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We investigate the influence of large-scale stellar feedback on the formation of molecular clouds in the Large Magellanic Cloud (LMC). Examining the relationship between HI and 12CO(J=1-0) in supergiant shells (SGSs), we find that the molecular fraction in the total volume occupied by SGSs is not enhanced with respect to the rest of the LMC disk. However, the majority of objects (~70% by mass) are more molecular than their local surroundings, implying that the presence of a supergiant shell does on average have a positive effect on the molecular gas fraction. Averaged over the full SGS sample, our results suggest that ~12-25% of the molecular mass in supergiant shell systems was formed as a direct result of the stellar feedback that created the shells. This corresponds to ~4-11% of the total molecular mass of the galaxy. These figures are an approximate lower limit to the total contribution of stellar feedback to molecular cloud formation in the LMC, and constitute one of the first quantitative measurements of feedback-triggered molecular cloud formation in a galactic system.
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We investigate the effects of Supergiant Shells (SGSs) and their interaction on dense molecular clumps by observing the Large Magellanic Cloud (LMC) star forming regions N48 and N49, which are located between two SGSs, LMC 4 and LMC 5. $^{12}$CO ($J$=3-2, 1-0) and $^{13}$CO ($J$=1-0) observations with the ASTE and Mopra telescopes have been carried out towards these regions. A clumpy distribution of dense molecular clumps is revealed with 7 pc spatial resolution. Large velocity gradient analysis shows that the molecular hydrogen densities ($n({rm H}_2)$) of the clumps are distributed from low to high density ($10^3$-$10^5$ cm$^{-3}$) and their kinetic temperatures ($T_{rm kin}$) are typically high (greater than $50$ K). These clumps seem to be in the early stages of star formation, as also indicated from the distribution of H$alpha$, young stellar object candidates, and IR emission. We found that the N48 region is located in the high column density HI envelope at the interface of the two SGSs and the star formation is relatively evolved, whereas the N49 region is associated with LMC 5 alone and the star formation is quiet. The clumps in the N48 region typically show high $n({rm H}_2)$ and $T_{rm kin}$, which are as dense and warm as the clumps in LMC massive cluster-forming areas (30 Dor, N159). These results suggest that the large-scale structure of the SGSs, especially the interaction of two SGSs, works efficiently on the formation of dense molecular clumps and stars.
We examine the recent star formation associated with four supergiant shells (SGSs) in the Large Magellanic Cloud (LMC): LMC 1, 4, 5, and 6, which have been shown to have simple expanding-shell structures. H II regions and OB associations are used to infer star formation in the last few Myr, while massive young stellar objects (YSOs) reveal the current ongoing star formation. Distributions of ionized, H I, and molecular components of the interstellar gas are compared with the sites of recent and current star formation to determine whether triggering has taken place. We find that a great majority of the current star formation has occurred in gravitationally unstable regions, and that evidence of triggered star formation is prevalent at both large and local scales.
We present the identification of the previously unnoticed physical association between the Corona Australis molecular cloud (CrA), traced by interstellar dust emission, and two shell-like structures observed with line emission of atomic hydrogen (HI) at 21 cm. Although the existence of the two shells had already been reported in the literature, the physical link between the HI emission and CrA was never highlighted before. We use both Planck and Herschel data to trace dust emission and the Galactic All Sky HI Survey (GASS) to trace HI. The physical association between CrA and the shells is assessed based both on spectroscopic observations of molecular and atomic gas and on dust extinction data with Gaia. The shells are located at a distance between 140 and 190 pc, comparable to the distance of CrA, which we derive as 150.5 +- 6.3 pc. We also employ dust polarization observations from Planck to trace the magnetic-field structure of the shells. Both of them show patterns of magnetic-field lines following the edge of the shells consistently with the magnetic-field morphology of CrA. We estimate the magnetic-field strength at the intersection of the two shells via the Davis-Chandrasekhar-Fermi (DCF) method. Albeit the many caveats that are behind the DCF method, we find a magnetic-field strength of 27 +- 8 $mu$G, at least a factor of two larger than the magnetic-field strength computed off of the HI shells. This value is also significantly larger compared to the typical values of a few $mu$G found in the diffuse HI gas from Zeeman splitting. We interpret this as the result of magnetic-field compression caused by the shell expansion. This study supports a scenario of molecular-cloud formation triggered by supersonic compression of cold magnetized HI gas from expanding interstellar bubbles.
We present the discovery of expanding spherical shells around low to intermediate-mass young stars in the Orion A giant molecular cloud using observations of $^{12}$CO (1-0) and $^{13}$CO (1-0) from the Nobeyama Radio Observatory 45-meter telescope. The shells have radii from 0.05 to 0.85 pc and expand outward at 0.8 to 5 km/s. The total energy in the expanding shells is comparable to protostellar outflows in the region. Together, shells and outflows inject enough energy and momentum to maintain the cloud turbulence. The mass-loss rates required to power the observed shells are two to three orders of magnitude higher than predicted for line-driven stellar winds from intermediate-mass stars. This discrepancy may be resolved by invoking accretion-driven wind variability. We describe in detail several shells in this paper and present the full sample in the online journal.
Nine supergiant shells (SGSs) have been identified in the Large Magellanic Cloud (LMC) based on H-alpha images, and twenty-three SGSs have been reported based on HI 21-cm line observations, but these sets do not always identify the same structures. We have examined the physical structure of the optically identified SGSs using HI channel maps and P-V diagrams to analyze the gas kinematics. There is good evidence for seven of the nine optically identified SGSs to be true shells. Of these seven H-alpha SGSs, four are the ionized inner walls of HI SGSs, while three are an ionized portion of a larger and more complex HI structure. All of the H-alpha SGSs are identified as such because they have OB associations along the periphery or in the center, with younger OB associations more often found along the periphery. After roughly 12 Myrs, if no new OB associations have been formed a SGS will cease to be identifiable at visible wavelengths. Thus, the presence and location of ionizing sources is the main distinction between shells seen only in HI and those also seen in H-alpha. Based on our analysis, H-alpha observations alone cannot unambiguously identify SGSs, especially in distant galaxies.
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