Multiwavelength observations of NaSt1 (WR 122): equatorial mass loss and X-rays from an interacting Wolf–Rayet binary

• Published in: Monthly notices of the Royal Astronomical Society Vol. 450; no. 3; p. 2551
• Main Authors: Mauerhan, Jon, Smith, Nathan, Van Dyk, Schuyler D, Morzinski, Katie M, Close, Laird M, Hinz, Philip M, Males, Jared R, Rodigas, Timothy J
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 01.07.2015
• Subjects: Astronomy; Double stars; Ellipsoids; Emission; Hubble Space Telescope; Imaging; Mass transfer; Morphology; Nebulae; Optics; Stars; Symbols; Wind; X-ray astronomy; X-rays
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stv257

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 ... 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 degree from edge-on, composed of a bright central ellipsoid surrounded by a larger clumpy ring. Ground-based spectroscopy reveals radial velocity structure ( plus or minus 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 K ...-band emission aligned with the larger [Nii] nebula, which we suspect is the result of scattered He i line emission (...2.06 mu m). 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. (ProQuest: ... denotes formulae/symbols omitted.)