Classical Oe Stars in the Field of the Small Magellanic Cloud

TL;DR

This study identifies and characterizes classical Oe stars in the Small Magellanic Cloud, revealing their spectral types, disk properties, and higher frequency compared to the Milky Way, supporting the viscous decretion disk model.

Contribution

First comprehensive spectroscopic survey of SMC field Oe stars, demonstrating their earlier spectral types and higher occurrence rate than in the Milky Way, confirming the role of metallicity and rotation in Oe star formation.

Findings

SMC Oe stars extend to earlier spectral types than in the Milky Way.

Oe/O frequency in SMC is significantly higher than in the Milky Way.

He II disk emission observed in the hottest O stars, supporting theoretical predictions.

Abstract

We present $29\pm1$ 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, $0.26\pm0.04$, is much greater than the MW value, $0.03\pm0.01$. 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 $J-[3.6] \geq 0.1$.