arXiv subjects

Yuichi Ito

Publications and source records attributed to Yuichi Ito.

At least 19 recordsLinked to original sources

arXiv2026

A Search for helium in the atmospheres of three sub-Neptunes and a super-Earth around M-dwarfs

Thousands of sub-Neptunes have been discovered mainly through space-based surveys such as Kepler and TESS. Their bulk compositions and internal structures are thought to reflect their formation and evolutionary pathways, and atmospheric observations provide constraints on these processes. The near-infrared helium triplet is a potential tracer of extended, escaping H/He atmospheres. Recent models that include geometric effects suggest that planets orbiting nearby late M dwarfs may offer favorable conditions for detecting this signal. Nevertheless, helium has been reported for only three planets around M dwarfs to date. We conducted high-resolution transmission spectroscopy of three sub-Neptunes (TOI-2136b, TOI-654b, and LP 791-18c) and a super-Earth (TOI-1634b) orbiting M dwarfs with the InfraRed Doppler (IRD) spectrograph on the Subaru Telescope. We find no statistically significant helium absorption in any target; accordingly, we derive 95% confidence upper limits on the helium line depth of 1.36%, 0.60%, 2.07%, and 3.00%, and on the equivalent width of 7.3, 2.1, 7.4, and 9.1 m\r{A}, for TOI-2136b, TOI-1634b, TOI-654b, and LP 791-18c, respectively. We further explored constraints on the upper-atmospheric temperature and mass-loss rate by comparing these results with isothermal Parker-wind models. While we have compared with self-consistent ATES models of primordial H/He atmospheres spanning a range of assumed X-ray luminosities, changes in the assumed XUV flux do not appear to account for the non-detections. The results suggest that these planets have metal-enriched H/He primary atmospheres or non-primordial atmospheres, such as water-rich envelopes. Future observations of other absorption lines, such as Lyman-$\alpha$, H-$\alpha$, and H$_2$O, may provide further constraints on these atmospheres.

astro-ph.EP↗

arXiv2026

Coupling magma-ocean and atmospheres in spectral retrievals of sub-Neptunes

Recent high-precision atmospheric observations with JWST is enabling detailed characterization of sub-Neptune atmospheres and motivating efforts to understand and constrain their interiors. Theoretical studies suggest that sub-Neptunes possibly host hydrogen-dominated atmospheres that are chemically coupled with an underlying magma ocean. However, a quantitative retrieval framework directly linking atmospheric spectra to magma ocean properties has yet to be established. Here we introduce MELTYQ, a coupled magma-atmosphere retrieval framework that links transmission spectra to the oxidation state and volatile inventory of underlying magma oceans. MELTYQ combines a magma-atmosphere equilibrium model, which includes the solubility of H-/O-/C-/N-bearing species in the melt and redox reactions, with a Bayesian spectral retrieval scheme. Using simulated retrieval tests, we validate the approach and show that magma redox state and volatile content can be constrained under favorable observational conditions. As a proof of concept, we apply MELTYQ to JWST transmission spectra of the benchmark sub-Neptunes K2-18 b and TOI-270 d. We find that coupled magma-atmosphere retrievals are generally capable of reproducing the observed spectra of these planets. However, we identify several key limitations in the current framework. Specifically: more flexible free-retrieval approaches remain statistically preferred; the CO/CO$_2$ absorption feature near 4.5 $\mu$m for TOI-270 d is not fully captured; and a number of underlying model assumptions may not be strictly valid. Nevertheless, embedding coupled magma-atmosphere models directly within Bayesian retrievals enables quantitative assessment of degeneracies and sensitivities, establishing a pathway for directly connecting atmospheric spectra to magma composition in this underexplored exoplanet regime.

astro-ph.EP↗

arXiv2025

On the synergetic use of Ariel and JWST for exoplanet atmospheric science

This paper explores the potential for strategic synergies between the JWST and the Ariel telescopes, two flagship observatories poised to revolutionise the study of exoplanet atmospheres. Both telescopes have the potential to address common fundamental questions about exoplanets-especially concerning their nature and origins-and serve a growing scientific community. With their operations now anticipated to overlap, starting from 2030, there is a unique opportunity to enhance the scientific outputs of both observatories through coordinated efforts. In this report, authored by the Ariel-JWST Synergy Working Group, part of the Ariel Consortium Science Team, we summarise the capabilities of JWST and Ariel; we highlight their key differences, similarities, synergies, and distinctive strengths. Ariel is designed to conduct a broad survey of exoplanet atmospheres but remains highly flexible, allowing the mission to integrate insights from JWST's discoveries. Findings from JWST, including data from initiatives shaped by NASA's decadal survey priorities and community-driven research themes, will inform the development of Ariel's core survey strategy. Conversely, Ariel's ability to perform broad-wavelength coverage observations for bright targets provides complementary avenues for exoplanet researchers, particularly those interested in time-domain observations and large-scale atmospheric studies. This paper identifies key pathways for fostering JWST-Ariel synergies, many of which can be initiated even before Ariel's launch. Leveraging their complementary designs and scopes, JWST and Ariel can jointly address fundamental questions about the nature, formation, and evolution of exoplanets. Such strategic collaboration has the potential to maximise the scientific returns of both observatories and lay the foundation for future facilities in the roadmap to exoplanet exploration.

astro-ph.IM↗

arXiv2025

Exoplanet Atmospheric Escape Observations with the Habitable Worlds Observatory

The Decadal Survey on Astronomy and Astrophysics 2020 highlights the importance of advancing research focused on discovering and characterizing habitable worlds. In line with this priority, our goal is to investigate how planetary systems evolve through atmospheric escape and to develop methods for identifying potentially Earth-like planets. By leveraging the ultraviolet (UV) capabilities of the Habitable Worlds Observatory (HWO), we can use transit spectroscopy to observe atmospheric escape in exoplanets and explore the processes that shape their evolution, assess the ability of small planets to retain their atmospheres, and search for signs of Earth-like atmospheres. To achieve this, we support the development of a UV spectrograph with moderate- to high-resolution capabilities for point-source observations, coverage of key spectral features in the 100-300 nm range, and detectors that can register high count rates reliably. This article is an adaptation of a science case document developed for the Characterizing Exoplanets Steering Committee within HWO's Solar Systems in Context Working Group.

astro-ph.IM↗

arXiv2025

Monosilane Worlds: Sub-Neptunes with Atmospheres Shaped by Reduced Magma Oceans

High-precision infrared spectroscopic measurements now enable detailed characterization of sub-Neptune atmospheres, potentially providing constraints on their interiors. Motivated by this, atmospheric models have been developed to explore chemical interactions between hydrogen-dominated atmospheres and possibly underlying magma oceans with various redox states. Recent models have predicted monosilane (SiH$_4$) as a potential atmospheric species derived from magma oceans in sub-Neptunes, but suggested that it is highly depleted in the observable atmospheric layers. Here, we propose that SiH$_4$ can persist throughout the atmospheres of sub-Neptunes with FeO-free reduced magma oceans by considering the dissolution of H$_2$O into the magma oceans, a factor not accounted for in previous models. We construct a one-dimensional atmospheric model to simulate the chemical equilibrium composition of hydrogen-dominated atmospheres overlying FeO-free magma oceans, incorporating H-O-Si chemistry. Our results show that the dissolution of H$_2$O enhances the SiH$_4$ molar fraction to levels of 0.1--10~\%, preventing it from reverting to silicates in the upper atmospheric layers. We find that SiH$_4$-rich atmospheres can exist across a broad parameter space with ground temperatures of 2000--6000~K and hydrogen pressures of 10$^2$--10$^5$~bar. We discuss that SiH$_4$-rich atmospheres could contain the other silanes but lack C-/N-/O-bearing species. The detection of SiH$_4$ in future observations of sub-Neptunes would provide compelling evidence for the presence of a rocky core with a reduced magma ocean. However, the accuracy of our model is limited by the lack of data on the non-ideal behavior and radiative properties of SiH$_4$, highlighting the need for further numerical and laboratory investigations.

astro-ph.EP↗

arXiv2024

Role of magma oceans in controlling carbon and oxygen of sub-Neptune atmospheres

Most exoplanets with a few Earth radii are more inflated than bare-rock planets with the same mass, indicating a substantial volatile amount. Neither the origin of the volatiles nor the planet's bulk composition can be constrained from the mass-radius relation alone, and the spectral characterization of their atmospheres is needed to solve this degeneracy. Previous studies showed that chemical interaction between accreted volatile and possible molten rocky surface (i.e., magma ocean) can greatly affects the atmospheric composition. However, a variety in the atmospheric compositions of such planets with different properties remains elusive. In this work, we examine the dependence of atmospheric H, O, and C on planetary parameters (atmospheric thickness, planetary mass, equilibrium temperature, and magma properties such as redox state) assuming nebula gas accretion on an Earth-like core, using an atmosphere-magma chemical equilibrium model. Consistent with previous work, we show that atmospheric $\rm H_{2}O$ fraction on a fully molten rocky interior with an Earth-like redox state is on the order of $10^{-2}$-$10^{-1}$ regardless of other planetary parameters. Despite the solubility difference between H- and C-bearing species, C/H increases only a few times above the nebula value except for atmospheric pressure $\lesssim$1000 bar and $\rm H_{2}O$ fraction $\gtrsim$10\%. This results in a negative O/H-C/O trend and depleted C/O below one-tenth of the nebula gas value under an oxidized atmosphere, which could provide a piece of evidence of rocky interior and endogenic water. We also highlight the importance of constraints on the high-pressure material properties for interpreting the magma-atmospheric interaction of inflated planets.

astro-ph.EP↗

arXiv2024

A secondary atmosphere on the rocky exoplanet 55 Cancri e

Characterizing rocky exoplanets is a central endeavor of astronomy, and yet the search for atmospheres on rocky exoplanets has hitherto resulted in either tight upper limits on the atmospheric mass or inconclusive results. The 1.95-REarth and 8.8-MEarth planet 55 Cnc e, with a predominantly rocky composition and an equilibrium temperature of ~2000 K, may have a volatile envelope (containing molecules made from a combination of C, H, O, N, S, and P elements) that accounts for up to a few percent of its radius. The planet has been observed extensively with transmission spectroscopy, and its thermal emission has been measured in broad photometric bands. These observations disfavor a primordial H2/He-dominated atmosphere but cannot conclusively determine whether the planet has a secondary atmosphere. Here we report a thermal emission spectrum of the planet obtained by JWST's NIRCam and MIRI instruments from 4 to 12 μm. The measurements rule out the scenario where the planet is a lava world shrouded by a tenuous atmosphere made of vaporized rock, and indicate a bona fide volatile atmosphere likely rich in CO2 or CO. This atmosphere can be outgassed from and sustained by a magma ocean.

astro-ph.EP↗

arXiv2024

Towards atmospheric retrievals of panchromatic light-curves: ExPLOR-ing generalized inversion techniques for transiting exoplanets with JWST and Ariel

Conventional atmospheric retrieval codes are designed to extract information, such as chemical abundances, thermal structures and cloud properties, from fully "reduced" spectra obtained during transit or eclipse. Reduced spectra, however, are assembled by fitting a series of simplified light-curves to time series observations, wavelength-by-wavelength. Thus, spectra are a post-processed summary statistics of the original data, which by definition does not encode all the available information (i.e., astrophysical signal, model covariance, instrumental noise). Here, we explore an alternative inversion strategy where the atmospheric retrieval is performed on the light-curve directly -- i.e., closer to the data. This method is implemented in ExPLOR (EXoplanet Panchromatic Light-curve Observation and Retrieval), a novel atmospheric retrieval code inheriting from the TauREx project. By explicitly considering time in the model, ExPLOR naturally handles transit, eclipse, phase-curve and other complex geometries for transiting exoplanets. In this paper, we have validated this new technique by inverting simulated panchromatic light-curves. The model was tested on realistic simulations of a WASP-43b like exoplanet as observed with the James Webb Space Telescope (JWST) and Ariel telescope. By comparing our panchromatic light-curve approach against conventional spectral retrievals on mock scenarios, we have identified key breaking points in information and noise propagation when employing past literature techniques. Throughout the paper, we discuss the importance of developing "closer-to-data" approaches such as the method presented in this work, and highlight the inevitable increase in model complexity and computing requirements associated with the recent JWST revolution.

astro-ph.EP↗

arXiv2024

Numerical performance of correlated-k distribution method in atmospheric escape simulation

Atmospheric escape is crucial to understand the evolution of planets in and out of the Solar system and to interpret atmospheric observations. While hydrodynamic escape simulations have been actively developed incorporating detailed processes such as UV heating, chemical reactions, and radiative cooling, the radiative cooling by molecules has been treated as emission from selected lines or rotational/vibrational bands to reduce its numerical cost. However, ad hoc selections of radiative lines would risk estimating inaccurate cooling rates because important lines or wavelengths for atmospheric cooling depend on emitting conditions such as temperature and optical thickness. In this study, we apply the correlated-k distribution (CKD) method to cooling rate calculations for H$_2$-dominant transonic atmospheres containing H$_2$O or CO as radiative species, to investigate its numerical performance and the importance of considering all lines of the molecules. Our simulations demonstrate that the sum of weak lines, which provides only 1 % of the line emission energy in total at optically thin conditions, can become the primary source of radiative cooling in optically thick regions, especially for H$_2$O-containing atmospheres. Also, in our hydrodynamic simulations, the CKD method with a wavelength resolution of 1000 is found to be effective, allowing the calculation of escape rate and temperature profiles with acceptable numerical cost. Our results show the importance of treating all radiative lines and the usefulness of the CKD method in hydrodynamic escape simulations. It is particularly practical for heavy-element-enriched atmospheres considered in small exoplanets, including super-Earths, without any prior selections for effective lines.

astro-ph.EP↗

arXiv2023

YunMa: Enabling Spectral Retrievals of Exoplanetary Clouds

In this paper, we present YunMa, an exoplanet cloud simulation and retrieval package, which enables the study of cloud microphysics and radiative properties in exoplanetary atmospheres. YunMa simulates the vertical distribution and sizes of cloud particles and their corresponding scattering signature in transit spectra. We validated YunMa against results from the literature. When coupled to the TauREx 3 platform, an open Bayesian framework for spectral retrievals, YunMa enables the retrieval of the cloud properties and parameters from transit spectra of exoplanets. The sedimentation efficiency ($f_{\mathrm{sed}}$), which controls the cloud microphysics, is set as a free parameter in retrievals. We assess the retrieval performances of YunMa through 28 instances of a K2-18 b-like atmosphere with different fractions of H$_2$/He and N$_2$, and assuming water clouds. Our results show a substantial improvement in retrieval performances when using YunMa instead of a simple opaque cloud model and highlight the need to include cloud radiative transfer and microphysics to interpret the next-generation data for exoplanet atmospheres. This work also inspires instrumental development for future flagships by demonstrating retrieval performances with different data quality.

astro-ph.EP↗

arXiv2021

Ariel Planetary Interiors White Paper

The recently adopted Ariel ESA mission will measure the atmospheric composition of a large number of exoplanets. This information will then be used to better constrain planetary bulk compositions. While the connection between the composition of a planetary atmosphere and the bulk interior is still being investigated, the combination of the atmospheric composition with the measured mass and radius of exoplanets will push the field of exoplanet characterisation to the next level, and provide new insights of the nature of planets in our galaxy. In this white paper, we outline the ongoing activities of the interior working group of the {\it Ariel} mission, and list the desirable theoretical developments as well as the challenges in linking planetary atmospheres, bulk composition and interior structure.

astro-ph.IM↗

arXiv2021

Detectability of Rocky-Vapour Atmospheres on Super-Earths with Ariel

Ariel will mark the dawn of a new era as the first large-scale survey characterising exoplanetary atmospheres with science objectives to address fundamental questions about planetary composition, evolution and formation. In this study, we explore the detectability of atmospheres vaporised from magma oceans on dry, rocky Super-Earths orbiting very close to their host stars. The detection of such atmospheres would provide a definitive piece of evidence for rocky planets but are challenging measurements with currently available instruments due to their small spectral signatures. However, some of the hottest planets are believed to have atmospheres composed of vaporised rock, such as Na and SiO, with spectral signatures bright enough to be detected through eclipse observations with planned space-based telescopes. In this study, we find that rocky super-Earths with a irradiation temperature of 3000 K and a distance from Earth of up to 20 pc, as well as planets hotter than 3500 K and closer than 50 pc, have SiO features which are potentially detectable in eclipse spectra observed with Ariel.

astro-ph.EP↗

arXiv2020

H$_2$O$_2$-induced Greenhouse Warming on Oxidized Early Mars

The existence of liquid water within an oxidized environment on early Mars has been inferred by the Mn-rich rocks found during recent explorations on Mars. The oxidized atmosphere implied by the Mn-rich rocks would basically be comprised of CO$_2$ and H$_2$O without any reduced greenhouse gases such as H$_2$ and CH$_4$. So far, however, it has been thought that early Mars could not have been warm enough to sustain water in liquid form without the presence of reduced greenhouse gases. Here, we propose that H$_2$O$_2$ could have been the gas responsible for warming the surface of the oxidized early Mars. Our one-dimensional atmospheric model shows that only 1 ppm of H$_2$O$_2$ is enough to warm the planetary surface because of its strong absorption at far-infrared wavelengths, in which the surface temperature could have reached over 273~K for a CO$_2$ atmosphere with a pressure of 3~bar. A wet and oxidized atmosphere is expected to maintain sufficient quantities of H$_2$O$_2$ gas in its upper atmosphere due to its rapid photochemical production in slow condensation conditions. Our results demonstrate that a warm and wet environment could have been maintained on an oxidized early Mars, thereby suggesting that there may be connections between its ancient atmospheric redox state and possible aqueous environment.

astro-ph.EP↗

arXiv2020

Hydrodynamic Escape of Mineral Atmosphere from Hot Rocky Exoplanet. I. Model Description

Recent exoplanet statistics indicate that photo-evaporation has a great impact on the mass and bulk composition of close-in low-mass planets. While there are many studies addressing photo-evaporation of hydrogen-rich or water-rich atmospheres, no detailed investigation regarding rocky vapor atmospheres (or mineral atmospheres) has been conducted. Here, we develop a new 1-D hydrodynamic model of the UV-irradiated mineral atmosphere composed of Na, Mg, O, Si, their ions and electrons, includin molecular diffusion, thermal conduction, photo-/thermo-chemistry, X--ray and UV heating, and radiative line cooling (i.e., the effects of the optical thickness and non-LTE). The focus of this paper is on describing our methodology but presents some new findings. Our hydrodynamic simulations demonstrate that almost all of the incident X-ray and UV energy from the host-star is converted into and lost by the radiative emission of the coolant gas species such as Na, Mg, Mg$^+$, Si$^{2+}$, Na$^{3+}$ and Si$^{3+}$. For an Earth-size planet orbiting 0.02~AU around a young solar-type star, we find that the X-ray and UV heating efficiency is as small as $1 \times 10^{-3}$, which corresponds to 0.3~$\Mearth$/Gyr of the mass loss rate simply integrated over all the directions. Because of such efficient cooling, the photo-evaporation of the mineral atmosphere on hot rocky exoplanets with masses of $1\Mearth$ is not massive enough to exert a great influence on the planetary mass and bulk composition. This suggests that close-in high-density exoplanets with sizes larger than the Earth radius survive in the high-UV environments.

astro-ph.EP↗

arXiv2020

AIM 2020 Challenge on Learned Image Signal Processing Pipeline

This paper reviews the second AIM learned ISP challenge and provides the description of the proposed solutions and results. The participating teams were solving a real-world RAW-to-RGB mapping problem, where to goal was to map the original low-quality RAW images captured by the Huawei P20 device to the same photos obtained with the Canon 5D DSLR camera. The considered task embraced a number of complex computer vision subtasks, such as image demosaicing, denoising, white balancing, color and contrast correction, demoireing, etc. The target metric used in this challenge combined fidelity scores (PSNR and SSIM) with solutions' perceptual results measured in a user study. The proposed solutions significantly improved the baseline results, defining the state-of-the-art for practical image signal processing pipeline modeling.

cs.CV↗

arXiv2020

Development of next-generation timing system for the Japan Proton Accelerator Research Complex

A precise and stable timing system is necessary for high intensity proton accelerators such as the J-PARC. The original timing system, which was developed during the construction period of the J-PARC, has been working without major issues since 2006. After a decade of operation, the optical modules, which are key components for signal transfer, were discontinued already. Thus, the next-generation timing system for the J-PARC has been developed and built. The new system is designed to be compatible with the original system in terms of the operating principle. The new system utilizes modern high speed serial communication for the transfer of the clock, reference trigger, and type code. We describe the system configuration of the next-generation timing system and current status.

physics.acc-ph↗

arXiv2020

Exploring Super-Earth Surfaces: Albedo of Near-Airless Magma Ocean Planets and Topography

In this paper we propose an analytic function for the spherical albedo values of airless and near-airless magma ocean planets (AMOPs). We generated 2-D fractal surfaces with varying compositions onto which we individually threw 10,000 light rays. Using an approximate form of the Fresnel equations we measured how much of the incident light was reflected. Having repeated this algorithm on varying surface roughnesses we find the spherical albedo as a function of the Hurst exponent, the geochemical composition of the magma, and the wavelength. As a proof of concept, we used our model on Kepler-10b to demonstrate the applicability of our approach. We present the spherical albedo values produced from different lava compositions and multiple tests that can be applied to observational data in order to determine their characteristics. Currently, there is a strong degeneracy in the surface composition of AMOPs due to the large uncertainties in their measured spherical albedos. In spite of this, when applied to Kepler-10b we show that its high albedo could be caused by a moderately wavy ocean that is rich in oxidised metallic species such as FeO, $\rm Fe_{2}O_{3}$, $\rm Fe_{3}O_{4}$. This would imply that Kepler-10b is a coreless or near-coreless body.

astro-ph.EP↗

arXiv2015

Theoretical Emission Spectra of Atmospheres of Hot Rocky Super-Earths

Motivated by recent detection of transiting high-density super-Earths, we explore the detectability of hot rocky super-Earths orbiting very close to their host stars. In the environment hot enough for their rocky surfaces to be molten, they would have the atmosphere composed of gas species from the magma oceans. In this study, we investigate the radiative properties of the atmosphere that is in the gas/melt equilibrium with the underlying magma ocean. Our equilibrium calculations yield Na, K, Fe, Si, SiO, O, and O$_2$ as the major atmospheric species. We compile the radiative-absorption line data of those species available in literature, and calculate their absorption opacities in the wavelength region of 0.1--100~$\mathrm{μm}$. Using them, we integrate the thermal structure of the atmosphere. Then, we find that thermal inversion occurs in the atmosphere because of the UV absorption by SiO. In addition, we calculate the ratio of the planetary to stellar emission fluxes during secondary eclipse, and find prominent emission features induced by SiO at 4~$\mathrm{μm}$ detectable by Spitzer, and those at 10 and 100~$\mathrm{μm}$ detectable by near-future space telescopes.

astro-ph.EP↗