5 Conclusions

Solar observations have shown a pattern of low-FIP material being overabundant relative to the high-FIP elements in the solar corona, in its wind, and also in solar energetic particles (Meyer 1985; Feldman 1992; Laming et al. 1995). Previous observations of stellar coronae have displayed a marked depletion of the metal abundance (essentially Fe) in active stars (e.g. Schmitt et al. 1996a), and either the absence of a FIP bias or a solar-like FIP effect in inactive stars (Drake et al. 1997). Recent results from XMM-Newton and Chandra have suggested the presence of a FIP bias at variance with the solar pattern; highly active stars appear to display a depletion of low-FIP elements relative to the high-FIP elements (Brinkman et al. 2001; Drake et al. 2001). In this paper, we have shown that a sample of RS CVn binary systems shows a FIP bias that appears to change from a marked inverse FIP effect in highly active binaries to an absence of a trend (or a possible solar-like FIP effect) in the intermediately active star Capella (Fig. 4). Although such a transition is only weakly suggested (since RS CVn binary systems often populate the highest levels of coronal activity), it resembles a similar transition found in solar-like stars (Güdel et al. 2002). The latter sample covers a wide range of activity levels and of ages. The highly active, young classical T Tauri star TW Hydrae displays a high depletion of the low-FIP Fe relative to O, whereas the high-FIP Ne is enhanced (Kastner et al. 2002). Preliminary results from the Chandra and XMM-Newton grating data of the extremely active YY Mensae show a similar marked depletion of low-FIP elements, with almost no emission lines and a well-developed continuum (Audard et al., in preparation). The above observations strongly suggest that the FIP bias in stellar coronae is correlated with the activity level. However, the absence of FIP bias in the inactive Procyon (Drake et al. 1995; Raassen et al. 2002) is a challenge. Procyon's coronal heating mechanism has been debated, with acoustic heating being a possibility, although this mechanism seems unlikely to play a major role in this star (Mullan & Cheng 1994; Schmitt et al. 1996b). A larger sample will eventually allow us to identify stars that may not fit into a simplistic activity-abundance correlation.

We have investigated whether currently used values of the solar photospheric abundances (Holweger 2001; Allende Prieto et al. 2001, 2002) have a significant effect on the observed FIP bias. Indeed, tables of solar abundance have changed significantly since the publication of the Anders & Grevesse (1989) standards. However, we have shown that their use does not remove the observed inverse FIP effect in highly active RS CVn binaries (Fig. 5). Although solar abundances are commonly used, it is preferential to use the stellar photospheric abundances to compare with the coronal abundances. In $\lambda$ And, we found no FIP bias either on the basis of solar or stellar photospheric abundances, suggesting that the absence of any FIP-related effect in this wide binary is real. However, $\lambda$ And may be a special case and a similar test would be more insightful if applied to a binary with a marked FIP bias, such UX Ari. However, the presently uncertain photospheric abundances in the most active stars does not allow such a procedure. Güdel et al. (2002) circumvented this problem for main-sequence stars by using solar analogs with known stellar photospheric abundances (similar to solar). Since their sample showed a transition from a solar FIP effect to an inverse FIP effect with decreasing age (or increasing activity level), the similar trend observed in RS CVn binaries at higher activity levels appears to be real.

The presence of the inverse FIP effect in highly active stars, and the suggested transition of the FIP bias as a function of the activity level could open up new views of the fractionation mechanisms in stellar atmospheres. Previous studies have focused on explaining the FIP effect in the Sun (see Hénoux 1995, for a review). One category of enrichment models suggests that the magnetic field plays no active role in the separation process. In these models, fractionation of ions from neutral species results from diffusion along the magnetic field lines, through collisions (e.g. Marsch et al. 1995; Peter 1996, 1998; Wang 1996). Another category of models proposes that the magnetic field does play an active role for the separation of elements. Fractionation occurs thanks to the difference between the drift velocities of ionized and neutral elements moving across the magnetic field (e.g. Vauclair & Meyer 1985; Vauclair 1996; von Steiger & Geiss 1989; Antiochos 1994; Hénoux & Somov 1997). Güdel et al. (2002) proposed to explain the inverse FIP effect seen in active stars by high-energy electrons detected by their gyrosynchrotron emission. The particles propagate downward and could prevent chromospheric ions - mostly low-FIP elements - from escaping along the magnetic field lines up into the corona by creating a downward-pointing electric field. In less active stars, the radio flux (therefore the density of high-energy electrons) is smaller, thus quenching the inverse FIP effect. This simplistic model could also account for the increase in the low-FIP abundances during large flares, as observed (Güdel et al. 1999; Audard et al. 2001a). However, more sophisticated models are timely to explain the inverse FIP effect in active stars.

Acknowledgements

We thank the XMM-Newton SOC team for allowing us to use data from the commissioning and calibration phases. We also thank the referee, Dr. A. Maggio, whose comments have improved the paper considerably. The PSI group acknowledges support from the Swiss National Science Foundation (grant 2000-058827 and fellowship 81EZ-67388). MA is grateful to Dr. S. White and the University of Maryland for their hospitality after the tragic events of 9/11, 2001. SRON is supported financially by NWO.