A SEARCH FOR CHROMOSPHERIC EMISSION IN A-TYPE STARS USING THE GODDARDHIGH-RESOLUTION SPECTROGRAPH

We have used the Goddard High-Resolution Spectrograph on the Hubble Sp ace Telescope to search for chromospheric emission in the ultraviolet C II 1335 angstrom lines of eight A-type stars. The B - V colors of th ese stars range from 0.12 to 0.22. We have detected emission in the sp ectrum of the A7 V star Altair, but in no other star. We present tenta tive evidence that the emission from Altair is blueshifted by approxim ately 60 km s-1 or more with respect to the photospheric spectrum of t his star. We interpret this as expansion and suggest that the base of the A star winds that we have proposed in earlier work may lie within the chromosphere. Together with IUE spectra, our observations indicate that in ordinary main-sequence stars the onset of convection zones ca pable of supporting detectable chromospheres occurs in the close vicin ity of B - V = 0.22.