NGC 6334 and NGC 6357

Abstract

Aims. The formation of high-mass stars is still debated. For this reason, several projects such as 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 environments of these cores we need to understand the previous massive star formation history. To better characterise the environment of these massive dense cores we study the previous high-mass star formation and how these stars act on their environments. Methods. This study is based on the 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 were 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. Results. We confirm that NGC 6334 and NGC 6357 belong to the Saggitarius-Carina arm which, in this direction, extends from 1 kpc to 2.2 kpc. From the location of the stars in Hertzprung-Russell diagram we show that stars older than ∼10 Myr 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 runaway stars. We evaluate a SFE of 0.019 +0.008 −0.007 and 0.021 +0.004 −0.003 and a SFR of 1.1 × 10 3 ± 300 M Myr −1 and 1.7 × 10 3 ± 400 M Myr −1 for NGC 6334 and NGC 6357, respectively. We note that 29 OB stars have X-ray counterparts, most of them belonging to NGC 6357. This suggests that molecular clouds in NGC 6357 are more impacted by X-ray flux and stellar winds than in NGC 6334. Finally, from the analysis of nebular lines (Hα, [NII], and [SII]) from spectra from several regions of ionised gas, we confirm that the filaments in NGC 6357 are shock heated.

Herschel-HOBYS study of the earliest phases of high-mass star formation in NGC 6357

“…(Russeil et al 2016) where dynamical interaction of the ionised gas with the molecular cloud can occur. There are also young stars and OB stars (Getman et al 2014;Russeil et al 2017) which can participate to the local turbulence by their feedback. Assuming that the −9 and the +2 km s −1 are the extreme velocities and −4 km s −1 the systemic velocity, we can estimate an expansion velocity of ∼5 km s −1 for the NGC 6357 central cavity.…”

“…Assuming that the −9 and the +2 km s −1 are the extreme velocities and −4 km s −1 the systemic velocity, we can estimate an expansion velocity of ∼5 km s −1 for the NGC 6357 central cavity. Finally, the patchy +6 km s −1 emission is usually attributed to a foreground layer (Russeil et al 2017).…”

“…Such statistical lifetime is estimated from the relative number of a given MDC phase to the OB stars. For NGC 6357 we estimate a total number of 60 O-B3 stars from Russeil et al (2012Russeil et al ( , 2017 and Povich et al (2017). We assume a median age of the O-B3 stars to be 1 × 10 6 yr according to Fang et al (2012) and Getman et al (2014).…”

“…However, because the Wolf-Rayet phase is expected to occur at a stellar age of 3 Myr (e.g. Sokal et al 2016) and because OB stars with ages around 4.6 Myr were also reported towards NGC 6357 (Russeil et al 2017), we can suspect that starformation in NGC 6357 has been active for at least 5 Myr. The large (radius 15 pc) Hα ring (as defined by Massi et al 2015) could even have been shaped by a previous event as Russeil et al (2017) note shock-heated gas towards the filaments on its northeast side.…”

OB stars and YSO populations in the region of NGC 6334–NGC 6357 as seen withGaiaDR2

“…1. By combining the distance of the young clusters and the spectro-photometric distance of the more disagreggated OB stars, a mean distance of 1.75 kpc was found by Russeil et al (2017), however, based on the maser parallax of the very young massive star-forming NGC 6334I(N), Chibueze et al (2014) suggest a distance of 1.35 kpc. In NGC 6357, young stellar objects (YSOs) are mainly found in clusters, the most numerous being Pismis 24 (e.g., Fang et al 2012), while in NGC 6334, YSOs are distributed throughout the ridge, being more numerous toward its north-east end (Willis et al 2013).…”