Coronal Abundances in Orion Nebula Cluster Stars

Following the Chandra Orion Ultradeep Project (COUP) observation, we havestudied the chemical composition of the hot plasma in a sample of 146 X-raybright pre-main sequence stars in the Orion Nebula Cluster. We reportmeasurements of individual element abundances for a subsample of 86slightly-absorbed and bright X-ray sources, using low resolution X-ray spectraobtained from the Chandra ACIS instrument. The X-ray emission originates from aplasma with temperatures and elemental abundances very similar to those ofactive coronae in older stars. A clear pattern of abundances vs. FirstIonization Potential (FIP) is evident if solar photospheric abundances areassumed as reference. The results are validated by extensive simulations. Theobserved abundance distributions are compatible with a single pattern ofabundances for all stars, although a weak dependence on flare loop size may bepresent. The abundance of calcium is the only one which appears to varysubstantially between stars, but this quantity is affected by relatively largeuncertainties. The ensemble properties of the X-ray bright COUP sources confirmthat the iron in the emitting plasma is underabundant with respect to both thesolar composition and to the average stellar photospheric values. Comparison ofthe present plasma abundances with those of the stellar photospheres and thoseof the gaseous component of the nebula, indicates a good agreement for all theother elements with available measurements, and in particular for the high-FIPelements (Ne, Ar, O, and S) and for the low-FIP element Si. We conclude thatthere is evidence of a significant chemical fractionation effect only for iron,which appears to be depleted by a factor 1.5--3 with respect to the stellarcomposition.Comment: 18 pages, 17 figure