We have considered three SFRs: the ONC, NGC 2264 and Chamaeleon I. After a critical re-analysis of the optical and X-ray data published in recent literature, we have tried to answer the question of whether stars with different circumstellar properties have different observed X-ray emission, both in absolute terms and in relation to their bolometric luminosities.
In all of the analyzed cases we find that CTTS are underluminous respect to WTTS. This result is found in spite of large differences in the selection of members and in the optical and X-ray data used. We believe that it indicates a difference either in the intrinsic properties of X-ray emission or, alternatively, in the radiation transport (e.g. absorption) in the proximity of the stellar system. However we tend to prefer the first option: a difference in the relation between optical and X-ray circumstellar extinction, for example, might explain our result, but no such indication has been found to date.
When we could investigate the matter, i.e. in the cases of the ONC and NGC 2264, we found that the difference holds both when we discriminate stars on the basis of accretion and disk presence indicators. Therefore we are not able to say which of these two related aspects is most relevant for the mechanism responsible for the difference.
Having established the reality of this effect, work remains to be done to better characterize it and to identify its physical source. Thanks to sensitive X-ray data from modern X-ray space-borne telescopes and high-throughput optical/IR instruments this goal seems well within the reach of near future research.
Acknowledgements
The authors wish to acknowledge support from the Italian Space Agency (ASI) and MURST.
Copyright ESO 2003