arXiv subjects

Yasuo Fukui

Publications and source records attributed to Yasuo Fukui.

At least 19 recordsLinked to original sources

arXiv2026

ALMA Observations of DEM L241/LMC P3 in the Large Magellanic Cloud: Evidence for the Formation of Cool Molecular Jets Driven by a Microquasar

We present ALMA observations of DEML 241/LMC P3, the most luminous $\gamma$-ray binary consisting of a compact object and an O star, in CO emission. We have found an one-sided jet-like CO feature of 8 pc length and 1 pc width, which accompanies another weaker CO jet candidate with slightly different orientation. The one-sided CO jet exhibits striking alignment with LMC P3, suggesting that the jet was driven by LMC P3. We have determined kinetic temperature of the CO jet to be significantly high at 33$-$60 K as compared with $\sim$15 K in the nearby non-jet CO cloud whereas no radiative heat source is found. We interpret that the high temperatures are due to shock heating of a microquasar jet driven by the $\gamma$-ray binary, where the compact object has an accretion disk fed by the O star winds. The CO jet matches existing predictions from magneto-hydrodynamical simulations, which show that CO jet can form from the interaction of the microquasar jet and an ambient ISM cloud. These results provide strong evidence that CO jets are a signature sculptured by microquasar jets, lending support for mass accretion in LMC P3 as the $\gamma$-ray origin. The results suggest a second case of CO jets potentially driven by a microquasar along with the CO jets in the microquasar candidate HESS J1023-575 recently identified in the Milky Way. Further, our results suggest the use of sub-mm observations for identifying microquasars, opening a new possible window for their discovery and study.

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arXiv2026

An ALMA view of the Jet-Arc CO clouds toward the TeV $\gamma$-ray source HESS J1023-575 and Westerlund 2; Evidence for the footprints of microquasar jets, the very powerful cosmic-ray accelerator in the Galactic disk

The TeV $\gamma$-ray source HESS J1023-575 (HESSJ 1023 hereafter) is one of the brightest H.E.S.S. sources near the young massive cluster Westerlund 2. HESS J1023 shows a remarkable positional alignment with the Jet and Arc CO clouds on its eastern and western sides over 170 pc length. We have carried out sub-pc scale observations of the CO clouds with ALMA and have discovered that the clouds consist of numerous thin filamentary features of $\sim$0.5 pc width and 10--20 pc length at distance of 7.5 kpc, which are well aligned with the Jet-Arc axis. Based on the magneto-hydrodynamical model of microquasar jets launched from {the center of the $\gamma$-ray source} HESS J1023-575, we present an interpretation that the thin filamentary clouds are the footprints of the microquasar jets on the HI gas. The model also explains the dissimilar Jet vs. Arc clouds in terms of HI density difference on each side. By using the density of the CO and HI gas and the $\gamma$-ray luminosity, we have calculated the cosmic ray proton energy $W_{\rm p}$ to be 7$\times$10$^{48}$ erg under the hadronic scheme, which is ten times larger than those derived in the TeV $\gamma$-ray SNRs RX J1713.7-3946 and RX J0852.0-4622. It is likely that HESS~J1023 has been active over 1-10 Myr, which is significantly longer than the duration of cosmic ray acceleration of the SNRs. HESS~J1023 is therefore an outstanding source of cosmic rays equivalent to at least 1000 SNRs, and is possibly the most powerful CR accelerator in the Galactic disk. A high energy compact source in HESS~J1023, which is likely a Myr-old black hole or neutron star, remains veiled due to heavy extinction.

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arXiv2026

Discovery of CO Clouds Associated with the X-ray Jets of SS 433: Evidence for Shock-Cloud Interaction Enhancing Nonthermal X-ray Emission

We report the first identification of molecular clumps directly associated with the re-brightening regions of the large-scale X-ray jets of SS 433, based on $^{12}$CO ($J$ = 1--0) observations with the Nobeyama 45-m Radio Telescope. Multiple clumps are detected toward the eastern and western jet heads, showing clear spatial correlation with the X-ray emission. The X-ray emission peaks immediately downstream of the molecular clumps, while the hardness ratio is enhanced at their surfaces, indicating that the observed structures cannot be explained by absorption effects. These results provide direct evidence for shock--cloud interactions between the jets and the surrounding interstellar medium. We suggest that turbulence generated at the jet--cloud interface amplifies magnetic fields, producing the observed non-thermal X-ray emission. Our findings highlight the importance of jet--ISM interactions in shaping the X-ray properties of microquasar jets.

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arXiv2026

Optical observations and atomic environment of supernova remnant G25.1-2.3

The supernova remnant (SNR) G25.1-2.3 was identified in the radio band during the Sino-German $\lambda$6 cm survey of the Galactic plane. We present a detailed investigation of the optical, HI, and CO emission towards the G25.1-2.3 to better understand its characteristics and environment. In this study, optical spectroscopic data of the remnant and its environment have been analysed for the first time, providing new insights into their emission properties. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and 1.5-m Russian-Turkish Telescope (RTT150) data show variations across the observed regions, with [SII]/H$\alpha$ ranging from 0.16 to 0.83. We identified shock-heated gas in the northern and southern regions and several photoionized regions around the SNR based on their [SII]/H$\alpha$ ratios derived from spectra. The [SII]$\lambda$6716/$\lambda$6731 ratio observed in the northern region suggests electron densities ($n_{\rm e}$) ranging from 120 to 1030 cm$^{-3}$, whereas the southern regions show higher values, between 490 and 4500 cm$^{-3}$. The variations in the observed H$\alpha$/H$\beta$ ratios indicate significant differences in extinction across the regions. H$\alpha$ images obtained using the 1-m Turkish Telescope (T100) reveal optical emission in the northern and southern, characterized by filamentary and diffuse structures. We newly found a hole-like distribution of HI, whose spatial extent is roughly consistent with the diameter of the SNR. Based on radio data, we examine the evolutionary stage of G25.1-2.3 using the surface brightness-diameter ($\Sigma-D$) relation and the equipartition method.

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arXiv2026

A multi-scale molecular and atomic gas view on the HII region N113 in the Large Magellanic Cloud:Evidence for high-mass star formation triggered by supersonically-colliding HI flows

The Large Magellanic Cloud (LMC) exhibits vigorous high-mass star formation, including the HII regions 30~Dor that is the most active site of star formation in the local group. The present paper focuses on the Giant Molecular Cloud (GMC) in the HII region N113 in the central part of the LMC. Based on the $^{12}$CO($J$ =1-0) and $^{13}$CO($J$ = 1-0) data at a resolution of approximately 0.2 pc taken with ALMA+APEX, we reveal that the GMC consists of two filamentary structures each of approximately 10 pc in length, forming a V-shape pattern with a vertex angle of 90 degrees. The filamentary structures host high-mass young stellar objects in gravitationally bound dense gas. Large-scale HI gas data covering 100 pc reveal two distinct velocity components separated by more than 40 km s$^{-1}$, that correspond to the low velocity (L-) and disk (D-) HI components of the LMC. The L-component appears to be located in a cavity-like distribution of the D-component, and the CO filaments are positioned at the cavity's edge. We find evidence for the L-component to fit the cavity by a 53 pc displacement, and suggest that collisional compression of the HI gas during the last 1.3 Myr triggered the GMC formation and the high-mass star formation. This lends support for the large scale collision driven by the tidal interaction is playing a role in evolution of interstellar medium in N113.

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arXiv2026

ALMA Band 9 CO(6--5) Reveals a Warm Ring Structure Associated with the Embedded Protostar in the Cold Dense Core MC 27/L1521F

Infall and outflows, coupled with magnetic fields, rapidly structure the gas around newborn protostars. Shocks from interacting components encode the temperature and density distribution, offering a direct probe of the earliest evolution history. However, interferometric observations characterizing warm envelopes using high-excitation lines remain scarce. We present ALMA Band 9 observations of the Taurus dense core MC 27/L1521F, which hosts a Class 0 protostar, targeting the CO($J$=6-5) line at an angular resolution of $\sim$2\arcsec\ ($\approx$300 au). We detect an off-centered ring-like structure with a diameter of $\sim$1000 au that was not identifiable in previous low-$J$ CO data, where emission close to the systemic velocity is strongly affected by optical depth. The ring shows a typical peak brightness temperature of $\sim$3 K at our resolution. Excitation considerations indicate that the detected CO($J$=6-5) emission likely arises from relatively warm ($T \gtrsim 20$ K) and dense ($n({\rm H_2}) \gtrsim 10^{5}$ cm$^{-3}$) gas embedded within the surrounding cold, dense core. The morphology and kinematics suggest an energetic and localized shock-heating event, potentially linked to dynamical gas--magnetic-field interactions in the earliest protostellar phase. Our results demonstrate that high-$J$ CO observations provide a powerful new window on warm and dense gas components, enabling a more direct view of the physical processes operating at the onset of star formation.

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arXiv2026

Gas Kinematics and Cosmic-Ray Acceleration in the Gamma-ray SNRs W41 and G22.7-0.2

We present a study of the interstellar medium associated with the two middle-aged supernova remnants (SNRs) W41 and G22.7-0.2, both detected in TeV gamma-rays. Using high-angular-resolution $^{12}$CO($J$ = 1-0) data from the Nobeyama 45-m telescope and HI data from the VLA, we investigated the spatial and kinematic properties of molecular and atomic gas that interact with the SNRs. We identified associated clouds in the velocity ranges of +50-+80 km s$^{-1}$ for W41 and +76-+110 km s$^{-1}$ for G22.7-0.2. Column density analysis indicates that target protons are dominated by molecular hydrogen, while atomic hydrogen contributes less than $\sim$10-15% even after correction for self-absorption. The mean proton densities are $\sim$1.2$\times$10$^{3}$ cm$^{-3}$ for W41 and $\sim$5.3$\times$10$^{2}$ cm$^{-3}$ for G22.7-0.2. From the gamma-ray luminosities, we estimate the total energy of accelerated cosmic-ray protons as $W_\mathrm{p}$ $\sim$3$\times$10$^{47}$~erg for W41 and $\sim$1$\times$10$^{48}$ erg for G22.7-0.2, corresponding to 0.03-0.1% of the canonical supernova explosion energy. hese $W_\mathrm{p}$ values agree with the decreasing trend in $W_\mathrm{p}$ observed in the middle-aged SNRs within the previously reported SNR age-$W_\mathrm{p}$ relation.

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arXiv2026

The Supernova Remnant G284.3$-$1.8 and Its Relation to the Gamma-ray Binary 1FGL J1018.6$-$5856

G284.3$-$1.8 is a supernova remnant with a radio shell and thermal X-ray emission. Located near its center is the gamma-ray binary 1FGL J1018.6$-$5856, although the physical association between the two systems is not clear yet. Our X-ray spectroscopy with Suzaku reveals that G284.3$-$1.8 and 1FGL J1018.6$-$5856 have compatible absorption column densities of $N_\mathrm{H} = 6\textrm{--}7 \times 10^{21}~\mathrm{cm}^{-2}$, indicating that the two systems have similar distances. The actual distance is determined as $3~\mathrm{kpc}$ using $\mathrm{^{12}CO}$ ($J=1\textrm{--}0$) data obtained with NANTEN. The X-ray spectrum of G284.3$-$1.8 shows a strong K-shell emission line of Mg, confirming that the earlier claim that the SNR is one of the few Mg-rich SNRs. Comparing recent stellar models taking into account the "shell merger" processes, we find that the obtained Mg-to-Ne mass ratio of $M_\mathrm{Mg}/M_\mathrm{Ne} = 0.73^{+0.07}_{-0.03}$ and Si-to-Mg mass ratio of $M_\mathrm{Si}/M_\mathrm{Mg} = 0.44\pm0.03$ suggest a supernova explosion that would have left a neutron star. The characteristics of 1FGL J1018.6$-$5856, on the other hand, are better explained with a model in which its compact object is neutron star. The present results, therefore, would suggest a possible scenario where G284.3$-$1.8 and 1FGL J1018.6$-$5856 are both remnants of a common supernova explosion although further observational tests are necessary.

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arXiv2025

Head-tail molecular clouds falling onto the Milky Way disk

We report discovery of two CO clouds which are likely falling down to the Galactic plane at more than $35$ km s$^{-1}$. The clouds show head-tail distributions elongated perpendicular to the Galactic plane at $l=331.6^{\circ}$ and $b=0^{\circ}$ as revealed by an analysis of the Mopra CO $J=$1-0 survey data. We derived the distance of the clouds to be $2.46 \pm 0.18$ kpc based on the Gaia Data Release 3. The CO clouds have molecular masses of $4.8\times 10^3\ M_{\odot}$ and $3.5\times 10^3\ M_{\odot}$, respectively, and show kinetic temperature of 30-50 K as derived from the line intensities of the $^{13}$CO $J$=2-1, $^{12}$CO $J$=1-0, and $^{13}$CO $J$=1-0 emission. The temperature in the heads of the clouds is significantly higher than 10 K of the typical molecular clouds, although no radiative heat source is found inside or close to the clouds. Based on the results, we interpret that the present clouds are falling onto the Milky Way disk and are significantly heated up by the strong shock interaction with the disk HI gas. We suggest that the clouds represent part of the HI intermediate velocity clouds falling to the Galactic plane which were converted into molecular clouds by shock compression. This is the first case of falling CO clouds having direct observed signatures of the falling motion including clear directivity and shock heating. Possible implications of the CO clouds in the evolution of the Galactic interstellar medium are discussed.

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arXiv2025

Evidence for protons accelerated and escaped from the Puppis A region using \textit{Fermi}-LAT observations

Supernova remnants (SNRs) interacting with molecular clouds are interesting laboratories to study the acceleration of cosmic rays and their propagation in the dense ambient medium. We analyze 14 years of Fermi-LAT observations of the supernova remnant Puppis A to investigate its asymmetric $\gamma$-ray morphology and spectral properties. This middle-aged remnant ($\sim$4 kyr) is evolving in an inhomogeneous environment, interacting with a dense molecular cloud in the northeast and a lower-density medium in the southwest. We find clear differences in both $\gamma$-ray luminosity and spectral energy distribution between these two regions. The emission from both sides is consistent with a hadronic origin. However, while the southwestern emission can be explained by standard Diffusive Shock Acceleration (DSA), the northeastern side may involve re-acceleration of pre-existing cosmic rays or acceleration via reflected shocks in the dense cloud environment. Additionally, we identify two significant $\gamma$-ray excesses outside the remnant, including a previously unreported source to the south. These features are likely produced by cosmic rays that have escaped Puppis A and are interacting with nearby dense molecular material. From this extended emission, we estimate the total energy in escaping cosmic rays to be $W_{CR} \sim 1.5 \times 10^{49}$ erg, providing important constraints on cosmic-ray propagation around the remnant.

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arXiv2025

A catalog of molecular clouds possibly associated with Galactic infrared bubbles I. The Southern Galactic plane

We have carried out a morphological search for molecular clouds possibly associated with 48 Galactic infrared bubbles with angular radii of $>1'$ in the southern Galactic plane of $295^\circ \le l\le 350^\circ$ and $|b|\le 1^\circ$ presented by Hanaoka et al. (2019). 116 molecular clouds in the $(l,b,V_{\rm LSR})$ space are identified from the archival $^{12}$CO~$J$~=~1--0 line data obtained by the Mopra Southern Galactic plane survey, where $V_{\rm LSR}$ is the CO-line radial velocity. The kinematic distances are derived from $V_{\rm LSR}$ using the most accurate rotation curve of the Milky Way. We also present measurements of velocity dispersion, size parameter, molecular mass, and virial mass of 116 molecular clouds. This catalog is presented to investigate star formation and the origin of molecular shells and cavities, possibly associated with infrared bubbles. CO line intensity maps and position-velocity diagrams of the molecular clouds are available online as supplementary data.

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arXiv2025

Studying the multi-phase interstellar medium in the Large Magellanic Cloud with SRG/eROSITA -- I. Analysis of diffuse X-ray emission

The Large Magellanic Cloud (LMC), being a nearby and actively star-forming satellite galaxy of the Milky Way, is an ideal site to observe the multi-phase interstellar medium (ISM) of a galaxy across the electromagnetic spectrum. We aim to exploit the available SRG/eROSITA all-sky survey data to study the distribution, composition and properties of the diffuse X-ray emitting hot gas in the LMC. We construct multi-band X-ray images of the LMC, reflecting the morphology and temperatures of the diffuse hot gas. By performing spatially resolved X-ray spectroscopy of 175 regions, we constrain the distribution, physical state, and composition of the hot ISM phase throughout the LMC. We combine our constraints with multiwavelength data to obtain a comprehensive view of the different ISM phases. We measure a total X-ray luminosity of the hot ISM phase of $1.9\times10^{38}\,\mathrm{erg\,s^{-1}}$ ($0.2-5.0\,\mathrm{keV}$ band), and constrain its thermal energy to around $5\times10^{54}\,\mathrm{erg}$. The typical density and temperature of the X-ray emitting plasma are around $5\times10^{-3}\,\mathrm{cm^{-3}}$ and $0.25\,\mathrm{keV}$, respectively, with both exhibiting broad peaks in the southeast of the LMC. The observed degree of X-ray absorption correlates strongly with the distribution of foreground HI gas, whereas a spatial anticorrelation between the hot and cold ISM phases is visible on sub-kpc scales within the disk. The abundances of light metals show a strong gradient throughout the LMC, with the north and east exhibiting a strong $\alpha$-enhancement, as expected from observed massive stellar populations there. In contrast, the enigmatic ``X-ray spur'' exhibits a local deficit in $\alpha$-elements, and a peak in hot-gas pressure at $P/k\sim10^5\,\mathrm{K\,cm^{-3}}$, consistent with a dominant energy input through tidally driven gas collisions.

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arXiv2025

Study of the origin of the azimuthal variation of synchrotron X-ray spectrum from SNR RX J0852.0-4622

We report the azimuthal distribution of the X-ray energy spectrum of non-thermal dominant supernova remnant RX J0852.0$-$4622. The X-rays from the shock region observed by the X-ray astronomy satellite Suzaku/XIS in the energy range of 2-8 keV are well described by the absorbed power-law model and can be parameterized with flux and photon index. The X-ray flux and photon index are bimodally distributed in relation to the azimuthal angle. To understand its origin, we examined three possible causes: azimuthal variation by (1) the galactic magnetic field, (2) cloud density, and (3) shock velocity. From the polarization observations of stars near the SNR, we find that the Galactic magnetic field around the SNR is not aligned. This result leads us to conclude that the azimuthal variation of the X-ray spectrum is most likely not caused by the Galactic magnetic field. The X-ray fluxes are positively correlated with the cloud density with a significance of $\sim 5\sigma$, and the azimuthal distributions of these physical quantities are particularly pronounced in the northern part of the SNR. In addition, the X-ray fluxes on the southern part of the SNR are positively correlated with the shock velocity. This phenomenon can be qualitatively explained by the increase in roll-off energy due to the amplification of the magnetic field by (A) the interaction between the shock and dense clouds in the north and (B) the fast shock velocity in the south of the SNR. Since the shock velocity is likely related to the cloud density interacting with the shock, we conclude that the azimuthal variation of cloud density most likely causes the azimuthal variations of the X-ray flux and photon index.

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arXiv2025

Giant Molecular Clouds in RCW 106 (G333): Galactic Mini-starbursts and Massive Star Formation Induced by Supersonic Cloud-Cloud Collisions

To reveal the origin of the mini-starbursts in the Milky Way, we carried out large-scale CO observations toward the RCW 106 giant molecular cloud (GMC) complex using the NANTEN2 4-m radio telescope operated by Nagoya University. We also analyzed the Mopra Southern Galactic plane CO survey and Herschel infrared continuum archival data. The RCW 106 GMC complex contains the radial velocity components of $-68$ km s$^{-1}$ and $-50$ km s$^{-1}$ reported by Nguyen et al. (2015). Focusing on the RCW 106 East and West region with the massive star formation having the bright infrared dust emission, we found that these regions have three different velocity components with $\sim 10$ km s$^{-1}$ differences. The two out of three velocity components show morphological correspondence with the infrared cold dust emission and connect with the bridge feature on a position-velocity diagram. Therefore, two molecular clouds (MCs) with $\sim 10$ km s$^{-1}$ differences are likely to be physically associated with massive star-forming regions in the GMC complex. Based on these observational results, we argue that mini-starbursts and massive star/cluster formation in the RCW 106 GMC complex are induced by supersonic cloud-cloud collisions in an agglomerate of molecular gas on the Scutum-Centaurus arm.

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arXiv2025

ALMA 0.1 pc View of Molecular Clouds Associated with High-Mass Protostellar Systems in the Small Magellanic Cloud: Are Low-Metallicity Clouds Filamentary or Not?

Filamentary molecular clouds are an essential intermediate stage in the star formation process. To test whether these structures are universal throughout cosmic star formation history, it is crucial to study low-metallicity environments within the Local Group. We present an ALMA analysis of the ALMA archival data at the spatial resolution of $\sim$0.1 pc for 17 massive young stellar objects (YSOs) in the Small Magellanic Cloud (SMC; Z $\sim$0.2 $Z_{\odot}$). This sample represents approximately 30% of the YSOs confirmed by Spitzer spectroscopy. Early ALMA studies of the SMC have shown that the CO emission line traces an H$_2$ number density of $\gtrsim$10$^4$ cm$^{-3}$, an order of magnitude higher than in the typical Galactic environments. Using the CO($J$ = 3-2) data, we investigated the spatial and velocity distribution of molecular clouds. Our analysis shows that about 60% of the clouds have steep radial profiles from the spine of the elongated structures, while the remaining clouds have a smooth distribution and are characterized by lower brightness temperatures. We categorized the former as filaments and the latter as non-filaments. Some of the filamentary clouds are associated with YSOs with outflows and exhibit higher temperatures, likely reflecting their formation conditions, suggesting that these clouds are younger than non-filamentary ones. This indicates that even if filaments form during star formation, their steep structures may become less prominent and transit to a lower-temperature state. Such transitions in structure and temperature have not been reported in metal-rich regions, highlighting a key behavior for characterizing the evolution of the interstellar medium and star formation in low-metallicity environments.

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arXiv2024

ACA CO(J=2-1) Mapping of the Nearest Spiral Galaxy M33. II. Exploring the Evolution of Giant Molecular Clouds

The evolution of giant molecular clouds (GMCs), the main sites of high-mass star formation, is an essential process to unravel the galaxy evolution. Using a GMC catalogue of M33 from ALMA-ACA survey, we classified 848 GMCs into three types based on the association with HII regions and their H$\alpha$ luminosities $\textit{L}$(H$\alpha$): Type I is associated with no HII regions; Type II with HII regions of $\textit{L}$(H$\alpha$) $<$ 10$^{37.5}$ erg s$^{-1}$; and Type III with HII regions of $\textit{L}$(H$\alpha$) $\geqq$ 10$^{37.5}$ erg s$^{-1}$. These criteria yield 224 Type I GMCs, 473 Type II GMCs, and 151 Type III GMCs. GMCs show changes in their physical properties according to the types; mass, radius, velocity dispersion, and $^{13}$CO detection rate of GMCs systematically increase from Type I to Type III, and additionally, Type III GMCs are closest to virial equilibrium. Type III GMCs show the highest spatial correlation with clusters younger than 10 Myr, Type II GMCs moderate correlation, and Type I GMCs are almost uncorrelated. We interpret that these types indicate an evolutionary sequence from Type I to Type II, and then to Type III with timescales of 4 Myr, 13 Myr, and 5 Myr, respectively, indicating the GMC lifetime of 22 Myr by assuming that Type II GMC has the same timescale as the Large Magellanic Cloud. The evolved GMCs concentrate on the spiral arms, while the younger GMCs are apart from the arm both to the leading and trailing sides. This indicated that GMCs collide with each other by the spiral potential, leading to the compression of GMCs and the triggering of high-mass star formation, which may support the dynamic spiral model. Overall, we suggest that the GMC evolution concept helps illuminate the galaxy evolution, including the spiral arm formation.

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arXiv2024

The Impact of Stellar Radiative Feedback on Formation of Young Massive Clusters via Fast HI Gas Collisions

Young massive clusters (YMCs) are dense aggregates of young stars and are often speculated as potential precursors to globular clusters. However, the formation mechanism of massive and compact gas clumps that precede YMCs remains unknown. In this paper, we study the formation of such massive clumps via fast HI gas collisions (~100 km/s) as suggested by recent observations and their subsequent evolution into YMCs by using three-dimensional magnetohydrodynamics simulations involving self-gravity and detailed thermal/chemical processes. In particular, the impact of ionization feedback from stellar radiation is included in an approximate fashion where the temperature within the HII regions is elevated to 10,000 K, while supernova feedback is not included. We examine whether the resulting massive clumps can survive this ionization feedback and evolve into YMCs. Our simulations reveal the emergence of gas clumps that do not only possess substantial mass (~10^5 M_sun) but also sufficient compactness (~5 pc). Notably, these clumps exhibit significantly higher escape velocities compared to the sound speed of the HII region, indicating effective gravitational retention of gas against feedback-induced evaporation. Consequently, these conditions foster efficient star formation within the massive gas clumps, ultimately leading to their evolution into YMCs. We also perform simulations involving lower-velocity gas collisions, approximately 15 km/s, typical shock velocities induced by galactic superbubbles.

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arXiv2024

CO Observations of the Type Ia Supernova Remnant 3C 397 by the Nobeyama 45-m Radio Telescope: Possible Evidence for the Single-Degenerated Explosion

We present a new CO observation toward the Type Ia supernova remnant (SNR) 3C 397 using the Nobeyama 45-m radio telescope at an unprecedent angular resolution of $\sim$18''. We newly found that the CO cloud at $V_{\mathrm{LSR}}$ = 55.7-62.2 km s$^{-1}$ (60 km s$^{-1}$ cloud) shows a good spatial correspondence with the radio continuum shell. We also found an expanding gas motion of the 60 km s$^{-1}$ cloud with an expansion velocity of $\sim$3 km s$^{-1}$, which is thought to be formed by the pre-and/or post-supernova feedback. By considering the positions of Galactic spiral arms and the X-ray/HI absorption studies, we concluded that 3C 397 is physically associated with the 60 km s$^{-1}$ cloud rather than the previously known CO cloud at $V_{\mathrm{LSR}}$ $\sim$30 km s$^{-1}$. Given that the previously measured pre-shock density is $\sim$2-5 cm$^{-3}$, the expanding motion of the 60 km s$^{-1}$ cloud was likely formed by the pre-supernova feedback known as optically thick wind. The scenario is consistent with that 3C 397 exploded inside a wind-blown bubble as a single degenerate system.

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