IUE-Ultraviolet and optical chromospheric studies of late-type giants in the Hyades Cluster

Ultraviolet and optical observations of four bright, late-type giants in the Hyades cluster detected with IUE are presented in order to study chromospheric and coronal activity in stars of the same age. Two of the giants, 77 Tau and gamma Tau, clearly exhibit emission in the high temperature ions such as N V, C IV, and Si IV at levels several times larger than the upper limits for the other two giants, delta Tau and epsilon Tau. Comparison of the Mg II h and k fluxes and the Ca II K emission strengths shows that 77 Tau and gamma Tau have larger chromospheric radiative losses than delta Tau, epsilon Tau, and beta Gem, a field giant which also displays low upper limits to emission from high temperature ions. Coronal X-ray emission indicates that the surface flux in X-rays is an order of magnitude brighter in 77 Tau than in delta Tau. The results indicate that a parameter other than age, temperature, gravity or metallicity determines the amount of chromospheric and coronal emission in late-type giants.