Spectral Response of the Pulsationally-Induced Shocks in the Atmosphere of BW Vulpeculae

The star BW Vul excites an extremely strong radial pulsation that grows in its envelope and is responsible for visible shock features in the continuum flux and spectral line profiles emerging in the atmosphere At two phases separated by 0.8 cycles. Material propelled upwards in the atmosphere from the shock returns 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 5 pulsation cycles (P = 0.201 days) in order to evaluate the effects of on a number of important lines in the spectrum, including the HeI 5875A and 6678A lines. These data were supplemented by archival high-dispersion IUE (UV) data from 1994. A comparison of profiles of the two HeI lines during the peak of the infall activity suggests that differences in the development of the blue wing at this time are due to heating and short-lived formations of an optically thin layer above the atmospheric region compressed by the infall. This discovery and the well-known decreases in equivalent widths of the CII 6578-83A doublet at the two shock phases, suggests that shock flattens the temperature gradient and produces heating in heating the upper atmosphere. Except for absorptions in the blue wings of the UV resonance lines, we find no evidence for sequential shock delays arriving at various regions of line formation of the photosphere (a "Van Hoof effect"). Phase lags cited by some former observers may be false indicators arising from varying degrees of desaturation of multiple lines, such as for the red HeI lines. In addition, an apparent lag in the equivalent width curve of lines arising from less excited atomic levels could instead be caused by post-shock cooling, followed by a rebound shock.Comment: 12 pages in Latex/MNRAS format, 9 eps-format figure

TESS uncloaks the secondaries in hydrogen-deficient binaries

$\upsilon$ Sgr is the prototype of four known hydrogen-deficient binary (HdB) systems. These are characterised by a hydrogen-deficient A-type primary, variable hydrogen emission lines, and a normally unseen secondary presumed to be an upper main-sequence star. Orbital periods range from tens of days to 360 d. TESS observations of all four HdBs show a flux variation with well-defined period in the range 0.5 -- 0.9 d, too short to be associated with the supergiant primary, and more likely to be the rotation period of the secondary and associated with a chemical surface asymmetry or a low-order non-radial oscillation. The observed rotation period supports a recent analysis of the $\upsilon$ Sgr secondary. The observations give a direct glimpse of the secondary in all four systems, and should help to explain how the primary has been stripped to become a low-mass hydrogen remnant.Comment: MNRAS: Accepted 2022 November 05. Submitted 2022 October 22, 5 page