Thomas Preibisch, Eric D. Feigelson

The evolution of X-ray emission in young stars

Astrophysical Journal Supplements, COUP Special Issue, 160, 390-400 (2005)

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 pre-main 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 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. When the fractional X-ray luminosity $L_{\rm X}/L_{\rm bol}$ 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 $0.1 < M < 0.4~M_\odot$ 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.

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