Neon and Chemical Fractionation Trends in Late-type Stellar Atmospheres

Abstract

A survey of Ne, O and Fe coronal abundances culled from the recent literature for about 60 late-type stars confirms that the Ne/O ratio of stellar outer atmospheres is about two times the value recently recommended by Asplund et al. The mean Ne/O remains flat from the most active stars down to at least intermediate activity levels (-5L_X/L_bol<-2), with some evidence for a decline toward the lowest activity levels sampled. The abundances surveyed are all based on emission measure distribution analyses and the mean Ne/O is about 0.1 dex lower than that found from line ratios in the seminal study of mostly active stars by Drake & Testa (2005), but is within the systematic uncertainties of that study. We also confirm a pattern of strongly decreasing Fe/O with increasing stellar activity. The observed abundance patterns are reminiscent of the recent finding of a dependence of the solar Ne/O and Fe/O ratios on active region plasma temperature and indicate a universal fractionation process is at work. The firm saturation in stellar Ne/O at higher activity levels combined with variability in the solar coronal Ne/O leads us to suggest that Ne is generally depleted in the solar outer atmosphere and photospheric values are reflected in active stellar coronae. The solution to the recent solar model problem would then appear to lie in a combination of the Asplund et al O abundance downward revision being too large, and the Ne abundance being underestimated for the Sun by about a factor of 2.