Ultraviolet Flux Decrease Under a Grand Minimum from IUE Short-wavelength Observation of Solar Analogs

Abstract

We have identified a sample of 33 Sun-like stars observed by the International Ultraviolet Explorer (IUE) with the short-wavelength spectrographs that have ground-based detections of chromospheric Ca II H+K activity. Our objective is to determine if these observations can provide an estimate of the decrease in ultraviolet (UV) surface flux associated with a transition from a normal stellar cycle to a grand-minimum state. The activity detections, corrected to solar metallicity, span the range -5.16< {log}{R}_{HK}^{\prime }< -4.26, and eight stars have log {R}_{HK}^{\prime }< -5.00. The IUE-observed flux spectra are integrated over the wavelength range 1250-1910 Å, transformed to surface fluxes, and then normalized to solar B - V. These normalized surface fluxes show a strong linear relationship with activity {R}_{HK}^{\prime } (R ² = 0.857 after three outliers are omitted). From this linear regression we estimate a range in UV flux of 9.3% over solar cycle 22 and a reduction of 6.9% below solar cycle minimum under a grand minimum. The 95% confidence interval in this grand-minimum estimate is 5.5%-8.4%. An alternative estimate is provided by the IUE observations of τ Cet (HD 10700), a star having strong evidence of being in a grand-minimum state, and this star’s normalized surface flux is 23.0 ± 5.7% lower than solar cycle minimum.