Abstract
Aims. Our goal is to better understand the origin and the star-formation history of regions NGC 6334 and NGC 6357. We focus our study on the kinematics of young stars (young stellar objects and OB stars) in both regions mainly on the basis of the Gaia DR2 data. Methods. For both regions, we compiled catalogs of OB stars and young stellar objects from the literature and complemented them using VPHAS+ DR2 and Spitzer IRAC/GLIMPSE photometry catalogues. We applied a cross-match with the Gaia DR2 catalog to obtain information on the parallax and transverse motion. Results. We confirm that NGC 6334 and NGC 6357 are in the far side of the Saggitarius-Carina arm at a distance of 1.76 kpc. For NGC 6357, OB stars show strong clustering and ordered star motion with Vlon ∼–10.7 km s−1 and Vlat ∼3.7 km s−1, whereas for NGC 6334, no significant systemic motion was observed. The OB stars motions and distribution in NGC 6334 suggest that it should be classified as an association. Ten runaway candidates may be related to NGC 6357 and two to NGC 6334, respectively. The spatial distributions of the runaway candidates in and around NGC 6357 favor a dynamical (and early) ejection during the cluster(s) formation. Because such stars are likely to be ejected during a cluster’s formation, the fact that not as many such stars are observed towards NGC 6334 suggests different formation conditions than have been assumed for NGC 6357.
Highlights
NGC 6334 and NGC 6357 are two well-studied Galactic, highmass, star-forming regions
Because young stars are expected to still keep the imprint of their birthplace kinematics, we focus our study on the young stars’ kinematics in
Among the 84 not paired spectroscopic OB stars, 38 have no u, g, or r magnitudes, which naturally explains why they are not found in the photometric catalog, while the remaining 46 stars are all B1 to B3 stars and were missed during the first step selection because they fall below the reddening law due to the u-band photometric uncertainty
Summary
NGC 6334 and NGC 6357 are two well-studied Galactic, highmass, star-forming regions (see Fig. 1) Because they share the same velocity (Caswell & Haynes 1987), it has been proposed that they can be found at the same distance. In NGC 6357, no such molecular ridge has been observed, but a large cavity of ionized gas is present, suggesting that the parental molecular cloud has largely been consumed or impacted by OB stars (e.g., Lortet et al 1984) These differences among both regions, despite their formation from a common filamentary structure, suggest they have evolved in different ways. Both regions mainly based on the Gaia DR2 data.
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