| Abstract
| - We investigate the evolution of X-ray stellar activity from the age of the youngest known star forming regions (SFR), $\la$1 Myr, to about 100 Myr, i.e. the zero age main sequence (ZAMS) for a ~1 $M_odot$ 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 LX and $L_{\rm X}/L_{\rm bol}$ as a function of stellar mass and association age. For low mass stars ( $M \la 1~M_odot$) we observe an increase in $L_{\rm X}/L_{\rm bol}$ in the first 3-4 Myr and a subsequent leveling off at the saturation level ( $L_{\rm X}/L_{\rm bol} \sim -3$). 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 ~10 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 $L_{\rm X}/L_{\rm bol}$ with time.
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