Astronomy & Astrophysics

Free access article

A&A 408, 1087-1102 (2003)
DOI: 10.1051/0004-6361:20031025

Three years in the coronal life of AB Dor

I. Plasma emission measure distributions and abundances at different activity levels

J. Sanz-Forcada, A. Maggio and G. Micela

INAF - Osservatorio Astronomico di Palermo G. S. Vaiana, Piazza del Parlamento 1, Palermo 90134, Italy
(Received 17 April 2003 / Accepted 27 June 2003 )

Abstract
The young active star AB Dor (K1 IV-V) has been observed 16 times in the last three years with the XMM-Newton and Chandra observatories, totalling 650 ks of high-resolution X-ray spectra. The XMM/RGS observations with the highest and lowest average emission levels have been selected to study the coronal properties of AB Dor in two different activity levels. We compare the results based on the XMM data with those obtained from a higher resolution Chandra/HETG spectrum, using the same line-based analysis technique. We have reconstructed the plasma Emission Measure Distribution vs. temperature (EMD) in the range $\log T({\rm K})\sim6.1$ -7.6, and we have determined the coronal abundances of AB Dor, obtaining consistent results between the two instruments. The overall shape of the EMD is also consistent with the one previously inferred from EUVE data. The EMD shows a steep increase up to the peak at $\log T({\rm K})\sim6.9$ and a substantial amount of plasma in the range $\log T({\rm K})\sim6.9$ -7.3. The coronal abundances show a clear trend of increasing depletion with respect to solar photospheric values, for elements with increasing First Ionization Potential (FIP), down to the Fe value ([Fe/H] = -0.57), followed by a more gradual recovery of the photospheric values for elements with higher FIP. He-like triplets and $\ion{xxi}$ and $\ion{xxii}$ lines ratios indicate electron densities $\log n_{\rm e} \sim 10.8$ cm ^-3 at $\log T({\rm K})\sim 6.3$ and $\log n_{\rm e}\sim 12.5$ cm ^-3 at $\log T({\rm K})\sim7$ , implying plasma pressures steeply increasing with temperature. These results are interpreted in the framework of a corona composed of different families of magnetic loop structures, shorter than the stellar radius and in isobaric conditions, having pressures increasing with the maximum plasma temperature, and which occupy a small fraction ( $f \sim 10^$ - 10^-6) of the stellar surface.

Key words: stars: coronae -- stars: individual: AB Dor -- X-rays: stars -- stars: late-type -- stars: abundances -- line: identification

Offprint request: J. Sanz-Forcada, jsanz@astropa.unipa.it

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