Time evolution of X-ray coronal activity in PMS stars; a possible relation with the evolution of accretion disks

We investigate the evolution of X-ray stellar activity from the age of the youngest known star forming regions (SFR), less than or similar to 1 Myr, to about 100 Myr, i.e. the zero age main sequence (ZAMS) for a similar to 1 M. star. We consider five SFR of varying age (rho Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I, and eta 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 less than or similar to 1 M.) we observe an increase in L-X/L-bol in the first 3-4 Myr and a subsequent leveling off at the saturation level (L-X/L-bol similar to -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 similar to10(7) 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.