IUE observations of rapidly rotating low-mass stars in young clusters - The relation between chromospheric activity and rotation

If the rapid spindown of low-mass stars immediately following their arrival on the ZAMS results from magnetic braking by coronal winds, an equally sharp decline in their chromospheric emission may be expected. To search for evidence of this effect, the IUE spacecraft was used to observe the chromospheric Mg II emission lines of G-M dwarfs in the nearby IC 2391, Alpha Persei, Pleiades, and Hyades clusters. Similar observations were made of a group of X-ray-selected 'naked' T Tauri stars in Taurus-Auriga. The existence of a decline in activity cannot be confirmed from the resulting data. However, the strength of the chromospheric emission in the Mg II lines of the cluster stars is found to be correlated with rotation rate, being strongest for the stars with the shortest rotation periods and weakest for those with the longest periods. This provides indirect support for such an evolutionary change in activity. Chromospheric activity may thus be only an implicit function of age.