Open AccessTime‐dependent structure in ultraviolet absorption lines of the rapid rotators HD 64760 (B0 Ib) and HD 93521 (O9.5 V)
Author(s) -
Howarth Ian D.
Publication year - 1998
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.1998.01437.x
Subject(s) - physics , astrophysics , spectral line , line (geometry) , absorption (acoustics) , absorption spectroscopy , phase (matter) , computational physics , optics , astronomy , geometry , mathematics , quantum mechanics
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 ( 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 (HR 3090; 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 λ eff = 1575 Å, with 95 per cent confidence), but this may itself be a consequence of wave leakage.