Hubble Space Telescope Observations of Chromospheric Emission from the Population II Red Giant HD 216143

Abstract

Spectra of the Hα and Ca ii K line of Population II red giants exhibit evidence for chromospheric mass outflows only among stars brighter than M_V = -1.7. In order to determine whether this phenomenon is indicative of a true physical onset in mass outflow, spectra of the 2800 Å Mg ii lines of the metal-poor halo red giant HD 216143 have been obtained with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. This star has shown no Hα emission when observed on two separate occasions and has an absolute magnitude of M_V = -1.5, placing it below the intrinsic brightness level at which Hα emission is seen among other Population II giants. In addition, a high-resolution Ca ii K line spectrum shows no significant K_3 velocity shift. Thus the Hα and Ca ii K lines show no evidence of chromospheric outflow. Nonetheless, HD 216143 exhibits clear Mg ii h and k emission profiles. These profiles are asymmetric in the sense expected for mass outflow from the chromosphere. The combined surface flux of these 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 ~20 km s^-1. The h_3 feature shows a smaller blueshift, evidence possibly for an accelerated outflow in the outer atmosphere of HD 216143. It appears that outflows set in among halo red giants at brightnesses comparable to or fainter than M_V = -1.5. Halo giants fainter than this tend to show no Hα emission and no significant velocity shifts in the K_3 central absorption component of the Ca ii K line. It seems that the search for the onset of mass outflows from such stars is best conducted using the Mg ii line. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.