The chromospheres and coronae of five G-K main-sequence stars

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Abstract
Five main-sequence stars, χ1 Ori (G0V), α Cen A (G2V), ξ Boo A (G8V), α Cen B (K0V) and ε Eri (K2V) have been observed at low and high dispersion with the International Ultraviolet Explorer (IUE) satellite. The data obtained and X-ray observations reported in the literature are used to make models of the structure of the atmospheres of these stars, from the high chromosphere to the corona. The electron pressures and coronal temperatures in these stars range from being similar to those in the quiet solar atmosphere (α Cen A) to the higher values found more typically in solar active regions (e.g. χ1 Ori, ξ Boo A). The models are used to examine the energy lost by radiation and transferred by thermal conduction, in order to establish the heating requirements. The results are similar to those found for the solar atmosphere. It seems unlikely that acoustic waves can provide sufficient flux above 2 × 104 K, but MHD modes cannot be excluded. Indeed, the observed emission measure distribution below 105 K can be matched in a model where Alfvén wave energy input, observed through non-thermal line broadening, is balanced by radiation losses. There are difficulties in disposing of energy conducted down from the corona – as in the Sun – difficulties which could in principle be resolved by restricting UV emission to supergranulation boundaries. There are not at present sufficient X-ray data to separate any time varying active region components from the ‘average’ corona.

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