Time-dependent structure in ultraviolet absorption lines of the rapid rotators HD 64760 (B0Ib) and HD 93521 (O9.5V)

Cross-correlation provides an effective line-averaging function for spectra containing many line features, a result which can be exploited in order to perform time-series and spatial-domain analyses of absorption-line variability in data of (relatively) poor quality. We apply this method to high-resolution IUE spectra. For the known non-radial pulsator HD 93521 (O9.5V), time-series analysis of the cross-correlation function recovers two periods that are confirmed in independent optical data (P_1=1.77 h, P_2=2.90 h); there is no statistically significant excess power at these frequencies in lines formed in the stellar wind. By comparing phase information from the time-series analysis with results from pulsation models we estimate l~=10+/-1 and 6+/-1 as the harmonic degrees for P_1 and P_2, respectively, with l+m<~2 for each mode (where m is the azimuthal order of the mode). We also present evidence for absorption-line variability in HD 64760 (HR3090 B0.5 Ib), finding marginally significant signals with P_1=8.9 h (or, possibly, 14.2 h) and P_2=29 h. The longer period is present, with a strong signal, in wind-formed lines. We consider possible circumstellar and (quasi-)photospheric origins for P_2, and conclude that this signal probably does not arise through rotational modulation (with the corollary that the stellar-wind signal also does not arise in corotating structures, contrary to previous suggestions). The phase behaviour of the signals is consistent with non-radial pulsation models characterized by l=5+/-1 (P_1) and 2+/-1 (P_2), with l+m<=1, supporting a previous suggestion by Baade that both low- and higher-order modes are present; the wind modulation at P_2 may then result from leakage of pulsation energy into the supersonic outflow. The lack of significant photometric variability is a serious difficulty for this model (any sinusoidal photometric variability at P_2 has semiamplitude <2 mmag at lambda_eff=1575Angstroms, with 95 per cent confidence), but this may itself be a consequence of wave leakage.