An Orbit for a Massive Wolf-Rayet Binary in the LMC: An Example of Binary Evolution

Abstract

Wolf-Rayet (WR) stars are helium-burning, evolved massive stars which have had most of their hydrogen-rich outer layers removed either through stellar winds and/or binary stripping. Here we report on LMC173-1, a WN3+O binary located in the Large Magellanic Cloud (LMC). Using spectra obtained from the Magellan and Gemini-S telescopes, we have derived system parameters for this intriguing binary. The WR star's mass is only 43% that of its companion, and we argue that this requires binary evolution rather than mass loss by stellar winds alone, given the metallicity of the LMC. The stars are close enough to each other with their 3.52 day period that the O star is actually orbiting within the wind of the WR star, as is the case for other well-known WR+O systems, such as V444 Cyg. As a result, high precision OGLE photometry reveals a WR atmospheric eclipse, as well as a 7-8 millimag ellipsoidal modulation due primarily to the tidal distortion of the O star. Modeling the light curve allows us to estimate the orbital inclination. Derivation of stellar parameters suggests neither component is filling its Roche-lobe surface today. The O star is spinning much faster than synchronous rotation. Using BPASS v2.2 binary models, we discuss the probable evolutionary history of the system. The WR progenitor likely underwent Case A Roche-lobe overflow (RLOF) before leaving the main-sequence. As it lost its H-rich envelope, it became a WN-type WR. The resulting system is a binary with similar luminosities but very different radii, representing a post-RLOF phase.

Figures (6)

• Figure 1: Blue spectral region of LMC173-1. The upper spectrum was taken on 2013 Oct 16 at a phase of 0.95, and the lower spectrum on 2016 Jan 6, at a phase of 0.20. Note the difference in the line strength of He ii$\lambda$4542, due to the relative shifting of weak broad WR emission with the narrower absorption from the O star. The prominent lines are labeled.
• Figure 2: EW variations with orbital phase. (a) The EWs of He i$\lambda$4471 are shown by black open circles, and those of He ii$\lambda$4542 are shown by filled red circles. The line shows the median He i EW value of 0.40 Å. (b) The ratios of the EWs were used by 1973ApJ...179..181C to define the O-type spectral classes, with the ranges indicated in the figure. We adopted the median He i EW value in these calculations. The star's spectral type varies from O5.5 to O7.5 during its cycle, but the constancy of the He i EW shows that the temperature itself is not changing.
• Figure 3: Radial velocity comparison of the O and WN3 stars. The data points are plotted showing the typical velocity uncertainties for each component, 6.9 km s$^{-1}$ for the O star, and 41 km s$^{-1}$ for the WN3 component. The line shows the best fit for the data points, taking their uncertainties into account.
• Figure 4: Orbit solutions for LMC173-1. The radial velocities of the O star are shown as green squares, and its radial velocity curve by a green solid line. The radial velocities of the WN3 star are shown by blue circles, and the corresponding radial velocity curve by a blue line. We have included the error bars of the typical velocity uncertainties, 6.9 km s$^{-1}$ for the O star primary, and 41 km s$^{-1}$ for the WN3 secondary. We have offset the radial velocities of the WR star by 206 km s$^{-1}$ in order to match their $\gamma$-velocities for the purposes of illustration. The data from phases 0.8-1.0 are repeated as -0.2 to 0.0, and the data from phases 0.0-0.2 are repeated as 1.0-1.2.
• Figure 5: Phased I-band photometry of LMC173-1. The upper figure shows the unbinned data; no error bars are shown to avoid crowding, but are typically 2-3 millimag. The lower figure shows the data binned by 0.01 phases; the error bars show the resulting standard deviations of the mean within each bin, typically 0.2-0.3 millimag.