arXiv subjects

R. I. Yamada

Publications and source records attributed to R. I. Yamada.

5 recordsLinked to original sources

arXiv2026

Discovery of Molecular and Atomic Gas associated with HESS J1646-458 (Westerlund 1): Spatial TeV Gamma-Ray and Interstellar Proton Correspondence

We report CO and HI studies of molecular and atomic gas toward the TeV gamma-ray source HESS J1646$-$458, widely considered to be associated with the young massive cluster Westerlund 1 (Wd1). We found that molecular clouds at $V_\mathrm{LSR} \sim$$-32$ km s$^{-1}$ coincide with arc-like structures seen at 8 $μ$m, likely illuminated by strong FUV radiation from Wd1. $^{12}$CO($J$ = 3-2) emission at the same velocity reveals a cavity-like structure with an expansion velocity of $\sim$$5$ km s$^{-1}$ toward the central region of Wd1, suggesting a recently formed wind-blown bubble driven by the cluster. We also identify a complementary spatial distribution between the $V_\mathrm{LSR} \sim$$-55$ and $\sim$$-32$ km s$^{-1}$ clouds, connected by an intermediate-velocity component at $V_\mathrm{LSR} \sim$$-44$ km s$^{-1}$. These characteristics are consistent with signatures of triggered star formation through a cloud-cloud collision and imply that both clouds are physically associated with Wd1. On larger scales, the total interstellar proton column density at $V_\mathrm{LSR}$ $\sim$$-36$-$-23$ km s$^{-1}$ shows a moderate spatial correspondence with the TeV gamma-ray shell. Together with this correlation, a substantial gas mass of $\sim$$1.6 \times 10^6$ $M_\odot$, and the absence of bright synchrotron X-rays, the TeV gamma-ray emission surrounding Wd1 is consistent with the hadronic origin. The present finding allows us to calculate the total energy of accelerated cosmic-ray protons to be $\sim$$6 \times 10^{49}$ erg.

astro-ph.HE↗

arXiv2024

Cloud-Cloud Collision: Formation of Hub-Filament Systems and Associated Gas Kinematics; Mass-collecting cone: A new signature of Cloud-Cloud Collision

Massive star-forming regions (MSFRs) are commonly associated with hub-filament systems (HFSs) and sites of cloud-cloud collision (CCC). Recent observational studies of some MSFRs suggest a possible connection between CCC and the formation of HFSs. To understand this connection, we analyzed the magneto-hydrodynamic simulation data from Inoue et al. (2018). This simulation involves the collision of a spherical turbulent molecular cloud with a plane-parallel sea of dense molecular gas at a relative velocity of about 10 km/s. Following the collision, the turbulent and non-uniform cloud undergoes shock compression, rapidly developing filamentary structures within the compressed layer. We found that CCC can lead to the formation of HFSs, which is a combined effect of turbulence, shock compression, magnetic field, and gravity. The collision between the cloud components shapes the filaments into a cone and drives inward flows among them. These inward flows merge at the vertex of the cone, rapidly accumulating high-density gas, which can lead to the formation of massive star(s). The cone acts as a mass-collecting machine, involving a non-gravitational early process of filament formation, followed by gravitational gas attraction to finalize the HFS. The gas distribution in the position-velocity (PV) and position-position spaces highlights the challenges in detecting two cloud components and confirming their complementary distribution if the colliding clouds have a large size difference. However, such CCC events can be confirmed by the PV diagrams presenting gas flow toward the vertex of the cone, which hosts gravitationally collapsing high-density objects, and by the magnetic field morphology curved toward the direction of the collision.

astro-ph.GA↗

arXiv2024

Giant molecular clouds and their Type classification in M74: Toward understanding star formation and cloud evolution

We investigated the giant molecular clouds (GMCs) in M74 (NGC 628), using data obtained from the PHANGS project. We applied the GMC Types according to the activity of star formation: Type I without star formation, Type II with H$α$ luminosity ($L_\mathrm{Hα}$) less than $10^{37.5}~\mathrm{erg~s^{-1}}$, and Type III with $L_\mathrm{Hα}$ greater than $10^{37.5}~\mathrm{erg~s^{-1}}$. A total of 432 GMCs were identified, with 59, 201, and 172 GMCs, for Type I, II, and III, respectively. The size and mass of the GMCs range from 23 to 238 pc and $10^{4.9}$ to $10^{7.1}$ M$_{\odot}$, indicating that the mass and radius increase from Type I to III. Clusters younger than 4 Myr and HII regions are concentrated within 150 pc of a GMC, indicating a tight association between these young objects and GMCs. The virial ratio decreases from Type I to Type III, indicating that Type III GMCs are the most gravitationally relaxed among the three. We interpret that the GMCs evolve from Type I to Type III, as previously observed in the LMC. Based on a steady-state assumption, the estimated evolutionary timescales of Type I, II, and III are 1, 5, and 4 Myr, respectively. We assume that the timescale of Type III is equal to the age of the associated clusters, indicating a GMC lifetime of 10 Myr or longer. Although Chevance et al. (2020, MNRAS, 493, 2872) investigated GMCs using the same PHANGS dataset of M74, they did not define a GMC, reaching an evolutionary picture with a 20 Myr duration of the non-star-forming phase, which was five times longer than 4 Myr. We compare the present results with those of Chevance et al. (2020) and argue that defining individual GMCs is essential for understanding GMC evolution.

astro-ph.GA↗

arXiv2024

A Kinematic Analysis of the Giant Molecular Complex W3; Possible Evidence for Cloud-Cloud Collisions that Triggered OB Star Clusters in W3 Main and W3(OH)

W3 is one of the most outstanding regions of high-mass star formation in the outer solar circle, including two active star-forming clouds, W3 Main and W3(OH). Based on a new analysis of the $^{12}$CO data obtained at 38$^{\prime\prime}$ resolution, we have found three clouds having molecular mass from 2000 to 8000~$M_\odot$ at velocities, $-50$~km s$^{-1}$, $-43$~km s$^{-1}$, and $-39$~km s$^{-1}$. The $-43$~km s$^{-1}$ cloud is the most massive one, overlapping with the $-39$~km s$^{-1}$ cloud and the $-50$~km s$^{-1}$ cloud toward W3 Main and W3(OH), respectively. In W3 Main and W3(OH), we have found typical signatures of a cloud-cloud collision, i.e., the complementary distribution with/without a displacement between the two clouds and/or a V-shape in the position-velocity diagram. We frame a hypothesis that a cloud-cloud collision triggered the high-mass star formation in each region. The collision in W3 Main involves the $-39$~km s$^{-1}$ cloud and the $-43$~km s$^{-1}$ cloud. The collision likely produced a cavity in the $-43$~km s$^{-1}$ cloud having a size similar to the $-39$~km s$^{-1}$ cloud and triggered the formation of young high-mass stars in IC~1795 2 Myr ago. We suggest that the $-39$~km s$^{-1}$ cloud is still triggering the high-mass objects younger than 1 Myr embedded in W3 Main currently. On the other hand, another collision between the $-50$~km s$^{-1}$ cloud and the $-43$~km s$^{-1}$ cloud likely formed the heavily embedded objects in W3(OH) within $\sim$0.5 Myr ago. The present results favour an idea that cloud-cloud collisions are common phenomena not only in the inner solar circle but also in the outer solar circle, where the number of reported cloud-cloud collisions is yet limited (Fukui et al. 2021, PASJ, 73, S1).

astro-ph.GA↗

arXiv2021

Evidence for a Cloud-Cloud Collision in Sh2-233 Triggering the Formation of the High-mass Protostar Object IRAS 05358+3543

We have carried out a new kinematical analysis of the molecular gas in the Sh2-233 region by using the CO $J$ = 2-1 data taken at $\sim$0.5 pc resolution. The molecular gas consists of a filamentary cloud of 5-pc length with 1.5-pc width where two dense cloud cores are embedded. The filament lies between two clouds, which have a velocity difference of 2.6 km s$^{-1}$ and are extended over $\sim$5 pc. We frame a scenario that the two clouds are colliding with each other and compressed the gas between them to form the filament in $\sim$0.5 Myr which is perpendicular to the collision. It is likely that the collision formed not only the filamentary cloud but also the two dense cores. One of the dense cores is associated with the high-mass protostellar candidate IRAS 05358+3543, a representative high-mass protostar. In the monolithic collapse scheme of high mass star formation, a compact dense core of 100 $M_\odot$ within a volume of 0.1 pc radius is assumed as the initial condition, whereas the formation of such a core remained unexplained in the previous works. We argue that the proposed collision is a step which efficiently collects the gas of 100 $M_\odot$ into 0.1 pc radius. This lends support for that the cloud-cloud collision is an essential process in forming the compact high-mass dense core, IRAS 05358+3543.

astro-ph.GA↗