An observational study of rotation and binarity of Galactic O-type runaway stars

Gaia DR3 data have revealed new massive runaway stars, while spectroscopic surveys enable detailed characterization. The relative contributions of binary supernova (BSS) and dynamical ejection (DES) scenarios to explain their runaway origin remain poorly constrained, particularly in the Milky Way. We aim to characterize the largest sample of Galactic O-type runaway stars ever investigated through their kinematics, rotation, and binarity to shed light into their origins. We use the GOSC-Gaia DR3 catalog, and IACOB spectroscopic information to build a sample with 214 O-type stars with projected rotational velocities ($v \sin{i}$), and a subsample of 168 O-type stars with additional information about their likely single (LS) or single-lined (SB1) spectroscopic binary nature. We also consider an additional sample of 65 double-lined (SB2) spectroscopic binaries. We find that among our sample of Galactic O-type runaways, most (74%) have $v \sin{i}<200$ km/s, whereas for normal stars this fraction is slightly higher (82%). There are no fast-moving runaways being fast rotators, except for HD 124 979. Runaways show lower SB1 fractions than normal stars, with no runaway SB1 fast-rotating systems; on average, runaways rotate faster than normal stars; and their runaway fraction is higher among fast rotators (44%) vs. the slow rotators (34%). This is consistent with BSS dominance for fast rotators. We also found that SB2 systems hardly reach runaway velocities with a low runaway fraction (10%). Runaways with 2D velocities > 60 km/s are mostly single and interpreted as DES products, while runaways with 2D velocities > 85 km/s are also interpreted as two-step products. Three of 12 runaway SB1 systems are HMXBs. Our study reveals that most Galactic O-type runaways are slow rotators, suggests a dominance of BSS among fast-rotating runaways, and of DES and two-step among the high-velocity ones. (Abridged)

💡 Research Summary

This paper presents the most extensive observational study to date of Galactic O‑type runaway stars, combining Gaia DR3 astrometry (via the GOSC‑Gaia catalog) with high‑resolution spectroscopic data from the IACOB project. From an initial set of 417 O‑type stars, 106 were identified as runaways using a statistically robust “E” parameter that quantifies the significance of their 2‑D peculiar velocities (V₂D_PEC) relative to Galactic rotation. The authors then cross‑matched these objects with IACOB measurements of projected rotational velocity (v sin i) and binary status (likely single LS, single‑lined SB1, and double‑lined SB2). After removing peculiar spectral types and clear SB2 systems, a clean “rotation sample” of 214 O‑type stars (78 runaways, 136 normal) was assembled, of which 168 have reliable LS/SB1 classifications.

Key findings include: (i) The majority of runaways (74 %) are slow rotators (v sin i < 200 km s⁻¹), a fraction slightly lower than that for normal O stars (82 %). Fast‑rotating runaways are rare; only HD 124979 is a high‑velocity, high‑rotation outlier. (ii) Runaway stars exhibit a lower SB1 fraction than normal stars, and no fast‑rotating runaway is found to be an SB1. Conversely, the overall runaway fraction is higher among fast rotators (44 %) than among slow rotators (34 %). This pattern aligns with the binary supernova scenario (BSS), in which pre‑supernova mass transfer spins up the surviving star, producing fast‑rotating runaways. (iii) Double‑lined spectroscopic binaries (SB2) rarely achieve runaway velocities; their runaway fraction is only ~10 %, supporting the view that dynamical ejection (DES) mainly produces single stars or wide binaries, while high‑velocity ejections are uncommon for SB2 systems. (iv) Runaways with V₂D_PEC > 60 km s⁻¹ are predominantly single (LS) and are interpreted as DES products. Those with V₂D_PEC > 85 km s⁻¹ may involve a two‑step process (DES followed by BSS), exemplified by the V479 Sct/LS 5039 system. (v) Among the 12 runaway SB1 systems, three are identified as high‑mass X‑ray binaries (HMXBs), providing direct evidence of BSS remnants evolving into X‑ray emitting binaries.

Overall, the study concludes that most Galactic O‑type runaways are slow rotators, that BSS dominates the production of fast‑rotating runaways, and that DES (and a two‑step DES + BSS channel) accounts for the highest‑velocity runaways. These observational constraints offer valuable benchmarks for population‑synthesis models, cluster dynamical simulations, and the evolutionary pathways of massive binaries after supernova explosions.