Astronomy & Astrophysics

Free access article

A&A 402, 277-292 (2003)
DOI: 10.1051/0004-6361:20030203

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

E. Flaccomio, G. Micela and S. Sciortino

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
(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), $\la$ 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 ( $\rho$ 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 $L_{\rm X}$ 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 ~ 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 $L_{\rm X}/L_{\rm bol}$ with time.

Key words: stars: activity -- stars: pre-main sequence -- open clusters and associations: individual: $\rho$ Ophiuchi, Orion Nebula Cluster, Chamaeleon I, $\eta$ Chamaeleontis

Offprint request: E. Flaccomio, ettoref@oapa.astropa.unipa.it

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