The ALMA Survey of 70 µm dark High-mass clumps in Early Stages (ASHES) has been designed to systematically characterize the earliest stages and to constrain theories of high-mass star formation. A deep understanding of highmass star formation requires the study of the clustered mode, which is the most commonly found in nature. A total of 12 massive (>500 M ), cold (≤15 K), 3.6-70 µm dark prestellar clump candidates, embedded in infrared dark clouds (IRDCs), were carefully selected in the pilot survey to be observed with the Atacama Large Millimeter/sub-millimeter Array (ALMA). Exploiting the unique capabilities of ALMA, we have mosaiced each clump (∼1 arcmin 2 ) in dust continuum and line emission with the 12 m, 7 m, and Total Power arrays at 224 GHz (1.34 mm), resulting in ∼1. 2 angular resolution (∼4800 AU at the average source distance of 4 kpc). As the first paper of the series, we concentrate on the dust continuum emission to reveal the clump fragmentation. We have detected a total of 294 cores, from which 84 (29%) are categorized as protostellar based on outflow activity or "warm core" line emission. The remaining 210 (71%) are considered prestellar core candidates. The number of detected cores is independent of the mass sensitivity range of the observations and, on average, more massive clumps tend to form more cores. We find no correlation between the mass of the host clump and the most massive embedded core. We find a large population of low-mass (<1 M ) cores and no high-mass (>30 M ) prestellar cores. The most massive prestellar core has a mass of 11 M . From the prestellar core mass function, we derive a power law index of 1.17 ± 0.10, slightly shallower than the Salpeter index of 1.35. We have used the minimum spanning tree technique to characterize the separation between cores and their spatial distribution, and to derive mass segregation ratios. While there is a range of core masses and core separations detected in the sample, the mean separation and mean mass of cores per clump are well explained
Based on the 850 µm dust continuum data from SCUBA-2 at James Clerk Maxwell Telescope (JCMT), we compare overall properties of Planck Galactic Cold Clumps (PGCCs) in the λ Orionis cloud to those of PGCCs in the Orion A and B clouds. The Orion A and B clouds are well known active star-forming regions, while the λ Orionis cloud has
Tycho's supernova remnant (SNR), as one of the few historical SNRs, has been widely studied in various wave bands. Observations show evidence that Tycho is expanding in a medium with density gradient and possibly interacting with a dense ambient medium toward the northeast direction. From the FCRAO CO survey of the outer Galaxy, we have identified a patch of molecular clouds in this area and have conducted a follow-up observation with the Nobeyama 45 m radio telescope. The high-resolution (16Љ) Nobeyama data show that a large molecular cloud surrounds the SNR along the northeastern boundary. We suggest that Tycho's SNR and the molecular cloud are located in the Perseus arm and that the dense medium interacting with the SNR is possibly the molecular cloud. We also discuss the possible connection between the molecular cloud and the Balmer-dominated optical filaments and suggest that the preshock gas may be accelerated within the cosmic-ray and/or fast neutral precursor.
We have carried out submillimeter 12 CO(J ¼ 3Y2) observations of six giant molecular clouds (GMCs) in the Large Magellanic Cloud (LMC) with the ASTE 10 m submillimeter telescope at a spatial resolution of 5 pc and very high sensitivity. We have identified 32 molecular clumps in the GMCs and revealed significant details of the warm and dense molecular gas with n(H 2 ) $ 10 3 Y10 5 cm À3 and T kin $ 60 K. These data are combined with 12 CO(J ¼ 1Y0) and 13 CO(J ¼ 1Y0) results and compared with LVG calculations. The results indicate that clumps that we detected are distributed continuously from cool ($10Y30 K) to warm (k30Y200 K), and warm clumps are distributed from less dense ($10 3 cm À3 ) to dense ($10 3.5 Y10 5 cm À3 ). We found that the ratio of 12 CO(J ¼ 3Y2) to 12 CO(J ¼ 1Y0) emission is sensitive to and is well correlated with the local H flux. We infer that differences of clump properties represent an evolutionary sequence of GMCs in terms of density increase leading to star formation. Type I and II GMCs (starless GMCs and GMCs with H ii regions only, respectively) are at the young phase of star formation where density does not yet become high enough to show active star formation, and Type III GMCs (GMCs with H ii regions and young star clusters) represent the later phase where the average density is increased and the GMCs are forming massive stars. The high kinetic temperature correlated with H flux suggests that FUV heating is dominant in the molecular gas of the LMC.
A full understanding of high-mass star formation requires the study of one of the most elusive components of the energy balance in the interstellar medium: magnetic fields. We report ALMA 1.2 mm, high-resolution (700 au) dust polarization and molecular line observations of the rotating hot molecular core embedded in the high-mass star-forming region IRAS 18089−1732. The dust continuum emission and magnetic field morphology present spiral-like features resembling a whirlpool. The velocity field traced by the H 13 CO + (J=3-2) transition line reveals a complex structure with spiral filaments that are likely infalling and rotating, dragging the field with them. We have modeled the magnetic field and find that the best model corresponds to a weakly magnetized core with a mass-tomagnetic-flux ratio (λ) of 8.38. The modeled magnetic field is dominated by a poloidal component, but with an important contribution from the toroidal component that has a magnitude of 30% of the poloidal component. Using the Davis-Chandrasekhar-Fermi method, we estimate a magnetic field strength of 3.5 mG. At the spatial scales accessible to ALMA, an analysis of the energy balance of the system indicates that gravity overwhelms turbulence, rotation, and the magnetic field. We show that high-mass star formation can occur in weakly magnetized environments, with gravity taking the dominant role.
Filaments are ubiquitous structures in molecular clouds and play an important role in the mass assembly of stars. We present results of dynamical stability analyses for filaments in the infrared dark cloud G14.225−0.506, where a delayed onset of massive star formation was reported in the two hubs at the convergence of multiple filaments of parsec length. Full-synthesis imaging is performed with the Atacama Large Millimeter/submillimeter Array (ALMA) to map the N 2 H + (1 − 0) emission in two hub-filament systems with a spatial resolution of ∼ 0.034 pc. Kinematics are derived from sophisticated spectral fitting algorithm that accounts for line blending, large optical depth, and multiple velocity components. We identify five velocity coherent filaments and derive their velocity gradients with principal component analysis. The mass accretion rates along the filaments are up to 10 −4 M yr −1 and are significant enough to affect the hub dynamics within one free-fall time (∼ 10 5 yr). The N 2 H + filaments are in equilibrium with virial parameter α vir ∼ 1.2. We compare α vir measured in the N 2 H + filaments, NH 3 filaments, 870 µm dense clumps, and 3 mm dense cores. The decreasing trend in α vir with decreasing spatial scales persists, suggesting an increasingly important role of gravity at small scales. Meanwhile, α vir also decreases with decreasing non-thermal motions. In combination with the absence of high-mass protostars and massive cores, our results are consistent with the global hierarchical collapse scenario.
We present new 3 mm continuum and molecular lines observations from the ATOMS survey towards the massive protostellar clump, MM1, located in the filamentary infrared dark cloud (IRDC), G034.43+00.24 (G34). The lines observed are the tracers of either dense gas (e.g. HCO+/H13CO+ J=1–0) or outflows (e.g. CS J=2–1). The most complete picture to date of seven cores in MM1 is revealed by dust continuum emission. These cores are found to be gravitationally bound, with virial parameter, αvir < 2. At least four outflows are identified in MM1 with a total outflowing mass of ∼45 M⊙, and a total energy of 1 × 1047 ergs, typical of outflows from a B0-type star. Evidence of hierarchical fragmentation, where turbulence dominates over thermal pressure, is observed at both the cloud and the clump scales. This could be linked to the scale-dependent, dynamical mass inflow/accretion on clump and core scales. We therefore suggest that the G34 cloud could be undergoing a dynamical mass inflow/accretion process linked to the multi-scale fragmentation, which leads to the sequential formation of fragments of the initial cloud, clumps, and ultimately dense cores, the sites of star formation.
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