Time-dependent structure in ultraviolet absorption lines of the rapid rotators HD 64760 (B0 Ib) and HD 93521 (O9.5 V)

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 (O9.5 V), time-series analysis of the cross-correlation function recovers two periods that are confirmed in independent optical data (P1 = 1.77 h, P2 = 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 P1 and P2, 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 (HR 3090; B0.5 Ib), finding marginally significant signals with P1 = 8.9 h (or, possibly, 14.2 h) and P2 = 29 h. The longer period is present, with a strong signal, in wind-formed lines. We consider possible circumstellar and (quasi-)photospheric origins for P2, 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 (P1) and 2 ± 1 (P2), with ℓ + m ≤ 1, supporting a previous suggestion by Baade that both low- and higher-order modes are present; the wind modulation at P2 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 P2 has semiamplitude < 2 mmag at λeff = 1575 Å, with 95 per cent confidence), but this may itself be a consequence of wave leakage.