Spectral response of the pulsationally induced shocks in the atmosphere of BW Vulpeculae

Abstract

BW Vulpeculae (BW Vul) is remarkable for exciting an extremely strong radial pulsation mode which grows through its outer envelope and forms visible shock features in the atmosphere. Material propelled upwards by the shock returns violently to the lower photosphere where it creates a second shock just before the start of the next cycle. We have obtained three nights of echelle data for this star over about five pulsation cycles (P= 0.201 d) in 2000 September in order to investigate the effects of atmospheric shocks on important lines in the optical red spectrum. These lines include He iλ5875 and λ6678, C iiλλ6578–83 doublet, and other moderate and high excitation lines. To these data we have added 37 archival IUE/SWP echelle spectra obtained in 1994. We have investigated the equivalent widths and shapes of the optical lines for evidence of inter alia lags and have compared our results to the IUE fluxes extracted from the far‐ultraviolet continuum, He iiλ1640, and several resonance lines. A comparison of He iλ5875 and λ6678 line profiles during the peak of the infall activity suggests that differences in the development of a second blue lobe in the profile at this time are due to heating and a short‐lived formation of an optically thin layer above the region compressed by the infall. This discovery and the well‐known decreases in equivalent widths of the C ii doublet at the two shock phases further suggest that shock heating flattens the atmospheric temperature gradient. Except for evidence of wind absorption in the far blue wings of the ultraviolet resonance lines, we find no evidence for a shock delay arriving at different photospheric strata (i.e., a ‘Van Hoof effect’). Line‐to‐line differences in the relative strengths of double lobes can be false indicators arising from varying degrees of desaturation of multiple lines, such as for the red He i lines.

Attempts to measure the magnetic field of the pulsating B starνEridani

“…This is similar to what is observed in the magnetic stars, implying that the same low-velocity part of the stellar wind is affected, even though different mechanisms are involved. This phenomenon, where the low-velocity part of the stellar wind is influenced by strong pulsations has previously been observed in strong pulsators, such as BW Vul (Burger et al 1982;Smith & Jeffery 2003).…”

“…DJ found that the asymmetry of this triplet, as measured from data of van Hoof (1961), could only partly be explained by the quadratic effects of rotation (see, e.g., Saio 1981) and suggested that a strong magnetic dipole field of the order of 5-10 kG could explain this discrepancy. In a more recent analysis (Paper I) the asymmetry was found to be a factor of 2 smaller than before, and Pamyatnykh et al (2004) could entirely account for the asymmetry in terms of quadratic rotational effects.…”

Time-dependent spectral-feature variations of stars displaying the B[e] phenomenon

“…It is a radial pulsator, whose amplitude is the largest known in the Galaxy. The shock waves have been proved to propagate through its atmosphere (Smith & Jeffery 2003). MWC 342 can fall into the range of β Cephei variable according to the errorbars of derived stellar parameters.…”

Hydrodynamic models for βCephei variables

“…For instance, Young et al (1981) suggest that a stationary layer is generated during the infalling atmospheric motion by the strong increase in temperature and gas pressure, and that the line doubling is not the result of the propagation of a shock front. Smith & Jeffery (2003) use thermodynamical considerations, especially temperature variations, to explain the Van Hoof effect, since no phase-lag is detected between the different optical lines they considered. In MGFC we were able to measure a phase-lag between the Si  and Hα variations which was interpreted in terms of the presence of progressive waves (Mathias & Gillet 1993).…”