The evolution of X-ray emission in young stars

The evolution of magnetic activity in late-type stars is part of the intertwined rotation-age-activity relation, which provides an empirical foundation to the theory of magnetic dynamos. We study the age-activity relation in the premain-sequence (PMS) regime, 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% of the stars in this sample are detected as X-ray sources. Within the PMS phase for stellar ages in the range ∼0.1-10 Myr, we establish a mild decay in activity with stellar age τ roughly as L _X ∝ τ ^-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 _⊙ < M < 1.2 M _⊙ mass range is more rapid with L _X ∝ τ ^-0.75 over the wide range of ages 5 yr < log τ < 9.5 yr. When the fractional X-ray luminosity L _X/L _bol and the X-ray surface flux are considered as activity indicators, the decay law index is similarly slow for the first 1-100 Myr but accelerates for older stars. The magnetic activity history for M stars with masses 0.1 M _⊙ < M < 0.4 M _⊙ is distinctly different. Only a mild decrease in X-ray luminosity, and even a mild increase in L _X/L _bol and F _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.