Extended optical spectroscopic monitoring of wind structure in HD 152408^*

¹ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
² Landessternwarte Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
³ European Southern Observatory, Alonso de Cordova 3107, Santiago 19, Chile

Corresponding author: R. K. Prinja, rkp@star.ucl.ac.uk

Received: 25 August 2000
Accepted: 14 December 2000

Abstract

New perspectives are provided on significant spatial structure and temporal variability in the near-star wind regions (i.e. ) of the massive luminous star HD 152408 (classified as O8:Iafpe or WN9ha). This study is primarily based on the analysis of high-quality échelle spectra secured during 21 nights between 1999 July to August, using the Landessternwarte-developed (fibre-fed) FEROS instrument on the ESO 1.52-m telescope. These extended time-series data, with a total simultaneous wavelength coverage of Å, were exploited to monitor absorption and emission fluctuations (of ∼ 5- of the line flux) in several He i and Balmer lines, together with more deep-seated (near-photosphere) disturbances in weaker metallic emission and absorption lines. Organised large-scale wind structure in HD 152408 is principally betrayed by sequential episodes of discrete absorption and emission features, which migrate from near zero velocity to almost the wind terminal velocity. This evolution is extremely slow, however, typically spanning ∼4 days for an individual episode. We demonstrate that the blue-shifted absorption episodes in He i are very closely mirrored (in velocity and time) by absorption features (i.e. reduced not enhanced flux) in the blue wings of the mainly recombination formed broad Hα emission line. The implication is that there is detailed balancing between ground state photoionization and recombination in the substantially optically thick Balmer lines. Surprisingly, the velocity behaviour of the red-ward and blue-ward migrating features is highly asymmetric, such that the mean acceleration of the former is less than 50% of the latter. Fourier analysis reveals a modulation time-scale for the wind activity of ∼7.7 days, plus its harmonic at 3.9 days. The longer period is ∼28 times greater than the characteristic radial wind flow time of HD 152408. We also detect a ∼1.5 day periodic variation in the radial velocity of the weak C iv 5801, 5812 absorption lines, which are the closest approximation to "pure"photospheric lines in the optical spectrum of HD 152408. The wind-formed optical lines of HD 152408 are also affected by fluctuations in the central peak emission, particularly evident in Hα where the equivalent width may vary by up to 20% . Data secured between 1995 and 1999 reveal, however, that the epoch-to-epoch mean profiles are remarkably similar. Non-LTE steady-state stellar atmosphere models are used to synthesis profiles to match representative Hα and He i λ5876 line profiles. Only a slow (tailored) velocity law (compared to ) provides a good match to the Hα emission peak and wings, but the models predict excess He i absorption. The observed extreme Hα emission variations can be reproduced by the synthetic profiles with an implied ±10% variation in mass-loss rate. The results on optical line profile variability in HD 152408 are discussed in the context of models for co-rotating interaction regions (CIRs) in the wind. Several constraints are provided that argue against simple velocity fields in such streams, including (i) the slow acceleration of features to high velocities, within ∼ , (ii) the strong asymmetry in projected acceleration of the approaching and receding stream material, (iii) Balmer line absorption effects in the approaching material, (iv) the rise of localised features from very low velocities, and (v) the stability of the large-scale CIRs against turbulent small-scale wind structure. We suggest that it may be worth exploring hydrodynamic simulations of CIRs that incorporate different velocity fields on the leading (faster accelerating; blue-ward absorption) and trailing (slower accelerating; red-ward emission) edges of the spiral structures.