Figure 3.
Gemini South GMOS
spectrum of the
[WN3] central star
of IC 4663 (black),
with our model
atmosphere (red).
The main emission
lines are labeled.
to six magnitudes fainter than a massive WN3
star assuming reasonable distances to IC 4663.
All of the above leaves no doubt that IC 4663
is a planetary nebula, rather than an ejecta
nebula around a massive WN star.
Stellar Properties
Wolf-Rayet stars have dense, expanding atmospheres that require specialized models
to reproduce their observed spectra. We used
the CMFGEN code to build a model atmosphere of the [WN3] star that takes into account metal-line blanketing and wind clumping (see Miszalski et al., 2012).
Figure 3 shows our best model that provides
a satisfactory fit to the GMOS spectrum. The
model parameters at our adopted distance of
11,400 light-years include an extremely hot
effective temperature of 140,000 K, a relatively fast wind expanding at a terminal speed
of 1900 km/s, a radius 0.11 times the solar
radius, a luminosity 4000 times greater than
the Sun, and a mass loss rate of 1.8 x 10-8 solar
masses per year. These parameters are comparable to the hottest [WC]-type Wolf-Rayet
central stars.
In stark contrast to [WC]-type central stars,
whose atmospheres are made up of a uniform
pattern of 30-50 percent helium, 30-60 per-
19
GeminiFocus
cent carbon, and 2-20 percent
oxygen, the atmosphere of IC
4663 is made up almost entirely
of helium (≥ 95 percent) along
with 0.8 percent nitrogen; it is
also depleted in carbon (< 0.1
percent) and oxygen (0.05 percent). This most unusual abundance pattern suggests that as
the [WN3] star’s wind starts to
dissipate, it will evolve into an
O(He) star whose known compositions closely match the
abundance pattern of IC 4663,
in the same way that [WC] central stars are thought to evolve
into PG 1159 stars (see Figure 4).
These stages are the penultimate phase before the formation of a hydrogen-deficient
DO white dwarf. The discovery of the [WN]
nature of IC 4663 has clarified the uncertain
evolutionary position of the O(He) stars, of
which only four are known, and two of these
have planetary nebulae (Rauch et al., 1998), as
the helium-rich equivalents of carbon-rich PG
1159 stars. This discovery in IC 4663 provides
the best evidence so far for a second pathway
for a subset of Sun-like stars to lose their hydrogen, one that is helium-rich in addition to
the more common carbon-rich pathway (see
Werner and Herwig, 2006).
Unexplained Origins
Most classical Wolf-Rayet stars (especially
WC types) are very hydrogen-deficient, with
hydrogen usually making up no more than a
few percent of their atmospheres. In massive
Wolf-Rayet stars, this can be explained by their
strong wind peeling off the outer layers of hydrogen. In contrast, it is thought that an AGB
precursor to a Wolf-Rayet central star experiences either a late or very late thermal pulse,
reigniting helium-shell burning to burn up or
mix away the remaining hydrogen. Although
this scenario can reproduce the chemical sig-
June2012