Wolf-Rayet 98A: Pinwheels in the Sky!
The second spiral Pinwheel Wolf-Rayet star discovered by us is known as WR 98a. As it is not as bright as WR 104, it was more difficult to make images with our high-resolution techniques. Despite this, we have seen this star also has a spiral outflow tail, and we have made a movie showing its motion (right).
The extra lumps and irregularities in the map are partly due to the lower quality data. This system is not quite as close to being face-on in viewing angle to the Earth. You can see this from the more elliptical shape of the tail as it curves away from the bright core. The 1.5 year period of rotation for WR 98a is longer than for WR 104, and interestingly in this case, it corresponds almost exactly to a previously-seen regular dimming and brightening.
There might be two reasons for this variability, or (as Astronomers call it) lightcurve of WR 98a. Firstly, more or less dust might be formed at different sectors as the binary star goes about its orbit. This would be likely if the orbit of the stars was not circular, but somewhat elliptical (as is the orbit of the Earth around the Sun, for example). A second reason might simply be due to our viewing angle. During certain parts of the orbit, our view of the hot inner parts might be blocked by dust, resulting in a variation in the lightcurve.
The simulations above show how this geometry works out when you wrap a conical shaped shock into a spiral, then inflate the whole thing in a spherical wind outflow. Although the almost face-on version looks like a nice spiral (left), the others which are closer to edge-on display more complex cusps and brightenings due to various line-of-sight effects through the spiral tail.
With two separate objects both exhibiting similar characteristics, we have established that the pinwheel nebulae are a class of object, not an isolated case. We hope that there might be even more of these rare exotic spirals, twirling out there in the night sky, waiting to be discovered.
WR 98a pinwheel was first published in the Astrophysical Journal Letters 1999, 525, L97.
This research was supported by grants from the Australian Research Council and the US National Science Foundation Stellar Astronomy and Astrophysics Program.
For more information, contact: Dr Peter Tuthill