Time Variability of FUV Emission from Cool Stars on Multi-year Timescales

The physical and chemical properties of planetary atmospheres are affected by temporal evolution of ultraviolet (UV) radiation inputs from their host stars at all time scales. This includes the short-lived flares ranging from minutes to hours, to medium-scaled stellar rotation and cycles ranging from days to years, and long-term stellar evolutionary changes on timescales of millions to billions of years. These varying energy inputs have great influence on the photochemical equilibrium of exoplanetary atmospheric conditions and their susceptibility to atmospheric escape. While studies of X-ray/UV flare properties and longterm stellar evolution of exoplanet host stars have provided new constraints regarding stellar inputs to exoplanetary systems, the UV temporal variability of cool stars on the timescale of stellar cycles remains largely unexplored. To address this concern, we analyze far-ultraviolet (FUV) emission lines of ions that trace the chromosphere and transition region of nearby stars (C II, Si III, Si IV, and N V; formation temperatures similar to 20 - 150kK) using data from the HST and IUE archives spanning temporal baselines of months to years. We select 33 unique stars of spectral types F - M with observing campaigns spanning over a year, and create ionic light curves to evaluate the characteristic variability of cool stars on such timescales. Screening for large flare events, we observe that the relative variability of FUV light curves, with such timescales, decreases with increasing stellar effective temperature, from 30 - 70% variability for M-type stars to < 30% variability for F and G-type stars. We also observe a weak trend in the temporal variability with the Ca II R ' stellar activity HK indicator, suggesting that stars with lower Ca II activity exhibit a smaller range of FUV flux variability. Screening for data sets with optimal temporal spread, and a sufficient number of individual observations, we select 5 data sets for further periodicity analysis (HST Alpha Centauri A, HST Alpha Centauri B, IUE Alpha Centauri B, IUE Epsilon Eri, IUE Xi Boo). Various periodic structures within the FUV flux were detected, with most significant being a 79 day frequency present within IUE Xi Boo, with a significance of 6-sigma. Additional periodic structure of high significance was detected within Alpha Centauri B, for both HST and IUE measurements being a 210 day frequency with significance of 3-sigma and 3.7-sigma, respectively. Periodicities detected require further examination to identify potential false-periodic sources, such as measurement frequency. Our results suggest that extreme ultraviolet (EUV) flux from cool stars varies by less than a factor of two on decade timescales, significantly smaller than variations on flare or stellar evolutionary timescales.