Guinan and senior N. Morgan studied Proxima Centauri using both photometric and IUE data. At a distance of 4.3 LY, Proxima Centauri (= Cen C; V645 Cen) is the nearest known star to the Sun. This M5 V flare star is the faintest member of the Cen triple star system (or moving group) and lies about 1400 AU nearer to the Earth than its brighter G2V and K2V companions. Because of its proximity and its membership in the triple system, Proxima has well determined physical properties that include an age of 5-6 Gyr. In spite of its old age, Proxima is a chromospherically active star with strong Mg II h+k (280nm) emission as well as being a flare star. This star is of great importance to magnetic dynamo theory because it is expected to have a fully convective envelope. One quantity, not well determined yet vital to understanding Proxima's magnetic behavior, is its rotation period.
During May-August 1995, Proxima was observed about twice a week with IUE. Low resolution LWP (200-320nm) spectra were obtained chiefly to observe the chromospheric Mg II emission and use it to measure Proxima's rotation period as active plage regions on the star's surface rotate in and out of view. The IUE data have been analyzed and the Mg II emission shows variations with a period of days. This period is assumed to be the star's rotation period. In addition, several flare events were observed and evidence was found for rather fast changes (on a time-scale of weeks) in the plage activity and distribution. Also, the analysis of the IUE archival data indicates the probable presence of a long-term activity cycle.
This study is supported from NASA grant NAG 5-2160 and NSF grant AST-9315365 which we gratefully acknowledge.