RCW 58: A Rarely Observed “Kinematic Structure” Hiding in the Stars

OTA: ……………….CDK17

Camera:………….SBIG STXL11002 with AOX and FW8G (0.63 arsec/pxl)

Observatory: … Heaven’s Mirror, Chile

EXPOSURES:

--Red:…….9 x 900 sec.

--Blue:......8 x 900

--Green:...8 x900

--HII:….…23 x 1800

--OIII:……21 x 1800

Total exposure 30.5 hours

Image Width: 36 arcmin

Processed by Alex Woronow (2021) using PixInsight, Topaz, Skylum, SWT



Only four other images of RCW 58 appear on Astrobin (https://www.astrobin.com/search/?q=RCW+58) and they well illustrate the difficulty of processing this target: A faint ‘planetary nebula’ very well hidden within a dense starfield. Bringing out the nebula with any detail poses one mean task! Several false starts, shelving the project for a couple of months, then a foggy idea opened a path that got me to the image I show above. I hope you like it.

The central star of RCW 58 is WN 8, a Wolf-Rayet (WR) star, a young supermassive star with an unstable nature that has led to the off-casting of an HII and OIII cloud. Lortet, et al. (1982) note that RCW 58 is quite “clumpy” and contains arcs or curls—not the usual geometry for planetaries. Furthermore, different clumps appear to be traveling at different speeds. Undoubtedly, the intense radiation and solar winds from the WN 8 tear at the gases and sculpt the streamers and clumps, giving the forms we observe.

Real planetary nebulae form when a star goes supernova, ejects its outer atmosphere, and then collapses to a white dwarf. That scenario does not apply here. So, in a sensu stricto, this cannot be called a “planetary nebula.” Smith et al. (1988) offer an alternative origin story to Lortet’s original one: they postulate that it is a “wind-blown nebula” that arose before WN 8 ignited. This rapidly expanding wind entraps slower-moving materials that formed the clumps. You-Hua Chu (1988) baptized this nebula a “Kinematic Structure.”

WN 8 is the class archetype for “WN8” stars, which may be examples of Thorne-Zytkow Objects (TZOs) (Foelimi and Moffat, 2006). WN8’s are commonly “run-away” stars having very high velocity. Some of these have binary partners, as evidence suggests WN 8 itself may. These smaller partners may siphon material from the larger partner Wolf-Rayet star until it reaches a critical mass, and goes supernova, leaving behind a white dwarf, black hole, or neutron star. As the WN8 stars evolve, they may expand and gobble up their compact partner, grow even larger--this may be the birth of an exotic TZO. Some other suspected attributes of TZOs are that they have neutron cores and an outer envelope that appears as that of a Red Supergiant. They probably develop intense stellar winds that carry away large volumes of the WR’s mass. Until researching this image, I never heard of WN8 stars! But now we both know about at least one…WN 8 itself!