Ultraviolet emission lines in Ba and non-Ba giants

With the Hubble Space Telescope (HST) and the Goddard High Resolution Spect rograph we have observed four barium and three weak barium stars in the ult raviolet spectral region, together with two nonpeculiar giant standard star s. An additional suspected Ba star was observed with HST and the Space Tele scope Imaging Spectrograph. In the H-R diagram, three of the observed Ba st ars lie on the same evolutionary tracks as the Hyades giants. Using Interna tional Ultraviolet Explorer (IUE) spectra of previously studied giants toge ther with our HST spectra, we investigate whether the chromospheric and tra nsition layer emission-line spectra of the Ba stars are different from thos e of nonpeculiar giants and from those of giants with peculiar carbon and/o r nitrogen abundances. Except for the Ba star HD 46407 and the suspected Ba star HD 65699, the Ba star and mild Ba star emission-line fluxes are, for a given effective temperature and for a given luminosity, lower than those for the nonpeculiar giants observed with IUE. In comparison with the HST-ob served standard stars, the C IV lambda 1550-to-C II lambda 1335 line flux r atios are smaller, but not necessarily so in comparison with all IUE-observ ed nonpeculiar giants. However, the C IV-to-C II line flux ratios for the B a stars decrease with increasing carbon abundances. This shows that the ene rgy balance in the lower transition layer is influenced by the carbon abund ance. The temperature gradient appears to be smaller in the C II line-emitt ing region. There does not seem to be a difference in chromospheric electro n densities for the Ba and non-Ba stars, though this result is rather uncer tain.