SIMBAD references

We analyze six ROSAT HRI observations pointed toward the Star Forming Region (SFR) NGC 2264. Three are pointed to the southern star formation core, the other three about 20' to the north. We detect 169 X-ray sources, ∼95% of which are likely to be Pre Main Sequence (PMS) stars, significantly enlarging the known population of the SFR in the area covered by the observations. Using published BVRI photometry we place the X-ray sources with well defined optical counterparts on the HRI diagram and estimate their masses and ages. Our comparison of the mass function and age distribution of the X-ray sources with results previously obtained for NGC 2264, demonstrates that deep X-ray observations provide, at least in this case, a very efficient method of selecting SFR members and does not introduce stronger biases than other methods. Since the observation cover a time span of ∼5 years, we are able to study in detail the X-ray variability of our sample of PMS stars. We find that: 1) a large fraction of our sources are variable on several time scales and 2) Classical T Tauri Systems (i.e. stars surrounded by disks) are significantly more variable than the rest of our sample, suggesting a role of accretion disks in the emission and/or in the absorption of the X-ray radiation.