|Affiliation||Osservatorio Astronomico G.S. Vaiana di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy|
|To appear in||Ph.D. dissertation abstract|
|Links||Orion Nebula Cluster / NGC 2264 / NGC 2516|
In the ONC I find no correlation between activity and stellar rotation. Instead I observe that median LX correlates with mass, from the brown dwarf regime, (M = 0.03-0.08 MO, Log(LX) ~ 1028 ergs/s) to about 3 MO (Log(LX) >~ 1031) and then drops for stars that, according to models, do not have a convective region. The relation of LX with mass can be also imputed to the fact that most stars have Log(LX/Lbol) close, even though on average lower, to the saturation value (~ -3) and to the direct relation between Lbol and mass for stars of a given age. I observe that median Log(LX/Lbol) also increases with mass by about 0.4 dex for 0.1 <~ M <~ 0.5MO. A dependence of LX and LX/Lbol on circumstellar accretion and/or presence of circumstellar disks is apparent in the data, accreting stars showing significantly lower average levels respect to non accreting ones.
Low mass NGC 2264 members also display saturated emission levels, Log(LX/Lbol) ~ -3. Investigating the X-ray variability of the sources detected in our HRI data, I find that Classical T Tauri Stars are significantly more variable than Weak Lined T Tauri Stars.
I derive activity levels for members of the young ZAMS NGC 2516 cluster and compare them with those of Pleiades stars, of roughly the same age, but with different metallicity. I find that G and K stars appear to be subluminous in NGC 2516 respect to the Pleiades. This result hints toward a role of metallicity in the determination of coronal activity.
Finally, by complementing the original observations with published data on two star forming regions, rho Ophiuchi, and Chamaeleon I, and on the Pleiades cluster, all partially re-analyzed here in a consistent way, I try to derive a coherent description of activity evolution from the early PMS to the ZAMS. Log(LX/Lbol), for stars of a given mass, is observed to increase in the first 3-5 Myrs and to reach the saturation level, -3. This early evolution is slower for lower mass stars. Following this phase, low mass stars keep saturated activity levels up to the ZAMS, while more massive stars de-saturate at some point between the age of the Chamaeleon I region and that of the Pleiades. I find that the PMS data is compatible with the following picture: PMS stars have Log(LX/Lbol) ~ -3, i.e. at the same level of small Rossby number MS stars, with the exception that accretion and/or the presence of disks acts to reduce their X-ray emission. Modern PMS evolutionary models and measured rotational periods indeed suggest that young stars should, have saturated emission according to the activity - Rossby number relation observed for main sequence stars. As the number of accreting/disked stars decreases with time, the fraction of saturated stars of a given mass increases and the mean activity level approaches the saturation level.