SIMBAD references

We investigate the evolution of X-ray stellar activity from the age of the youngest known star forming regions (SFR), ≲1Myr, to about 100Myr, i.e. the zero age main sequence (ZAMS) for a ∼1M_☉ star. We consider five SFR of varying age (ρ Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I, and η Chamaeleontis) and two young clusters (the Pleiades and NGC 2516). Optical and X-ray data for these regions are retrieved both from archival observations and recent literature, and reanalyzed here in a consistent manner so as to minimize systematic differences in the results. We study trends of L_X and L_X/L_bol as a function of stellar mass and association age. For low mass stars (M≲1M_☉) we observe an increase in L_X/L_bol in the first 3-4Myr and a subsequent leveling off at the saturation level (L_X/L_bol~-3). Slowly evolving very low mass stars then retain saturated levels down to the oldest ages here considered, while for higher mass stars activity begins to decline at some age after ∼10⁷ years. We find our data consistent with the following tentative picture: low mass PMS stars with no circumstellar accretion disk have saturated activity, consistently with the activity-Rossby number relation derived for MS stars. Accretion and/or the presence of disks somehow lowers the observed activity levels; disk dissipation and/or the decrease of mass accretion rate in the first few Myrs of PMS evolution is therefore responsible for the observed increase of L_X/L_bol with time.