Coronal Emission Measures and Abundances for Moderately Active K Dwarfs Observed byChandra

We have used Chandra to resolve the nearby 70 Oph (K0 V+K5 V) and 36 Oph (K1V+K1 V) binary systems for the first time in X-rays. The LETG/HRC-S spectra ofall four of these stars are presented and compared with an archival LETGspectrum of another moderately active K dwarf, Epsilon Eri. Coronal densitiesare estimated from O VII line ratios and emission measure distributions arecomputed for all five of these stars. We see no substantial differences incoronal density or temperature among these stars, which is not surprisingconsidering that they are all early K dwarfs with similar activity levels.However, we do see significant differences in coronal abundance patterns.Coronal abundance anomalies are generally associated with the first ionizationpotential (FIP) of the elements. On the Sun, low-FIP elements are enhanced inthe corona relative to high-FIP elements, the so-called "FIP effect." Differentlevels of FIP effect are seen for our stellar sample, ranging from 70 Oph A,which shows a prominent solar-like FIP effect, to 70 Oph B, which has no FIPbias at all or possibly even a weak inverse FIP effect. The strong abundancedifference exhibited by the two 70 Oph stars is unexpected considering howsimilar these stars are in all other respects (spectral type, age, rotationperiod, X-ray flux). It will be difficult for any theoretical explanation forthe FIP effect to explain how two stars so similar in all other respects canhave coronae with different degrees of FIP bias. Finally, for the stars in oursample exhibiting a FIP effect, a curious difference from the solar version ofthe phenomenon is that the data seem to be more consistent with the high-FIPelements being depleted in the corona rather than a with a low-FIP enhancementComment: 35 pages, 8 figures, AASTEX v5.0 plus EPSF extensions in mkfig.sty; accepted by Ap