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

INAF - Osservatorio Astronomico di Palermo Giuseppe S. Vaiana, Palazzo dei Normanni, 90134 Palermo, Italy e-mail: giusi@oapa.astropa.unipa.it, sciorti@oapa.astropa.unipa.it

Corresponding author: E. Flaccomio, ettoref@oapa.astropa.unipa.it

Received: 24 September 2002
Accepted: 6 February 2003

Abstract

We investigate the evolution of X-ray stellar activity from the age of the youngest known star forming regions (SFR), 1 Myr, to about 100 Myr, i.e. the zero age main sequence (ZAMS) for a ~1  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 as a function of stellar mass and association age. For low mass stars () we observe an increase in in the first 3–4 Myr and a subsequent leveling off at the saturation level (). 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  with time.