Coronal Structure and Abundances in Young Fast Rotators

Abstract

AB Dor, Speedy Mic and Rst137B are in their early post-T Tauri evolutionary phase (<100Myr), at the age of fastest rotation in the life of late-type stars. They straddle the coronal saturation-supersaturation boundary first defined by young stars in open clusters. High resolution Chandra X-ray spectra have been analysed to study their coronal properties as a function of coronal activity parameters Rossby number, $L_X/L_{bol}$ and a coronal temperature index. Plasma emission measure distributions as a function of temperature show broad peaks at T~10e7K. Differences between stars suggest that as supersaturation is reached the DEM slope below the temperature of peak DEM becomes shallower, while the DEM drop-off above this temperature becomes more pronounced. A larger sample comprising our three targets and 22 active stars studied in the recent literature reveals a general increase of plasma at T>10e7 toward the saturated-supersaturated boundary but a decline beyond this among supersaturated stars. All three of the stars studied in detail here show lower coronal abundances of the low FIP elements Mg, Si and Fe, relative to the high FIP elements S, O and Ne, as compared to the solar mixture. The coronal Fe abundances of the stellar sample are inversely correlated with Lx/Lbol, declining slowly with rising Lx/Lbol, but with a much more sharp decline at Lx/Lbol>3x10e-4. For dwarfs the Fe abundance is also well-correlated with Rossby number. The coronal O/Fe ratios for dwarfs show a clear increase with decreasing Rossby number, apparently reaching saturation at [O/Fe]=0.5 at the coronal supersaturation boundary. Similar increases in O/Fe with increasing coronal temperature and $L_X/L_{bol}$ are seen.