The Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4)

TL;DR

The RIOTS4 survey provides a comprehensive spectroscopic analysis of field OB stars in the SMC, revealing high binary fractions, a steep upper IMF, and evidence for in-situ star formation, enriching understanding of massive star populations in low-metallicity environments.

Contribution

This study offers the first detailed spectroscopic catalog of field OB stars in an external galaxy, including binary statistics, IMF characterization, and insights into isolated star formation.

Findings

High binary fraction (~60%) among massive stars.

Large fraction (42%) of classical Oe/Be stars, higher than in the Galaxy.

Confirmation of a steep upper IMF in the SMC field.

Abstract

We present the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4), a spatially complete survey of uniformly selected field OB stars that covers the entire star-forming body of the SMC. Using the IMACS multislit spectrograph and MIKE echelle spectrograph on the Magellan telescopes, we obtained spectra of 374 early-type field stars that are at least 28 pc from any other OB candidates. We also obtained spectra of an additional 23 field stars in the SMC bar identified from slightly different photometric criteria. Here, we present the observational catalog of stars in the RIOTS4 survey, including spectral classifications and radial velocities. For three multi-slit fields covering 8% of our sample, we carried out monitoring observations over 9-16 epochs to study binarity, finding a spectroscopic, massive binary frequency of at least $\sim$60% in this subsample. Classical Oe/Be stars represent a large fraction of RIOTS4 (42%), occurring at much higher frequency than in the Galaxy, consistent with expectation at low metallicity. RIOTS4 confirmed a steep upper IMF in the field, apparently caused by the inability of the most massive stars to form in the smallest clusters. Our survey also yields evidence for in-situ field OB star formation, and properties of field emission-line star populations, including sgB[e] stars and classical Oe/Be stars. We also discuss the radial velocity distribution and its relation to SMC kinematics and runaway stars. RIOTS4 presents a first quantitative characterization of field OB stars in an external galaxy, including the contributions of sparse, but normal, star formation; runaway stars; and candidate isolated star formation.