FUSE Observations of the Unprecedentedly Deep "Quiescent" Magnetic Activity State of alpha Centauri A

Since 1990 we have been engaged in the “Sun-in-Time” project -a program of coordinated multiwavelength observations of solar-type stars. These stars, which all have similar physical properties (mass, radius, temperature, and depth of convection zone), have been selected as proxies for the Sun at stages throughout its main-sequence lifetime, differing only in age, or equivalently, rotation period. An important component of this program is the investigation of magnetic activity cycles that occur within these solar proxies, and the changes in high energy X-ray coronal and FUV transition region emissions.

Recently, X-ray emission fluxes for one of our program stars, the approximately solar-aged α Cen A, was observed to diminish by a factor of 25 in only two years' time, a phenomenon never before observed for any solar-type star. The XMM observations imply that the chromospheric through coronal regions of α Cen A can be highly variable, with changes in both mean temperatures and the emission measures necessary to address these recent observations. Presented here are FUSE Cycle 7 observations obtained during this unprecedented, very deep magnetic inactivity state of this solar proxy. These new observations are compared to the previously existing Cycle 2 spectra, secured before this dramatic magnetic recession began. We find that in this short span of five years α Cen A has exhibited a significant decrease of approximately 2.5-3× in the key FUV emissions, which may indicate an activity cycle. Examination of other high energy measures, such as X-ray (coronal; Einstein, ROSAT, and XMM), UV (transition region; IUE), and NUV (chromospheric; IUE) emissions of α Cen A, along with comparisons to the Sun (Solar EUV Experiment) are presented.

We gratefully acknowledge support by NASA Grant NNX06AC45G and the Villanova University Research for Undergraduates Award Program.