Classical Oe Stars in the Field of the Small Magellanic Cloud

Abstract

We present $29\pm 1$ classical Oe stars from RIOTS4, a spatially complete, spectroscopic survey of Small Magellanic Cloud (SMC) field OB stars. The two earliest are O6e stars, and four are earlier than any Milky Way (MW) Oe stars. We also find ten Ope stars, showing He~\textsc{i} infill and/or emission; five appear to be at least as hot as $\sim$O7.5e stars. The hottest, star 77616, shows He~\textsc{ii} disk emission, suggesting that even the hottest O stars can form decretion disks, and offers observational support for theoretical predictions that the hottest, fastest rotators can generate He${}^{+}$-ionizing atmospheres. Our data also demonstrate that Ope stars correspond to Oe stars earlier than O7.5e with strong disk emission. We find that in the SMC, Oe stars extend to earlier spectral types than in the MW, and our SMC Oe/O frequency, $0.26\pm 0.04$, is much greater than the MW value, $0.03\pm 0.01$. These results are consistent with angular momentum transport by stronger winds suppressing decretion disk formation at higher metallicity. In addition, our SMC field Oe star frequency is indistinguishable from that for clusters, which is consistent with the similarity between rotation rates in these environments, and contrary to the pattern for MW rotation rates. Thus, our findings strongly support the viscous decretion disk model and confirm that Oe stars are the high-mass extension of the Be phenomenon. Additionally, we find that Fe~\textsc{ii} emission occurs among Oe stars later than O7.5e with massive disks, and we revise a photometric criterion for identifying Oe stars to $J-[3.6]\ge 0.1$.

18 pages, 8 Figures, Accepted for publication in ApJ

References in corpus (8)

• Southern Massive Stars at High Angular Resolution: Observational Campaign and Companion Detection
• Effects of gravitational darkening on the determination of fundamental parameters in fast rotating B-type stars
• Spitzer SAGE-SMC Infrared Photometry of Massive Stars in the Small Magellanic Cloud
• Rotational Velocities For B0-B3 Stars in 7 Young Clusters: Further Study of the Relationship between Rotation Speed and Density in Star-Forming Regions
• Spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud
• The Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4)
• The remarkable light and colour variability of Small Magellanic Cloud Be stars
• Stellar Rotation in Field and Cluster B-Stars

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• Swift J011511.0-725611: Discovery of a rare Be Star / White Dwarf binary system in the SMC
• Ultraviolet Spectropolarimetry: on the origin of rapidly rotating B stars
• Intriguing X-ray and optical variations of the gamma Cas analog HD45314
• Binarity at Low Metallicity (BLOeM) -- Multiplicity properties of Oe and Be stars
• New Oe stars in LAMOST DR5
• Populations of evolved massive binary stars in the Small Magellanic Cloud I: Predictions from detailed evolution models
• An extensive spectroscopic time series of three Wolf-Rayet stars -- II. A search for wind asymmetries in the dust-forming WC7 binary WR137
• Strong Variability in AzV 493, an Extreme Oe-Type Star in the SMC
• New Field OB and OBe Binaries of the SMC Wing: Observational Properties and Population Modeling
• Astrophysical parameters of LS 437 and the nature of X0726-260