The WR+OB Progenitor RY Scuti: Intensive Spectroscopy of Its Compact Double‐Ring Nebula

Abstract

We present a detailed spectroscopic analysis of the peculiar nebulaaround RY Scuti using data from the Space Telescope Imaging Spectrograph(STIS) and various ground-based observatories. This massive contactbinary may represent a rapid evolutionary phase in transition to ashort-period WR+OB system, and it is surrounded by a young nebula withunusual geometry. The ionized nebula is only about 1'' across, so STIShas allowed us to spatially resolve the spectrum of the star from itscircumstellar nebula for the first time. Combining STIS data withground-based spectra at optical and infrared wavelengths solves somegeometric ambiguities posed by previous images and reveals the kinematicrelationship between the nebular geometry and RY Scuti's complexemission-line profiles. We offer a new determination of the systemicvelocity of 20+/-3 km s^{-1}, and we estimate the distance to RYScuti as 1.8+/-0.1 kpc. Nebular lines show subtle phase-dependentvariability in addition to expected changes in equivalent width due toeclipses of stellar continuum. These changes in equivalent width yieldan updated ephemeris, but a previously suggested period change is stilluncertain after considering these new data. The nebula contains roughly0.003 M_{solar} of material that is He- and N-rich and probablyO-deficient compared to solar abundances, indicating that CNO-processedmaterial has reached the surface in at least one component of the binarysystem. Diagnostic line ratios suggest that the electron density andtemperature in the nebula are 2{\times}10^{5} cm^{-3} and~9500 K, respectively. There are concentrated regions of higher densityas well; emission knots seen around the rings suggest that commonenvelope mass loss during massive contact binary evolution may becharacterized by strong azimuthal asymmetry and sporadic mass ejectionevents. Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.