arXiv subjects

Shinsuke Takasao

Publications and source records attributed to Shinsuke Takasao.

At least 19 recordsLinked to original sources

arXiv2026

Asymmetric, variable H$α$ line profile in planetary mass object SR 12 c

Young, forming planetary-mass objects often exhibit clear signatures of ongoing mass accretion and are thought to accrete material through processes analogous to those operating in young stars. In this study, we present high-spectral-resolution observations of asymmetric and time-variable H$α$ line profiles from the planetary-mass companion SR~12~c. The H$α$ line was observed at a resolving power of $R \sim 49,000$--40{,}000 (corresponding to 6.1--7.5~km~s$^{-1}$) using the High Dispersion Spectrograph (HDS) on the 8.2 m Subaru Telescope. Strong H$α$ emission is clearly detected, while higher-order Balmer lines (H$β$, H$γ$, and H$δ$) are not detected due to their faintness. The H$α$ line profiles are well spectrally resolved and exhibit blueshifted emission peaks, which can be interpreted as arising from either (a) emission partially absorbed by redshifted accreting material along the line of sight and/or (b) geometric occultation by the inner circumplanetary disk. Moreover, the H$α$ flux shows significant variability at 43.6~$\pm$~6.4~\% relative to the peak flux on hourly timescales. During a continuous 2.5-hour observing sequence, the emission component peaking at approximately $-30$~km~s$^{-1}$ weakened over the first hour. Subsequently, an emission component centered near $-10$~km~s$^{-1}$ became dominant and remained stable for the remaining 1.5 hours. We discuss possible interpretations of this behavior. Overall, these results support that magnetospheric accretion is operating in the planetary-mass object SR~12~c while a scenario combining boundary-layer accretion with a failed wind cannot be ruled out.

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arXiv2026

Cosmic ray heating of cold streams: Implications for the gas supply and growth of massive galaxies

Recent observations have demonstrated the presence of cosmic rays (CRs) in cosmic-web filaments. Cold streams supply gas inflows from these filaments into massive galaxies during the cosmic noon. As these streams are expected to be magnetised, external cosmic-web CRs may become entrained with this inflowing gas. We aim to determine whether this externally-supplied CR population can deposit energy to alter or disrupt the supply of cold gas to galaxies. We couple a spectrally-resolved CR transport calculation to a redshift-dependent analytical model of magnetised cold streams in galaxy haloes and investigate whether externally-supplied CRs can modify gas supply through this channel. We find CR energy deposition can alter the thermal state of cold streams. Dense stream cores remain largely resilient and only experience weak heating. Their temperature is raised by less than a factor of 10, which is insufficient to overcome radiative cooling at the stream-CGM interface. In more diffuse streams, and in partially mixed interface gas of the most massive haloes near the virial radius, CR heating becomes strong enough that radiative cooling can no longer balance it, and the gas is heated toward or above the mixing-layer temperature. This weakens the stability of the stream, making it more susceptible to disruption. Complete evaporation is possible only in extreme cases. Cold streams are therefore more vulnerable to CR heating at larger galactocentric radii, higher halo masses, and in more diffuse or partially-mixed stream material. By preferentially heating diffuse gas, externally supplied CRs may introduce additional selectivity into cold-gas accretion that modifies the gas supply and growth of massive galaxies. These CRs weaken fragile streams and erode their cold envelope, and may cause surviving cold flow components to appear thinner and more sharply confined far into galaxy haloes.

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arXiv2026

Systematic Comparison between Constrained Transport and Mixed Divergence Cleaning Methods for Astrophysical Magnetohydrodynamic Simulations

Magnetohydrodynamic (MHD) simulations are indispensable research infrastructure in astrophysics today. In order to satisfy the solenoidal constraint of the MHD equations on discretized grids, modern simulation codes often employ either constrained transport (CT) with a staggered grid or divergence cleaning using an additional variable. We compare CT and Dedner's mixed divergence cleaning schemes systematically, and find that the divergence cleaning scheme can produce substantial artifacts in certain situations. Through numerical experiments including both idealized tests and practical applications, we show that the original implementation of Dedner's scheme becomes inaccurate when magnetic fields are strongly localized or when the timestep suddenly changes. We find that some previous results, such as the extremely rapid growth of magnetic fields during star formation in the early Universe, may be affected by the spurious behavior of the divergence cleaning scheme. We propose a few modifications to improve the robustness of the divergence cleaning method. Nevertheless, we find that the CT scheme is more accurate and reliable in many situations.

astro-ph.IM↗

arXiv2026

Numerical Investigation of Efficient Electron Acceleration at an Unsteady Solar Flare Loop-Top

Using magnetohydrodynamic (MHD) fields and guiding-center test-particle calculations, this study investigates how time-dependent loop-top dynamics modulate the adiabatic energization of electrons in a solar flare. Our results indicate that a time-varying loop-top structure enhances acceleration efficiency compared to a quasi-steady one. In the quasi-steady velocity field, the net acceleration is suppressed due to the decelerating effect of betatron cooling. Conversely, in the unsteady velocity field, the betatron mechanism readily accelerates electrons within the compressed magnetic field at the edge of the loop-top. These findings suggest that the acceleration of electrons at the loop-top is driven not only by the static shape of the magnetic structure but also by dynamic events such as plasmoid collisions. While previous studies have primarily focused on acceleration processes within the reconnection outflow, such as at termination shocks or within plasmoids, our research highlights the importance of the acceleration and deceleration processes at the exit point where electrons escape from the loop-top.

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arXiv2026

A Magnetized Black Hole Envelope Model for Little Red Dots

Recent observations have revealed a unique class of active galactic nuclei (AGNs), termed little red dots (LRDs). These objects are hypothesized to be powered by massive black holes rapidly accreting in dense gaseous environments. Theoretical studies suggest that the circum-nuclear gas can form an optically thick black hole envelope (BHE), whose structure resembles the atmospheres of convective stars near the Hayashi limit. Given that such cool stars typically generate magnetic fields, we propose a dynamical and spectral model for an LRD enshrouded by a magnetized BHE. Assuming spherical free-fall accretion onto a rotating, magnetized BHE, our model accounts for key observational properties of LRDs. We propose that the Doppler component of broad emission lines originates from plasma clumps co-rotating within the BHE magnetosphere. Including additional broadening due to electron scattering allows the resulting line profile to be fitted by a combination of a Gaussian core and an exponential tail. This model can reproduce Doppler components up to a few thousand ${\rm km~s^{-1}}$. We suggest that conventional black hole mass estimation methods based on the virial relation may yield erroneous results. Furthermore, our model is consistent with X-ray non-detections in LRDs. We evaluate the X-ray luminosities of two potential sources: the post-shock region of accretion shocks and a magnetically heated corona. We find that these X-ray luminosities are constrained to $\lesssim 10^{41}~{\rm erg~s^{-1}}$ across a wide range of black hole masses ($10^5 M_\odot \lesssim M_{\rm BH}\lesssim 10^7M_\odot$) and accretion rates, consistent with current upper limits on X-ray emission.

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arXiv2026

The hydrodynamical response of cold circumgalactic clouds to quasar radiation

Recent simulations increasingly resolve the small-scale structure of the circumgalactic medium (CGM), but the dynamical impact of ionising radiation on its cold $10^4$ K component remains poorly understood. We investigate the evolution of cold gas structures exposed to quasars' EUV radiation. We develop an analytical framework to describe the evolution of such clouds, introducing a new threshold that defines when a cloud becomes radiation-shielded. The framework is validated using radiation-hydrodynamic simulations of single static clouds. It predicts three evolutionary paths: (i) an optically thin regime, in which radiation uniformly ionises the cloud; (ii) a radiation-shielded regime, where the cloud remains largely unaffected; and (iii) a rocket-effect regime, in which the propagation of the ionisation front ionises the illuminated side while compressing the opposite side, later accelerating the surviving cold clump. In the latter regime, the cloud's Ly$α$ luminosity can be up to one order of magnitude higher than the optically thin case. Such luminosities are as high as $70\%$ of the values obtained from a fluorescent regime without considering hydrodynamical response. Unless the cloud is shielded, at least $\sim 50$-$60\,\%$ of Ly$α$ emission arises from recombination. Applying this framework to both a ray crossing a population of clouds, and a ray propagating inside a cold stream, we find that the cold CGM around bright quasars ($L_{\mathrm{ν,LL}} \sim 10^{31.6} \, \mathrm{erg\, s^{-1}\, Hz^{-1}}$) is likely fully ionised, whereas the one around faint quasars ($L_{\mathrm{ν,LL}} \sim 10^{28.6} \, \mathrm{erg\, s^{-1}\, Hz^{-1}}$) predominantly experiences a rocket-effect regime. These results imply that the hydrodynamical response of cold CGM structures to quasar radiation must be considered when deriving their physical properties, particularly for faint quasars.

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arXiv2025

The mass distribution of clumpy accretion onto the nearby young star TW Hya

The proliferation of high time-resolution and decades-long monitoring of classical T Tauri stars provides a vast opportunity to test the variability of the star-disk connections. However, most monitoring surveys use single broad-band filters, which makes the conversion of photometric variability into accretion rate difficult. In this study, we analyze accretion bursts onto the nearby young star TW Hya over short (hours, days) and long (months, years) timescales by calibrating TESS and ASAS-SN $g$-band photometry to accretion rates with simultaneous spectroscopy. The high cadence TESS light curve shows bursts of accretion in clumps with masses from a sensitivity limit of $\sim10^{-13}$~M$_\odot$ up to $3\times 10^{-11}$\,M$_\odot$. The average burst duration of 1.8 days is longer than a simple estimate of the thermal response timescale, supporting the interpretation that the photometric variability probes the instantaneous accretion rate. The reset timescale of 1.2--2 days derived from the structure function and previously reported quasi-periods of 3.5--4 days are consistent with bursts that may be related to the different rotation between the stellar magnetosphere and inner disk or with azimuthal asymmetries in the inner disk. The near-daily ASAS-SN light curve across 8 years reveals some seasonal changes in brightness with a standard deviation of $\sim 0.13$ mag, about half of the scatter seen on short timescales. This study demonstrates the importance of coordinating contemporaneous multi-epoch spectroscopy with time domain surveys to interpret light curves of young stars.

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arXiv2025

A Necessary Condition for the Submergence of Proto-Neutron Star Magnetic Fields by Supernova Fallback

Central compact objects (CCOs) are a subclass of neutron stars with a dipole magnetic field strength considerably weaker than those of radio pulsars and magnetars. One possible explanation for such weak magnetic fields in the CCOs is the hidden magnetic field scenario, in which supernova fallback submerges the magnetosphere of a proto-neutron star beneath a newly formed crust. However, the fallback mass and timescale required for this submergence process remain uncertain. We perform one-dimensional general relativistic magnetohydrodynamic simulations of the supernova fallback onto a magnetized proto-neutron star, while considering neutrino cooling. In our simulations, the infalling material compresses the magnetic field and drives a strong shock. The shock initially expands outward, but eventually stalls and recedes as neutrino cooling becomes significant. After the shock stalls, the gas density above the magnetosphere increases rapidly, potentially leading to the formation of a new crust. To understand the shock dynamics, we develop semi-analytic models that describe the resulting magnetospheric and shock radii when the shock stalls. By comparing the fallback time scale with the shock stalling time scale, corresponding to the waiting time for the new crust formation, we derive a necessary condition for the submergence of the PNS magnetic field. Our results will provide guidance for investigating the diversity of young isolated neutron stars through multidimensional simulations.

astro-ph.HE↗

arXiv2025

X-ray emission of F-type stars and its analogy with G-type stars

We conducted a systematic spectral study for single F-type main-sequence (MS) stars without significant X-ray outbursts to investigate X-ray spectral properties such as temperature, emission measure (EM), and luminosity (Lx). To this end, 33 single stars with relatively rich X-ray photon statistics were selected by cross-matching large astronomical catalogs of the XMM-Newton source catalog and the Tycho-2 spectral type catalog. A positive correlation was found in the observed EM--EM-weighted temperature relationship as seen in late-type stars and it is also found in the relationship that our single F-type MS star samples have a plasma with an EM-weighted temperature of $\lesssim$1 keV and an EM of $\lesssim$10$^{53}$ cm$^{-3}$ corresponding to Lx of $\lesssim$10$^{30}$~erg~s$^{-1}$. These observational features for the single F-type MS stars are consistent with those of the single G dwarf stars, suggesting that there are no significant differences in their X-ray coronal properties. Additionally, the obtained relationship between the X-ray activity and the Rossby number reinforces this suggestion in the literature. Moreover, the upper bounds in EM and Lx were found to be unique signatures for single stars and not valid for binary stars. Our results suggest that the planetary evolution in terms of the X-ray properties around F-type MS stars can be understood by extending the frameworks developed for G-type stars.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2025

Connecting a Magnetized Disk to a Convective Low-mass Protostar: A Global Three-dimensional Model of Boundary Layer Accretion

In the early stages of star formation, boundary layer accretion, where protostars accrete material from disks extending down to their surfaces, plays a crucial role. Understanding how a magneto-rotational-instability (MRI)-active disk connects to a protostar's surface remains a significant challenge. To investigate the mechanisms of mass and angular momentum transfer, we develop a global, three-dimensional magnetohydrodynamic model of boundary layer accretion around a magnetized, convective low-mass protostar. Our results reveal that angular momentum transport mechanisms transition significantly from the outer MRI-active disk to the protostellar surface. Various mechanisms--MRI, spiral shocks, coronal accretion, jets, and disk winds--contribute to angular momentum transfer, resulting in three distinct disk structures: (1) the MRI-active disk, (2) the transition layer, and (3) the boundary layer. The simulated protostar is strongly magnetized due to the accumulation of the disk fields, wrapping by disk toroidal fields, and stellar dynamo activity. Magnetic concentrations analogous to starspots form on the protostar and interact with the rotating disk gas to generate spiral shocks. These shocks play a key role in driving accretion. These findings demonstrate the necessity of global MHD models for a comprehensive understanding of angular momentum transport. Additionally, we identify explosive events triggered by magnetic reconnection in both the protostar and the disk atmosphere. We also find decretion flows in the disk midplane, which may be important for the radial transport of refractory materials, such as Calcium-Aluminium-rich Inclusions (CAIs) precursor gas, to the outer disk.

astro-ph.SR↗

arXiv2025

Evidence for Variable Accretion onto PDS 70 c and Implications for Protoplanet Detections

Understanding the processes of planet formation and accretion in young systems is essential to unraveling the initial conditions of planetary systems. The PDS 70 system, which hosts two directly imaged protoplanets, provides a unique laboratory for studying these phenomena, particularly through H-alpha emission a commonly used accretion tracer. We present multi-epoch observations and examine the variability in accretion signatures within this system, focusing on PDS 70 b and c. Using Hubble Space Telescope narrowband H-alpha imaging from 2020 and 2024, we achieve high signal-to-noise ratio detections of these planets and reveal significant changes in H-alpha flux. For PDS 70 c, the H-alpha flux more than doubled between 2020 and 2024. The trend is consistent with the one identified in recently published MagAO-X data, further confirming that PDS 70 c has become significantly brighter in H between 2023 March and 2024 May. The observed variability suggests dynamic accretion processes, possibly modulated by circumplanetary disk properties or transient accretion bursts. High-amplitude variability in PDS 70 c motivates simultaneous monitoring of multiple accretion tracers to probe the mechanisms of mass growth of gas giant planets. We quantify the impact of variability on the detectability of protoplanets in imaging surveys and emphasize the need for continued and regular monitoring to accurately assess the occurrence and characteristics of young, forming planets.

astro-ph.EP↗

arXiv2025

NuSTAR detection of a hot stellar superflare with a temperature of 95 MK in hard X-rays

A search of the hard X-ray archive data of NuSTAR found a transient source, NuSTAR J230059+5857.4, during an observation of 1E 2259+586 on 2013 April 25. A multi-wavelength analysis using X-ray, optical, and IR data, mostly taken in its quiescent phase, was conducted to identify the origin of NuSTAR J230059+5857.4 and elucidate the phenomena associated with the flare activity. The results indicated that NuSTAR J230059+5857.4 was a stellar flare that occurred on a single M-dwarf, M-dwarf binary, or pre-main-sequence star. NuSTAR J230059+5857.4 exhibited the higher emission measure and higher temperature, 8.60+2.15/-1.73x10^54 cm^-3 and 8.21+2.71/-1.86 keV, respectively, on average than the nominal values of stellar flares reported in the past. The flare loop size estimated on the basis of the model to balance the plasma and magnetic pressures was larger than the stellar radius by a factor of several. Since based on solar flare loops, this flare loop scale is excessively large, we conjecture that the observed large emission measure is possible to be attributed to the observation of mutually-associated multiple flares simultaneously occurring on the stellar surface, known as sympathetic flares. Thanks to the large effective area of NuSTAR in the hard X-ray band, we can conduct detailed discussion about a temperature variation associated with the flare. Investigation of the temperature variation during the flare revealed that the temperature remained significantly higher than during the quiescent phase even after the count rate dropped to around 5% of the peak. The sustained high temperature over the long duration is consistent with the idea of sympathetic flares. We found that it is essential to use theoretical models to evaluate loops and assess temporal changes in temperature as done in this study to determine whether there are multiple flares or not when analyzing flare observation data.

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arXiv2024

Spin-down of solar-mass protostars in magnetospheric accretion paradigm

Stellar spin is one of the fundamental quantities that characterize a star itself and its planetary system. Nevertheless, stellar spin-down mechanisms in protostellar and pre-main-sequence stellar phases have been a long-standing problem in the star formation theory. To realize the spin-down, previous axisymmetric models based on the conventional magnetospheric paradigm have to assume massive stellar winds or produce highly time-variable magnetospheric ejections. However, this picture has been challenged by both numerical simulations and observations. With a particular focus on the propeller regime for solar-mass stars, we propose a new picture of stellar spin-down based on our recent three-dimensional (3D) magnetohydrodynamic simulation and stellar evolution calculation. We show that failed magnetospheric winds, unique to 3D models, significantly reduce the spin-up accretion torque, which make it easier for the star to spin-down. Additionally, the amplitude of time variability associated with magnetospheric ejections is reduced by 3D effects. Our simulation demonstrates that the star spins down by generating a conical disk wind, driven by a rotating stellar magnetosphere. Our theoretical estimates, inspired by the numerical model, suggest that the conical disk wind is likely to play a crucial role in extracting stellar angular momentum during the protostellar phase. As magnetospheric accretion is expected to occur in other accreting objects such as proto-giant planets, this study will also contribute to the understanding of the angular momentum of such objects.

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arXiv2024

Magnetising galaxies with cold inflows

High-redshift ($z\sim2-3$) galaxies accrete circumgalactic gas through cold streams. Recent high-resolution MHD simulations of these streams showed a significant amplification of the intergalactic magnetic field in the shear layer around them. In this work we estimate the magnetisation of high-redshift galaxies that would result purely due to the accretion of already magnetised gas from cold streams. We use the mass inflow rates and saturated magnetic field values from cold stream simulations as input to a simple analytic model that calculates the galactic magnetic field purely from mass accretion. Our model predicts average magnetic field strengths that exceed $\rmμG$ values at $z\sim 2-3$ for inflow rates above $0.1 \, \rm{M_{\odot} yr^{-1}}$. For high inflow rates, our model results are consistent with the recent detection of a strong magnetic field in $z\gtrsim 2.6$ galaxies. Within the assumptions of our simple model, magnetised cold streams emerge as a viable mechanism for seeding a dynamically important galactic magnetic field.

astro-ph.GA↗

arXiv2024

Effect of time-varying X-ray emission from stellar flares on the ionization of protoplanetary disks

X-rays have significant impacts on cold, weakly ionized protoplanetary disks by increasing the ionization rate and driving chemical reactions. Stellar flares are explosions that emit intense X-rays and are the unique source of hard X-rays with an energy of $\gtrsim10$ keV in the protoplanetary disk systems. Hard X-rays should be carefully taken into account in models as they can reach the disk midplane as a result of scattering in the disk atmospheres. However, previous models are insufficient to predict the hard X-ray spectra because of simplifications in flare models. We develop a model of X-ray spectra of stellar flares based on observations and flare theories. The flare temperature and nonthermal electron emissions are modeled as functions of flare energy, which allows us to better predict the hard X-ray photon flux than before. Using our X-ray model, we conduct radiative transfer calculations to investigate the impact of flare hard X-rays on disk ionization, with a particular focus on the protoplanetary disk around a T Tauri star. We demonstrate that for a flare with an energy of $ 10^{35}$ erg, X-ray photons with $\gtrsim 5$ keV increase the ionization rates more than galactic cosmic rays down to $z \approx 0.1R$. The contribution of flare X-rays to the ionization at the midplane depends on the disk parameters such as disk mass and dust settling. We also find that the 10-year-averaged X-rays from multiple flares could certainly contribute to the ionization. These results emphasize the importance of stellar flares on the disk evolution.

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arXiv2024

Assessing the capability of a model-based stellar XUV estimation

Stellar XUV (X-ray and extreme ultraviolet) emission drives the heating and chemical reactions in planetary atmospheres and protoplanetary disks, and therefore, a proper estimation of a stellar XUV spectrum is required for their studies. One proposed solution is to estimate stellar atmospheric heating using numerical models, although the validation was restricted to the Sun over a limited parameter range. In this study, we extend the validation of the model by testing it with the Sun and three young, nearby solar-type stars with available XUV observational data. We first test the model with the solar observations, examining its accuracy in activity minimum and maximum phases, its dependence on loop length, the effect of loop length superposition, and its sensitivity to elemental abundance. We confirm that the model spectrum is mostly accurate both in activity minimum and maximum, although the high-energy X-rays (< 1 nm) are underestimated in the activity maximum. Applying the model to young solar-type stars, we find that it can reproduce the observed XUV spectra within a factor of 3 in the range of 1-30 nm for stars with magnetic flux up to 100 times that of the Sun. For a star with 300 times the solar magnetic flux, although the raw numerical data show a systematically lower spectrum than observed, the spectra are in good agreement once corrected for the effect of insufficient resolution in the transition region. For all young solar-type stars, high-energy X-rays (< 1 nm) are significantly underestimated, with the deviation increasing with stellar magnetic activity. Our findings indicate that the stellar XUV spectrum can be reasonably estimated through a numerical model, given that the essential input parameters (surface magnetic flux and elemental abundance) are known.

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arXiv2024

Upper Limit on the Coronal Cosmic Ray Energy Budget in Seyfert Galaxies

The IceCube collaboration has reported possible detections of high-energy neutrinos from nearby Seyfert galaxies. While central hot coronae are proposed as the primary neutrino production site, the exact coronal cosmic-ray energy budget has been loosely constrained. In this study, we propose a new stringent upper bound on the coronal cosmic-ray energy budget of Seyfert galaxies, considering both accretion dynamics and observed properties of radio-quiet Seyfert galaxies. Notably, even under the calorimetric condition where cosmic rays lose all their energies, our limit indicates that the coronal neutrino flux of NGC~1068 is about an order of magnitude fainter than the observed levels. This discrepancy suggests the need for further theoretical and observational investigations on the IceCube signals from Seyfert galaxies.

astro-ph.HE↗