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Description
The formation of high mass stars is still debated. For this reason, several projects, as the Herschel-HOBYS are focussed on the study of the earliest phases of massive star formation. As a result, massive star-forming complexes such as NGC 6334 and NGC 6357 have been observed in the far-infrared to study their massive dense cores where massive stars are expected to form. However, to better characterise the environment of these cores we need to understand the previous massive star formation history. To better characterize the environment of these massive dense cores we propose to study the previous high mass star formation and how these stars act on their environment. This study is based on spectral classification of the OB stars identified towards NGC 6334 and NGC 6357 with spectra taken with the AAOmega spectrograph on the Anglo-Australian Telescope (AAT). From the subsequent spectral classification of 109 stars across these regions we have been able to evaluate the following: distance, age, mass, global star-forming efficiency (SFE) and star-formation rate (SFR) of the regions. The physical conditions of the ionised gas for both complexes was also derived. We confirm that NGC 6334 and NGC 6357 belong to the Saggitarius-Carina arm which, in this direction, extends from 1kpc to 2.2kpc. From the location of the stars in Hertzprung-Russell diagram we show that stars older than ~10Myr are broadly spread across these complexes while younger stars are mainly located in the H ii regions and stellar clusters. Our data also suggests that some of the young stars can be considered as runaway stars. We evaluate a SFE of 0.019 and 0.021 and a SFR of 1.1x10^3^M_{sun}/Myr^ and 1.7x10^3^M_{sun}_/Myr for NGC 6334 and NGC 6357 respectively. We note that 25 OB stars have X-ray counterparts, most of them belonging to NGC 6357. This suggests that molecular clouds in NGC 6357 is more impacted by X-ray flux and stellar winds than for NGC 6334. Finally, from analysis of nebular lines (H{alpha}, [NII] and [SII]) from spectra from several regions of ionised gas, we confirm that the filaments in NGC 6357 are shock heated.
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