arXiv subjects

Riouhei Nakatani

Publications and source records attributed to Riouhei Nakatani.

At least 19 recordsLinked to original sources

arXiv2025

Finding New Debris Discs at Sub-millimetre Wavelengths

Debris discs reveal the architectures and dynamical histories of planetary systems. Sub-millimetre observations trace large dust grains within debris discs, revealing their bulk properties. Debris discs have so far only been detected around ~20% of stars, representing the bright end of the population. A new facility is required to reach fainter discs, overcoming the confusion limit, with multiwavelength capabilities for characterisation, sensitivity to large-scale emission for nearby targets and a large field of view for surveying distant populations. All of this is made possible with the Atacama Large Aperture Submillimetre Telescope (AtLAST).

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arXiv2025

Model of X-ray and extreme-UV emission from magnetically heated atmospheres in classical T Tauri stars: Case study of TW Hya

Photoevaporation caused by X-rays and ultraviolet radiation from the central star has attracted attention as a key process driving the dispersal of protoplanetary discs. Although numerous models have been used to investigate the photoevaporation process, their conclusions vary, partly due to differences in the adopted radiation spectra of the host star in particular in the extreme ultraviolet (EUV) and soft X-ray bands. This study aims to construct the EUV and (soft) X-ray emission spectrum from pre-main-sequence stars using a physics-based model. While the high-energy radiation sources of pre-main-sequence stars include accretion shocks and magnetically heated coronae, this study focuses on the latter. An MHD model capable of reproducing the coronal emission of main-sequence stars is applied to a pre-main-sequence star TW Hya, and its feasibility is assessed by comparing the predicted and observed emission-line intensities. We find that the emission lines formed at coronal temperatures ($T = 4-13 \times 10^6$ K) are reproduced in intensity within a factor of three. Emission lines from lower-temperature ($T < 4 \times 10^6$ K) plasmas are systematically underestimated, with typical intensities at 10-30% of observed values, consistent with previous findings that these emissions predominantly originate from accretion shocks. Emission lines emitted at extremely high temperatures ($T > 13 \times 10^6$ K) account for only about 1-10% of the observed values, likely due to the neglect of transient heating associated with flares. These results indicate that the quiescent coronal emission of pre-main-sequence stars can be adequately modeled using a physics-based approach.

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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.

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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.

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arXiv2024

Water vapor as a probe of the origin of gas in debris disks

Debris disks embrace the formation and evolution histories of planetary systems. Recent detections of gas in these disks have received considerable attention, as its origin ties up ongoing disk evolution and the present composition of planet-forming materials. Observations of the CO gas alone, however, cannot reliably differentiate between two leading, competing hypotheses: (1) the observed gas is the leftover of protoplanetary disk gas, and (2) the gas is the outcome of collisions between icy bodies. We propose that such differentiation may become possible by observing cold water vapor. Order-of-magnitude analyses and comparison with existing observations are performed. We show that different hypotheses lead to different masses of water vapor. This occurs because, for both hypotheses, the presence of cold water vapor is attributed to photodesorption from dust particles by attenuated interstellar UV radiation. Cold water vapor cannot be observed by current astronomical facilities as most of its emission lines fall in the far-IR (FIR) range. This work highlights the need for a future FIR space observatory to reveal the origin of gas in debris disks and the evolution of planet-forming disks in general.

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arXiv2024

Broadening the Canonical Picture of EUV-Driven Photoevaporation of Accretion Disks

Photoevaporation driven by hydrogen-ionizing radiation, also known as extreme-ultraviolet (EUV), profoundly shapes the lives of diverse astrophysical objects. Focusing here mainly on the dispersal of protoplanetary disks, we construct an analytical model accounting for the finite timescales of photoheating and photoionization. The model offers improved estimates for the ionization, temperature, and velocity structures versus distance from the central source, for a given EUV emission rate and spectral hardness. Compared to the classical picture of fully-ionized and isothermal winds with temperatures $\approx 10^4{\rm \,K}$ and speeds $\approx 10{\rm \,km\,s^{-1}}$, our model unveils broader hydrodynamical and thermochemical states of photoevaporative winds. In contrast to the classical picture, T~Tauri stars with EUV luminosities around $10^{30}{\rm \,erg\,s^{-1}}$ have non-isothermal ionized winds at lower temperatures than the classical value if the spectrum is soft, with an average deposited energy per photoionization less than about 3.7\,eV. Conversely, if the spectrum is hard, the winds tend to be atomic and isothermal at most radii in the disk. For lower EUV intensities, even with soft spectra, atomic winds can emerge beyond $\sim 10{\, \rm au}$ through advection. We demonstrate that the analytical model's predictions are in general agreement with detailed radiation-hydrodynamics calculations. The model furthermore illustrates how the energy efficiency of photoevaporation varies with the intensity and spectral hardness of the EUV illumination, as well as addressing discrepancies in the literature around the effectiveness of X-ray photoevaporation. These findings highlight the importance of considering the finite timescales of photoheating and photoionization, both in modeling and in interpreting observational data.

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arXiv2023

A Primordial Origin for the Gas-Rich Debris Disks Around Intermediate-Mass Stars

While most debris disks consist of dust with little or no gas, a fraction has significant amounts of gas detected via emission lines of CO, ionized carbon, and/or atomic oxygen. Almost all such gaseous debris disks known are around A-type stars with ages up to 50 Myr. We show, using semi-analytic disk evolution modeling, that this can be understood if the gaseous debris disks are remnant protoplanetary disks that have become depleted of small grains compared to the interstellar medium. Photoelectric heating by the A-stars' FUV radiation is then inefficient, while the stars' EUV and X-ray emissions are weak owing to a lack of surface convective zones capable of driving magnetic activity. In this picture, stars outside the range of spectral types from A through early B are relatively hard to have such long-lived gas disks. Less-massive stars have stronger magnetic activity in the chromosphere, transition region, and corona with resulting EUV and X-ray emission, while more-massive stars have photospheres hot enough to produce strong EUV radiation. In both cases, primordial disk gas is likely to photoevaporate well before 50 Myr. These results come from 0D disk evolution models where we incorporate internal accretion stresses, MHD winds, and photoevaporation by EUV and X-ray photons with luminosities that are functions of the stellar mass and age. A key issue this work leaves open is how some disks become depleted in small dust so that FUV photoevaporation slows. Candidates include grains' growth, settling, radial drift, radiation force, and incorporation into planetary systems.

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arXiv2022

Formation of dust clumps with sub-Jupiter mass and cold shadowed region in gravitationally unstable disk around Class 0/I protostar in L1527 IRS

We have investigated the protostellar disk around a Class 0/I protostar, L1527 IRS, using multi-wavelength observations of the dust continuum emission at $λ=0.87$, 2.1, 3.3, and 6.8 mm obtained by the Atacama Large Millimeter/submillimeter Array (ALMA) and the Jansky Very Large Array (VLA). Our observations achieved a spatial resolution of $3-13$ au and revealed an edge-on disk structure with a size of $\sim80-100$ au. The emission at 0.87 and 2.1 mm is found to be optically thick within a projected disk radius of $ r_{\rm proj}\lesssim50$ au. The emission at 3.3 and 6.8 mm shows that the power-law index of the dust opacity ($β$) is $β\sim1.7$ around $ r_{\rm proj}\sim 50$ au, suggesting that grain growth has not yet begun. The dust temperature ($T_{\rm dust}$) shows a steep decrease with $T_{\rm dust}\propto r_{\rm proj}^{-2}$ outside of the VLA clumps previously identified at $r_{\rm proj}\sim20$ au. Furthermore, the disk is gravitationally unstable at $r_{\rm proj}\sim20$ au, as indicated by a Toomre {\it Q} parameter value of $Q\lesssim1.0$. These results suggest that the VLA clumps are formed via gravitational instability, which creates a shadow on the outside of the substructure, resulting in the sudden drop in temperature. The derived dust masses for the VLA clumps are $\gtrsim0.1$ $M_{\rm J}$. Thus, we suggest that Class 0/I disks can be massive enough to be gravitationally unstable, which might be the origin of gas-giant planets in a 20 au radius. Furthermore, the protostellar disks can be cold due to shadowing.

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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.

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arXiv2022

Rapid growth of seed black holes during early bulge formation

We study the early growth of massive seed black holes (BHs) via accretion in protogalactic nuclei where the stellar bulge component is assembled, performing axisymmetric two-dimensional radiation hydrodynamical simulations. We find that when a seed BH with $M_\bullet \sim 10^5~M_\odot$ is embedded in dense metal-poor gas ($Z=0.01~Z_\odot$) with a density of $\gtrsim 100~{\rm cm}^{-3}$ and bulge stars with a total mass of $M_\star \gtrsim 100~M_\bullet$, a massive gaseous disk feeds the BH efficiently at rates of $\gtrsim 0.3-1~M_\odot~{\rm yr}^{-1}$ and the BH mass increases nearly tenfold within $\sim 2$ Myr. This rapid accretion phase lasts until a good fraction of the gas bounded within the bulge accretes onto the BH, although the feeding rate is regulated owing to strong outflows driven by ionizing radiation emitted from the accreting BH. The transient growing mode can be triggered for seed BHs formed in massive dark-matter halos with masses of $\gtrsim 10^9~M_\odot$ at $z\sim 15-20$ (the virial temperature is $T_{\rm vir}\simeq 10^5~{\rm K}$). The host halos are heavier and rarer than those of typical first galaxies, but are more likely to end up in quasar hosts by $z\simeq 6$. This mechanism naturally yields a mass ratio of $M_\bullet/M_\star >0.01$ higher than the value seen in the local universe and the existence of such overmassive BHs provides us a unique opportunity of detecting highly accreting seed BHs at $z\sim 15$ with AB magnitude of $m_{\rm AB} \sim26 - 29$ mag at $2~μ{\rm m}$ (rest-frame 10 eV) by the upcoming observations by the James Webb Space Telescope and Nancy Grace Roman Space Telescope.

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arXiv2022

FAUST III. Misaligned rotations of the envelope, outflow, and disks in the multiple protostellar system of VLA 1623$-$2417

We report a study of the low-mass Class-0 multiple system VLA 1623AB in the Ophiuchus star-forming region, using H$^{13}$CO$^+$ ($J=3-2$), CS ($J=5-4$), and CCH ($N=3-2$) lines as part of the ALMA Large Program FAUST. The analysis of the velocity fields revealed the rotation motion in the envelope and the velocity gradients in the outflows (about 2000 au down to 50 au). We further investigated the rotation of the circum-binary VLA 1623A disk as well as the VLA 1623B disk. We found that the minor axis of the circum-binary disk of VLA 1623A is misaligned by about 12 degrees with respect to the large-scale outflow and the rotation axis of the envelope. In contrast, the minor axis of the circum-binary disk is parallel to the large-scale magnetic field according to previous dust polarization observations, suggesting that the misalignment may be caused by the different directions of the envelope rotation and the magnetic field. If the velocity gradient of the outflow is caused by rotation, the outflow has a constant angular momentum and the launching radius is estimated to be $5-16$ au, although it cannot be ruled out that the velocity gradient is driven by entrainments of the two high-velocity outflows. Furthermore, we detected for the first time a velocity gradient associated with rotation toward the VLA 16293B disk. The velocity gradient is opposite to the one from the large-scale envelope, outflow, and circum-binary disk. The origin of its opposite gradient is also discussed.

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arXiv2021

Photoevaporation of Grain-Depleted Protoplanetary Disks around Intermediate-Mass Stars: Investigating Possibility of Gas-Rich Debris Disks as Protoplanetary Remnants

Debris disks are classically considered to be gas-less systems, but recent (sub)millimeter observations have detected tens of those with rich gas content. The origin of the gas component remains unclear; namely, it can be protoplanetary remnants and/or secondary products deriving from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for $\gtrsim10{\,\rm Myr}$. However, previous models predict $\lesssim 10{\,\rm Myr}$ lifetimes because of efficient photoevaporation at the late stage of disk evolution. In the present study, we investigate photoevaporation of gas-rich, optically-thin disks around intermediate-mass stars at a late stage of the disk evolution. The evolved system is modeled as those where radiation force is sufficiently strong to continuously blow out small grains ($\lesssim 4 {\,\rm μm}$), which are an essential component for driving photoevaporation via photoelectric heating induced by stellar far-ultraviolet (FUV). We find that the grain depletion reduces photoelectric heating, so that FUV photoevaporation is not excited. Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-loss rate of $2$--$5\times10^{-10}(Φ_{\rm EUV}/10^{41}{\,\rm s}^{-1})^{1/2}\,M_\odot\,{\rm yr}^{-1}$, where $Φ_{\rm EUV}$ is the EUV emission rate. The estimated lifetimes of the gas component are $\sim 50 (M_{\rm disk}/10^{-2}\,M_\odot)(Φ_{\rm EUV}/10^{41}\,{\rm s}^{-1})^{1/2}\,{\rm Myr}$ and depend on the ``initial'' disk mass at the point small grains have been depleted in the system. With an order estimation, we show that the gas component can survive for a much longer time around A-type stars than lower-mass stars. This trend is consistent with the higher frequency of gas-rich debris disks around A-type stars, implying the possibility of the gas component being protoplanetary remnants.

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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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arXiv2021

Radiation hydrodynamics simulations of protoplanetary disks: Stellar mass dependence of the disk photoevaporation rate

Recent multi-wavelength observations suggest that inner parts of protoplanetary disks (PPDs) have shorter lifetimes for heavier host stars. Since PPDs around high-mass stars are irradiated by strong ultra-violet radiation, photoevaporation may provide an explanation for the observed trend. We perform radiation hydrodynamics simulations of photoevaporation of PPDs for a wide range of host star mass of $M_* =0.5$-$7.0 M_{\odot}$. We derive disk mass-loss rate $\dot{M}$, which has strong stellar dependence as $\dot{M} \approx 7.30\times10^{-9}(M_{*}/M_{\odot})^{2}M_{\odot}\rm{yr}^{-1}$. The absolute value of $\dot{M}$ scales with the adopted far-ultraviolet and X-ray luminosities. We derive the surface mass-loss rates and provide polynomial function fits to them. We also develop a semi-analytic model that well reproduces the derived mass-loss rates. The estimated inner disk lifetime decreases as the host star mass increases, in agreement with the observational trend. We thus argue that photoevaporation is a major physical mechanism for PPD dispersal for a wide range of the stellar mass and can account for the observed stellar mass dependence of the inner disk lifetime.

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arXiv2021

FAUST II. Discovery of a Secondary Outflow in IRAS 15398-3359: Variability in Outflow Direction during the Earliest Stage of Star Formation?

We have observed the very low-mass Class 0 protostar IRAS 15398-3359 at scales ranging from 50 au to 1800 au, as part of the ALMA Large Program FAUST. We uncover a linear feature, visible in H2CO, SO, and C18O line emission, which extends from the source along a direction almost perpendicular to the known active outflow. Molecular line emission from H2CO, SO, SiO, and CH3OH further reveals an arc-like structure connected to the outer end of the linear feature and separated from the protostar, IRAS 15398-3359, by 1200 au. The arc-like structure is blue-shifted with respect to the systemic velocity. A velocity gradient of 1.2 km/s over 1200 au along the linear feature seen in the H2CO emission connects the protostar and the arc-like structure kinematically. SO, SiO, and CH3OH are known to trace shocks, and we interpret the arc-like structure as a relic shock region produced by an outflow previously launched by IRAS 15398-3359. The velocity gradient along the linear structure can be explained as relic outflow motion. The origins of the newly observed arc-like structure and extended linear feature are discussed in relation to turbulent motions within the protostellar core and episodic accretion events during the earliest stage of protostellar evolution.

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arXiv2020

Ring formation by coagulation of dust aggregates in early phase of disk evolution around a protostar

Ring structures are observed by (sub-)millimeter dust continuum emission in various circumstellar disks from early stages of Class 0 and I to late stage of Class II young stellar objects (YSOs). In this paper, we study one of the possible scenarios of such ring formation in early stage, which is coagulation of dust aggregates. The dust grains grow in an inside-out manner because the growth timescale is roughly proportional to the orbital period. The boundary of the dust evolution can be regarded as the growth front, where the growth time is comparable to the disk age. With radiative transfer calculations based on the dust coagulation model, we find that the growth front can be observed as a ring structure because dust surface density is sharply changed at this position. Furthermore, we confirm that the observed ring positions in the YSOs with an age of $\lesssim1$ Myr are consistent with the growth front. The growth front could be important to create the ring structure in particular for early stage of the disk evolution such as Class 0 and I sources.

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arXiv2020

Photoevaporation of Minihalos during Cosmic Reionization: Primordial and Metal-Enriched Halos

The density distribution of the inter-galactic medium is an uncertain but highly important issue in the study of cosmic reionization. It is expected that there are abundant gas clouds hosted by low-mass "minihalos" in the early universe, which act as photon sinks until photoevaporated by the emerging ultra-violet background (UVB) radiation. We perform a suite of radiation hydrodynamics simulations to study the photoevaporation of minihalos. Our simulations follow hydrodynamics, non-equilibrium chemistry, and the associated cooling processes in a self-consistent manner. We conduct a parametric study by considering a wide range of gas metallicity ($0\,Z_\odot \leq Z \leq 10^{-3}\,Z_\odot$), halo mass ($10^3 M_\odot \leq M \leq 10^8 M_\odot$), UVB intensity ($0.01 \leq J_{21} \leq 1$), and turn-on redshift of ionizing sources ($10\leq z_{\rm IN} \leq 20$). We show that small halos are evaporated in a few tens million years, whereas larger mass halos survive for ten times longer. We show that the gas mass evolution of a minihalo can be characterized by a scaling parameter that is given by a combination of the halo mass, background radiation intensity, and redshift. Efficient radiative cooling in metal-enriched halos induces fast condensation of the gas to form a dense, self-shielded core. The cold, dense core can become gravitationally unstable in halos with high metallicities. Early metal enrichment may allow star formation in minihalos during cosmic reionization.

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arXiv2020

Substructure Formation in a Protostellar Disk of L1527 IRS

We analyze multi-frequency, high-resolution continuum data obtained by ALMA and JVLA to study detailed structure of the dust distribution in the infant disk of a Class~0/I source, L1527 IRS. We find three clumps aligning in the north-south direction in the $7 {\rm \,mm}$ radio continuum image. The three clumps remain even after subtracting free-free contamination, which is estimated from the $1.3{\rm \,cm}$ continuum observations. The northern and southern clumps are located at a distance of $\sim 15{\rm \,au}$ from the central clump and are likely optically thick at $7{\rm \,mm}$ wavelength. The clumps have similar integrated intensities. The symmetric physical properties could be realized when a dust ring or spiral arms around the central protostar is projected to the plane of the sky. We demonstrates for the first time that such substructure may form even in the disk-forming stage, where the surrounding materials actively accrete toward a disk-protostar system.

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