arXiv subjects

Kento Masuda

Publications and source records attributed to Kento Masuda.

At least 55 records · Page 3Linked to original sources

arXiv2021

Auto-Differentiable Spectrum Model for High-Dispersion Characterization of Exoplanets and Brown Dwarfs

We present an auto-differentiable spectral modeling of exoplanets and brown dwarfs. This model enables a fully Bayesian inference of the high--dispersion data to fit the ab initio line-by-line spectral computation to the observed spectrum by combining it with the Hamiltonian Monte Carlo in recent probabilistic programming languages. An open source code, exojax, developed in this study, was written in Python using the GPU/TPU compatible package for automatic differentiation and accelerated linear algebra, JAX (Bradbury et al. 2018). We validated the model by comparing it with existing opacity calculators and a radiative transfer code and found reasonable agreements of the output. As a demonstration, we analyzed the high-dispersion spectrum of a nearby brown dwarf, Luhman 16 A and found that a model including water, carbon monoxide, and $\mathrm{H_2/He}$ collision induced absorption was well fitted to the observed spectrum ($R=10^5$ and 2.28-2.30 $μ$m). As a result, we found that $T_0=1295_{-32}^{+35}$ K at 1 bar and C/O $=0.62 \pm 0.03$, which is slightly higher than the solar value. This work demonstrates the potential of full Bayesian analysis of brown dwarfs and exoplanets as observed by high-dispersion spectrographs and also directly-imaged exoplanets as observed by high-dispersion coronagraphy.

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arXiv2021

Tidal Effects on the Radial Velocities of V723 Mon: Additional Evidence for a Dark $3\,M_\odot$ Companion

Jayasinghe et al. (2021) identified a dark $\approx 3\,M_\odot$ companion on a nearly edge-on $\approx 60\,\mathrm{day}$ orbit around the red giant star V723 Monoceros as a black hole candidate in the mass gap. This scenario was shown to explain most of the data presented by Jayasinghe et al. (2021), except for periodic radial velocity (RV) residuals from the circular Keplerian model. Here we show that the RV residuals are explained by orbital phase-dependent distortion of the absorption line profile associated with changing visible fractions of the approaching and receding sides of the red giant star, whose surface is tidally deformed by and rotating synchronously with the dark companion. Our RV model constrains the companion mass $M_\bullet = 2.95\pm0.17\,M_\odot$ and orbital inclination $i=82.9^{+7.0}_{-3.3}\,\mathrm{deg}$ (medians and 68.3% highest density intervals of the marginal posteriors) adopting the radius of the red giant $24.0\pm0.9\,R_\odot$ as constrained from its SED and distance. The analysis provides independent support for the companion mass from ellipsoidal variations and the limits on the companion's luminosity from the absence of eclipses both derived by Jayasinghe et al. (2021). We also show that a common scheme to evaluate the tidal RV signal as the flux-weighted mean of the surface velocity field can significantly underestimate its amplitude for RVs measured with a cross-correlation technique, and present a modified prescription that directly models the distorted line profile and its effects on the measured RVs. The formulation will be useful for estimating the component masses and inclinations in other similar binaries.

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arXiv2021

First Detection of Hydroxyl Radical Emission from an Exoplanet Atmosphere: High-dispersion Characterization of WASP-33b using Subaru/IRD

We report the first detection of a hydroxyl radical (OH) emission signature in the planetary atmosphere outside the solar system, in this case, in the day-side of WASP-33b. We analyze high-resolution near-infrared emission spectra of WASP-33b taken using the InfraRed Doppler spectrograph on the 8.2-m Subaru telescope. The telluric and stellar lines are removed using a de-trending algorithm, SysRem. The residuals are then cross-correlated with OH and H$_{2}$O planetary spectrum templates produced using several different line-lists. We check and confirm the accuracy of OH line-lists by cross-correlating with the spectrum of GJ 436. As a result, we detect the emission signature of OH at $K_\mathrm{p}$ of 230.9$^{+6.9}_{-7.4}$ km s$^{-1}$ and $v_{\mathrm{sys}}$ of $-$0.3$^{+5.3}_{-5.6}$ km s$^{-1}$ with S/N of 5.4 and significance of 5.5$σ$. Additionally, we marginally detect H$_{2}$O emission in the H-band with S/N of 4.0 and significance of 5.2$σ$ using the POKAZATEL line-list. However, no significant signal is detected using the HITEMP 2010, which might be due to differences in line positions and strengths, as well as the incompleteness of the line-lists. Nonetheless, this marginal detection is consistent with the prediction that H$_{2}$O is mostly thermally dissociated in the upper atmosphere of the ultra-hot Jupiters. Therefore, along with CO, OH is expected to be one of the most abundant O-bearing molecules in the day-side atmosphere of ultra-hot Jupiters and should be considered when studying their atmosphere.

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arXiv2020

Hot Stars With Kepler Planets Have High Obliquities

It has been known for a decade that hot stars with hot Jupiters tend to have high obliquities. Less is known about the degree of spin-orbit alignment for hot stars with other kinds of planets. Here, we re-assess the obliquities of hot Kepler stars with transiting planets smaller than Neptune, based on spectroscopic measurements of their projected rotation velocities (vsini). The basis of the method is that a lower obliquity -- all other things being equal -- causes sini to be closer to unity and increases the value of vsini. We sought evidence for this effect using a sample of 150 Kepler stars with effective temperatures between 5950 and 6550K and a control sample of 101 stars with matching spectroscopic properties and random orientations. The planet hosts have systematically higher values of vsini than the control stars, but not by enough to be compatible with perfect spin-orbit alignment. The mean value of sini is 0.856 +/- 0.036, which is 4-sigma away from unity (perfect alignment), and 2-sigma away from pi/4 (random orientations). There is also evidence that the hottest stars have a broader obliquity distribution: when modeled separately, the stars cooler than 6250K have = 0.928 +/- 0.042, while the hotter stars are consistent with random orientations. This is similar to the pattern previously noted for stars with hot Jupiters. Based on these results, obliquity excitation for early-G and late-F stars appears to be a general outcome of star and planet formation, rather than being exclusively linked to hot Jupiter formation.

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arXiv2020

Dippers from the TESS Full-Frame Images I: The Results of the first 1 year data and Discovery of A Runaway dipper

We present a comprehensive catalog of the dippers---young stellar objects that exhibit episodic dimming---derived from the one year's worth of data of Transiting Exoplanet Survey Satellite ($ TESS$) full-frame images. In the survey, we found 35 dippers using the convolutional neural network, most of them newly discovered. Although these dippers are widely distributed over the first half-hemisphere that $TESS$ surveyed, we identified the majority's membership with the nearest association Scorpius--Centaurus, Velorum OB2, and nearby Orion molecular cloud complex. However, several dippers are likely to be located in the field. We also found three old dippers whose age exceeds ten million year, which is considered as the disk dissipation time. The color-color diagram indicates that these old dippers are likely to have an extreme debris disk. In particular, we found a runaway old dipper having a large three-dimensional velocity of $72 \mathrm{km\ s}^{-1}$. The dippers in the field, which were probably escaping from their birth molecular clouds or were born outside the current area of star forming regions, are more common than previously considered.

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arXiv2020

Revisiting the Architecture of the KOI-89 System

While high stellar obliquities observed in exoplanetary systems may be attributed to processes that tilt the planetary orbits, it is also possible that they reflect misalignments between protoplanetary disks and stellar spins. This latter hypothesis predicts the presence of co-planar multi-planetary systems misaligned with their central stars. Here we re-evaluate the evidence of such an architecture that has been claimed for the KOI-89 system. KOI-89 is an early-type star with one validated transiting planet KOI-89.01/Kepler-462b (period 84.7 days, radius $3.0\,R_\oplus$) and one transiting planet candidate KOI-89.02 (period 207.6 days, radius $4.0\,R_\oplus$), where the latter exhibits transit timing variations (TTVs). A previous modeling of the stellar gravity-darkening effect in the transit light curves inferred a high stellar obliquity of $\approx70^\circ$. We perform a photodynamical modeling of the Kepler transit light curves, and use the resulting constraints on the orbital configuration and transit times to update the gravity-darkened transit model. As a result, we find no firm evidence for gravity darkening effect in the transit shapes and conclude that stellar obliquity is not constrained by the data. Given evidence for low orbital eccentricities from the dynamical analysis, the system architecture can thus be consistent with many other multi-transiting systems with flat, near-circular orbits aligned with the stellar spin. We find that the TTVs imparted on its neighbor imply that KOI-89.01 has a mass $\gtrsim20\,M_\oplus$. This would render it one of the densest known sub-Neptunes, mostly composed of a solid core. Lower masses are possible if the TTVs are instead due to an unseen third planet.

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arXiv2020

Bayesian Dynamic Mapping of an Exo-Earth from Photometric Variability

Photometric variability of a directly imaged exo-Earth conveys spatial information on its surface and can be used to retrieve a two-dimensional geography and axial tilt of the planet (spin-orbit tomography). In this study, we relax the assumption of the static geography and present a computationally tractable framework for dynamic spin-orbit tomography applicable to the time-varying geography. First, a Bayesian framework of static spin-orbit tomography is revisited using analytic expressions of the Bayesian inverse problem with a Gaussian prior. We then extend this analytic framework to a time-varying one through a Gaussian process in time domain, and present analytic expressions that enable efficient sampling from a full joint posterior distribution of geography, axial tilt, spin rotation period, and hyperparameters in the Gaussian-process priors. Consequently, it only takes 0.3 s for a laptop computer to sample one posterior dynamic map conditioned on the other parameters with 3,072 pixels and 1,024 time grids, for a total of $\sim 3 \times 10^6$ parameters. We applied our dynamic mapping method on a toy model and found that the time-varying geography was accurately retrieved along with the axial-tilt and spin rotation period. In addition, we demonstrated the use of dynamic spin-orbit tomography with a real multi-color light curve of the Earth as observed by the Deep Space Climate Observatory. We found that the resultant snapshots from the dominant component of a principle component analysis roughly captured the large-scale, seasonal variations of the clear-sky and cloudy areas on the Earth.

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arXiv2020

On the Inference of a Star's Inclination Angle from its Rotation Velocity and Projected Rotation Velocity

It is possible to learn about the orientation of a star's rotation axis by combining measurements of the star's rotation velocity ($v$) and its projection onto our line of sight ($v\sin i$). This idea has found many applications, including the investigation of the obliquities of stars with transiting planets. Here, we present a method for the probabilistic inference of the inclination of the star's rotation axis based on independent data sets that constrain $v$ and $v\sin i$. We also correct several errors and misconceptions that appear in the literature.

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arXiv2020

Four New Self-lensing Binaries from Kepler: Radial Velocity Characterization and Astrophysical Implications

In Kawahara et al. (2018) and Masuda et al. (2019), we reported the discovery of four self-lensing binaries consisting of F/G-type stars and (most likely) white dwarfs whose masses range from 0.2 to 0.6 solar masses. Here we present their updated system parameters based on new radial velocity data from the Tillinghast Reflector Echelle Spectrograph at the Fred Lawrence Whipple Observatory, and the Gaia parallaxes and spectroscopic parameters of the primary stars. We also briefly discuss the astrophysical implications of these findings.

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arXiv2020

Mutual Orbital Inclinations Between Cold Jupiters and Inner Super-Earths

Previous analyses of Doppler and Kepler data have found that Sun-like stars hosting "cold Jupiters" (giant planets with $a\gtrsim 1\,\mathrm{AU}$) almost always host "inner super-Earths" (1-$4\,R_\oplus$, $a\lesssim1\,\mathrm{AU}$). Here, we attempt to determine the degree of alignment between the orbital planes of the cold Jupiters and the inner super-Earths. The key observational input is the fraction of Kepler stars with transiting super-Earths that also have transiting cold Jupiters. This fraction depends on both the probability for cold Jupiters to occur in such systems, and on the mutual orbital inclinations. Since the probability of occurrence has already been measured in Doppler surveys, we can use the data to constrain the dispersion of the mutual inclination distribution. We find $σ=11.8^{+12.7}_{-5.5}\,\mathrm{deg}$ (68% confidence) and $σ>3.5\,\mathrm{deg}$ (95% confidence), where $σ$ is the scale parameter of the Rayleigh distribution. This suggests that planetary orbits in systems with cold Jupiters tend to be coplanar - although not quite as coplanar as those in the Solar System, which have a mean inclination from the invariable plane of $1.8\,\mathrm{deg}$. We also find evidence that cold Jupiters have lower mutual inclinations relative to inner systems with higher transit multiplicity. This suggests a link between the dynamical excitation in the inner and outer systems. For example, perturbations from misaligned cold Jupiters may dynamically heat or destabilize systems of inner super-Earths.

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arXiv2019

The Featureless Transmission Spectra of Two Super-Puff Planets

The Kepler mission revealed a class of planets known as ''super-puffs,'' with masses only a few times larger than Earth's but radii larger than Neptune, giving them very low mean densities. All three of the known planets orbiting the young solar-type star Kepler 51 are super-puffs. The Kepler 51 system thereby provides an opportunity for a comparative study of the structures and atmospheres of this mysterious class of planets, which may provide clues about their formation and evolution. We observed two transits each of Kepler 51b and 51d with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. Combining new WFC3 transit times with re-analyzed Kepler data and updated stellar parameters, we confirmed that all three planets have densities lower than 0.1 g/cm$^{3}$. We measured the WFC3 transmission spectra to be featureless between 1.15 and 1.63 $μ$m, ruling out any variations greater than 0.6 scale heights (assuming a H/He dominated atmosphere), thus showing no significant water absorption features. We interpreted the flat spectra as the result of a high-altitude aerosol layer (pressure $<$3 mbar) on each planet. Adding this new result to the collection of flat spectra that have been observed for other sub-Neptune planets, we find support for one of the two hypotheses introduced by Crossfield and Kreidberg (2017), that planets with cooler equilibrium temperatures have more high-altitude aerosols. We strongly disfavor their other hypothesis that the H/He mass fraction drives the appearance of large amplitude transmission features.

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arXiv2019

Prospects of Finding Detached Black Hole-Star Binaries with TESS

We discuss prospects of identifying and characterizing black hole (BH) companions to normal stars on tight but detached orbits, using photometric data from the Transiting Exoplanet Survey Satellite (TESS). We focus on the following two periodic signals from the visible stellar component: (i) in-eclipse brightening of the star due to gravitational microlensing by the BH (self-lensing), and (ii) a combination of ellipsoidal variations due to tidal distortion of the star and relativistic beaming due to its orbital motion (phase-curve variation). We evaluate the detectability of each signal in the light curves of stars in the TESS input catalog, based on a pre-launch noise model of TESS photometry as well as the actual light curves of spotted stars from the prime Kepler mission to gauge the potential impact of stellar activity arising from the tidally spun-up stellar components. We estimate that the self-lensing and phase-curve signals from BH companions, if exist, will be detectable in the light curves of effectively $\mathcal{O}(10^5)$ and $\mathcal{O}(10^6)$ low-mass stars, respectively, taking into account orbital inclination dependence of the signals. These numbers could be large enough to actually detect signals from BHs: simple population models predict some 10 and 100 detectable BHs among these "searchable" stars, although the latter may be associated with a comparable number of false-positives due to stellar variabilities and additional vetting with radial velocity measurements would be essential. Thus the TESS data could serve as a resource to study nearby BHs with stellar companions on shorter-period orbits than will potentially be probed with Gaia.

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arXiv2019

Homogeneous Analysis of Hot Earths: Masses, Sizes, and Compositions

Terrestrial planets have been found orbiting Sun-like stars with extremely short periods --- some as short as 4 hours. These "ultra-short-period planets" or "hot Earths" are so strongly irradiated that any initial H/He atmosphere has probably been lost to photoevaporation. As such, the sample of hot Earths may give us a glimpse at the rocky cores that are often enshrouded by thick H/He envelopes on wider-orbiting planets. However, the mass and radius measurements of hot Earths have been derived from a hodgepodge of different modeling approaches, and include several cases of contradictory results. Here, we perform a homogeneous analysis of the complete sample of 11 known hot Earths with an insolation exceeding 650 times that of the Earth. We combine all available data for each planet, incorporate parallax information from {\it Gaia} to improve the stellar and planetary parameters, and use Gaussian Process regression to account for correlated noise in the radial-velocity data. The homogeneous analysis leads to a smaller dispersion in the apparent composition of hot Earths, although there does still appear to be some intrinsic dispersion. Most of the planets are consistent with an Earth-like composition (35\% iron and 65\% rock), but two planets (K2-141b and K2-229b) show evidence for a higher iron fraction, and one planet (55\,Cnc\,e) has either a very low iron fraction or an envelope of low-density volatiles. All of the planets are less massive than 8\,$M_\oplus$, despite the selection bias towards more massive planets, suggesting that 8\,$M_\oplus$ is the critical mass for runaway accretion.

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arXiv2019

Self-lensing Discovery of a $0.2\,M_\odot$ White Dwarf in an Unusually Wide Orbit Around a Sun-like Star

We report the discovery of the fifth self-lensing binary in which a low-mass white dwarf (WD) gravitationally magnifies its 15th magnitude G-star companion, KIC 8145411, during eclipses. The system was identified from a pair of such self-lensing events in the Kepler photometry, and was followed up with the Tillinghast Reflector Echelle Spectrograph (TRES) on the 1.5m telescope at the Fred Lawrence Whipple Observatory and the High-Dispersion Spectrograph (HDS) on the Subaru 8.2m telescope. A joint analysis of the TRES radial velocities, the HDS spectrum, and the Kepler photometry of the primary star determines the WD mass $0.20\pm0.01\,M_\odot$, orbital semi-major axis $1.28\pm0.03\,\mathrm{au}$, and orbital eccentricity $0.14\pm0.02$. Because such extremely low-mass WDs cannot be formed in isolation within the age of the Galaxy, their formation is believed to involve binary interactions that truncated evolution of the WD progenitor. However, the observed orbit of the KIC 8145411 system is at least ten times wider than required for this scenario to work. The presence of this system in the Kepler sample, along with its similarities to field blue straggler binaries presumably containing WDs, may suggest that some 10% of post-AGB binaries with Sun-like primaries contain such anomalous WDs.

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arXiv2019

It takes two planets in resonance to tango around K2-146

K2-146 is a cool, 0.358 M_sun dwarf that was found to host a mini-Neptune with a 2.67-days period. The planet exhibited strong transit timing variations (TTVs) of greater than 30 minutes, indicative of the presence of a further object in the system. Here we report the discovery of the previously undetected outer planet, K2-146 c, in the system using additional photometric data. K2-146 c was found to have a grazing transit geometry and a 3.97-day period. The outer planet was only significantly detected in the latter K2 campaigns presumably because of precession of its orbital plane. The TTVs of K2-146 b and c were measured using observations spanning a baseline of almost 1200 days. We found strong anti-correlation in the TTVs, suggesting the two planets are gravitationally interacting. Our TTV and transit model analyses revealed that K2-146 b has a radius of 2.25 $\pm$ 0.10 \R_earth and a mass of 5.6 $\pm$ 0.7 M_earth, whereas K2-146 c has a radius of $2.59_{-0.39}^{+1.81}$ R_earth and a mass of 7.1 $\pm$ 0.9 M_earth. The inner and outer planets likely have moderate eccentricities of $e = 0.14 \pm 0.07$ and $0.16 \pm 0.07$, respectively. Long-term numerical integrations of the two-planet orbital solution show that it can be dynamically stable for at least 2 Myr. The evaluation of the resonance angles of the planet pair indicates that K2-146 b and c are likely trapped in a 3:2 mean motion resonance. The orbital architecture of the system points to a possible convergent migration origin.

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arXiv2019

Transiting Planets near the Snow Line from Kepler. I. Catalog

We present a comprehensive catalog of cool (period $P\gtrsim 2\,\mathrm{yr}$) transiting planet candidates in the four-year light curves from the prime \kepler\ mission. Most of the candidates show only one or two transits and have largely been missed in the original Kepler Object of Interest catalog. Our catalog is based on all known such candidates in the literature as well as new candidates from the search in this paper, and provides a resource to explore the planet population near the snow line of Sun-like stars. We homogeneously performed pixel-level vetting, stellar characterization with GAIA parallax and archival/Subaru spectroscopy, and light-curve modeling to derive planet parameters and to eliminate stellar binaries. The resulting clean sample consists of 67 planet candidates whose radii are typically constrained to 5\%, in which 23 are newly reported. The number of Jupiter-sized candidates (29 with $r>8\,R_\oplus$) in the sample is consistent with the Doppler occurrence. The smaller candidates are more prevalent (23 with $4 4\,R_\oplus$ have systematically higher [Fe/H] than the Kepler field stars, providing evidence that giant planet--metallicity correlation extends to $P>2\,\mathrm{yr}$.

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arXiv2019

The misaligned orbit of the Earth-sized planet Kepler-408b

Kepler-408 is one of the 33 planet-hosting {\it Kepler} stars for which asteroseismology has been used to investigate the orientation of the stellar rotation axis relative to the planetary orbital plane. The transiting "hot Earth," Kepler-408b, has an orbital period of 2.5 days and a radius of $0.86$~$R_\oplus$, making it much smaller than the planets for which spin-orbit alignment has been studied using the Rossiter-McLaughlin effect. Because conflicting asteroseismic results have been reported in the literature, we undertake a thorough re-appraisal of this system and perform numerous checks for consistency and robustness. We find that the conflicting results are due to the different models for the low-frequency noise in the power spectrum. A careful treatment of the background noise resolves these conflicts, and shows that the stellar inclination is $\is=42^{+5}_{-4}$ degrees. Kepler-408b is, by far, the smallest planet known to have a significantly misaligned orbit.

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arXiv2018

Larger mutual inclinations for the shortest-period planets

The {\it Kepler} mission revealed a population of compact multiple-planet systems with orbital periods shorter than a year, and occasionally even shorter than a day. By analyzing a sample of 102 {\it Kepler} and {\it K2} multi-planet systems, we measure the minimum difference $ΔI$ between the orbital inclinations, as a function of the orbital distance of the innermost planet. This is accomplished by fitting all the planetary signals simultaneously, constrained by an external estimate of the stellar mean density. We find $ΔI$ to be larger when the inner orbit is smaller, a trend that does not appear to be a selection effect. We find that planets with $a/R_\star$<5 have a dispersion in $ΔI$ of $6.7\pm 0.6$~degrees, while planets with $5 < a/R_\star < 12$ have a dispersion of $2.0\pm 0.1$~degrees. The planetary pairs with higher mutual inclinations also tend to have larger period ratios. These trends suggest that the shortest-period planets have experienced both inclination excitation and orbital shrinkage.

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