arXiv subjects

Hiroto Mitani

Publications and source records attributed to Hiroto Mitani.

6 recordsLinked to original sources

arXiv2026

Analytical modeling of helium absorption signals of isothermal atmospheric escape

Atmospheric escape driven by extreme ultraviolet (EUV) radiation is a critical process shaping the evolution of close-in exoplanets. Recent observations have detected helium triplet absorption in numerous (>20) close-in exoplanets, highlighting the importance of understanding upper atmospheric thermo-chemical structure. While super-solar metallicity has been observed in the atmospheres of some close-in exoplanets, the impact of metal species on both atmospheric escape dynamics and observed absorption features remains poorly understood. In this study, we derive a simplified yet accurate formula for the equivalent width of helium absorption in the limit of an isothermal temperature for the upper atmosphere. Our results demonstrate that planets with lower temperature (metal-rich atmosphere) exhibit lower mass-loss rate although the equivalent width of helium triplet absorption remains largely independent of atmospheric temperature (metallicity) because the low temperatures in these atmospheres enhance the fraction of helium in its triplet state. Additionally, we present a hydrodynamic model based on radiation-hydrodynamics simulations that incorporates the effects of metal cooling. Our analytical model can predict the helium triplet equivalent width of the atmosphere of simulations. The analytical model provides a comprehensive framework for understanding how metal cooling in the upper atmosphere influences the thermo-chemical structure and observable helium features of close-in exoplanetary atmospheres, offering valuable insights for interpreting current and future observational data.

astro-ph.EP↗

arXiv2025

Secret of Longevity: Protoplanetary Disks as a Source of Gas in Debris Disks

While protoplanetary disks (PPDs) are generally thought to disperse within several million years, recent observations have revealed gas in their older counterparts, debris disks. The origin of this gas remains uncertain, with one possibility being the unexpectedly long survival of PPDs (the primordial-origin scenario). To explore the plausibility of this scenario, we conduct 1D disk evolution simulations, varying parameters like stellar mass, disk mass, turbulent stress, and the model of magnetohydrodynamic winds, while incorporating stellar evolution to account for time-varying photoevaporation rates. Our focus is on disks where small grains are depleted, as these are potentially long-lived due to reduced far-ultraviolet photoevaporation. Our results show that gas in these disks can survive beyond 10 Myr regardless of the stellar mass, provided they are initially massive ($M_{\mathrm{disk}}\approx 0.1M_*$) with relatively weak turbulent stress ($α\ll 10^{-2}$). The longest lifetimes are consistently found for $M_* = 2 M_{\odot}$ across a wide parameter space, with gas typically persisting at $\sim 10$--$10^3$ au. Roughly estimated CO masses for these disks fall within the observed range for the most massive gas-rich debris disks around early A~stars. These alignments support the plausibility of the primordial-origin scenario. Additionally, our model predicts that accretion persists for as long as the disk survives, which could explain the accretion signatures detected in old disks hosted by low-mass stars, including Peter Pan disks. Our finding also suggests that ongoing accretion may exist in gas-rich debris disks. Thus, searching for accretion signatures could be a key to determining the origins of gas in debris disks.

astro-ph.EP↗

arXiv2025

Physically motivated analytic model of energy efficiency for EUV-driven atmospheric escape of close-in exoplanets

Extreme Ultraviolet (EUV) driven atmospheric escape is a key process in the atmospheric evolution of close-in exoplanets. In many evolutionary models, the energy-limited mass-loss rate with a constant efficiency (typically $\sim10\%$) is assumed for calculating the mass-loss rate. However, hydrodynamic simulations have demonstrated that this efficiency depends on various stellar and planetary parameters. Comprehending the underlying physics of the efficiency is essential for understanding planetary atmospheric evolution and recent observations of the upper atmosphere of close-in exoplanets. We introduce relevant temperatures and timescales derived from physical principles to elucidate the mass-loss process. Our analytical mass-loss model is based on phenomenology and consistent across a range of planetary parameters. We compare our mass-loss efficiency and the radiation hydrodynamic simulations. The model can predict efficiency in both energy-limited and recombination-limited regimes. We further apply our model to exoplanets observed with hydrogen absorption (Ly$α$ and H$α$). Our findings suggest that Ly$α$ absorption is detectable in planets subjected to intermediate EUV flux; under these conditions, the escaping outflow is insufficient in low-EUV environments, while the photoionization timescale remains short in high-EUV ranges. Conversely, H$α$ absorption is detectable under high EUV flux conditions, facilitated by the intense Ly$α$ flux exciting hydrogen atoms. According to our model, the non-detection of neutral hydrogen can be explained by a low mass-loss rate and is not necessarily due to stellar wind confinement or the absence of a hydrogen-dominated atmosphere in many cases. This model assists in identifying future observational targets and explicates the unusual absorption detection/non-detection patterns observed in recent studies.

astro-ph.EP↗

arXiv2022

Stellar Wind Effect on the Atmospheric Escape of Hot Jupiters and their Ly-$α$ and H$α$ transits

Atmospheric escape of close-in exoplanets can be driven by high energy radiation from the host star. The planetary outflows interacting with the stellar wind may generate observable transit signals that depend on the strength of the stellar wind. We perform detailed radiation-hydrodynamics simulations of the atmospheric escape of hot Jupiters with including the wind from the host star in a self-consistent, dynamically coupled manner. We show that the planetary outflow is shaped by the balance between its thermal pressure and the ram pressure of the stellar wind. We use the simulation outputs to calculate the Lyman-$α$ and H$α$ transit signatures. Strong winds can confine the outflow and decrease the Lyman-$α$ transit depth. Contrastingly, the wind effect on H$α$ is weak because of the small contribution from the uppermost atmosphere of the planet. Observing both of the lines is important to understand the effect of the UV radiation and wind from the host. The atmospheric mass-loss rate is approximately independent of the strength of the wind. We also discuss the effect of the coronal mass ejections on the signatures. We argue that around M dwarfs the effect can be significant in every transit.

astro-ph.EP↗

arXiv2021

Atmospheric Escape of Close-in Giants around Hot Stars: Far-Ultraviolet Radiation and Photoelectric Heating Effect

Atmospheric escape is an important process that controls the long-term evolution of close-in planets. We perform radiation hydrodynamics simulations of photo-evaporation of exoplanets' atmospheres to study the effect of photoelectric heating by far-ultraviolet (FUV) radiation. Specifically, we consider a close-in hot Jupiter around a hot A-star. Hot main-sequence stars emit not only extreme ultraviolet radiation but also FUV radiation, and thus can drive strong atmospheric escape by photoelectric heating. We show that the planetary atmosphere escapes at a rate as large as $\dot{M}\sim10^{14}\, \mathrm{g}~{\rm sec}^{-1}$ if the atmosphere contains a small amount of dust grains with the level of ten percent of the local interstellar medium. Close-in planets around hot stars can lose a significant fraction of the atmosphere during the long-term evolution. We quantify the amount of dust necessary for causing photoevaporation. The dust-to-gas mass ratio of $10^{-4}$ is sufficient to drive stronger atmospheric escape by FUV photoelectric heating than in the case with only extreme ultraviolet radiation. We also explore the metallicity dependence of the FUV-driven escape. The mass-loss rate increases with increasing the atmosphere's metallicity because of the enhanced photoelectric heating, but the stellar FUV flux decreases with increasing stellar metallicity. We derive an accurate estimate for the mass-loss rate as a function of FUV flux and metallicity, and of the planet's characteristics. The FUV driven atmospheric escape may be a key process to understand and explain the so-called sub-Jovian desert.

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arXiv2019

Spectral Energy Distribution of the First Galaxies: Contribution from Pre-Main-Sequence Stars

One of the major goals of next-generation space-borne and ground-based telescopes is to detect and characterize the first galaxies that were in place in the first few hundred million years after the big bang. We study the spectral energy distribution (SED) of the first galaxies and discuss the prospects for detection and identification. We consider very young star-forming galaxies at $z=15$ and incorporate the contribution from pre-main-sequence (PMS) stars. Unlike in the present-day galaxies, primordial protostars are not embedded in dusty gas clouds, and hence the light from them can be visible at a wide range of wavelengths. We use MESA code to follow the PMS evolution and use the BT-Settl model to calculate the SED of individual PMS stars. We show that PMS stars contribute to boost the flux in the mid-infrared, and that the galaxy SED at very early evolutionary phases is overall redder than at later phases. The infrared flux contribution is comparable to that caused by emission lines powered by massive stars. We argue that the contribution from PMS stars is important for characterizing young galaxies in the early Universe and also for target selection with future deep galaxy surveys.

astro-ph.GA↗