arXiv subjects

Yusuke Tsukamoto

Publications and source records attributed to Yusuke Tsukamoto.

At least 19 recordsLinked to original sources

arXiv2026

An Adjoint Projection Formulation for Enforcing the divergence-free Constraint in Smoothed Particle Magnetohydrodynamics

We present a projection method for controlling numerical $\nabla\cdot\B$ errors in smoothed particle magnetohydrodynamics (SPMHD). The method corrects the magnetic field after an MHD update by solving an elliptic projection problem constructed from the same discrete divergence operator used to measure the error. A key ingredient is to use the adjoint gradient associated with a volume-weighted metric. With this choice, the projection gives an energy-minimizing correction, does not increase the discrete magnetic energy, and leads to a symmetric positive semidefinite linear system that can be solved by the conjugate-gradient method without explicitly assembling the matrix. We test the method using two-dimensional Dedner-type divergence tests and three-dimensional magnetized collapse calculations. With sufficiently many iterations, the projection reduces the divergence error to the floating-point roundoff level in both test problems. In realistic collapse runs, practical stopping criteria designed to reduce the divergence error generated by the underlying SPMHD update suppress the normalized divergence error well below that obtained in the divergence-cleaning run, with a projection cost of only about $1$--$10\%$ of the SPMHD update cost. The density and plasma-$\beta$ structures remain consistent when the projection interval is varied, whereas the divergence-cleaning run shows quantitative differences. These results indicate that the projection method is a robust and attractive alternative to divergence cleaning for controlling $\nabla\cdot\B$ errors in SPMHD and related particle or meshless MHD schemes.

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arXiv2026

Probing the ion-neutral drift velocity towards the L1544 prestellar core: Detection of ambipolar diffusion using N$_2$D$^+$ and para-NH$_2$D

The dynamical role of the magnetic field in the star formation process is tightly linked to the coupling between matter and the field. This coupling is due to the interaction between ions and neutrals in the partially ionized interstellar medium. When the ionization degree drops in the dense environment of prestellar cores, the magnetic field and the matter may decouple, leading to differences in the infalling velocities of ions and neutrals known as ambipolar diffusion. The onset of gravitational collapse resulting from ion-neutral decoupling has never been observed. The aim of this work is to search for signatures of ambipolar diffusion within a prestellar core. We observed the deuterated N$_2$D$^+$ ion and the neutral para-NH$_2$D species towards the prototypical prestellar core L1544. These two species are ideal tracers of prestellar cores sampling the same high densities in the core interior. We compared the velocity centroid and linewidth maps of the ion-neutral pair. We find a mean ion-neutral velocity difference of $\sim$0.05 km/s towards the core. By comparing with predictions from self-consistent calculations of the ambipolar resistivity including dust grain growth, we interpret the observed ion-neutral velocity difference in L1544 as a signature of ambipolar diffusion. We do not detect a significant ion-neutral linewidth difference that may be attributed to the subsonic infall motions of the gas in L1544 and geometrical effects in the presence of inclination. These results emphasize the role of dust grain growth at the prestellar core stage in setting the ambipolar resistivity and regulating the dynamical evolution of dense cores towards their collapse into protostars. We propose that measurements of ion-neutral drift velocities provide new constraints on the total magnetic field strength and the dust size distribution within prestellar cores.

astro-ph.GA↗

arXiv2026

Nonthermal Velocity Dispersion in the Outer Disk of HL Tau

Turbulence in protoplanetary disks plays a crucial role in the evolution of disk structures and the planet formation process therein. However, the strength of the turbulence remains unclear in young, embedded disks surrounded by infalling envelopes. In this paper, we present the first direct measurement of the nonthermal velocity dispersion within the embedded disk around HL Tau, which possesses a dusty disk with multiple rings and gap structures but is still associated with infalling gas flows from an envelope. Using ALMA archival data of the $\mathrm{H_2CO}$ emission, we measured the local line width through a parametric model fitting that accounts for the contribution of Keplerian shear motion. After subtracting the thermal component, the nonthermal velocity dispersion is $\sim\!\!0.15~\mathrm{km~s^{-1}}$ on average over radii of $80$-$180~\mathrm{au}$, and it slightly increases with radius. The estimated nonthermal motions correspond to a turbulent mach number of $\mathcal{M}\!\!\sim\!\!0.4$ or a viscous $\alpha$ value of $\alpha \!\!\sim\!\!0.16$, assuming that it is entirely caused by turbulence and $\alpha \!\!\sim \!\! \mathcal{M}^2$. Our analysis also suggests that the $\mathrm{H_2CO}$ emission traces near the disk midplane ($z\lesssim 0.1 R$). Turbulence driven by the gravitational instability or infall from the envelope most naturally explains the large nonthermal motions, considering the large disk mass and associated infalling streamers. The strong turbulence measured in the outer disk, in contrast to the vertically settled inner dusty disk, suggests a pronounced radial variation in the turbulence strength and/or an anisotropic nature of the turbulence within the disk.

astro-ph.EP↗

arXiv2026

Early Planet Formation in Embedded Disks (eDisk). XVIII. Indication of a possible spiral structure in the dust-continuum emission of the protostellar disk around IRAS 16544-1604 in CB 68

We performed numerical simulations along with radiative transfer calculations to reproduce an intriguing asymmetric shoulder feature in the dust-continuum emission of the protostellar disk around one of the eDisk targets, the Class 0 protostar IRAS 16544-1604 in CB 68. This is our first attempt to bridge the theoretical works of protostellar disk evolution and the eDisk observations. We found that while our hydrodynamic simulations form spiral structures caused by gravitational instability, they become less discernible after the disk is inclined and convolved with the telescope beam. The widths of the spiral structure as obtained by our numerical simulations are ~0.1-0.8 times the eDisk beam size of 4.5 au. Our modeling effor implies that the apparent absence of spiral features in the eDisk observations does not necessarily indicate the real absence of internal substructures and gravitational instability. We also found that the asymmetric shoulder structure of the continuum profile along the major axis appears when the disk is massive enough with a Toomre parameter Q~1. This mechanism offers a potential explanation for the observed, asymmetric shoulder features in the disks surrounding IRAS 16544-1604 and the other eDisk sources.

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arXiv2026

Ion-Neutral Drift Velocity as a Diagnostic of Dust Growth and Magnetic Field in Star-Forming Environments

Recent observations have revealed that the ion-neutral drift velocity in star-forming molecular clouds and dense cores is on the order of 100 m s^-1. Theoretical studies have shown that, in ambipolar diffusion, the process responsible for the differential motion between ions and neutrals, the dust size distribution has a significant impact on the magnetic resistivities. In this study, we perform simulations to investigate how dust growth through accretion and coagulation affects the ion-neutral drift velocity in molecular clouds and cores. We find that, on core scales, both dust growth and a magnetic field strength of 200 microgauss are required to reproduce the observed drift velocity. We suggest that measurements of ion-neutral drift velocity, particularly on core scales, may serve as a new diagnostic to constrain the dust size distribution and magnetic field strength in such environments.

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arXiv2025

Development of 1-D non-ideal MHD simulation code towards understanding Long-term Evolution of Protoplanetary Disk

We developed a one-dimensional magnetohydrodynamic (MHD) simulation code to investigate the long-term evolution of protoplanetary disks with low computational cost. In this simulation code, the physical processes necessary for protostellar formation and protoplanetary disk evolution, such as magnetic braking, non-ideal MHD effects, and angular momentum transport due to viscosity, are implemented. Using this simulation code, we performed the simulations of the long-term evolution of protoplanetary disks starting from the molecular cloud. Our simulation results suggest that the disk size and mass are a few tens of au and $\sim 0.01 M_\odot$ at $10^5$ years after protostellar formation. These values were relatively consistent with observations. The disk evolves through magnetic braking, and its radial profiles are consistent with the analytical solutions of previous studies. Our simulation code will be an important tool for studying the long-term evolution of protoplanetary disks.

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arXiv2025

"Ashfall" Induced by Molecular Outflow in Protostar Evolution. II. Analytical Study on the Maximum Size of Dust Grains Lifted by Outflows

In this paper, we study the dust dynamics in the molecular outflow using an analytical magnetohydrodynamical outflow model. Specifically, we investigate the maximum size of dust grains $a_{\rm d,max}$ that can be lifted by the outflow and whether they can escape into interstellar space. We also investigate the dependence of the maximum size of the dust grains on various outflow parameters, such as the mass ejection rate of the outflow $\dot{M}$, the disk size $r_{\rm disk}$, the mass of the central protostar $M_{*}$ and the internal dust density $\rho_{\rm mat}$. We find the empirical formula for the maximum dust size as a function of the outflow parameters. $a_{\rm d,max}$ depends on the parameters as $a_{\rm d,max}$ $\propto$ $\dot{M}^{1.0}$ $r_{\rm disk}^{-0.44}$ $M_{*}^{-0.82}$ $\rho_{\rm mat}^{-1.0}$. We also find that the dust grains with size of 100 $\rm \mu$m to 1 mm can be lifted by the outflow and can distribute in the well outside of the disk when the mass ejection rate of the outflow has the value of $10^{-7}\,M_{\odot}\rm \,yr^{-1}$ to $10^{-6}\,M_{\odot}\rm \,yr^{-1}$. This result is consistent with recent observations that show a correlation between the mass ejection rate of the outflow and the spectral index $\beta$ of the dust opacity at the envelope scale.

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arXiv2025

Observationally derived magnetic field strength and 3D components in the HD 142527 disk

In protoplanetary disks around young stars, magnetic fields play an important role for disk evolution and planet formation. Polarized thermal emission from magnetically aligned grains is one of the reliable methods to trace magnetic fields. However, it has been difficult to observe magnetic fields from dust polarization in protoplanetary disks because other polarization mechanisms involving grown dust grains become efficient. Here, we report multi-wavelength (0.87 mm, 1.3 mm, 2.1 mm, and 2.7 mm) observations of polarized thermal emission in the protoplanetary disk around HD 142527, showing the lopsided dust distribution. We revealed that the smaller dust still exhibits magnetic alignment in the southern part of the disk. Furthermore, angular offsets between the observed magnetic field and the disk azimuthal direction were discovered, which can be used as a method to measure the relative strengths of each component (radial ($B_r$), azimuthal ($B_\phi$), and vertical ($B_z$)) of 3D magnetic field. Applying this method, we derived the magnetic field around a 200-au radius from the protostar as $|B_r |:|B_\phi |:|B_z | \sim 0.26:1:0.23$ and a strength of $\sim 0.3$ milli-Gauss. Our observations provide some key parameters of magnetic activities including the plasma beta, which have only been assumed in theoretical studies. In addition, the radial and vertical angular momentum transfer are found to be comparable, which poses a challenge to theoretical studies of protoplanetary disks.

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arXiv2024

A statistical approach for interpreting polarized dust emission of the filamentary molecular clouds toward the estimate of 3D magnetic field structure

In this study, we perform 3D magnetohydrodynamics (MHD) simulations of filamentary molecular clouds. We then generate synthetic observations based on the simulation results. Using these, we investigate how the new polarization data analysis method recently introduced by Doi et al. (2021) reflects the magnetic field structure in turbulent filamentary molecular clouds. Doi et al. (2021) proposed that the $R_{\rm{FWHM}}$, the ratio of the Full Width at Half Maximum (FWHM) of the polarized intensity ($PI$) to that of the total intensity ($I$) can be used to probe the three-dimensional structure of the magnetic field. We calculate the $R_{\rm{FWHM}}$ from the density and magnetic field structure obtained in the 3D-MHD simulations. We find that the mean and variance of $R_{\rm{FWHM}}$ within a filament are smaller and larger, respectively, with a larger inclination of the magnetic field to the plane-of-sky. We also find that both small-scale ($<0.1~\rm{pc}$) and large-scale ($\gtrsim 0.1~\rm{pc}$) turbulence affect the polarized intensity of the dust thermal emission. We conclude that future extensive observations of $R_{\rm{FWHM}}$ may be able to quantify the inclination of the magnetic field to the plane-of-sky in the filamentary molecular clouds.

astro-ph.GA↗

arXiv2024

Co-evolution of dust grains and protoplanetary disks II: structure and evolution of protoplanetary disks; an analytical approach

In our previous study (Tsukamoto {\it et al.} 2023), we investigated formation and early evolution of protoplanetary disks with 3D non-ideal magnetohydrodynamics simulations considering dust growth, and found that the modified equations of the conventional steady accretion disk model which consider the magnetic braking, { dust growth} and ambipolar diffusion reproduce the disk structure obtained from simulations very well. In this paper, as a sequel of the our previous study, we analytically investigate the structure and evolution of protoplanetary disks corresponding to Class 0/I young stellar objects using the modified steady accretion disk model combining an analytical model of envelope accretion. We estimate that the disk radius is several AU at disk formation epoch and increases to several 100 AU at the end of the accretion phase. The disk mass is estimated to be $0.01 M_\odot \lesssim M_{\rm disk} \lesssim 0.1 M_\odot$ for a disk with radius of several 10 AU and mass accretion rate of $\dot{M}_{\rm disk} \sim 10^{-6} M_\odot {\rm yr^{-1}}$. We also found that, with typical disk ionization rates and moderate mass accretion rate ($\dot{M}_{\rm disk}\gtrsim10^{-8} M_\odot {\rm yr^{-1}}$), magneto-rotational instability is suppressed in the disk because of low plasma $β$ and efficient ambipolar diffusion. We argue that the radial profile of specific angular momentum (or rotational velocity) at the disk outer edge should be continuously connected to that of the envelope if the disk evolves by magnetic braking, and should be discontinuous if the disk evolves by internal angular momentum transport process such as gravitational instability or magneto-rotational instability. Future detailed observations of the specific angular momentum profile around the disk outer edge are important for understanding the angular momentum transport mechanism of protoplanetary disks.

astro-ph.SR↗

arXiv2024

Formation of unipolar outflow and $\textit{protostellar rocket effect}$ in magnetized turbulent molecular cloud cores

Observed protostellar outflows exhibit a variety of asymmetrical features, including remarkable unipolar outflows and bending outflows. Revealing the formation and early evolution of such asymmetrical protostellar outflows, especially the unipolar outflows, is essential for a better understanding of the star and planet formation because they can dramatically change the mass accretion and angular momentum transport to the protostars and protoplanetary disks. Here, we perform the three-dimensional non-ideal magnetohydrodynamics simulations to investigate the formation and early evolution of the asymmetrical protostellar outflows in magnetized turbulent isolated molecular cloud cores. We find, for the first time to our knowledge, that the unipolar outflow forms even in the single low-mass protostellar system. The results show that the unipolar outflow is driven in the weakly magnetized cloud cores with the dimensionless mass-to-flux ratios of $μ=8$ and $16$. Furthermore, we find the $\textit{protostellar rocket effect}$ of the unipolar outflow, which is similar to the launch and propulsion of a rocket. The unipolar outflow ejects the protostellar system from the central dense region to the outer region of the parent cloud core, and the ram pressure caused by its ejection suppresses the driving of additional new outflows. In contrast, the bending bipolar outflow is driven in the moderately magnetized cloud core with $μ=4$. The ratio of the magnetic to turbulent energies of a parent cloud core may play a key role in the formation of asymmetrical protostellar outflows.

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arXiv2023

Evolution of the Angular Momentum of Molecular Cloud Cores in Magnetized Molecular Filaments

The angular momentum of molecular cloud cores plays a key role in the star formation process. However, the evolution of the angular momentum of molecular cloud cores formed in magnetized molecular filaments is still unclear. In this paper, we perform three-dimensional magnetohydrodynamics simulations to reveal the effect of the magnetic field on the evolution of the angular momentum of molecular cloud cores formed through filament fragmentation. As a result, we find that the angular momentum decreases by 30% and 50% at the mass scale of 1Msun in the case of weak and strong magnetic field, respectively. By analyzing the torques exerted on fluid elements, we identify the magnetic tension as the dominant process for angular momentum transfer for mass scales < 3Msun for the strong magnetic field case. This critical mass scale can be understood semi-analytically as the timescale of magnetic braking. We show that the anisotropy of the angular momentum transfer due to the presence of strong magnetic field changes the resultant angular momentum of the core only by a factor of two. We also find that the distribution of the angle between the rotation axis and the magnetic field does not show strong alignment even just before the first core formation. Our results also indicate that the variety of the angular momentum of the cores are inherited from the difference of the phase of the initial turbulent velocity field. The variety could contribute to the diversity in size and other properties of protoplanetary disks recently reported by observations.

astro-ph.GA↗

arXiv2023

Co-evolution of dust grains and protoplanetary disks

We propose a new evolutionary process of protoplanetary disks "co-evolution of dust grains and protoplanetary disks", revealed by dust-gas two-fluid non-ideal magnetohydrodynamics simulations considering the growth of dust and associated changes in magnetic resistivity. We found that the dust growth significantly affects disk evolution by changing the coupling between the gas and magnetic field. Moreover, once the dust grains sufficiently grow and the adsorption of charged particles on dust grains becomes negligible, the physical quantities (e.g., density and magnetic field) of the disk are well described by characteristic power laws. In this disk structure, the radial profile of density is steeper and the disk mass is smaller than those of the model ignoring dust growth. We analytically derive these power laws from the basic equations of non-ideal magnetohydrodynamics. The analytical power laws are determined only by observable physical quantities, e.g., central stellar mass and mass accretion rate, and do not include difficult-to-determine parameters e.g., viscous parameter $α$. Therefore, our model is observationally testable and this disk structure is expected to provide a new perspective for future studies on protostar and disk evolution.

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arXiv2023

Cosmic-ray ionization rate versus Dust fraction: Which plays a crucial role in the early evolution of the circumstellar disk?

We study the formation and early evolution of young stellar objects (YSOs) using three-dimensional non-ideal magnetohydrodynamic (MHD) simulations to investigate the effect of cosmic ray ionization rate and dust fraction (or amount of dust grains) on circumstellar disk formation. Our simulations show that a higher cosmic ray ionization rate and a lower dust fraction lead to (i) a smaller magnetic resistivity of ambipolar diffusion, (ii) a smaller disk size and mass, and (iii) an earlier timing of outflow formation and a greater angular momentum of the outflow. In particular, at a high cosmic ray ionization rate, the disks formed early in the simulation are dispersed by magnetic braking on a time scale of about 104 years. Our results suggest that the cosmic ray ionization rate has a particularly large impact on the formation and evolution of disks, while the impact of the dust fraction is not significant.

astro-ph.EP↗

arXiv2022

The role of magnetic fields in the formation of protostars, disks, and outflows

We present our current understanding of the formation and early evolution of protostars, protoplanetary disks, and the driving of outflows as dictated by the interplay of magnetic fields and partially ionized gas in molecular cloud cores. In recent years, the field has witnessed enormous development through sub-millimeter observations which in turn have constrained models of protostar formation. As a result of these observations % that the observations provided, the state-of-the-art theoretical understanding of the formation and evolution of young stellar objects is described. In particular, we emphasize the importance of the coupling, decoupling, and re-coupling between weakly ionized gas and the magnetic field on appropriate scales. This highlights the complex and intimate relationship between gravitational collapse and magnetic fields in young protostars.

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arXiv2022

Impact of dust size distribution including large dust grains on magnetic resistivity: an analytical approach

This paper investigates the impact of dust size distribution on magnetic resistivity. In particular, we focus on its impact when the maximum dust size significantly increases from sub-micron. The first half of the paper describes our calculation method for magnetic resistivity based on the model of \citet{1987ApJ...320..803D} and shows that the method reproduces the results of a more realistic chemical reaction network calculations reasonably well. Then, we describe the results of the resistivity calculations for dust distributions with large maximum dust grains. Our results show that resistivity tends to decrease with dust growth, which is particularly true when the dust size power exponent $q$ is $q=2.5$. On the other hand, the decrease is less pronounced when the dust size power exponent $q$ is $q=3.5$, i.e., when the small dust is also responsible for the dust cross-section. Our results suggest that detailed dust coagulation and fragmentation processes play a vital role in the magnetic resistivities in protostar formation.

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arXiv2022

Calibration and performance of the readout system based on switched capacitor arrays for the Large-Sized Telescope of the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is the next-generation ground-based very-high-energy gamma-ray observatory. The Large-Sized Telescope (LST) of CTA is designed to detect gamma rays between 20 GeV and a few TeV with a 23-meter diameter mirror. We have developed the focal plane camera of the first LST, which has 1855 photomultiplier tubes (PMTs) and the readout system which samples a PMT waveform at GHz with switched capacitor arrays, Domino Ring Sampler ver4 (DRS4). To measure the precise pulse charge and arrival time of Cherenkov signals, we developed a method to calibrate the output voltage of DRS4 and the sampling time interval, as well as an analysis method to correct the spike noise of DRS4. Since the first LST was inaugurated in 2018, we have performed the commissioning tests and calibrated the camera. We characterised the camera in terms of the charge pedestal under various conditions of the night sky background, the charge resolution of each pixel, the charge uniformity of the whole camera, and the time resolutions with a test pulse and calibration laser.

astro-ph.IM↗

arXiv2021

"Ash-fall" induced by molecular outflow in protostar evolution

Dust growth and its associated dynamics play key roles in the first phase of planet formation in young stellar objects (YSOs). Observations have detected signs of dust growth in very young protoplanetary disks. Furthermore, signs of planet formation, gaps in the disk at a distance of several 10 astronomical units (AU) from the central protostar are also reported. From a theoretical point of view, however, it is not clear how planet form at the outer region of a disk despite the difficulty due to rapid inward drift of dust so called radial drift barrier. Here, on the basis of three-dimensional magneto-hydrodynamical simulations of disk evolution with the dust growth, we propose a mechanism named "ash-fall" phenomenon induced by powerful molecular outflow driven by magnetic field which may circumvent the radial drift barrier. We found that the large dust which grows to a size of $\sim \cm$ in the inner region of a disk is entrained by an outflow from the disk. Then large dust decoupled from gas is ejected from the outflow due to centrifugal force, enriching the grown dust in the envelope and is eventually fall onto the outer edge of the disk. The overall process is similar to behaviour of ash-fall from volcanic eruptions. In the ash-fall phenomenon, the Stokes number of dust increases by reaccreting to the less dense disk outer edge. This may make the dust grains overcome the radial drift barrier. Consequently, the ash-fall phenomenon can provide a crucial assist for making the formation of the planetesimals in outer region of the disk possible, and hence the formation of wide-orbit planets and the formation of the gaps.

astro-ph.SR↗