Old Dogs, New Tricks: Re-examining Photographic Spectra of Plaskett’s Star

Abstract Photographic spectra of Plaskett’s Star (HR 2420, HD 47129, V640 Monocerotis) that were recorded at the Dominion Astrophysical Observatory have been digitized with a flatbed scanner. Many of the spectra were recorded during campaigns in 1922 and 1937, and cover wavelengths between 0.39 and 0.50 μ m. Spectra of poor quality are identified. Mean spectra near orbital phases 0.25 and 0.75 match many characteristics of synthetic spectra, although H γ and He i λ 4388 are exceptions. Evidence is presented that H γ was affected by transient activity in 1937, but not in 1922. Emission lines of N iii and He ii move with wavelength in a manner that is consistent with them tracking the motion of the secondary, indicating that an “f” spectral type designation should be assigned to the secondary. The location of the peak that is associated with the secondary in cross-correlation functions changes with time near phase 0.75, although the mean amplitude of the radial velocity curve of the secondary did not change between the two campaigns. There is also an offset in velocities of the primary measured from H γ and He i λ 4472 near phase 0.25. The velocity curves of the components suggest a mass ratio that is larger than previous estimates, although uncertainties associated with the spectroscopic features attributed to the secondary, coupled with the wavelength resolution of the spectra, complicate efforts to determine robust masses. We conclude that the peculiarities in the radial velocity curves of the components have thus been in place for over a thousand orbital cycles.