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.