SIMBAD references

A 56 ks soft X-ray image of the IC 2391 star cluster has been acquired with the ROSAT High Resolution Imager, resulting in the detection of 34 sources above a 3 σ detection level of L_X≃4x10²⁸ ergs.s^–1. We report source coordinates and fluxes plus identifications and broadband BVRI photometry for optical counterparts to each source. We compare these results with X-ray source fluxes from a ROSAT PSPC image of the same region, which we obtained 2 yr earlier. The largest change in X-ray brightness for any of the known or suspected G, K, and M cluster members is 0.43 dex, i.e., a factor of ∼3. Given this modest amplitude in X-ray variability, we conclude that the broad (factor of 20) dispersion in X-ray luminosities within IC 2391 is unrelated to short- or intermediate-term cycles of the stellar dynamo. We further show that the effects of source confusion on the PSPC X-ray luminosity function, for distance scales of 5"-30", are relatively small and are thus negligible for the purpose of comparing IC 2391 with older clusters and for studying the evolution of coronal X-ray emission in solar-type stars. The greater part of the observed scatter in X-ray luminosity within IC 2391 still awaits explanation.