Radial Velocity Evidence for a Post-Mass-Transfer Massive Binary System NaSt1

Abstract

We present multi-epoch high-resolution optical spectroscopy ($R\simeq 80,000$) 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 $\pm$ 5 d, 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 d. The RV-LC phase relationship and high-ionization lines favor binary interaction over pulsations. We model the 1--5~$\mu$m spectrum of NaSt1 and find two optically thin dust components: hot $T_{\mathrm{h}}\simeq 1230$ K, $M_{\mathrm{h}}\simeq 2\times 10^{-10}{M}_{\odot }$ and warm $T_{\mathrm{c}}\simeq 660$ K, $M_{\mathrm{c}}\simeq 3\times 10^{-8}{M}_{\odot }$. IFU spectroscopy spatially resolves the circumstellar medium in the [\ion{N}{2}] $\lambda 6548$ and $\lambda 6584$ emission lines, showing a deprojected expansion velocity of $\sim 31$ km~s${}^{-1}$, implying a dynamical age of $\sim 40$ 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.

Accepted for publication in The Astrophysical Journal. 19 pages, 11 figures, 2 Tables