Figure 1: Gemini South GMOS spectrum of the WN central star of IC4663 (black) with model atmosphere (red). The model has a very hot stellar temperature of 140,000 degrees. According to the standard classification scheme for massive WN stars, the subtype is WN3 due to the absence of carbon and neutral helium emission lines.
The international team, led by Brent Miszalski of the South African Astronomical Observatory and the Southern African Large Telescope, were searching for binary companions to central stars of planetary nebulae, when they stumbled across the rarest of stellar gems. Gemini observations of the nucleus of IC4663 (Figure 1) revealed a peculiar mix of helium and nitrogen emission lines, unique and entirely out of place for a planetary nebula, but nonetheless an apparent clone of high mass Wolf-Rayet stars.
Planetary nebulae nuclei are the extremely hot, inert cores of low mass stars like the Sun, which are not far away from retiring as Earth-sized white dwarfs. The atmospheres of most nuclei show absorption lines of hydrogen and helium similar to white dwarfs. Many other strange flavors also exist. Around 100 planetary nebulae with Wolf-Rayet type nuclei are known, uniquely displaying emission lines of carbon, oxygen and helium, as a result of their powerful winds. These are the low-mass cousins of massive carbon-rich (WC-type) Wolf-Rayet stars, the final pre-supernova phase of very massive stars. A second flavor of massive Wolf-Rayet star, rich in helium and nitrogen (WN-type), are also common, but no clear-cut counterparts amongst planetary nebulae nuclei have been identified to date. While a handful of candidates have been identified, most cannot be unequivocally distinguished apart from ejecta known to surround some massive WN stars. A well known example is the WN star WR124, and its nebula M1-67, which is a proven massive star.
Is IC4663 just another case of a massive star with a confused identity? Team member Professor Paul Crowther, from the University of Sheffield, explains, "IC4663 may walk and talk like a duck, but our analysis reveals a completely different beast." It would have likely taken billions of years for IC4663 to reach old age, whereas it’s heftier cousins could get there in just a few million years. According to Dr Miszalski, the properties of the central star were "just right" for the team to prove that it is the first low-mass counterpart to nitrogen-rich WN stars. Relative to its massive cousins, it is exceptionally faint, and possesses an elliptical inner nebula, exquisitely captured by the Hubble Space Telescope (Figure 2), that is characteristic of other planetary nebulae. In addition, Gemini images reveal the presence of a faint halo (also seen in Figure 2). Haloes are widely accepted as a telltale signature of a previous cool giant phase, one that massive Wolf-Rayet stars do not experience.
Dr Miszalski’s team discovered that IC4663 hosted a helium and nitrogren rich central star that unambiguously had a WN-type spectrum, a composition that had never previously been predicted by theoretical models that aim to trace the evolutionary steps of low mass stars like the Sun. Models are able to reproduce the composition of carbon-rich planetary nebulae nuclei, but not IC4663. According to Dr Miszalski, "if our understanding of Solar-type stars were complete, then the central star of IC4663 simply should not exist!".
It is hard enough to explain why Wolf-Rayet central stars lack hydrogen, but it is even harder to come to terms with the extremely helium-rich nature of IC4663. Its existence is the first solid evidence that there's a second way to make hydrogen poor central stars, producing a helium-rich atmosphere instead of the more common carbon-rich atmosphere. Further work is needed to identify the evolutionary origins of the helium-rich composition. Although IC4663 does not appear to be a binary at the present time, binary stellar evolution may be the answer, since it may be the product of a stellar merger. Binary central stars are the leading explanation for the perplexing variety of exquisite shapes of planetary nebulae and several new discoveries have been made by Dr Miszalski and colleagues in recent years such that we now know of around 50 such systems. This paper has been accepted for publication in Monthly Notices of Royal Astronomical Society. The pre-print is available at http://arxiv.org/abs/1203.3303.