arXiv subjects

Takayuki Kotani

Publications and source records attributed to Takayuki Kotani.

At least 19 recordsLinked to original sources

arXiv2026

Probing Two-dimensional Asymmetries of an Exoplanet Atmosphere from Chromatic Transit Variation

We propose a new method for investigating atmospheric inhomogeneities in exoplanets through transmission spectroscopy. Our approach links chromatic variations in conventional transit model parameters (central transit time, total and full durations, and transit depth) to atmospheric asymmetries. By separately analyzing atmospheric asymmetries during ingress and egress, we can derive clear connections between these variations and the underlying asymmetries of the planetary limbs. Additionally, this approach enables us to investigate differences between the limbs slightly offset from the terminator on the dayside and the nightside. We applied this method to JWST's NIRSpec/G395H observations of the hot Saturn exoplanet WASP-39 b. Our analysis suggests a higher abundance of CO2 on the evening limb compared to the morning limb and indicates a greater probability of SO2 on the limb slightly offset from the terminator on the dayside relative to the nightside. These findings highlight the potential of our method to enhance the understanding of photochemical processes in exoplanetary atmospheres.

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arXiv2026

Revealing α-Element's Past with Subaru/IRD: Oxygen Abundance of 35 Very Metal-Poor Stars from Near-IR OH lines

Oxygen abundances in very and extremely metal-poor (V/EMP) stars provide critical constraints on early massive stars' nucleosynthesis. An Oxygen abundance analysis is presented for 35 V/EMP stars (-4.0<[Fe/H]< -1.5) using near-infrared H-band OH vibro-rotational lines from high-resolution Subaru/IRD spectra. To examine the reliability of these NIR OH lines, the results are compared with the abundances obtained from the 3D/NLTE-insensitive forbidden [OI] 6300Å line using archival high-resolution optical spectra. After homogeneously rederiving stellar parameters and 1D/NLTE Fe abundances using Gaia photo-astrometry and literature optical Fe equivalent width data, oxygen abundance from OH and [OI] lines is determined through 1D/LTE spectral synthesis. A sensitivity analysis confirms that near-IR OH lines are highly sensitive to the adopted temperature compared to the forbidden line. A temperature-dependent discrepancy between the tracers is identified: in cool red giants (Teff <4600 K), OH-based abundances are systematically lower than [OI]-based abundance by 0.05 to 0.25 dex, while warmer red giants show higher OH-based abundances as expected from 3D effects. Despite this systematic offset, the numerous measurable NIR OH lines yield significantly smaller random abundance errors than that of the single, weak [OI] line. Leveraging this statistical precision, an empirical calibration as a function of Teff, log g, [Fe/H], and [C/Fe] is derived to align the 1D/LTE OH abundances onto the [OI] scale. Applying this correction substantially reduces the scatter and temperature dependence in the [O/Fe] versus [Fe/H] plane and flattens the trend, bringing the results into fairly good agreement with Galactic chemical evolution models.

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arXiv2026

Japan's Possible Contributions for Coronagraph of the Habitable Worlds Observatory (HWO)

In this paper, we describe Japan's possible contributions for coronagraph of the Habitable Worlds Observatory (HWO) based on our independent study. We are considering to contribute to the HOW coronagraph by science and hardware, based on Japan's experience for the SPICA coronagraph instrument, contributions to the Nancy Grace Roman Space Telescope, and SCExAO for the Subaru telescope. Currently, hardware contributions of various scales, from large-scale to small components, are considered. As an example of the large-scale hardware case, the optical and mechanical layout of the entire infrared coronagraph is presented. Several individual high-contrast technologies are also briefly introduced, for which research is ongoing in Japan. In discussions, it is pointed out that both the inner working angle (IWA) and sensitivity are particularly critical for the NIR coronagraph. In this situation, dedicated observations of a small number of targets close to the solar system can be one of key science program in this situation, and designing consolidating science objectives, requirements, observation targets, and survey plans is important. It is essential to push the development of advanced coronagraphs that provide small IWAs. On the other hand, it is also necessary to prepare solutions that adopt more robust coronagraphs in parallel. How to coexist visible and NIR coronagraphs within constraints of volume, mass, budget etc. is an important issue. The international sharing for the coronagraph development should be carefully decided by international agreement. Although all of our studies may not be realized in contributions to the first generation of HWO instruments, we are considering Japan's multigenerational participation in the HWO to maximize outcomes of the HWO.

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arXiv2026

The mass of TOI-1883 b: A low density super-Neptune in the ridge regime transiting an early-M dwarf

Recent large-scale transit surveys conducted by space telescopes such as Kepler and TESS have revealed a vast number of exoplanets, uncovering the diversity of their population. One of the remarkable findings is the presence of a deficiency region in the period-radius distribution of short-period (< 10 days) Neptune-sized planets (4-8 Earth radii). This region is classified into the Neptune desert (< 3.2 days), the ridge (3.2-5.7 days), and the savanna (> 5.7 days) based on orbital period, each likely reflecting distinct evolutionary pathways. In this study, we used the InfraRed Doppler (IRD) instrument on the Subaru Telescope to determine the mass of the super-Neptune TOI-1883 b, which resides in the ridge region (P ~ 4.51 days) orbiting an M dwarf. We measured a planetary mass of Mp = 13.7 +6.8/-6.5 Earth masses and a mean density of 0.4 +0.3/-0.2 g cm^-3, with 3-sigma upper limits of 34.1 Earth masses, and 5-sigma upper limits of 47.7 Earth masses. These results suggest that TOI-1883 b is likely a low density super-Neptune. We also find that the boundary of the Neptune desert defined by planets orbiting FGK-type stars exhibits a similar distribution for planets around M-type stars. According to the population-based argument of Bourrier et al. (2025), this suggests that TOI-1883 b may have undergone disk-driven migration to reach its current orbit and experienced early atmospheric photoevaporation driven by strong stellar XUV irradiation. The derived planetary mass is comparable to or exceeds the conventional critical core mass. We suggest that the high metallicity of the host star ([Fe/H] = 0.32 +/- 0.18) may have suppressed the onset of runaway gas accretion. Furthermore, TOI-1883 b has a high Transmission Spectroscopy Metric (TSM > 140), making it an excellent target for future atmospheric characterization via transmission spectroscopy.

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arXiv2026

Discovery of a Low-Mass Companion to the Accelerating Star HIP 53005 with Strongly Conflicting Mass Estimates

We present the discovery of a low-mass companion located at $ρ$ $\sim$ 0\farcs{}85 ($r_{\rm proj} \approx 62~au$) from the early-type 1.2 Gyr-old star HIP 53005 using direct imaging data from the Subaru and Keck Telescopes and astrometry from the Hipparcos-Gaia Catalog of Accelerations. The companion, HIP 53005 C, is a component of a multiple system also including a $\approx$ 12\farcs{}4-separation M dwarf companion inducing a negligible proper motion acceleration. HIP~53005 C's position on color-magnitude diagrams, the fit of its spectral energy distribution to atmosphere models, and its location on an empirical mass-magnitude diagram all suggest that it lies at the M/L transition and near the hydrogen-burning limit ($\sim80~M_{\rm Jup}$). However, our orbital fitting combining direct-imaging relative astrometry with proper motion acceleration favors a much higher dynamical mass of $\sim185\ M_{\rm Jup}$. An additional unseen, more closely-orbiting companion below the detection limit (at $ρ\lesssim0\farcs2$)) may explain this discrepancy. Alternatively, HIP~53005C could be a low-mass binary like Gliese~229Bab, making this system an intriguing laboratory for studying multiple star formation.

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arXiv2026

PyIRD: A Python-Based Data Reduction Pipeline for Subaru/IRD and REACH

PyIRD is a Python-based pipeline for reducing spectroscopic data obtained with IRD (InfraRed Doppler; Kotani et al. (2018)) and REACH (Rigorous Exoplanetary Atmosphere Characterization with High dispersion coronagraphy; Kotani et al. (2020)) on the Subaru Telescope. It is designed to process raw images into one-dimensional spectra in a semi-automatic manner. Unlike traditional methods, it does not rely on IRAF (Tody, 1986; 1993), a software used for astronomical data reduction. This approach simplifies the workflow while maintaining efficiency and accuracy. Additionally, the pipeline includes an updated method for removing readout noise patterns from raw images, enabling efficient extraction of spectra even for faint targets such as brown dwarfs. The code is open source and available at https://github.com/prvjapan/pyird .

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arXiv2025

Direct Imaging Explorations for Companions from the Subaru/IRD Strategic Program II; Discovery of a Brown-dwarf Companion around a Nearby mid-M~dwarf LSPM~J1446+4633

We report the discovery of a new directly-imaged brown dwarf companion with Keck/NIRC2+pyWFS around a nearby mid-type M~dwarf LSPM~J1446+4633 (hereafter J1446). The $L'$-band contrast ($4.5\times10^{-3}$) is consistent with a $\sim20-60\ M_{\rm Jup}$ object at 1--10~Gyr and our two-epoch NIRC2 data suggest a $\sim30\%$ ($\sim3.1σ)$ variability in its $L'$-band flux. We incorporated Gaia DR3 non-single-star catalog into the orbital fitting by combining the Subaru/IRD RV monitoring results, NIRC2 direct imaging results, and Gaia proper motion acceleration. As a result, we derive ${59.8}_{-1.4}^{+1.5}\ M_{\rm Jup}$ and $\approx4.3~{\rm au}$ for the dynamical mass and the semi-major axis of the companion J1446B, respectively. J1446B is one of the intriguing late-T~dwarfs showing variability at $L'$-band for future atmospheric studies with the constrained dynamical mass. Because the J1446 system is accessible with various observation techniques such as astrometry, direct imaging, and high-resolution spectroscopy including radial velocity measurement, it has a potential as a great benchmark system to improve our understanding for cool dwarfs.

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arXiv2025

Unveiling the Atmosphere of HR 7672 B from the Near-Infrared High-Resolution Spectrum Using REACH/Subaru

Characterizing the atmospheres of exoplanets and brown dwarfs is crucial for understanding their atmospheric physics and chemistry, searching for biosignatures, and investigating their formation histories. Recent advances in observational techniques, combining adaptive optics with high-resolution spectrographs, have enabled detailed spectroscopic analysis for directly imaged faint companions. In this paper, we report an atmospheric retrieval on the L-type brown dwarf HR 7672 B using a near-infrared high-contrast high-resolution spectrograph, REACH (Y, J, H band, $R\sim100,000$), which combines SCExAO with IRD at the Subaru Telescope. Our model, developed based on the ExoJAX spectrum code, simultaneously accounts for several factors, including the presence of clouds in the L dwarf's atmosphere as well as contamination from the host star's light and telluric absorption lines in the observed spectra. Our analysis identified H2O and FeH as the primary absorbers in the observed J- and H-band spectra. Additionally, the observed features were reproduced with a model that includes cloud opacity, assuming an optically thick cloud at the pressure $P_\mathrm{top}$. The resulting temperature at the cloud top pressure suggests the potential formation of clouds composed of TiO2, Al2O3, or Fe. This study is the first science demonstration for faint spectra obtained by REACH, providing a foundation for future investigations into the atmospheres of exoplanets and brown dwarfs.

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arXiv2025

The mass of TOI-654 b: A short-period sub-Neptune transiting a mid-M dwarf

Sub-Neptunes are small planets between the size of the Earth and Neptune. The orbital and bulk properties of transiting sub-Neptunes can provide clues for their formation and evolution of small planets. In this paper, we report on follow-up observations of a planetary system around the mid-M dwarf TOI-654, whose transiting sub-Neptune TOI-654 b ($P=1.53$ day) is validated as a suitable target for the atmospheric observation. We measure the planetary mass and stellar properties with the InfraRed Doppler instrument (IRD) mounted on the Subaru telescope and obtain the stellar and planetary properties from additional transit observations by the Transit Exoplanetary Survey Satellite (TESS) and a series of the Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT). As a result, the planetary mass of TOI-654 b is determined to be $M_{\rm p} = 8.71 \pm 1.25 M_{\oplus}$, and the radius is updated to be $R_{\rm p} = 2.378 \pm 0.089 R_{\oplus}$. The bulk density suggests that the planet is composed of a rocky and volatile-rich core or a rocky core surrounded by a small amount of H/He envelope.TOI-654 b is one of unique planets located around the radius valley and and also on the outer edge of the Neptune desert. The precise mass determination enables us to constrain the atmospheric properties with future spectroscopic observations especially for the emission by the James Webb Space Telescope and Ariel.

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arXiv2025

Atmospheric retrieval of Subaru/IRD high-resolution spectrum of the archetype T-type brown dwarf Gl 229 B

Brown dwarfs provide a unique opportunity to study atmospheres and their physical and chemical processes with high precision, especially in temperature ranges relevant to exoplanets. In this study, we performed high-resolution ($R \sim 70,000$) spectroscopy using Subaru/IRD ($Y$, $J$, $H$-bands) of the T7.0p-type object Gl 229 B, the first discovered T-type brown dwarf, which orbits an M1V host star at a separation of 33 au. We conducted atmospheric retrieval on the reduced $H$-band spectrum using the high-resolution spectrum model compatible with automatic differentiation and GPU, ExoJAX. In contrast to previous retrieval studies on medium-resolution spectra, we obtained a C/O ratio consistent with that of the host star, aligning with the expected formation process for such a massive brown dwarf. Additionally, based on the strong constraint on temperature from the high-resolution spectrum and previously measured photometric magnitude, our analysis indicates that Gl 229 B is a binary, which was also proposed by Brandt et al. (2021) and recently confirmed by Xuan et al. (2024). Finally, we validated current molecular line lists by leveraging the obtained high-resolution, high signal-to-noise ratio spectrum of this warm ($\sim 900$ K) atmosphere. This study highlights the importance of observing companion brown dwarfs as benchmark objects for establishing characterization techniques for low-mass objects and enhancing our understanding of their atmospheres, given the wealth of available information and the relative ease of observation.

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arXiv2025

Differentiable Modeling of Planet and Substellar Atmosphere: High-Resolution Emission, Transmission, and Reflection Spectroscopy with ExoJAX2

Modeling based on differentiable programming holds great promise for astronomy, enabling advanced techniques such as gradient-based posterior sampling and optimization. This paradigm motivated us to develop ExoJAX (Kawahara et al. 2022), the first auto-differentiable spectrum model of exoplanets and brown dwarfs. ExoJAX directly calculates cross-sections as functions of temperature and pressure to minimize interpolation errors in high-dispersion spectra, although initial work focused on narrowband emission spectroscopy. Here, we introduce a fast, memory-efficient opacity algorithm and differentiable radiative transfer for emission, transmission, and reflection spectroscopy. In the era of data-rich JWST observations, retrieval analyses are often forced to bin high-resolution spectra due to computational bottlenecks. The new algorithm efficiently handles native-resolution data, preserving the full information content and dynamic range. The advances proposed in this paper enable broader applications, demonstrated by retrievals of GL229 B's high-dispersion emission, WASP-39 b's JWST mid-resolution transmission at original resolution (R $\sim$ 2,700), and Jupiter's reflection spectrum. We derive a C/O ratio for GL229 B consistent with its host star, constrain WASP-39 b's radial velocity from molecular line structures, and infer Jupiter's metallicity in line with previous estimates.

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arXiv2025

Measurement of Methane Line Broadening in Hot Hydrogen/Helium Atmospheres at $λ$ = 1.60-1.63 μm for Substellar Object Spectroscopy

Recent high-dispersion spectroscopy from ground-based telescopes and high-precision spectroscopy from space observatories have enabled atmospheric observations of substellar objects, such as brown dwarfs and hot gaseous exoplanets, with sufficient precision to make ambient gas differences in molecular line broadening a significant factor. In this paper, we experimentally measured the pressure broadening of methane in a high-temperature hydrogen-helium background atmosphere in the H band, which had not been previously measured. The experiment used glass cells, inserted in a tube furnace, filled with methane in a hydrogen-helium background atmosphere or pure methane gas. Spectra were obtained at four temperatures ranging from room temperature to 1000 K, in the wavelength range 1.60-1.63 $μ$m, using a tunable laser, yielding eight high-resolution spectra in total. A full Bayesian analysis was performed on the obtained spectra, using the differentiable spectral model ExoJAX and the Hamiltonian Monte Carlo for inferring a large number of parameters, allowing us to infer the H2/He pressure broadening for 22 transitions mainly in the R-branch of the 2$ν_3$ band. As a result, we found a temperature exponent of approximately 0.27 and a reference width at 296 K of around 0.040 for $ J_{lower} $ = 13-20. This temperature dependency is much milder than that provided by the molecular database ExoMol, yielding a line width approximately 5-45% smaller than ExoMol at 296 K, but similar at 1000 K. Our results suggest the need for further accumulation of experimental data for spectral analysis of substellar objects with hydrogen-helium atmospheres.

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arXiv2025

Elemental abundances of 44 very metal-poor stars determined from Subaru/IRD near-infrared spectra

Abundances of five elements, Na, Mg, Al, Si, and Sr, are investigated for 44 very metal-poor stars (-4.0 < [Fe/H] < -1.5) in the Galactic halo system based on an Local Thermodinamic Equilibrium (LTE) analysis of high-resolution near-infrared spectra obtained with the Infrared Doppler instrument (IRD) on the Subaru Telescope. Mg and Si abundances are determined for all 44 stars. The Si abundances are determined from up to 29 lines, which provide reliable abundance ratios compared to previous results from a few optical lines. The Mg and Si of these stars are over-abundant, relative to iron, and are well-explained by chemical-evolution models. No significant scatter is found in the abundance ratios of both elements with respect to iron, except for a few outliers. The small scatter of the abundance ratios of these elements provides constraints on the variations of stellar and supernova's yields at very low metallicity. Al abundances are determined for 27 stars from near-infrared lines (e.g., 1312nm), which are expected to be less affected by non-LTE (NLTE) effects than optical resonance lines. The average of the [Al/Fe] ratios is close to the solar value, and no dependence on metallicity is found over -3.0 < [Fe/H] < -2.0. Na abundances are determined for 12 stars; they exhibit Solar abundance ratios and no dependence on metallicity. The Sr abundances determined from the Sr II triplet are significantly higher than those from the optical resonance lines obtained by previous studies for our sample. This discrepancy shows a clear dependence on temperature and surface gravity, supporting models that predict large NLTE effects on the near-infrared lines for metal-poor red giants.

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arXiv2024

Revisiting Near-Infrared Features of Kilonovae: The Importance of Gadolinium

The observation of the kilonova AT2017gfo and investigations of its light curves and spectra confirmed that neutron star mergers are sites of r-process nucleosynthesis. However, the identification of elements responsible for the spectral features is still challenging, particularly at the near-infrared wavelengths. In this study, we systematically searched for all possible near-infrared transitions of heavy elements using experimentally calibrated energy levels. Our analysis reveals that most candidate elements with strong absorption lines are lanthanides (Z=57-71) and actinides (Z=89-103). This is due to their complex structures leading to many low-lying energy levels, which results in strong transitions in the near-infrared range. Domoto et al. (2022) have shown that La III and Ce III can explain the absorption features at $λ\sim$ 12,000 - 15,000 A. While our results confirm that these two elements show strong infrared features, we additionally identify Gd III as the next most promising species. Due to its unique atomic structure involving the half-filled 4f and the outer 5d orbitals, Gd III has one of the lowest-lying energy levels, between which relatively strong transitions occur. We also find absorption lines caused by Gd III in the near-infrared spectrum of a chemically peculiar star HR 465, which supports their emergence in kilonova spectra. By performing radiative transfer simulations, we confirm that Gd III lines affect the feature at $\sim$ 12,000 A previously attributed to La III. Future space-based time-series observations of kilonova spectra will allow the identification of Gd III lines.

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arXiv2024

Spectroscopy using a visible photonic lantern at the Subaru telescope: Laboratory characterization and first on-sky demonstration on Ikiiki (α Leo) and `Aua (α Ori)

Photonic lanterns are waveguide devices enabling high throughput single mode spectroscopy and high angular resolution. We aim to present the first on-sky demonstration of a photonic lantern (PL) operating in visible light, to measure its throughput and assess its potential for high-resolution spectroscopy of compact objects. We used the SCExAO instrument (a double stage extreme AO system installed at the Subaru telescope) and FIRST mid-resolution spectrograph (R 3000) to test the visible capabilities of the PL on internal source and on-sky observations. The best averaged coupling efficiency over the PL field of view was measured at 51% +/- 10% with a peak at 80%. We also investigate the relationship between coupling efficiency and the Strehl ratio for a PL, comparing them with those of a single-mode fiber (SMF). Findings show that in the AO regime, a PL offers better coupling efficiency performance than a SMF, especially in the presence of low spatial frequency aberrations. We observed Ikiiki (alpha Leo - mR = 1.37) and `Aua (alpha Ori - mR = -1.17) at a frame rate of 200 Hz. Under median seeing conditions (about 1 arcsec measured in H band) and large tip/tilt residuals (over 20 mas), we estimated an average light coupling efficiency of 14.5% +/- 7.4%, with a maximum of 42.8% at 680 nm. We were able to reconstruct both star's spectra, containing various absorption lines. The successful demonstration of this device opens new possibilities in terms of high throughput single-mode fiber-fed spectroscopy in the Visible. The demonstrated on-sky coupling efficiency performance would not have been achievable with a single SMF injection setup under similar conditions, partly because the residual tip/tilt alone exceeded the field of view of a visible SMF (18 mas at 700 nm). Thus emphasizing the enhanced resilience of PL technology to such atmospheric disturbances. The additional

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arXiv2024

Weighing Single-lined Spectroscopic Binaries Using Tidal Effects on Radial Velocities: The Case of V723 Monocerotis

In single-lined spectroscopic binaries (SB1s) where flux variations due to tidal deformation of the primary star (ellipsoidal variations, EVs) are detected, the binary mass can be determined by combining EVs with the primary's radial velocity (RV) variations from orbital motion and information about the primary's radius. This method has been used for mass estimation in close binaries including X-ray systems, but it has been pointed out that contaminating light from sources other than the primary star could introduce systematic errors in the mass and inclination estimates. Here, we focus on the apparent RV variations caused by asymmetric distortion of the absorption lines of the tidally deformed primary star (tidal RV). Because this signal contains information equivalent to that from photometric EVs, it enables mass estimation of the binary system using only the primary star's absorption lines from high-resolution spectroscopic data, providing a potentially more robust approach against contaminating light. We apply the method to the binary system V723 Monocerotis, where both photometric EV and tidal RV signals are detected, and successfully determine the component masses using only the primary star's RVs and projected rotational velocity, without relying on absolute flux measurements or on stellar evolutionary models. The masses derived from the tidal RV model show a reasonable agreement with those obtained from EVs after carefully modeling the flux contamination from the secondary. This result demonstrates that tidal RVs provide a useful alternative means for mass estimation in SB1s.

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arXiv2024

Photonic beam-combiner for visible interferometry with SCExAO/FIRST: laboratory characterization and design optimization

Integrated optics are used to achieve astronomical interferometry inside robust and compact materials, improving the instrument's stability and sensitivity. In order to perform differential phase measurements at the H$α$ line (656.3 nm) with the 600-800 nm spectro-interferometer FIRST, a photonic integrated circuit (PIC) is being developed in collaboration with TEEM Photonics. This PIC performs the interferometric combination of the beams coming from sub-apertures selected in the telescope pupil, thus implementing the pupil remapping technique to restore the diffraction limit of the telescope. In this work, we report on the latest developments carried out within the FIRST project to produce a high performance visible PIC. The PICs are manufactured by TEEM Photonics, using their technology based on $K_+:Na_+$ ion exchange in glass. The first part of the study consists in the experimental characterization of the fundamental properties of the waveguides, in order to build an accurate model, which is the basis for the design of more complex functions. In the second part, theoretical designs and their optimization for three types of combiner architectures are presented: symmetric directional coupler, asymmetric directional couplers and ABCD cells including achromatic phase shifters.

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arXiv2024

Photonic chip for visible interferometry: laboratory characterization and comparison with the theoretical model

Integrated optics are used to achieve astronomical interferometry inside robust and compact materials, improving the instruments stability and sensitivity. In order to perform differential phase measurements at the H$α$ line (656.3 nm) with the 600-800 nm spectro-interferometer FIRST, a photonic integrated circuit (PIC) is being developed. This PIC performs the visible combination of the beams coming from the telescope pupil sub-apertures. In this work, TEEM Photonics waveguides fabricated by $K_+:Na_+$ ion exchange in glass are characterized in terms of single-mode range and mode field diameter. The waveguide diffused index profile is modeled on Beamprop software. FIRST beam combiner building blocks are simulated, especially achromatic directional couplers and passive $π/2$ phase shifters for a potential ABCD interferometric combination.

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