2005/06/02 by Thomas Preibisch, Eric D. Feigelson · 2 citations
Physics and Astronomy · #astro-ph
paper · pdf · doi:10.1086/432094
accepted for ApJS, COUP special issue
arxiv created 2005/06/02 · arxiv updated 2009/12/01
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 ∼ 0.1-10 Myr, we establish a mild decay in activity with stellar age τ roughly as L\rm 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 < 1.2 M_\odot mass range is more rapid with L\rm X ∝ τ-0.75 over the wide range of ages 5 < log τ< 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.