An evolved → hot star which is responsible for the ionization of a
→ planetary nebula. Planetary nebulae result from
mass ejection by evolved stars undergoing violent instabilities.
CSPNe are extremely hot, with → effective temperatures ranging from
30,000 to 120,000 K. They evolve rapidly toward the → white dwarf
stage, while the planetary nebulae continue expanding with a small
expansion velocity of about 25 km s-1, becoming progressively thinner and
thus eventually invisible after some 104 years. The initial masses of
these stars range from about 1 to 8 solar masses.
CSPNe are not a homogeneous group and present a large variety of spectral characteristics.
Many of CSPNe display emission lines, some of them with spectra
resembling those of → WC Wolf-Rayet.
Although superficially similar, they differ from classical W-R stars in
their degenerate structure, much lower masses, a wider range of
temperatures, and limitation almost exclusively to carbon-rich stars.
Some CSPNe show → weak emission-lines
(wels). A considerable fraction of both groups are hydrogen deficient.
However, some wels may be H-rich despite having emission lines.
The evolutionary status of the [WR]-type stars is still very
uncertain, and it is unclear whether there is any evolutionary
relation to the wels. CSPNe have a strong → stellar wind composed of
helium, carbon, and oxygen. Some of them are binary systems.
The study of CSPNe is important for
understanding of → post-AGB stellar evolution.
See also: CSPN, short for → Central, → Star of
→ Planetary Nebula.
