Radial Velocity Evidence for a Post-mass-transfer Massive Binary System NaSt1
October 2026 • 2026ApJ..1010...53W
Abstract • We present multiepoch high-resolution optical spectroscopy (R ≃ 80,000) of the emission-line object NaSt1 to test its proposed binary nature, along with long-term multiband photometry, mid-infrared spectroscopy, and spatially resolved integral field unit (IFU) spectroscopy to probe the circumstellar kinematics of the system. We detect two groups of 36 emission lines showing radial velocity (RV) variation with a mean period of 311 ± 5 days, but varying in opposite phase. We associate these two groups with the optically thick wind of the stripped primary star and the wind--wind collision region with the companion star, providing strong evidence for binarity. The RV and light curve (LC) periods are consistent within the uncertainties, ruling out ellipsoidal modulation, which would require an orbital period of about 620 days. The RV--LC phase relationship and high-ionization lines favor binary interaction over pulsations. We model the 1--5 μm spectrum of NaSt1 and find two optically thin dust components: hot: Th ≃ 1230 K and Mh ≃ 2 × 10-10 M⊙, and warm: Tc ≃ 660 K and Mc ≃ 3 × 10-8 M⊙. IFU spectroscopy spatially resolves the circumstellar medium in the [N II] λ6548 and λ6584 emission lines, showing a deprojected expansion velocity of ~31 km s-1, implying a dynamical age of ~40 yr. This short timescale suggests that the nebula was produced by recent mass loss. The system may represent a Galactic analog of a massive binary undergoing a mass-loss process to become a stripped-envelope supernova progenitor.
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