Abstract
Abstract Extreme-ultraviolet and X-ray emission from stellar coronae drives mass loss from exoplanet atmospheres, and ultraviolet emission from stellar chromospheres drives photochemistry in exoplanet atmospheres. Comparisons of the spectral energy distributions of host stars are, therefore, essential for understanding the evolution and habitability of exoplanets. The large number of stars observed with the MUSCLES, Mega-MUSCLES, and other recent Hubble Space Telescope observing programs has provided for the first time a large sample (79 stars) of reconstructed Lyα fluxes that we compare with X-ray fluxes to identify significant patterns in the relative emission from these two atmospheric regions as a function of stellar age and effective temperature. We find that as stars age on the main sequence, the emissions from their chromospheres and coronae follow a pattern in response to the amount of magnetic heating in these atmospheric layers. A single trend-line slope describes the pattern of X-ray versus Lyα emission for G and K dwarfs, but the different trend lines for M dwarf stars show that the Lyα fluxes of M stars are significantly smaller than those of warmer stars with the same X-ray flux. The X-ray and Lyα luminosities divided by the stellar bolometric luminosities show different patterns depending on stellar age. The L(Lyα)/L(bol) ratios increase smoothly to cooler stars of all ages, but the L(X)/L(bol) ratios show different trends. For older stars, the increase in coronal emission with decreasing is much steeper than that of chromospheric emission. We suggest a fundamental link between atmospheric properties and trend lines relating coronal and chromospheric heating,
Highlights
All stars with convective interiors from the A7 V star α Aql (Robrade & Schmitt 2009) to the late-M and perhaps L dwarfs (Berger et al 2010; Stelzer et al 2012; Hawley & Johns-Krull 2003)emit ultraviolet (91.2–300 nm, UV) and X-ray (0.1–10 nm) photons from plasmas at temperatures ranging from roughly 5,000 K to at least 106 K
∗ Based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS AR-09525.01A
Stellar age plays an important role in determining whether chromospheric emission becomes relatively weak or coronal emission becomes relatively strong with decreasing Teff
Summary
All stars with convective interiors from the A7 V star α Aql (Robrade & Schmitt 2009) to the late-M and perhaps L dwarfs (Berger et al 2010; Stelzer et al 2012; Hawley & Johns-Krull 2003). There are many examples of correlations of chromospheric emission (e.g., Lyman-α, Ca II H and K lines, Mg II h and k lines, Hα) with such stellar activity indicators as age, rotation, and magnetic field strength and coverage (e.g., Wood et al 2005; Guinan, Engle, & Durbin 2016; Newton et al 2017). These correlations generally show saturation at high activity levels and linear regressions in log-log plots with decreasing activity indicators such as age and rotation These correlations are usually described by power-law relations of the form log F (corona) = α log F (chromo) +β, where F (corona) is a coronal flux or luminosity diagnostic, usually the broad band X-ray emission, and F (chromo) is the flux or luminosity of a chromospheric diagnostic, generally an emission line such as the Ca II K line (393.3 nm), Mg II k line (279.6 nm) or H I Lyman-α line (121.56 nm).
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