Neon and oxygen in low activity stars: towards a coronal unification with the Sun

Abstract

Aims: The disagreement between helioseismology and a recent downward revision of solar abundances has resulted in a controversy about the true neon abundance of the Sun and other stars. We study the coronal Ne/O abundance ratios of nearby stars with modest activity levels and investigate a possible peculiarity of the Sun among the stellar population in the solar neighborhood.
Methods: We used XMM-Newton and Chandra data from a sample of weakly and moderately active stars with log L_X/L_bol ≈ -5...-7 to investigate high-resolution X-ray spectra to determine their coronal Ne/O abundance ratio. We applied two linear combinations of strong emission lines from neon and oxygen, as well as a global-fitting method for each dataset, and crosschecked the derived results.
Results: The sample stars show a correlation between their Ne/O ratio and stellar activity in the sense that stars with a higher activity level show a higher Ne/O ratio. We find that the Ne/O abundance ratio decreases in our sample from values of Ne/O ≈ 0.4 down to Ne/O ≈ 0.2-0.25, suggesting that ratios similar to “classical” solar values, i.e. Ne/O ≈0.2, are rather common for low activity stars. A significantly enhanced neon abundance as the solution to the solar modeling problem seems unlikely.
Conclusions: From the coronal Ne/O abundance ratios, we find no indications of a peculiar position of the Sun among other stars. The solar behavior appears to be rather typical of low activity stars.