We have used the Goddard High-Resolution Spectrograph on the Hubble Space Telescope to search for chromospheric emission in the ultraviolet C II 1335 Å lines of eight A-type stars. The B-V colors of these stars range from 0.12 to 0.22. We have detected emission in the spectrum of the A7 V star Altair, but in no other star. We present tentative evidence that the emission from Altair is blueshifted by ∼60 km s-1 or more with respect to the photospheric spectrum of this 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 capable of supporting detectable chromospheres occurs in the close vicinity of B-V = 0.22.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science