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.