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Astron. Astrophys. 339, 759-772 (1998)

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5. Conclusions

In this paper, we presented new observational data on G45, covering the near- and mid-infrared wavelengths. On our [FORMULA] and H band images taken with the adaptive optics system ADONIS, we detected several point sources eight of which could be identified as O and B stars from colour-magnitude measurements. This identification was only made possible by the extraordinary resolution of 0:004 provided by the adaptive optics. These identifications were checked against measurements of the extinction towards G45. The extinction was calculated from the comparison of our measured Br[FORMULA] flux and the prediction for Br[FORMULA] emission from 6 cm VLA maps. Additional extinction was found from a determination of the dust mass inside G45 from our MIR images. The derived high optical depths inside the region pose a significant problem for ionization by UV photons if the matter is distributed homogeneously inside G45. On our mid-infrared images, we detected two more sources which obviously represent young stellar objects. By combining these data, we argued that G45 is a young cluster of O- and B-stars similar to those being formed in the Orion regions in size but obviously younger and still in the formation process. Additionally, we argued on the history of star formation on the basis of an apparent age gradient from south (readily formed massive stars) to north (chain of stars along the ionization front and, further north, several objects either heavily reddened or with extended features). These findings confirm earlier speculations by WC89 on UCH II s being young OB clusters, as well as of W96 on sequential star formation in the area triggered by G45. For the first time, we have brought up direct evidence that one form of UCH II s is a cluster of massive stars. Our images suggest that sequential star formation does take place in these clusters.

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998