Duvvuri, Girish M.; Berta-Thompson, Zachory K.; Pineda, John Sebastian; France, Kevin C.; Brown, Alexander; Youngblood, Allison A.; Wilson, David J.; Froning, Cynthia S.; Schneider, Peter Christian; Ayres, Thomas R.; & Stassun, Keivan Guadalupe. (2025). Stellar library of differential emission measures and extreme ultraviolet spectra: Dwarf stars observed by the Extreme Ultraviolet Explorer. Astrophysical Journal, 993(1), 138. https://doi.org/10.3847/1538-4357/ae06a7
Extreme ultraviolet (EUV; 100–912 angstroms) radiation from stars plays a major role in shaping planets. EUV photons can ionize hydrogen and other atoms, which affects how planetary atmospheres form, change, and sometimes erode over time. However, for most stars that host exoplanets, their EUV radiation is difficult to measure directly and is therefore not well known.
In this study, we used a modeling method called the differential emission measure (DEM) technique to estimate the EUV spectra of eight nearby stars. These stars were previously observed with high-quality data by the Extreme Ultraviolet Explorer (EUVE) satellite between 1992 and 2002. The sample includes stars of different spectral types, from cooler M-type stars to hotter F-type stars, such as AD Leo, ε Eridani, κ¹ Ceti, Procyon, α Centauri A and B, and ξ Boötis A and B.
Our DEM-based model spectra closely match the original EUVE measurements. For most individual data points, the modeled values are within a factor of three of the observed flux densities, and for the total energy emitted between 100 and 300 angstroms, the agreement is within 30 percent. We provide the atomic data, X-ray, EUV, and far-ultraviolet observations used as inputs, along with the DEM models and the predicted EUV spectra. These predicted spectra extend beyond the original EUVE wavelength range of 90–510 angstroms.
We also found that different layers of a star’s outer atmosphere contribute differently to its EUV emission. The transition region and the corona both produce EUV radiation, but their relative contributions vary from star to star. The corona, in particular, is strongly affected by stellar flares, which cause temporary and unpredictable increases in EUV radiation at certain wavelengths. The amount and pattern of this variability depend on the star’s temperature structure, flare activity, and magnetic activity cycle.
These findings are important because many studies of planetary atmospheric evolution rely on estimates of stellar EUV radiation. Understanding how EUV emission varies over time helps improve models of how exoplanet atmospheres respond to their host stars.
Figure 1. Demonstration of the DEM technique applied to SIRS (T. N. Woods et al. 2009). The top-left panel shows the median DEM value across 105 draws from the posterior distribution (solid green line) with shading spanning the interval between the 16th–84th percentile of the DEM draw values and the horizontal bars are the flux constraints discussed in Section 2.2. The bars labeled with ion species correspond to measured integrated line fluxes from that species while the unlabeled bars correspond to spectral bins where the contribution function is a sum from unresolved blends of lines and continuum processes. The top-right panel and its associated color bar show the Gλ(T) contribution function matrix calculated using atomic data, with the position of dark patches along a single wavelength column corresponding to the plasma temperature that contributes more observed emission at that wavelength (assuming an equal distribution of plasma at all temperatures). The bottom panel compares the model-generated DEM spectrum (green) to the original data (black) with the shaded interval representing the model uncertainty determined by sampling from the posterior distribution of DEM shapes and systematic uncertainty inflation s-factors.