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

Upper panels: MagAO Ks , L’ , and M’ images of NaSt1 in false color. The images are 2.7 arcsec square in angular size and oriented in detector coordinates. Lower panels: the Ks-band image of NaSt1 with logarithmic intensity contours (left) and the HST [N II] narrowband image with the Ks contours overlaid (right). An ellipsoid of extended Ks emission is clearly resolved, and is approximately aligned with the major axis of the optical nebula. The lower images are oriented with north up and east toward the left.
Abstract: NaSt1 is a peculiar emission-line star embedded in an extended nebula of optical [N II] emission with a compact dusty core. This object has been characterized as a Wolf-Rayet (WR) star cloaked in an opaque nebula of material that shows signs of advanced nuclear processing in the core of a massive star. To discern the morphology of the [N II] nebula we performed narrowband imaging using the Hubble Space Telescope. The images reveal that the nebula has a disk-like geometry, composed of a bright central ellipsoid surrounded by a larger clumpy ring. Near-infrared imaging with Magellan AO resolved a compact ellipsoid of Ks-band emission aligned with the larger [N II] nebula. The source is more compact at L’ and M’, so we suspect the Ks-band emission result of neutral helium emission from the inner stellar wind scattered outward by reflection off of cool dust in the nebula. Observations with the Chandra X-ray Observatory have revealed a hard X-ray point source at the core of the nebula that is consistent with WR stars and massive binaries where powerful stellar winds collide and make X-rays. We suggest that NaSt1 is a WR binary embedded in an equatorial outflow that formed as the result of non-conservative mass transfer (the transfer is not completely efficient, and some material is lost from the binary system through the outer Lagrange points). NaSt1 thus appears to be a rare and important example of a stripped-envelope WR forming through binary interaction.

Mauerhan, J., et al. “Multiwavelength Observations of NaSt1 (WR 122): Equatorial Mass Loss and X-rays from an Interacting Wolf-Rayet Binary”

MNRAS 450, 3, 2551; arxiv preprint