WR 148: Identifying the companion of an extreme runaway massive binary

Abstract

WR 148 (HD 197406) is an extreme runaway system considered to be a potential candidate for a short-period (4.3173 d) rare WR + compact object binary. Provided with new high resolution, high signal-to-noise spectra from the Keck observatory, we determine the orbital parameters for both the primary WR and the secondary, yielding respective projected orbital velocity amplitudes of $88.1\pm 3.8$ km s${}^{-1}$ and $79.2\pm 3.1$ km s${}^{-1}$ and implying a mass ratio of $1.1\pm 0.1$. We then apply the shift-and-add technique to disentangle the spectra and obtain spectra compatible with a WN7ha and an O4-6 star. Considering an orbital inclination of $\sim 67^{\circ }$, derived from previous polarimetry observations, the system's total mass would be a mere 2-3 M${}_{\odot }$ , an unprecedented result for a putative massive binary system. However, a system comprising a 37 M${}_{\odot }$ secondary (typical mass of an O5V star) and a 33 M${}_{\odot }$ primary (given the mass ratio) would infer an inclination of $\sim 18^{\circ }$. We therefore reconsider the previous methods of deriving the orbital inclination based on time-dependent polarimetry and photometry. While the polarimetric results are inconclusive requiring better data, the photometric results favour low inclinations. Finally, we compute WR 148's space velocity and retrace the runaway's trajectory back to the Galactic plane (GP). With an ejection velocity of $198\pm 27$ km s${}^{-1}$ and a travel time of $4.7\pm 0.8$ Myr to reach its current location, WR 148 was most likely ejected via dynamical interactions in a young cluster.

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