arXiv subjects

Kento Masuda

Publications and source records attributed to Kento Masuda.

At least 73 records · Page 4Linked to original sources

arXiv2018

Kepler-730b is probably a hot Jupiter with a small companion

Here we draw attention to a candidate system with one hot Jupiter and one small, nearby companion, revealed by the recent Kepler data release (DR25). The hot Jupiter, Kepler-730b, has radius $R_{\rm p}=11.36^{+1.14}_{-0.98}~R_\oplus$ and orbital period $P=6.492$ d, and the newly discovered companion, KOI-929.02, has $R_{\rm p}=1.45^{+0.15}_{-0.20}~R_\oplus$ and $P=2.852$ d. No transit timing variation (TTV) was detected because of the marginal detection of the small companion transit, but this small companion passed all the validation tests. This system is probably another planetary system with hot Jupiter and small, nearby companion, after the outstanding WASP-47 system, and so far the only such system (out of 46) in the prime Kepler mission. The nature of this system, if confirmed, would suggest that hot Jupiters with small, nearby companions are probably more common than we used to believe. There remains a possibility that the small companion actually transits a star that is different from the hot Jupiter host, and follow-up observations with 10-m telescopes can potentially resolve this issue.

astro-ph.EP↗

arXiv2018

A Hubble constant measurement from superluminal motion of the jet in GW170817

The Hubble constant ($H_0$) measures the current expansion rate of the Universe, and plays a fundamental role in cosmology. Tremendous effort has been dedicated over the past decades to measure $H_0$. Notably, Planck cosmic microwave background (CMB) and the local Cepheid-supernovae distance ladder measurements determine $H_0$ with a precision of $\sim 1\%$ and $\sim 2\%$ respectively. A $3$-$σ$ level of discrepancy exists between the two measurements, for reasons that have yet to be understood. Gravitational wave (GW) sources accompanied by electromagnetic (EM) counterparts offer a completely independent standard siren (the GW analogue of an astronomical standard candle) measurement of $H_0$, as demonstrated following the discovery of the neutron star merger, GW170817. This measurement does not assume a cosmological model and is independent of a cosmic distance ladder. The first joint analysis of the GW signal from GW170817 and its EM localization led to a measurement of $H_0=74^{+16}_{-8}$ km/s/Mpc (median and symmetric $68\%$ credible interval). In this analysis, the degeneracy in the GW signal between the source distance and the weakly constrained viewing angle dominated the $H_0$ measurement uncertainty. Recently, Mooley et al. (2018) obtained tight constraints on the viewing angle using high angular resolution imaging of the radio counterpart of GW170817. Here we obtain a significantly improved measurement $H_0=68.9^{+4.7}_{-4.6}$ km/s/Mpc by using these new radio observations, combined with the previous GW and EM data. We estimate that 15 more localized GW170817-like events (comparable signal-to-noise ratio, favorable orientation), having radio images and light curve data, will potentially bring resolution to the tension between the Planck and Cepheid-supernova measurements, as compared to 50-100 GW events without such data.

astro-ph.CO↗

arXiv2018

Biases in Planet Occurrence Caused by Unresolved Binaries in Transit Surveys

Wide-field surveys for transiting planets, such as the NASA Kepler and TESS missions, are usually conducted without knowing which stars have binary companions. Unresolved and unrecognized binaries give rise to systematic errors in planet occurrence rates, including misclassified planets and mistakes in completeness corrections. The individual errors can have different signs, making it difficult to anticipate the net effect on inferred occurrence rates. Here we use simplified models of signal-to-noise limited transit surveys to try and clarify the situation. We derive a formula for the apparent occurrence rate density measured by an observer who falsely assumes all stars are single. The formula depends on the binary fraction; the mass function of the secondary stars; and the true occurrence of planets around primaries, secondaries, and single stars. It also takes into account the Malmquist bias by which binaries are over-represented in flux-limited samples. Application of the formula to an idealized Kepler-like survey shows that for planets larger than 2 $R_\oplus$, the net systematic error is of order 5%. In particular, unrecognized binaries are unlikely to be the reason for the apparent discrepancies between hot Jupiter occurrence rates measured in different surveys. For smaller planets the errors are potentially larger: the occurrence of Earth-sized planets could be overestimated by as much as 50%. We also show that whenever high-resolution imaging reveals a transit host star to be a binary, the planet is usually more likely to orbit the primary star than the secondary star.

astro-ph.EP↗

arXiv2018

Systematic Search for Rings around Kepler Planet Candidates: Constraints on Ring Size and Occurrence Rate

We perform a systematic search for rings around 168 Kepler planet candidates with sufficient signal-to-noise ratios that are selected from all the short-cadence data. We fit ringed and ringless models to their lightcurves, and compare the fitting results to search for the signatures of planetary rings. First, we identify 29 tentative systems, for which the ringed models exhibit statistically significant improvement over the ringless models. The lightcurves of those systems are individually examined, but we are not able to identify any candidate that indicates evidence for rings. In turn, we find out several mechanisms of false-positives that would produce ring-like signals, and the null detection enables us to place upper limits on the size of rings. Furthermore, assuming the tidal alignment between axes of the planetary rings and orbits, we conclude that the occurrence rate of rings larger than twice the planetary radius is less than 15 percent. Even though the majority of our targets are short-period planets, our null detection provides statistical and quantitative constraints on largely uncertain theoretical models of origin, formation, and evolution of planetary rings.

astro-ph.EP↗

arXiv2018

Discovery of three self-lensing binaries from Kepler

We report the discovery of three edge-on binaries with white dwarf companions that gravitationally magnify (instead of eclipsing) the light of their stellar primaries, as revealed by a systematic search for pulses with long periods in the Kepler photometry. We jointly model the self-lensing light curves and radial-velocity orbits to derive the white dwarf masses, all of which are close to 0.6 Solar masses. The orbital periods are long, ranging from 419 to 728 days, and the eccentricities are low, all less than 0.2. These characteristics are reminiscent of the orbits found for many blue stragglers in open clusters and the field, for which stable mass transfer due to Roche-lobe overflow from an evolving primary (now a white dwarf) has been proposed as the formation mechanism. Because the actual masses for our three white dwarf companions have been accurately determined, these self-lensing systems would provide excellent tests for models of interacting binaries.

astro-ph.SR↗

arXiv2017

High-Resolution Spectroscopic Detection of TiO and Stratosphere in the Day-side of WASP-33b

We report high-resolution spectroscopic detection of TiO molecular signature in the day-side spectra of WASP-33 b, the second hottest known hot Jupiter. We used High-Dispersion Spectrograph (HDS; R $\sim$ 165,000) in the wavelength range of 0.62 -- 0.88 $μ$m with the Subaru telescope to obtain the day-side spectra of WASP-33 b. We suppress and correct the systematic effects of the instrument, the telluric and stellar lines by using SYSREM algorithm after the selection of good orders based on Barnard star and other M-type stars. We detect a 4.8-$σ$ signal at an orbital velocity of $K_{p}$= +237.5 $^{+13.0}_{-5.0}$ km s$^{-1}$ and systemic velocity $V_{sys}$= -1.5 $^{+4.0} _{-10.5}$ km s$^{-1}$, which agree with the derived values from the previous analysis of primary transit. Our detection with the temperature inversion model implies the existence of stratosphere in its atmosphere, however, we were unable to constrain the volume-mixing ratio of the detected TiO. We also measure the stellar radial velocity and use it to obtain a more stringent constraint on the orbital velocity, $K_{p} = 239.0^{+2.0}_{-1.0}$ km s$^{-1}$. Our results demonstrate that high-dispersion spectroscopy is a powerful tool to characterize the atmosphere of an exoplanet, even in the optical wavelength range, and show a promising potential in using and developing similar techniques with high-dispersion spectrograph on current 10m-class and future extremely large telescopes.

astro-ph.EP↗

arXiv2017

Eccentric Companions to Kepler-448b and Kepler-693b: Clues to the Formation of Warm Jupiters

I report the discovery of non-transiting close companions to two transiting warm Jupiters (WJs), Kepler-448/KOI-12b (orbital period $P=17.9\,\mathrm{days}$, radius $R_{\rm p}=1.23^{+0.06}_{-0.05}\,R_{\rm Jup}$) and Kepler-693/KOI-824b ($P=15.4\,\mathrm{days}$, $R_{\rm p}=0.91\pm0.05\,R_{\rm Jup}$), via dynamical modeling of their transit timing and duration variations (TTVs and TDVs). The companions have masses of $22^{+7}_{-5}\,M_{\rm Jup}$ (Kepler-448c) and $150^{+60}_{-40}\,M_{\rm Jup}$ (Kepler-693c), and both are on eccentric orbits ($e=0.65^{+0.13}_{-0.09}$ for Kepler-448c and $e=0.47^{+0.11}_{-0.06}$ for Kepler-693c) with periastron distances of $1.5\,\mathrm{au}$. Moderate eccentricities are detected for the inner orbits as well ($e=0.34^{+0.08}_{-0.07}$ for Kepler-448b and $e=0.2^{+0.2}_{-0.1}$ for Kepler-693b). In the Kepler-693 system, a large mutual inclination between the inner and outer orbits ($53^{+7}_{-9}\,\mathrm{deg}$ or $134^{+11}_{-10}\,\mathrm{deg}$) is also revealed by the TDVs. This is likely to induce a secular oscillation of the inner WJ's eccentricity that brings its periastron close enough to the host star for tidal star-planet interactions to be significant. In the Kepler-448 system, the mutual inclination is weakly constrained and such an eccentricity oscillation is possible for a fraction of the solutions. Thus these WJs may be undergoing tidal migration to become hot Jupiters (HJs), although the migration via this process from beyond the snow line is disfavored by the close-in and massive nature of the companions. This may indicate that WJs can be formed in situ and could even evolve into HJs via high-eccentricity migration inside the snow line.

astro-ph.EP↗

arXiv2017

Reassessment of the Null Result of the HST Search for Planets in 47 Tucanae

We revisit the null result of the Hubble Space Telescope search for transiting planets in the globular cluster 47 Tucanae, in the light of improved knowledge of planet occurrence from the Kepler mission. Gilliland and co-workers expected to find 17 planets, assuming the 47 Tuc stars have close-in giant planets with the same characteristics and occurrence rate as those of the nearby stars that had been surveyed up until 1999. We update this result by assuming that 47 Tuc and Kepler stars have identical planet populations. The revised number of expected detections is $4.0^{+1.7}_{-1.4}$. When we restrict the Kepler stars to the same range of masses as the stars that were searched in 47 Tuc, the number of expected detections is reduced to $2.2^{+1.6}_{-1.1}$. Thus, the null result of the HST search is less statistically significant than it originally seemed. We cannot reject even the extreme hypothesis that 47 Tuc and Kepler stars have the same planet populations, with more than 2-3$σ$ significance. More sensitive searches are needed to allow comparisons between the planet populations of globular clusters and field stars.

astro-ph.EP↗

arXiv2017

Towards Detection of Exoplanetary Rings Via Transit Photometry: Methodology and a Possible Candidate

Detection of a planetary ring of exoplanets remains as one of the most attractive but challenging goals in the field. We present a methodology of a systematic search for exoplanetary rings via transit photometry of long-period planets. The methodology relies on a precise integration scheme we develop to compute a transit light curve of a ringed planet. We apply the methodology to 89 long-period planet candidates from the Kepler data so as to estimate, and/or set upper limits on, the parameters of possible rings. While a majority of our samples do not have a sufficiently good signal-to-noise ratio for meaningful constraints on ring parameters, we find that six systems with a higher signal-to-noise ratio are inconsistent with the presence of a ring larger than 1.5 times the planetary radius assuming a grazing orbit and a tilted ring. Furthermore, we identify five preliminary candidate systems whose light curves exhibit ring-like features. After removing four false positives due to the contamination from nearby stars, we identify KIC 10403228 as a reasonable candidate for a ringed planet. A systematic parameter fit of its light curve with a ringed planet model indicates two possible solutions corresponding to a Saturn-like planet with a tilted ring. There also remain other two possible scenarios accounting for the data; a circumstellar disk and a hierarchical triple. Due to large uncertain factors, we cannot choose one specific model among the three.

astro-ph.EP↗

arXiv2016

Possible Outcomes of Coplanar High-eccentricity Migration: Hot Jupiters, Close-in Super-Earths, and Counter-orbiting Planets

We investigate the formation of close-in planets in near-coplanar eccentric hierarchical triple systems via the secular interaction between an inner planet and an outer perturber (Coplanar High-eccentricity Migration, CHEM). We generalize the previous work on the analytical condition for successful CHEM for point masses interacting only through gravity by taking into account the finite mass effect of the inner planet. We find that efficient CHEM requires that the systems should have m_1<<m_0 and m_1<<m_2. In addition to the gravity for point masses, we examine the importance of the short-range forces, and provide an analytical estimate of the migration time scale. We perform a series of numerical simulations in CHEM for systems consisting of a sun-like central star, giant gas inner planet and planetary outer perturber, including the short-range forces and stellar and planetary dissipative tides. We find that most of such systems end up with a tidal disruption, a small fraction of the systems produce prograde hot Jupiters (HJs), but no retrograde one. In addition, we extend CHEM to super-Earth mass range, and show that the formation of close-in super-Earths in prograde orbits is also possible. Finally, we carry out CHEM simulation for the observed hierarchical triple and counter-orbiting HJ systems. We find that CHEM can explain a part of the former systems, but it is generally very difficult to reproduce counter-orbiting HJ systems.

astro-ph.EP↗

arXiv2016

Transiting Planet Candidates Beyond the Snow Line Detected by Visual Inspection of 7557 Kepler Objects of Interest

We visually inspected the light curves of 7557 Kepler Objects of Interest (KOIs) to search for single transit events (STEs) possibly due to long-period giant planets. We identified 28 STEs in 24 KOIs, among which 14 events are newly reported in this paper. We estimate the radius and orbital period of the objects causing STEs by fitting the STE light curves simultaneously with the transits of the other planets in the system or with the prior information on the host star density. As a result, we found that STEs in seven of those systems are consistent with Neptune- to Jupiter-sized objects of orbital periods ranging from a few to $\sim$ $20\,\mathrm{yr}$. We also estimate that $\gtrsim20\%$ of the compact multi-transiting systems host cool giant planets with periods $\gtrsim 3\,\mathrm{yr}$ on the basis of their occurrence in the KOIs with multiple candidates, assuming the small mutual inclination between inner and outer planetary orbits.

astro-ph.EP↗

arXiv2016

Transiting planets as a precision clock to constrain the time variation of the gravitational constant

Analysis of transit times in exoplanetary systems accurately provides an instantaneous orbital period, $P(t)$, of their member planets. A long-term monitoring of those transiting planetary systems puts limits on the variability of $P(t)$, which are translated into the constraints on the time variation of the gravitational constant $G$. We apply this analysis to $10$ transiting systems observed by the Kepler spacecraft, and find that $ΔG/G\lesssim 5\times10^{-6}$ for 2009-2013, or $\dot{G}/G \lesssim 10^{-6}\,\mathrm{yr}^{-1}$ if $\dot{G}$ is constant. While the derived limit is weaker than those from other analyses, it is complementary to them and can be improved by analyzing numerous transiting systems that are continuously monitored.

gr-qc↗

arXiv2015

Revisiting a gravity-darkened and precessing planetary system PTFO 8-8695: spin-orbit non-synchronous case

We reanalyse the time-variable lightcurves of the transiting planetary system PTFO 8-8695, in which a planet of 3 to 4 Jupiter mass orbits around a rapidly rotating pre-main-sequence star. Both the planetary orbital period of 0.448 days and the stellar spin period less than 0.671 days are unusually short, which makes PTFO 8-8695 an ideal system to check the model of gravity darkening and nodal precession. While the previous analysis of PTFO 8-8695 assumed that the stellar spin and planetary orbital periods are the same, we extend the analysis by discarding the spin-orbit synchronous condition, and find three different classes of solutions roughly corresponding to the nodal precession periods of 199$\pm$16, 475$\pm$21, and 827$\pm$53 days that reproduce the transit lightcurves observed in 2009 and 2010. We compare the predicted lightcurves of the three solutions against the photometry data of a few percent accuracy obtained at Koyama Astronomical Observatory in 2014 and 2015, and find that the solution with the precession period of 199$\pm$16 days is preferred even though preliminary. Future prospect and implications to other transiting systems are briefly discussed.

astro-ph.EP↗

arXiv2015

Absolute Dimensions of a Flat Hierarchical Triple System KIC 6543674 from the Kepler Photometry

Many of the Kepler close binaries are suggested to constitute hierarchical triple systems through their eclipse timing variations (ETVs). Eclipses by the third body in those systems, if observed, provide precise constraints on its physical and orbital properties, which are otherwise difficult to obtain. In this Letter, we analyze such a "tertiary event" observed only once in the KIC 6543674 system. The system consists of a short-period ($2.4\,\mathrm{days}$) inner eclipsing binary and a third body on a wide ($1100\,\mathrm{days}$) and eccentric ($e\simeq0.6$) orbit. Analysis of three tertiary eclipses around a single inferior conjunction of the third body yields the mutual inclination between the inner and outer binary planes to be $3.3^\circ\pm0.6^\circ$, indicating an extremely flat geometry. Furthermore, combining the timings and shapes of the tertiary eclipses with the phase curve and ETVs of the inner binary, we determine the mass and radius ratios of all three bodies in the system using the Kepler photometry alone. With the primary mass and temperature from the Kepler Input Catalog, the absolute masses, radii, and effective temperatures of the three stars are obtained as follows: $M_\mathrm{A}=1.2\pm0.3\,M_\odot$, $R_\mathrm{A}=1.8\pm0.1\,R_\odot$, $M_\mathrm{B}=1.1_{-0.2}^{+0.3}\,M_\odot$, $R_\mathrm{B}=1.4\pm0.1\,R_\odot$, $M_\mathrm{C}=0.50_{-0.08}^{+0.07}\,M_\odot$, $R_\mathrm{C}=0.50\pm0.04\,R_\odot$, $T_\mathrm{A} \simeq T_\mathrm{B}\simeq 6100\,\mathrm{K}$, and $T_\mathrm{C}<5000\,\mathrm{K}$. Implication for the formation scenario of close binaries is briefly discussed.

astro-ph.SR↗

arXiv2015

Spin-Orbit Angles of Kepler-13Ab and HAT-P-7b from Gravity-Darkened Transit Light Curves

Analysis of the transit light curve deformed by the stellar gravity darkening allows us to photometrically measure both components of the spin-orbit angle $ψ$, its sky projection $λ$ and inclination of the stellar spin axis $i_\star$. In this paper, we apply the method to two transiting hot Jupiter systems monitored with the Kepler spacecraft, Kepler-13A and HAT-P-7. For Kepler-13A, we find $i_\star=81^\circ\pm5^\circ$ and $ψ=60^\circ\pm2^\circ$ adopting the spectroscopic constraint $λ=58.6^\circ\pm2.0^\circ$ by Johnson et al. (2014). In our solution, the discrepancy between the above $λ$ and that previously reported by Barnes et al. (2011) is solved by fitting both of the two parameters in the quadratic limb-darkening law. We also report the temporal variation in the orbital inclination of Kepler-13Ab, $\mathrm{d} |\cos i_{\rm orb}|/\mathrm{d}t=(-7.0\pm0.4)\times10^{-6}\,\mathrm{day}^{-1}$, providing further evidence for the spin-orbit precession in this system. By fitting the precession model to the time series of $i_{\rm orb}$, $λ$, and $i_\star$ obtained with the gravity-darkened model, we constrain the stellar quadrupole moment $J_2=(6.1\pm0.3)\times10^{-5}$ for our new solution, which is several times smaller than $J_2=(1.66\pm0.08)\times10^{-4}$ obtained for the previous one. We show that the difference can be observable in the future evolution of $λ$, thus providing a possibility to test our solution with follow-up observations. The second target, HAT-P-7, is the first F-dwarf star analyzed with the gravity-darkening method. Our analysis points to a nearly pole-on configuration with $ψ=101^\circ\pm2^\circ$ or $87^\circ\pm2^\circ$ and the gravity-darkening exponent $β$ consistent with $0.25$. Such an observational constraint on $β$ can be useful for testing the theory of gravity darkening.

astro-ph.EP↗

arXiv2015

Precise Radial Velocity Measurements for Kepler Giants Hosting Planetary Candidates: Kepler-91 and KOI-1894

We present results of radial-velocity follow-up observations for the two Kepler evolved stars Kepler-91 (KOI-2133) and KOI-1894, which had been announced as candidates to host transiting giant planets, with the Subaru 8.2m telescope and the High Dispersion Spectrograph (HDS). By global modeling of the high-precision radial-velocity data taken with Subaru/HDS and photometric ones taken by Kepler mission taking account of orbital brightness modulations (ellipsoidal variations, reflected/emitted light, etc.) of the host stars, we independently confirmed that Kepler-91 hosts a transiting planet with a mass of 0.66 M_Jup (Kepler-91b), and newly detected an offset of ~20 m s$^{-1}$ between the radial velocities taken at ~1-yr interval, suggesting the existence of additional companion in the system. As for KOI-1894, we detected possible phased variations in the radial velocities and light curves with 2--3 sigma confidence level which could be explained as a reflex motion and ellipsoidal variation of the star caused by the transiting sub-saturn-mass (~0.18 M_Jup) planet.

astro-ph.EP↗

arXiv2014

Global Analysis of KOI-977: Spectroscopy, Asteroseismology, and Phase-curve Analysis

We present a global analysis of KOI-977, one of the planet host candidates detected by {\it Kepler}. Kepler Input Catalog (KIC) reports that KOI-977 is a red giant, for which few close-in planets have been discovered. Our global analysis involves spectroscopic and asteroseismic determinations of stellar parameters (e.g., mass and radius) and radial velocity (RV) measurements. Our analyses reveal that KOI-977 is indeed a red giant in the red clump, but its estimated radius ($\gtrsim 20R_\odot=0.093$ AU) is much larger than KOI-977.01's orbital distance ($\sim 0.027$ AU) estimated from its period ($P_\mathrm{orb}\sim 1.35$ days) and host star's mass. RV measurements show a small variation, which also contradicts the amplitude of ellipsoidal variations seen in the light-curve folded with KOI-977.01's period. Therefore, we conclude that KOI-977.01 is a false positive, meaning that the red giant, for which we measured the radius and RVs, is different from the object that produces the transit-like signal (i.e., an eclipsing binary). On the basis of this assumption, we also perform a light-curve analysis including the modeling of transits/eclipses and phase-curve variations, adopting various values for the dilution factor $D$, which is defined as the flux ratio between the red giant and eclipsing binary. Fitting the whole folded light-curve as well as individual transits in the short cadence data simultaneously, we find that the estimated mass and radius ratios of the eclipsing binary are consistent with those of a solar-type star and a late-type star (e.g., an M dwarf) for $D\gtrsim 20$.

astro-ph.EP↗

arXiv2014

Very Low-Density Planets around Kepler-51 Revealed with Transit Timing Variations and an Anomaly Similar to a Planet-Planet Eclipse Event

We present an analysis of the transit timing variations (TTVs) in the multi-transiting planetary system around Kepler-51 (KOI-620). This system consists of two confirmed transiting planets, Kepler-51b ($P_{\rm b} = 45.2\,\mathrm{days}$) and Kepler-51c ($P_{\rm c} = 85.3\,\mathrm{days}$), and one transiting planet candidate KOI-620.02 ($P_{02} = 130.2\,\mathrm{days}$), which lie close to a $1:2:3$ resonance chain. Our analysis shows that their TTVs are consistently explained by the three-planet model, and constrains their masses as $M_{\rm b} = 2.1_{-0.8}^{+1.5} M_{\oplus}$ (Kepler-51b), $M_{\rm c} = 4.0 \pm 0.4 M_{\oplus}$ (Kepler-51c), and $M_{02} = 7.6 \pm 1.1 M_{\oplus}$ (KOI-620.02), thus confirming KOI-620.02 as a planet in this system. The masses inferred from the TTVs are rather small compared to the planetary radii based on the stellar density and planet-to-star radius ratios determined from the transit light curves. Combining these estimates, we find that all three planets in this system have densities among the lowest determined, $ρ_p \lesssim 0.05\,{\rm g\,cm^{-3}}$. With this feature, the Kepler-51 system serves as another example of low-density compact multi-transiting planetary systems. We also identify a curious feature in the archived Kepler light curve during the double transit of Kepler-51b and KOI-620.02, which could be explained by their overlapping on the stellar disk (a planet-planet eclipse). If this is really the case, the sky-plane inclination of KOI-620.02's orbit relative to that of Kepler-51b is given by $ΔΩ= -25.3_{-6.8}^{+6.2}\mathrm{deg}$, implying significant misalignment of their orbital planes. This interpretation, however, seems unlikely because such an event that is consistent with all of the observations is found to be exceedingly rare.

astro-ph.EP↗