission using the AKARI all-sky survey data.

astro-ph.GA↗