by Brent Miszalski, Paul Crowther, and Anthony Moffat
A Second Flavor of
Wolf-Rayet Central Stars
of Planetary Nebulae
Gemini South optical observations of the planetary nebula IC 4663 reveal
the first proven case of a central star with a nitrogen-sequence WolfRayet spectrum. Its existence challenges the conventional view of how
certain solar-mass stars become hydrogen-deficient white dwarfs.
Population I classical Wolf-Rayet stars represent the short-lived, hydrogen-deficient, pre-supernova phase of very massive stars. These hot, high-luminosity bodies possess powerful, fast,
dense winds (for recent review see Crowther, 2007). They exhibit unique, broad emission lines
generated via Doppler expansion that are readily seen spectroscopically.
Wolf-Rayet stars come in two main flavors: nitrogen-rich WN-type stars and carbon-rich WCtype stars. These two classes reflect the products of hydrogen and helium-burning, respectively; whereas helium and nitrogen emission lines dominate in WN-types, the WC-types show
emission lines mostly of carbon, oxygen, and helium. Very high-mass stars are thought to end
their lives as WN or WC stars, although they are exceptionally rare, with only a few hundred
cases known in the Milky Way.
The Wolf-Rayet Star Phenomenon
A subset of low-mass, post-Asymptotic Giant Branch (AGB) stars are also hydrogen-deficient
(Werner Herwig, 2006). High temperature examples include He-rich subdwarf OB stars, O(He)
stars, and DO white dwarfs. In addition, around 100 hydrogen-deficient central stars of planetary nebulae also exhibit a Wolf-Rayet spectroscopic signature. To date, all Wolf-Rayet central
stars have been carbon-rich variants, with square brackets added to distinguish [WC]-type
central stars from WC stars.
17
GeminiFocus
June2012