Far‐Ultraviolet and Optical Observations of BW Vulpeculae

Abstract

We have compared the behavior of 39 Far-Ultraviolet Spectroscopic Explorer spectra through the pulsation cycle of the very large amplitude β Cephei BW Vul with optical and UV (IUE) spectra, as well as optical photometry. The FUSE light curve suggests a temperature amplitude of about 4000 K, while the radial velocity curve is similar to corresponding optical curves, except that the famous stillstand feature, due to a shock wave caused by returning infall to the atmosphere from a previous cycle, is slightly weakened. We have performed line syntheses on a number of features in various spectral regions. We find that profiles of the red He I lines (λ6678 and λ5876) at faint star and bright star phases can be synthesized with standard Kurucz model atmospheres by assuming moderate and high microturbulences, respectively. For the faint-star profile, the line profiles are too deep to be explained other than by lowering the boundary temperature of a standard Kurucz model by 1000-2000 K. This result is confirmed by line syntheses of the far-UV spectra at both phases, which show that fits can be best achieved with models having effective temperatures 4000 K hotter than models fitting optical lines. This result is robust against non-LTE effects. The combined optical and far-UV results suggest that the temperature distributions of both the lower and upper atmosphere are steeper than those computed in (static) Kurucz models. With ad hoc modifications to the atmospheric temperature distribution, we simulated the weakening of the temperature-sensitive C II λλ6578-6583 doublet during shock passages by introducing a uniform shallow temperature gradient through the line formation region. This result agrees with published hydrodynamic results that these shocks produce a plateau-like elevation in gas variables through the atmosphere.