Outer atmospheres of cool stars. IX - A survey of ultraviolet emission from F-K dwarfs and giants with IUE

Abstract

We report preliminary results of an ultraviolet survey of cool-star emission properties with IUE. We present 1150-2000 Á spectra of representative F-K dwarfs and giants and construct correlation diagrams that compare chromospheric (T<10 4 K) and transition-region (TaelO 5 K) emission line strengths, and broad-band coronal {T> 10 6 K) soft-X-ray fluxes. We find that the transition-region (TR) and coronal emission in the G-K dwarfs and G giants is well correlated with the Mg ii \2800 doublet emission strength, which in turn is symptomatic of chromospheric energy losses. However, the power-law slopes are steeper than unity, particularly for soft X-rays. We conclude that while the distinct atmospheric layers very likely are physically associated, the internal heating mechanisms must be quite different. Despite the apparent chromosphere-TR-corona correlations among the G-K dwarfs and G giants, the earlier (F) and later (K) giants exhibit anomalous behavior. The former appear to have brighter TRs for a given chromospheric emission level, while the latter are systematically deficient in T>10 5 K material. In fact, not only are TRs very weak among the red giants, but chromospheres and coronae are systematically weak as well (at least compared with the mean activity levels of F-G giants). We propose that the weakness of hot outer atmospheres in the red giants compared with the yellow giants can be understood as a consequence of stellar evolution since stars of slightly different spectral type in the giant branch likely have very different main-sequence progenitors.

On the Origin of the Basal Emission from Stellar Atmospheres: Analysis of Solar CiiLines

“…It is a strong emission feature in the Sun (e.g., Lites et al 1978; see our Fig. 1) and solar-type stars (e.g., Ayres et al 1981). The multiplet has been observed in many different regions on the Sun and many stars.…”

“…Studies of samples of stellar spectra have led to the identification of a '' basal '' level of stellar chromospheric heating that has since become almost synonymous with acoustic wave heating. Evidence for this basal component of chromospheric heating was first presented by Schrijver (1987), who followed work by Ayres, Marstad, & Linsky (1981) and Oranje (1986) in studying relationships between the radiative flux densities of pairs of emission features formed in stellar chromospheres, transition regions, and coronae. Using statistical methods, Schrijver found evidence for two components of heating in the outer atmospheres: the basal component, which depends only on fundamental stellar parameters (effective temperature, gravity), and another component which depends on rotation rates and convection zone properties, and hence on magnetic fields generated by a large-scale stellar dynamo.…”

Observation and modelling of main-sequence star chromospheres

“…In the solar case, this is about a factor of 5 in the lower chromospheric CaII and MgII resonance lines (e.g. Chapman 1980;Ayres et al 1981). In the HeI lines, the contrast is higher, typically a factor of 16 (e.g., Smith et al 2009) and may reach a factor of 30.…”

Stellar Chromospheric Activity

“…A critical early result was the discovery of tight power law correlations between emission in chromospheric, transition region, and coronal lines (e.g., Ayres et al, 1981). A few quotations from that paper loom large in the present-day literature; Ayres et al (1981) suggest that "the very existence of the correlations argues that coronae and chromospheres are physically associated," and later in their paper, discussing chromospheric and coronal heating mechanisms, they write that "the small-scale magnetic flux tubes thought to comprise the major component of the solar surface field [may] serve as conduits of wave energy (acoustic or Alfvén) into the outer atmosphere.…”