The Relative Emission from Chromospheres and Coronae: Dependence on Spectral Type and Age

We present a comprehensive picture of the relative emission and heating of the chromospheres and coronae of stars with spectral types F2 - M7.5 V as a function of effective temperature and age. At chromospheric temperatures the dominant emission is in the Lyman-alpha line, which we take as representative of the total emission from chromospheres. At coronal temperatures the dominant emission is by X-rays, which we take a representative of the total emission from coronae. We include in this study 79 dwarf stars for which there are X-ray fluxes and Lyman-alpha fluxes corrected for interstellar absorption. The Lyman-alpha fluxes are from HST/STIS spectra obtained with the MUSCLES, Mega-MUSCLES, and other surveys and programs. The X-ray fluxes are from XMM-Newton, Chandra, and ROSAT.

We find that Lyman-alpha and X-ray fluxes for F, G, and K stars lie on the same trend line with active stars near the top and inactive stars near the bottom. As stars evolve, they systematically descend the trend line. M stars depart from the FGK trend line in the sense of relatively weak Lyman-alpha emission. The ratio of the Lyman-alpha to bolometric luminosity, L(Lya)/L(bol), increases to lower effective temperature for stars at all ages. The ratio of X-ray to bolometric luminosity, L(X)/L(bol), follows a different pattern with young stars (t<450 Myr) showing X-ray saturation for effective temperatures less than 5200 K and stronger L(X)/L(bol) than L(Lya/L(bol). For older stars (t>4 Gyr), L(Lya)/L(bol) exceeds L(X)/L(bol), but L(X)/L(bol) increases to lower effective temperatures much faster than L(Lya)/L(bol). We discuss the implications of these results concerning the relative heating rates at coronal and chromospheric temperatures.

This work is supported by grants from the Space Telescope Science Institute for programs HST-GO-12475, 12596, 13650, 14640, and 15071.