Multiwavelength Observations of NaSt1 (WR 122): Equatorial Mass Loss and X-rays from an Interacting Wolf-Rayet Binary

TL;DR

NaSt1 is a unique Wolf-Rayet binary with an equatorial outflow, exhibiting complex nebular morphology, X-ray emission, and evidence of non-conservative mass transfer, providing insights into binary evolution and mass loss processes.

Contribution

This study provides detailed multiwavelength imaging and spectroscopy of NaSt1, revealing its nebular structure, X-ray properties, and binary nature, highlighting its status as a rare example of a stripped-envelope WR star formed via binary interaction.

Findings

Nebula has a disk-like, tilted geometry with a bright ellipsoid and clumpy ring.

X-ray observations show a heavily absorbed point source consistent with a WR binary.

Evidence suggests NaSt1 is a binary system undergoing Roche-lobe overflow, leading to equatorial mass loss.

Abstract

NaSt1 (aka Wolf-Rayet 122) is a peculiar emission-line star embedded in an extended nebula of [N II] emission with a compact dusty core. This object was characterized by Crowther & Smith (1999) as a Wolf-Rayet (WR) star cloaked in an opaque nebula of CNO-processed material, perhaps analogous to Eta Car and its Homunculus nebula, albeit with a hotter central source. To discern the morphology of the [N II] nebula we performed narrowband imaging using the Hubble Space Telescope and Wide-field Camera 3. The images reveal that the nebula has a disk-like geometry tilted 12 degrees 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 [N II] nebula, which we suspect is the result of scattered He I line emission (2.06 um). 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.