2015MNRAS.450.2551M

NaSt1 (aka Wolf-Rayet 122) is a peculiar emission-line star embedded in an extended nebula of [Nii] emission with a compact dusty core. The object was previously characterized as a Wolf-Rayet (WR) star cloaked in an opaque nebula of CNO-processed material, perhaps analogous to η Car and its Homunculus nebula, albeit with a hotter central source. To discern the morphology of the [Nii] nebula we performed narrow-band imaging using the Hubble Space Telescope and Wide-field Camera 3. The images reveal that the nebula has a disc-like geometry tilted ~12° from edge-on, composed of a bright central ellipsoid surrounded by a larger clumpy ring. Ground-based spectroscopy reveals radial velocity structure (±10 km/s) near the outer portions of the nebula's major axis, which is likely to be the imprint of outflowing gas. Near-infrared adaptive-optics imaging with Magellan AO has resolved a compact ellipsoid of Ks-band emission aligned with the larger [Nii] nebula, which we suspect is the result of scattered Hei line emission (λ2.06 µm). Observations with the Chandra X-ray Observatory have revealed an X-ray point source at the core of the nebula that is heavily absorbed at energies <1 keV and has properties consistent with WR stars and colliding-wind binaries. We suggest that NaSt1 is a WR binary embedded in an equatorial outflow that formed as the result of non-conservative mass transfer. NaSt1 thus appears to be a rare and important example of a stripped-envelope WR forming through binary interaction, caught in the brief Roche lobe overflow phase.