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

TL;DR

This study re-examines historical photographic spectra of Plaskett's Star, revealing spectral variability, potential activity effects, and implications for the star's mass ratio, using digitized archival data.

Contribution

It provides a new analysis of old spectra, identifying spectral features affected by transient activity and refining the understanding of the star's orbital dynamics.

Findings

Spectra from 1922 and 1937 show consistent features with synthetic spectra.

Evidence of transient activity affecting Hgamma line in 1937.

Secondary's motion indicated by NIII and HeII emission lines.

Abstract

Photographic spectra of Plaskett's Star (PS; HR2420, HD47129, V640 Mon) that were recorded at the Dominion Astrophysical Observatory (DAO) have been digitized with a flatbed scanner. Many of the spectra were recorded during campaigns in 1922 and 1937, and sample wavelengths between 0.39 and 0.50um. Spectra of poor quality are identified. Mean spectra near orbital phases 0.25 and 0.75 match many characteristics of synthetic spectra, although Hgamma and HeI 4388 are exceptions. Evidence is presented that Hgamma was affected by transient activity in 1937, but not in 1922. Emission lines of NIII and HeII 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 Hgamma and HeI 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 peculiarities in the radial velocity curves of the components have thus been in place for over a thousand orbital cycles.