The evolution of X-ray emission in young stars

We study the relation between age and magnetic activity in late-type pre-main sequence (PMS) stars, for the first time using mass-stratified subsamples. The effort is based on the Chandra Orion Ultradeep Project (COUP) which provides very sensitive and homogenous X-ray data on a uniquely large sample of 481 optically well-characterized low-extinction low-mass members of the Orion Nebula Cluster, for which individual stellar masses and ages could be determined. More than 98 percent of the stars in this sample are detected as X-ray sources. Within the PMS phase for stellar ages in the range $\sim 0.1-10$ Myr, we establish a mild decay in activity with stellar age $\tau$ roughly as $L_{\rm X} \propto \tau^{-1/3}$. On longer timescales, when the Orion stars are compared to main sequence stars, the X-ray luminosity decay law for stars in the $0.5 < M < 1.2$ M$_\odot$ mass range is more rapid with $L_{\rm X} \propto \tau^{-0.75}$ over the wide range of ages $5 < \log \tau < 9.5$ yr. The magnetic activity history for M stars with masses $0.1 < M < 0.4 M_\odot$ is distinctly different. Only a mild decrease in X-ray luminosity, and even a mild increase in $L_{\rm X}/L_{\rm bol}$ and $F_{\rm X}$, is seen over the 1-100 Myr range, though the X-ray emission does decay over long timescales on the main sequence. Together with COUP results on the absence of a rotation-activity relation in Orion stars, we find that the activity-age decay is strong across the entire history of solar-type stars but is not attributable to rotational deceleration during the early epochs. A combination of tachocline and distributed convective dynamos may be operative in young solar-type stars. The results for the lowest mass stars are most easily understood by the dominance of convective dynamos during both the PMS and main sequence phases.