Properties of Ap Stars in the Ultraviolet

The observational study of the “chemically peculiar (CP) stars of the upper main sequence” (Preston 1974) at wavelengths shortward of 3000 Å began early in this decade with Bernacca and Molnar's (1972) photometric observations of the helium-weak Bp stars and Molnar's (1973) investigation of α2CVn, both with the Wisconsin Experiment Package on 0A0-2. Thus, the present review deals with a subject still in its infancy, but growing rapidly. If we limit the discussion to Ap stars as traditionally defined, i.e. the magnetic Si and SrCrEu stars and the non-magnetic HgMn stars (Preston's groups CP-2 and CP-3, respectively), we find satellite data currently available for the forty-one stars listed in Table 1, and unpublished or unreduced data doubtless exist for other stars as well. In addition to the 0A0-2/Wisconsin experiment, which provided broad-band filter photometry from 1330 Å to 4250 Å and 10-20 Å bandpass spectrum scans from 1150 Å to longward of 3500 Å, data have been obtained with the TD-1A experiments S59 (1.8 Å resolution spectrum scans in the three wavelength intervals 2060 - 2155 Å, 2495 - 2590 Å and 2775 2865 Å) and S2/68 (35-40 Å bandpass spectrum scans over the interval 1350 - 2550 Å).