The WR+OB progenitor RY Scuti: Intensive spectroscopy of its compact double-ring nebula

We present a detailed spectroscopic analysis of the peculiar nebula around RY Scuti using data from the Space Telescope Imaging Spectrograph (STIS) and various ground-based observatories. This massive contact binary may represent a rapid evolutionary phase in transition to a short-period WR+OB system, and it is surrounded by a young nebula with unusual geometry. The ionized nebula is only about 1″ across, so STIS has allowed us to spatially resolve the spectrum of the star from its circumstellar nebula for the first time. Combining STIS data with ground-based spectra at optical and infrared wavelengths solves some geometric ambiguities posed by previous images and reveals the kinematic relationship between the nebular geometry and RY Scuti's complex emission-line profiles. We offer a new determination of the systemic velocity of 20 ± 3 km s^-1, and we estimate the distance to RY Scuti as 1.8 ± 0.1 kpc. Nebular lines show subtle phase-dependent variability in addition to expected changes in equivalent width due to eclipses of stellar continuum. These changes in equivalent width yield an updated ephemeris, but a previously suggested period change is still uncertain after considering these new data. The nebula contains roughly 0.003 M _⊙ of material that is He-and N-rich and probably O-deficient compared to solar abundances, indicating that CNO-processed material has reached the surface in at least one component of the binary system. Diagnostic line ratios suggest that the electron density and temperature in the nebula are 2 × 10⁵ cm^-3 and ∼9500 K, respectively. There are concentrated regions of higher density as well; emission knots seen around the rings suggest that common envelope mass loss during massive contact binary evolution may be characterized by strong azimuthal asymmetry and sporadic mass ejection events.