5 Summary and conclusions

Our XMM-Newton observation of the Lagoon Nebula (M 8) reveals a cluster of rather faint point-like X-ray sources in addition to the bright O4 star 9 Sgr. We have shown that most of these sources have optical counterparts inside the very young open cluster NGC 6530. We suggest that an important fraction of these sources are associated with low-mass pre-main sequence stars (most probably weak-line T Tauri stars) which were not previously identified as PMS objects because of the lack of strong H$\alpha$ emission. Our results suggest that the true number of low-mass PMS objects exceeds by more than a factor two the number of objects identified by Sung et al. (2000).

The X-ray selected PMS candidates in NGC 6530 are found to have rather hard spectra with kT of a few keV and their X-ray luminosities (of order of a few times 10³¹ erg s^-1) are apparently in the higher part of the luminosities of T Tauri stars in the literature. The large number of X-ray sources associated with PMS stars in NGC 6530 may shed some light on the origin of apparently diffuse emission in more distant starburst clusters (e.g. NGC 3603) where source confusion becomes a serious issue.

As to intermediate and high-mass young stellar objects, we found that at least one of the known Herbig Be stars in NGC 6530 exhibits a relatively strong X-ray emission, while most of the early-type main sequence stars of spectral type B1 and later are not detected.

We also report X-ray emission from the massive protostar Herschel 36 as well as probably diffuse emission from the Hourglass Region that might reveal a bubble of hot gas produced by the interaction of the stellar wind of Herschel 36 with the denser part of the molecular cloud. Higher spatial resolution observations of the HG with Chandra are needed to confirm the diffuse nature of this emission and to further constrain its morphology.

Acknowledgements

We thank the referee Dr. R. D. Jeffries for valuable comments that helped to significantly improve our manuscript. The Liège team acknowledges support from the Fonds National de la Recherche Scientifique (Belgium) and through the PRODEX XMM-OM and Integral Projects. This research is also supported in part by contracts P4/05 and P5/36 "Pôle d'Attraction Interuniversitaire'' (SSTC-Belgium). MCR acknowledges support from ESA-Prodex project No. 13346/98/NL/VJ(ic). This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France and NASA's Astrophysics Data System Abstract Service.