HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF CHROMOSPHERIC EMISSION FROM THE POPULATION-II RED GIANT HD-216143

Spectra of the H alpha and Ca II K Line of Population II red giants ex hibit evidence for chromospheric mass outflows only among stars bright er than M-V = -1.7. In order to determine whether this phenomenon is i ndicative of a true physical onset in mass outflow, spectra of the 280 0 Angstrom Mg II lines of the metal-poor halo red giant HD 216143 have been obtained with the Goddard High Resolution Spectrograph on the Hu bble Space Telescope. This star has shown no H alpha emission when obs erved on two separate occasions and has an absolute magnitude of M-V = - 1.5, placing it below the intrinsic brightness level at which H alp ha emission is seen among other Population II giants. In addition, a h igh-resolution Ca II K line spectrum shows no significant K-3 velocity shift. Thus the H alpha and Ca II K lines show no evidence of chromos pheric outflow. Nonetheless, HD 216143 exhibits clear Mg II h and k em ission profiles. These profiles are asymmetric in the sense expected f or mass outflow from the chromosphere. The combined surface flux of th ese two emission lines is similar to fluxes of other Population II red giants for which the Mg II lines have been observed. The central k(3) absorption feature is blueshifted with respect to the midpoint of the k emission profile by similar to 20 km s(-1). The h(3) feature shows a smaller blueshift, evidence possibly for an accelerated outflow in t he outer atmosphere of HD 216143. It appears that outflows set in amon g halo red giants at brightnesses comparable to or fainter than M-V = -1.5. Halo giants fainter than this tend to show no H alpha emission a nd no significant velocity shifts in the K-3 central absorption compon ent of the Ca II K line. It seems that the search for the onset of mas s outflows from such stars is best conducted using the Mg II line.