arXiv subjects

Shu-ichiro Inutsuka

Publications and source records attributed to Shu-ichiro Inutsuka.

At least 19 recordsLinked to original sources

arXiv2026

The Milky Way Atlas for Linear Filaments III: Giant filaments and magnetic fields as evidence of a bubbly Galactic disk

Linear filamentary structures are fundamental constituents of the interstellar medium and play a central role in star formation. Their relative orientation with respect to the ambient magnetic field (B-field) provides key constraints on filament formation mechanisms. We investigate the relative orientation between Milky Way linear filaments (MWLFs) and the plane-of-sky B-field using polarization observations from the Atacama Cosmology Telescope (ACT) DR6, complemented by Planck data. Filament orientations are compared with the local B-field and the Galactic plane, while projection effects and statistical significance are quantified using Monte Carlo simulations of vector pairs in three-dimensions. We find no strong preferential alignment between MWLFs and the ambient B-field. Although the B-field is preferentially aligned with the Galactic plane with relative angles $θ_{\rm BG} \sim0-25°$, filament orientations exhibit a bimodal distribution, being either parallel or perpendicular to the plane ($θ_{\rm FG} \sim0-15°$ and $\sim75-90°$). Filaments located far from the Galactic midplane ($|z|>90$ pc) preferentially show perpendicular alignment with both the plane and the B-field, whereas those near the midplane exhibit a bimodal orientation. These results indicate that large-scale B-fields do not dominate the formation of MWLFs and instead favor a super-Alfvénic regime in which magnetic forces are dynamically subdominant, as expected for filaments associated with supernova-driven shells. Overall, our findings suggest that a face-on view of the Milky Way would resemble nearby disk galaxies such as M74, as observed in JWST images, with its disk structured by a network of supernova-driven bubbles (i.e., a bubbly disk).

astro-ph.GA↗

arXiv2026

Formation of Isotopically Heterogeneous Molecular Cloud Cores in Filamentary Molecular Clouds

Meteorite analysis shows that the older solids of the solar system, such as the calcium-aluminum-rich inclusions (CAIs), have isotopic inhomogeneity. This indicates that the isotopic inhomogeneity could originate from parental molecular clouds. We investigate the evolution of the isotopically heterogeneous molecular cloud cores formed from filament fragmentation using the smoothed particle hydrodynamics method. We show that the effect of the variation of isotopic ratio along the minor axes of the filament is smaller than that along the longitudinal axis of the filament due to the filament geometry. Our results also suggest that isotopic inhomogeneities remain in the resulting cores, although the amounts of initial inhomogeneities are reduced by a factor of 100 from those over the initial filament length of 1 pc. This fraction corresponds to 1-10% of the maximum isotopic ratio that the core can acquire from the filament in each model. The origin of the isotopic inhomogeneity of the shells could be attributed to the initial difference in the center-of-mass of shells caused by the turbulent velocity field. Our model indicates that the isotopic inhomogeneity could survive even in the circumstellar disk.

astro-ph.GA↗

arXiv2026

From inter-filamentary gas to filaments and hubs: gas flows in the Mon R2 hub-filament system

Hub-filament systems (HFSs) play an important role in the formation of massive stars and star clusters. Although the velocity structures along dense filaments have been studied, the gas kinematics in the low density inter-filament regions has not been investigated. We use $^{13}$CO ($J$ = 1--0) and C$^{18}$O ($J$ = 1--0) observations obtained with the Nobeyama 45 m telescope to study the gas dynamics towards the Monoceros R2 (Mon R2) HFS. From the $^{13}$CO and C$^{18}$O data, tracing low- and high-density gas, respectively, we identify velocity coherent structures and divide them into filaments (Fs) and inter-filamentary regions (IFs). We estimate velocity gradients ($Δv$) and mass accretion rates ($\dot{M}$) along ($\parallel$) and across ($\perp$) the Fs and IFs. The mean ratio of $\dot{M}_\parallel$ to $\dot{M}_\perp$ in Fs is 6.8, while that in IFs is 1.5. These results show that the overall gas within both Fs and IFs flows directly into the hub and the gas flows faster along the Fs than the IFs. In addition, we found that at least 30\% of the gas mass in the IFs may flow towards the Fs replenishing the latter with new matter. Our study reveals the importance of considering the total gas mass reservoir, both low- and high-density, infalling into the hub and promoting the formation of massive stars, which are preferentially located in the hub of Mon R2.

astro-ph.GA↗

arXiv2026

Modeling the Accretion of High-Velocity Clouds from a Rotating Halo

High-Velocity Clouds (HVCs) are a major fuel reservoir for star formation in the Galactic disk. Determining their origin and kinematics is thus crucial for understanding Galactic evolution. In this paper, we employ simple test-particle simulations to model HVC kinematics, generating line-of-sight velocity maps and probability density functions (PDFs) for comparison with observational results. We find that models assuming low angular momentum and an initial scale of tens of kiloparsecs (kpc) successfully reproduce the observed kinematic trends for both blue-shifted and red-shifted components. This consistency may support the dominance of intermediate-halo dynamics (tens of kpc scale) in regulating Galactic evolution, consistent with HVC formation via thermal instability in metal-polluted gas in the halo. Furthermore, by considering the entire bulk mass involved in the continuous accretion process -- including diffuse or ionized components that often escape direct observation -- our theoretical estimates yield a total mass accretion rate of several solar masses per year. This indicates that HVC accretion has the potential to supply a sufficient amount of gas to the Galactic disk to sustain ongoing star formation over several Gyr. Our findings suggest that the Galactic baryon cycle and disk evolution are governed by dynamics within the intermediate halo, providing key kinematic constraints for future magnetohydrodynamical simulations that resolve spatial structures of high velocity clouds.

astro-ph.GA↗

arXiv2026

Interplay between Escaping Cosmic Rays and Interstellar Medium: Driving of Galactic Winds and Shaping the Local Proton Spectrum

We study the effects of escaping cosmic rays (CRs) on the interstellar medium (ISM) around their source with spherically symmetric CR-hydrodynamical simulations taking into account the evolution of the CR energy spectrum, radiative cooling, and thermal conduction. We show how the escaping CRs accelerate and heat the ISM depending on the CR diffusion coefficient. The CR heating effects are potentially responsible for the recent observations of the unexpected H$α$ and [OIII]$\lambda5007$ lines in old supernova remnants. The implied gas outflow rate by CRs can be comparable to the Galactic star formation rate, compatible with the Galactic wind required for polluting the halo gas with metals. Assuming a locally suppressed CR diffusion and a few nearby CR sources in the Local Bubble, we also propose alternative interpretations for the Galactic CR proton spectrum around the Earth measured with CALET, AMS02, and Voyager 1.

astro-ph.HE↗

arXiv2026

An Origin of Radially Aligned Filaments in Hub-Filament Systems

Recent observations have identified hub-filament systems (HFSs) as the primary formation sites of massive stars and star clusters. Some HFSs are characterized by multiple filaments aligned radially toward a central high-density hub. However, the physical origin of radially aligned filaments remains unknown. Here, we propose a new formation mechanism of HFSs driven by the interaction of a fast magnetohydrodynamic shock with a molecular cloud characterized by an hourglass-shaped magnetic field and density inhomogeneity. Our three-dimensional magnetohydrodynamic simulations show that the shock propagation leads to the formation of radially aligned filamentary structures with line masses slightly above the thermally critical line mass and lengths of $1$-$3\,\rm{pc}$, and widths of $0.06$-$0.08\,\rm{pc}$. High-density filamentary gas ($n_{\rm{H_2}} \sim 10^4 \, \rm{cm^{-3}}$) selectively exhibits inward velocities of $1-4\, \rm{km \, s^{-1}}$ that increase toward the hub center, while the ambient low-density inter-filament gas retains low velocities regardless of the radius. Mass accretion onto the hub is channeled through dense filaments. The filament formation is driven by oblique shocks generated at the bent magnetic field lines. The resulting post-shock amplification of the tangential magnetic field induces a magnetically guided inflow. The shock-interface interaction amplifies density perturbations, resembling Richtmyer--Meshkov instability modes, which promotes the fragmentation of the shocked layer into multiple filaments. The process studied in this Letter explains both the morphology of radially aligned filaments and the selective mass accretion observed in HFSs. In our simulation, the resulting star formation efficiency is $\sim4\%$, suggesting that the shock-driven evolution limits the SFE to only a few percent.

astro-ph.GA↗

arXiv2025

Linear Analyses of Thermal Instability in Stratified Medium

Thermal instability in the circum-galactic medium (CGM) can be responsible for the existence of cold clouds (e.g., high-velocity clouds) embedded in a hot diffuse medium (e.g., X-ray emitting gas). While many previous studies have analyzed thermal instability in uniform medium, the instability mechanism in gravitationally stratified medium like CGM has not been fully analyzed. This study investigates how gravity affects the behavior of thermal instability through linear perturbation analyses.We find that in stratified medium, thermal instability can drive over-stable modes, a behavior distinctly different from the monotonic growth of thermal instability in a uniformmedium. Furthermore, we find that the combination of buoyancy and thermal instability drives other two unstable modes. Applying our results to a simplified model of the CGM, we estimate the gas accretion rate from the CGM to the Galactic disk and the typical size of high-velocity cloud driven by thermal instability to be a few solar masses per year. This gas accretion rate is comparable to the observed star formation rate, and hence, the mass in the Galactic disk can be maintained. Our results provide a theoretical framework for understanding the formation of multi-phase gas, particularly in the CGM.

astro-ph.GA↗

arXiv2025

Fragmenting Filaments and Evolving Cores -- Insights from Dust Polarisation Study of a filament in Northern Orion B

We present an analysis of polarised dust emission at 850 micron for a parsec long filament in the northern part of the Orion B molecular cloud. The region was observed by the JCMT SCUBA-2/POL-2 polarimeter. The filament has a line mass (~80 Msun/pc) larger than the critical (magnetic) line mass (~37 Msun/pc); and hosts one starless, three prestellar, and four protostellar cores, with masses in the range 0.13 to 9.13 Msun. The mean (debiased) polarisation fraction of the filament and core pixels was calculated to be 5.3+/-0.3% and 3.2+/-0.3%, respectively, likely reflecting their distinct physical conditions. The polarisation fraction for the cores does not depend on the type of core, and was found to decrease with increasing column density, varying from 6-11% at the filament edges to 1$^{+0.7}_{-0.1}$% in the denser parts ($N_{H2}\gtrsim$2x10$^{22}$cm$^{-2}$). Magnetic field orientation of the protostellar cores, in contrast to prestellar cores, appears to be relatively aligned with the magnetic field orientation of the local filament in this region. Using the Davis-Chandrasekhar-Fermi formalism the plane-of-sky magnetic field strength for the protostellar cores (~39-110 microG) was found to be higher than that of the prestellar cores (~22-61 microG); and weakest for the starless core (~6 microG). The average value for the filament was found to be ~31 microG. The magnetic field-volume density relation for the prestellar/starless cores and protostellar cores suggests a transition from weak field case to strong field case as the cores evolve from prestellar to protostellar phase.

astro-ph.GA↗

arXiv2025

Modeling of Collisional Outcomes Based on Impact Simulations of Mars-sized Bodies

We investigate the outcomes of collisions between Mars-sized bodies through smooth particle hydrodynamics (SPH) simulations, focusing on the transitions among ``merging'', ``hit-and-run'', and catastrophic disruption. By systematically varying impact velocity, angle, and mass ratio, we characterize the dependence of collision outcomes on geometric and energetic parameters. A new analytic model is developed using characteristic energies -- particularly the energy deposited in overlapping regions of the colliding bodies -- to accurately describe the mass of the largest and second-largest remnants. The model successfully reproduces simulation results across a broad range of impact conditions and improves on previous models by better capturing the transitions between ``merging'', ``hit-and-run'', and disruption. We also derive outcome formulas averaged over impact-parameter-weighted angular distributions, enabling more realistic applications to integrated modeling of planet formation. The model further shows consistency with outcomes from dust aggregate collision simulations, highlighting its utility for modeling collisional processes not only for large planetesimals but also for smaller bodies.

astro-ph.EP↗

arXiv2025

The Dynamical Interaction between Low-mass Planets and Dust Coagulation

We investigate the impact of a low-mass planet on dust coagulation, and its consequent feedback on planetary migration, using a linear analysis of the coupled dust-gas hydrodynamic equations. Dust coagulation is incorporated via a single-size approximation. In the co-orbital region of the planet, we find that the growth of dust size is significantly suppressed by planet-induced coagulation modes (CMs). This effect are less pronounced with smaller stopping times, stronger gaseous turbulence or imperfect sticking. Regarding planetary migration, we find that CMs make outward migration require $τ\gtrsim 0.3$ ($τ$ is dimensionless stopping time) with typical turbulent strength and dust coagulation efficiency. We demonstrate that the torque variations are reasonable and arise from phase shifts between the density and stopping time perturbations in the coagulation modes.

astro-ph.EP↗

arXiv2025

Special Relativistic Smoothed Particle Hydrodynamics Based on Riemann Solver

This paper proposes a novel numerical method based on Godunov Smoothed Particle Hydrodynamics for special relativistic fluid dynamics. Our method utilizes a Riemann solver to describe shock, enhancing accuracy in strong shock waves. The formulation maintains conservation laws and achieves higher accuracy through convolution integrals that define physical quantities for SPH particles. We also propose the number density calculation method that uses a non-equal baryon number in each SPH particle and variable smoothing length in a way different from the conventional method. Numerical experiments demonstrate the method's robustness across one- and two-dimensional relativistic shock tube problems, as well as its ability to simulate Kelvin-Helmholtz instabilities accurately, validating SRGSPH as a reliable approach for high-resolution relativistic simulations.

physics.comp-ph↗

arXiv2025

Growth of Massive Molecular Cloud Filament by Accretion Flows. II. New Mechanism to Support a Supercritical Filament against Radial Collapse

Observations indicate that dense molecular filamentary clouds are sites of star formation. The filament width determines the most unstable scale for self-gravitational fragmentation and influences the stellar mass. Therefore, constraining the evolution of filaments and the origin of their properties are important for understanding star formation. Although some observations show a universal width of 0.1 pc, many theoretical studies predict the contraction of thermally supercritical filaments (> 17 Msun pc-1) due to radial collapse. Through non-ideal magnetohydrodynamics simulations with ambipolar diffusion, we explore the formation and evolution of filaments via slow-shock instability at the front of accretion flows. We reveal that ambipolar diffusion allows the gas in the filament to flow across the magnetic fields around the shock front, forming dense blobs behind the concave points of the shock front. The blobs transfer momentum that drives internal turbulence. We name this mechanism the "STORM" (Slow-shock-mediated Turbulent flOw Reinforced by Magnetic diffusion). The persistence and efficiency of the turbulence inside the filament are driven by the magnetic field and the ambipolar diffusion effect, respectively. The STORM mechanism sustains the width even when the filament reaches very large line masses (~ 100 Msun pc-1).

astro-ph.GA↗

arXiv2025

Analytical and Numerical Linear Analyses of Convection Revisited

We conduct linear analyses of convection in domains larger than the temperature scale height. We employ both analytical and numerical methods in these analyses. In the case excluding all dissipation, the typical time scale of convection is determined by the free fall time over the temperature scale height. We quantitatively show the condition for the Boussinesq and Wentzel-Kramers-Brillouin (WKB) approximations to be applicable. We provide a reassessment of the critical Rayleigh number, a key indicator of convection, and show that WKB approximation tends to underestimate the critical Rayleigh number, particularly when the temperature scale height is comparable to or smaller than the domain height. We show clear explanation why both thermal conduction and viscosity are required for stabilizing negative entropy gradient medium.

physics.flu-dyn↗

arXiv2025

The JCMT BISTRO Survey: Unveiling the Magnetic Fields around Galactic Center

We acquired 450 μm and 850 μm dust continuum polarization observations toward the inner region of the Central Molecular Zone (CMZ) as part of the B-Fields In Star-Forming Region Observations (BISTRO) survey using the POL-2 polarimeter on the James Clerk Maxwell Telescope. These observations encompassed three dense structures: the 20 km s{^{-1}} cloud (20MC), 50 km s{^{-1}} cloud (50MC), and circumnuclear disk (CND). Our aim is to investigate the magnetic field morphology and strength in the inner region of the CMZ using polarized dust continuum and the Davis-Chandrasekhar-Fermi method. The magnetic field morphology is highly ordered in all three dense regions. The plane-of-sky magnetic field strengths are {\sim}1 mG for the 20MC and the 50MC, and {\sim}2 mG for the CND. We compare the energy contributions of turbulence, gravity, and thermal motion with that of the magnetic field using the plasma β, mass-to-flux ratio, and Alfvén Mach number. The outcomes reveal the magnetic field stands out as the predominant factor within the inner region of the CMZ. The dominance of the magnetic field may explain the low star-forming rate in the CMZ. We further investigate the dust grain alignment efficiency by exploring the relationship between polarization fraction and total intensity. The results suggest that dust grains are well aligned with the magnetic fields.

astro-ph.GA↗

arXiv2025

ALMA observations of massive clouds in the central molecular zone: slim filaments tracing parsec-scale shocks

The central molecular zone (CMZ) of our Galaxy exhibits widespread emission from SiO and various complex organic molecules (COMs), yet the exact origin of such emission is uncertain. Here we report the discovery of a unique class of long ($>$0.5 pc) and narrow ($<$0.03 pc) filaments in the emission of SiO 5$-$4 and eight additional molecular lines, including several COMs, in our ALMA 1.3 mm spectral line observations toward two massive molecular clouds in the CMZ, which we name as slim filaments. However, these filaments are not detected in the 1.3 mm continuum at the 5$σ$ level. Their line-of-sight velocities are coherent and inconsistent with being outflows. The column densities and relative abundances of the detected molecules are statistically similar to those in protostellar outflows but different from those in dense cores within the same clouds. Turbulent pressure in these filaments dominates over self gravity and leads to hydrostatic inequilibrium, indicating that they are a different class of objects than the dense gas filaments in dynamical equilibrium ubiquitously found in nearby molecular clouds. We argue that these newly detected slim filaments are associated with parsec-scale shocks, likely arising from dynamic interactions between shock waves and molecular clouds. The dissipation of the slim filaments may replenish SiO and COMs in the interstellar medium and lead to their widespread emission in the CMZ.

astro-ph.GA↗

arXiv2024

A Tale of Three: Magnetic Fields along the Orion Integral-Shaped Filament as Revealed by JCMT BISTRO survey

As part of the BISTRO survey, we present JCMT 850 $μ$m polarimetric observations towards the Orion Integral-Shaped Filament (ISF) that covers three portions known as OMC-1, OMC-2, and OMC-3. The magnetic field threading the ISF seen in the JCMT POL-2 map appears as a tale of three: pinched for OMC-1, twisted for OMC-2, and nearly uniform for OMC-3. A multi-scale analysis shows that the magnetic field structure in OMC-3 is very consistent at all the scales, whereas the field structure in OMC-2 shows no correlation across different scales. In OMC-1, the field retains its mean orientation from large to small scales, but shows some deviations at small scales. Histograms of relative orientations between the magnetic field and filaments reveal a bimodal distribution for OMC-1, a relatively random distribution for OMC-2, and a distribution with a predominant peak at 90$^\circ$ for OMC-3. Furthermore, the magnetic fields in OMC-1 and OMC-3 both appear to be aligned perpendicular to the fibers, which are denser structures within the filament, but the field in OMC-2 is aligned along with the fibers. All these suggest that gravity, turbulence, and magnetic field are each playing a leading role in OMC-1, 2, and 3, respectively. While OMC-2 and 3 have almost the same gas mass, density, and non-thermal velocity dispersion, there are on average younger and fewer young stellar objects in OMC-3, providing evidence that a stronger magnetic field will induce slower and less efficient star formation in molecular clouds.

astro-ph.GA↗

arXiv2024

The JCMT BISTRO Survey: The Magnetic Fields of the IC 348 Star-forming Region

We present 850 $μ$m polarization observations of the IC 348 star-forming region in the Perseus molecular cloud as part of the B-fields In STar-forming Region Observation (BISTRO) survey. We study the magnetic properties of two cores (HH 211 MMS and IC 348 MMS) and a filamentary structure of IC 348. We find that the overall field tends to be more perpendicular than parallel to the filamentary structure of the region. The polarization fraction decreases with intensity, and we estimate the trend by power-law and the mean of the Rice distribution fittings. The power indices for the cores are much smaller than 1, indicative of possible grain growth to micron size in the cores. We also measure the magnetic field strengths of the two cores and the filamentary area separately by applying the Davis-Chandrasekhar-Fermi method and its alternative version for compressed medium. The estimated mass-to-flux ratios are 0.45-2.20 and 0.63-2.76 for HH 211 MMS and IC 348 MMS, respectively, while the ratios for the filament is 0.33-1.50. This result may suggest that the transition from subcritical to supercritical conditions occurs at the core scale ($\sim$ 0.05 pc) in the region. In addition, we study the energy balance of the cores and find that the relative strength of turbulence to the magnetic field tends to be stronger for IC 348 MMS than HH 211 MMS. The result could potentially explain the different configurations inside the two cores: a single protostellar system in HH 211 MMS and multiple protostars in IC 348 MMS.

astro-ph.GA↗

arXiv2024

Relative Alignments Between Magnetic Fields, Velocity Gradients, and Dust Emission Gradients in NGC 1333

Magnetic fields play an important role in shaping and regulating star formation in molecular clouds. Here, we present one of the first studies examining the relative orientations between magnetic ($B$) fields and the dust emission, gas column density, and velocity centroid gradients on the 0.02 pc (core) scales, using the BISTRO and VLA+GBT observations of the NGC 1333 star-forming clump. We quantified these relative orientations using the Project Rayleigh Statistic (PRS) and found preferential global parallel alignment between the $B$ field and dust emission gradients, consistent with large-scale studies with Planck. No preferential global alignments, however, are found between the $B$ field and velocity gradients. Local PRS calculated for subregions defined by either dust emission or velocity coherence further revealed that the $B$ field does not preferentially align with dust emission gradients in most emission-defined subregions, except in the warmest ones. The velocity-coherent structures, on the other hand, also showed no preferred $B$ field alignments with velocity gradients, except for one potentially bubble-compressed region. Interestingly, the velocity gradient magnitude in NGC 1333 ubiquitously features prominent ripple-like structures that are indicative of magnetohydrodynamic (MHD) waves. Finally, we found $B$ field alignments with the emission gradients to correlate with dust temperature and anticorrelate with column density, velocity dispersion, and velocity gradient magnitude. The latter two anticorrelations suggest that alignments between gas structures and $B$ fields can be perturbed by physical processes that elevate velocity dispersion and velocity gradients, such as infall, accretions, and MHD waves.

astro-ph.GA↗