Cloud–cloud collisions and triggered star formation Fukui, Yasuo; Habe, Asao; Inoue, Tsuyoshi ...
Publications of the Astronomical Society of Japan,
01/2021, Letnik:
73, Številka:
Supplement_1
Journal Article
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Abstract
Star formation is a fundamental process for galactic evolution. One issue over the last several decades has been determining whether star formation is induced by external triggers or ...self-regulated in a closed system. The role of an external trigger, which can effectively collect mass in a small volume, has attracted particular attention in connection with the formation of massive stellar clusters, which in extreme cases may lead to starbursts. Recent observations have revealed massive cluster formation triggered by cloud–cloud collisions in nearby interacting galaxies, including the Magellanic system and the Antennae Galaxies as well as almost all well-known high-mass star-forming regions in the Milky Way, such as RCW 120, M 20, M 42, NGC 6334, etc. Theoretical efforts are going into the foundation for the mass compression that causes massive cluster/star formation. Here, we review the recent progress on cloud–cloud collisions and the triggered star-cluster formation, and discuss future prospects for this area of study.
Abstract
The Sgr B region, including Sgr B1 and Sgr B2, is one of the most active star-forming regions in the Galaxy. Hasegawa et al. originally proposed that Sgr B2 was formed by a cloud–cloud ...collision (CCC) between two clouds with velocities of ∼45 km s
−1
and ∼75 km s
−1
. However, some recent observational studies conflict with this scenario. We have reanalyzed this region, by using recent, fully sampled, dense-gas data and by employing a recently developed CCC identification methodology, with which we have successfully identified more than 50 CCCs and compared them at various wavelengths. We found two velocity components that are widely spread across this region and that show clear signatures of a CCC, each with a mass of ∼10
6
M
⊙
. Based on these observational results, we suggest an alternative scenario, in which contiguous collisions between two velocity features with a relative velocity of ∼20 km s
−1
created both Sgr B1 and Sgr B2. The physical parameters, such as the column density and the relative velocity of the colliding clouds, satisfy a relation that has been found to apply to the most massive Galactic CCCs, meaning that the triggering of high-mass star formation in the Galaxy and starbursts in external galaxies can be understood as being due to the same physical CCC process.
Abstract
Recent large-area, deep CO surveys in the Galactic disk have revealed the formation of ~50 high-mass stars or clusters triggered by cloud–cloud collisions (CCCs). Although the Galactic ...Center (GC)—which contains the highest volume density of molecular gas—is the most favorable place for cloud collisions, systematic studies of CCCs in that region are still untouched. Here we report for the first time evidence of CCCs in the common foot point of molecular loops 1 and 2 in the GC. We have investigated the distribution of molecular gas toward the foot point by using a methodology for identifying CCCs, and we have discovered clear signatures of CCCs. Using the estimated displacements and relative velocities of the clouds, we find the elapsed time since the beginnings of the collisions to be 105–106yr. We consider possible origins for previously reported peculiar velocity features in the foot point and discuss star formation triggered by CCCs in the GC.
We review recent progress in elucidating the relationship between high-energy radiation and the interstellar medium (ISM) in young supernova remnants (SNRs) with ages of ∼2000 yr, focusing in ...particular on RX J1713.7−3946 and RCW 86. Both SNRs emit strong nonthermal X-rays and TeV
γ
-rays, and they contain clumpy distributions of interstellar gas that includes both atomic and molecular hydrogen. We find that shock–cloud interactions provide a viable explanation for the spatial correlation between the X-rays and ISM. In these interactions, the supernova shocks hit the typically pc-scale dense cores, generating a highly turbulent velocity field that amplifies the magnetic field up to 0.1–1 mG. This amplification leads to enhanced nonthermal synchrotron emission around the clumps, whereas the cosmic-ray electrons do not penetrate the clumps. Accordingly, the nonthermal X-rays exhibit a spatial distribution similar to that of the ISM on the pc scale, while they are anticorrelated at sub-pc scales. These results predict that hadronic
γ
-rays can be emitted from the dense cores, resulting in a spatial correspondence between the
γ
-rays and the ISM. The current pc-scale resolution of
γ
-ray observations is too low to resolve this correspondence. Future
γ
-ray observations with the Cherenkov Telescope Array will be able to resolve the sub-pc-scale
γ
-ray distribution and provide clues to the origin of these cosmic
γ
-rays.
Abstract
Stars are born in dense molecular filaments irrespective of their mass. Compression of the interstellar medium by shocks causes filament formation in molecular clouds. Observations show that ...a massive star cluster formation occurs where the peak of gas column density in a cloud exceeds 10
23
cm
−2
. In this study, we investigate the effect of the shock-compressed layer duration on filament/star formation and how the initial conditions of massive star formation are realized by performing three-dimensional isothermal magnetohydrodynamics simulations with gas inflow duration from the boundaries (i.e., shock-wave duration) as a controlling parameter. Filaments formed behind the shock expand after the duration time for short-shock-duration models, whereas long-duration models lead to star formation by forming massive supercritical filaments. Moreover, when the shock duration is longer than two postshock freefall times, the peak column density of the compressed layer exceeds 10
23
cm
−2
, and the gravitational collapse of the layer causes the number of OB stars expected to be formed in the shock-compressed layer to reach the order of 10 (i.e., massive cluster formation).
Abstract
Recent observations suggest an that intensive molecular cloud collision can trigger massive star/cluster formation. The most important physical process caused by the collision is a shock ...compression. In this paper, the influence of a shock wave on the evolution of a molecular cloud is studied numerically by using isothermal magnetohydrodynamics simulations with the effect of self-gravity. Adaptive mesh refinement and sink particle techniques are used to follow the long-time evolution of the shocked cloud. We find that the shock compression of a turbulent inhomogeneous molecular cloud creates massive filaments, which lie perpendicularly to the background magnetic field, as we have pointed out in a previous paper. The massive filament shows global collapse along the filament, which feeds a sink particle located at the collapse center. We observe a high accretion rate $\dot{M}_{\rm acc}> 10^{-4}\, M_{\odot }\:$yr−1 that is high enough to allow the formation of even O-type stars. The most massive sink particle achieves M > 50 M$_{\odot }$ in a few times 105 yr after the onset of the filament collapse.
Abstract
Understanding massive cluster formation is one of the important issues of astronomy. By analyzing the H i data, we have identified that the two H i velocity components (L- and D-components) ...are colliding toward the H i Ridge, in the southeastern end of the Large Magellanic Cloud (LMC), which hosts the young massive cluster R136 and ∼400 O/Wolf–Rayet stars (Doran et al. 2013, A&A, 558, A134) including the progenitor of SN 1987A. The collision is possibly evidenced by bridge features connecting the two H i components and by complementary distributions between them. We frame a hypothesis that the collision triggered the formation of R136 and the surrounding high-mass stars as well as the H i Ridge and the Molecular Ridge. Fujimoto and Noguchi (1990, PASJ, 42, 505) advocated that the last tidal interaction between the LMC and the Small Magellanic Cloud (SMC) induced collision of the L- and D-components about 0.2 Gyr ago. This model is consistent with numerical simulations (Bekki & Chiba 2007a, MNRAS, 381, L16). We suggest that a dense H i, cloud of 106 M
⊙ partly including CO, a precursor of R136, was formed at the shock-compressed interface between the colliding L- and D-components. We suggest that part of the low-metallicity gas from the SMC was mixed in the tidal interaction based on the Planck/IRAS data of dust optical depth (Planck Collaboration 2014, A&A, 571, A11).
Abstract
A supersonic cloud–cloud collision produces a shock-compressed layer which leads to formation of high-mass stars via gravitational instability. We carried out a detailed analysis of the ...layer by using the numerical simulations of magneto-hydrodynamics which deal with colliding molecular flows at a relative velocity of 20 km s−1 (Inoue & Fukui 2013, ApJ, 774, L31). Maximum density in the layer increases from 1000 cm−3 to more than 105 cm−3 within 0.3 Myr by compression, and the turbulence and the magnetic field in the layer are amplified by a factor of ∼5, increasing the mass accretion rate by two orders of magnitude to more than 10−4 $ M_{\odot } $ yr−1. The layer becomes highly filamentary due to gas flows along the magnetic field lines, and dense cores are formed in the filaments. The massive dense cores have size and mass of 0.03–0.08 pc and 8–$ 50\, M_{\odot } $ and they are usually gravitationally unstable. The mass function of the dense cores is significantly top-heavy as compared with the universal initial mass function, indicating that the cloud–cloud collision preferentially triggers the formation of O and early B stars. We argue that the cloud–cloud collision is a versatile mechanism which creates a variety of stellar clusters from a single O star like RCW 120 and M 20 to tens of O stars of a super star cluster like RCW 38 and a mini-starburst W 43. The core mass function predicted by the present model is consistent with the massive dense cores obtained by recent ALMA observations in RCW 38 (Torii et al. 2021, PASJ, in press) and W 43 (Motte et al. 2018, Nature Astron., 2, 478). Considering the increasing evidence for collision-triggered high-mass star formation, we argue that cloud–cloud collision is a major mechanism of high-mass star formation.
Young massive clusters (YMCs) are dense aggregates of young stars, which are essential to galaxy evolution, owing to their ultraviolet radiation, stellar winds, and supernovae. The typical mass and ...radius of YMCs are M ∼ 104 M and R ∼ 1 pc, respectively, indicating that many stars are located in a small region. The formation of YMC precursor clouds may be difficult because a very compact massive cloud should be formed before stellar feedback blows off the cloud. Recent observational studies suggest that YMCs can be formed as a consequence of the fast H i gas collision with a velocity of ∼100 km s−1, which is the typical velocity of the galaxy-galaxy interaction. In this study, we examine whether the fast H i gas collision triggers YMC formation using three-dimensional magnetohydrodynamics simulations, which includes the effects of self-gravity, radiative cooling/heating, and chemistry. We demonstrate that massive gravitationally bound gas clumps with M > 104 M and L ∼ 4 pc are formed in the shock compressed region induced by the fast H i gas collision, in which massive gas clumps can evolve into YMCs. Our results show that the YMC precursors are formed by the global gravitational collapse of molecular clouds, and YMCs can be formed even in low-metal environments, such as the Magellanic Clouds. Additionally, the very massive YMC precursor cloud, with M > 105 M , can be created when we consider the fast collision of H i clouds, which may explain the origin of the very massive stellar cluster R136 system in the Large Magellanic Cloud.
Abstract The clinical usefulness of presepsin for discriminating between bacterial and nonbacterial infections (including systemic inflammatory response syndrome) was studied and compared with ...procalcitonin (PCT) and interleukin-6 (IL-6) in a multicenter prospective study. Suspected sepsis patients ( n = 207) were enrolled into the study. Presepsin levels in patients with systemic bacterial infection and localized bacterial infection were significantly higher than in those with nonbacterial infections. In addition, presepsin, PCT, and IL-6 levels in patients with bacterial infectious disease were significantly higher than in those with nonbacterial infectious disease ( P < 0.0001, P < 0.0001, and P < 0.0001, respectively). The area under the receiver operating characteristic curve was 0.908 for presepsin, 0.905 for PCT, and 0.825 for IL-6 in patients with bacterial infectious disease and those with nonbacterial infectious disease. The cutoff value of presepsin for discrimination of bacterial and nonbacterial infectious diseases was determined to be 600 pg/ml, of which the clinical sensitivity and specificity were 87.8 % and 81.4 %, respectively. Presepsin levels did not differ significantly between patients with gram-positive and gram-negative bacterial infections. The sensitivity of blood culture was 35.4 %; that for presepsin was 91.9 %. Also there were no significant differences in presepsin levels between the blood culture-positive and -negative groups. Consequently, presepsin is useful for the diagnosis of sepsis, and it is superior to conventional markers and blood culture.
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