A protoplanetary nebula or preplanetary nebula
(Sahai, Sánchez Contreras & Morris 2005)
(PPN) is an astronomical object which is at
the short-lived episode during a star's rapid
evolution between the late asymptotic giant
branch (LAGB) phase and the subsequent planetary
nebula (PN) phase. A PPN emits strongly in
infrared radiation, and is a kind of reflection
nebula. It is the second-from-the-last high-luminosity
evolution phase in the life cycle of intermediate-mass
stars (1–8 M☉). (Kastner 2005)
== Naming ==
The name protoplanetary nebula is an unfortunate
choice due to the possibility of confusion
with the same term being sometimes employed
when discussing the unrelated concept of protoplanetary
disks. The name protoplanetary nebula is a
consequence of the older term planetary nebula,
which was chosen due to early astronomers
looking through telescopes and finding a similarity
in appearance of planetary nebula to the gas
giants such as Neptune and Uranus. To avoid
any possible confusion, Sahai, Sánchez Contreras
& Morris 2005 suggests employing a new term
preplanetary nebula which does not overlap
with any other disciplines of astronomy. They
are often referred to as post-AGB stars, although
that category also includes stars that will
never ionize their ejected matter.
== Evolution ==
=== Beginning ===
During the late asymptotic giant branch (LAGB)
phase, when mass loss reduces the hydrogen
envelope's mass to around 10−2 M☉ for
a core mass of 0.60 M☉, a star will begin
to evolve towards the blue side of the Hertzsprung–Russell
diagram. When the hydrogen envelope has been
further reduced to around 10−3 M☉, the
envelope will have been so disrupted that
it is believed further significant mass loss
is not possible. At this point, the effective
temperature of the star, T*, will be around
5,000 K and it is defined to be the end of
the LAGB and the beginning of the PPN. (Davis
et al. 2005)
=== Protoplanetary nebula phase ===
During the ensuing protoplanetary nebula phase,
the central star's effective temperature will
continue rising as a result of the envelope's
mass loss as a consequence of the hydrogen
shell's burning. During this phase, the central
star is still too cool to ionize the slow-moving
circumstellar shell ejected during the preceding
AGB phase. However, the star does appear to
drive high-velocity, collimated winds which
shape and shock this shell, and almost certainly
entrain slow-moving AGB ejecta to produce
a fast molecular wind. Observations and high-resolution
imaging studies from 1998 to 2001, demonstrate
that the rapidly evolving PPN phase ultimately
shapes the morphology of the subsequent PN.
At a point during or soon after the AGB envelope
detachment, the envelope shape changes from
roughly spherically symmetric to axially symmetric.
The resultant morphologies are bipolar, knotty
jets and Herbig–Haro-like "bow shocks".
These shapes appear even in relatively "young"
PPN. (Davis et al. 2005)
=== End ===
The PPN phase continues until the central
star reaches around 30,000 K and it is hot
enough (producing enough ultraviolet radiation)
to ionize the circumstellar nebula (ejected
gases) and it becomes a kind of emission nebula
called a PN. This transition must take place
in less than around 10,000 years or else the
density of the circumstellar envelope will
fall below the PN formulation density threshold
of around 100 per cm³ and no PN will result,
such a case is sometimes referred to as a
'lazy planetary nebula'. (Volk & Kwok 1989)
== Recent conjectures ==
In 2001, Bujarrabal et al. found that the
"interacting stellar winds" model of Kwok
et al. (1978) of radiatively-driven winds
is insufficient to account for their CO observations
of PPN fast winds which imply high momentum
and energy inconsistent with that model. This
has prompted theorists (Soker & Rappaport
2000; Frank & Blackmann 2004) to investigate
whether an accretion disk scenario, similar
to the model used to explain jets from active
galactic nuclei and young stars, could account
for both the point symmetry and the high degree
of collimation seen in many PPN jets. In such
a model, the accretion disk forms through
binary interactions. Magneto-centrifugal launching
from the disk surface is then a way to convert
gravitational energy into the kinetic energy
of a fast wind. If this model is correct and
magneto-hydrodynamics (MHD) do determine the
energetics and collimation of PPN outflows,
then they will also determine physics of the
shocks in these flows, and this can be confirmed
with high-resolution pictures of the emission
regions that go with the shocks. (Davis et
al. 2005)
== See also ==
Bipolar nebula
Bipolar outflow
List of protoplanetary nebulae
Planetary nebula
== Notes
