arXiv subjects

Shotaro Tada

Publications and source records attributed to Shotaro Tada.

5 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.

astro-ph.EP↗

arXiv2026

A Radon Transform Perspective on Exoplanet Transits

Transit light curves are usually analyzed under the assumption that the transiting planet has a circular sky-projected silhouette. However, planetary rotation, tides, rings, or atmospheric inhomogeneities can produce non-circular silhouettes. This raises the question of what information transit light curves can provide about the underlying two-dimensional attenuation map. In this paper, we show that, in a simple and transparent limit, the time derivative of the transit light curve during ingress or egress can be interpreted as a Radon-transform measurement of the planetary attenuation map, with the projection direction set by the local normal to the stellar limb. This viewpoint makes the information content of a single transit clear. Ingress and egress provide at most two projection angles, so the data constrain the Fourier transform of the attenuation map only along at most two radial slices, leaving a large null space. Physical constraints on the attenuation values and shape priors can reduce the range of viable solutions, but non-uniqueness generally remains. We further examine how realistic effects modify this picture. In particular, small stellar-limb curvature introduces weak sensitivity to transverse Fourier-space structure around the ideal slices, a sensitivity that is absent in the strict Radon-transform limit. These results provide a framework for understanding what transit light curves can and cannot reveal about non-circular planetary silhouettes.

astro-ph.EP↗

arXiv2026

DOLPHIN (Device of Line Photometry in Nasmyth): A Switchable Multiband Photometer for the ISAS 1.3-m Telescope

We present the design, construction, and commissioning of DOLPHIN (Device of Line Photometry in Nasmyth), a compact instrument for switchable broad- and narrow-band photometry at the Nasmyth focus of the ISAS 1.3-m telescope. DOLPHIN was developed as a cost-conscious platform for differential measurements of selected spectral features with modest-aperture telescopes. A motorized eight-position filter wheel provides access to Johnson-Bessell B, V, and R bands and narrow-band Na D and H$α$ filters. All filters share a common optical path and CMOS detector, and adjacent positions can be switched in approximately 0.8 s. The system uses commercially available components and provides an approximately 10-arcmin field of view. Laboratory characterization shows that the detector response is linear to within 0.1% below 10,000 ADU, with a conversion factor of 0.81 $e^-$/ADU, read noise of 9 $e^-$, and pixel-response non-uniformity of 0.37%. We also measure the in-system transmission profiles of all filters and identify a broader-than-specified Na D passband. Commissioning observations of HD 189733 are used to validate filter-sequence operation, image calibration, and relative photometric stability. The sequence used 1-s exposures in B, V, and R and 20-s exposures in Na D and H$α$. The unbinned RMS ranges from 0.34% in Na D to 0.94% in B; after eight-point binning, the Na D/R ratio reaches 0.16%. Positive inter-band correlations that increase with bin size demonstrate removal of part of the low-frequency common-mode noise. Recovery of the known broadband transit provides an end-to-end validation of the observing and reduction system. These results establish DOLPHIN as a practical platform for differential narrow-band monitoring and define the instrumental limitations to be addressed in future observations.

astro-ph.IM↗

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.

astro-ph.EP↗

arXiv2024

JASMINE: Near-Infrared Astrometry and Time Series Photometry Science

Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is the Galactic archaeology with Galactic Center Survey, which aims to reveal the Milky Way's central core structure and formation history from Gaia-level (~25 $μ$as) astrometry in the Near-Infrared (NIR) Hw-band (1.0-1.6 $μ$m). The other is the Exoplanet Survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information of the stars in the Galactic center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars and microlensing studies, including discovery of (intermediate mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions.

astro-ph.IM↗