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Hubble Space Telescope Observations of Young Planetary Nebulae
Abstract
We present results from an ongoing Hubble Space Telescope Hα
imaging survey of young planetary nebulae (PNe), selected on the basis
of their low excitation characteristics. All objects imaged so far show
highly aspherical morphology, with a majority characterised by
multipolar bubbles distributed roughly point-symmetrically around the
central star. In some objects, bipolar ansae or collimated radial
structures are seen, indicating the presence of jets, whereas in others
bright structures near the minor axes indicate the presence of disks or
torii. The complexity, organization and symmetry of the above structures
leads us to propose that the primary agent for shaping PNe are
high-speed collimated outflows or jets which operate during the late AGB
and/or early post-AGB evolutionary phase. These outflows carve out a
complex imprint within an intrinsically spherical AGB circumstellar
envelope (CSE). Subsequent expansion of a hot, tenuous stellar wind from
the post-AGB star inside the imprinted AGB CSE then produces the
observed PN, whose shape and structure depend in detail on how the
characteristics of the jets change with time.
... Collimated jets are produced by astrophysical bodies on many different scales; there is evidence for outflows associated with Planetary nebulae (PN, see [56,57]), young stellar objects (YSOs or protostars, which will be discussed in more detail later in this chapter) and active galactic nuclei (AGN, see [58]). ...
We report continued detailed spectroscopic studies of highly articulated structures, called ''microstructures,'' in NGC 6543, 6826, and 7009. Of particular interest of these PNs are FLIERs (fast, low-ionization emission regions) characterized by the following: ionization-much lower ionization than immediately adjacent gas; size-small size in at least one dimension (almost-equal-to 2'', or about 0.01 pc at a distance of 1 kpc); morphology-generally ansae, jets, knots, or strings of knots; distribution-pairs (or sets of pairs) on equal and opposite sides of the central nucleus located near a nebular symmetry axis; velocity-each partner has equal and opposite highly supersonic velocity (almost-equal-to +/- 50 km s-1); kinematic age-the kinematic age (projected distance/redshift) of some FLIERs is less than that of the rest of the PN; N abundance-apparent enhancement of nitrogen (only) relative to hydrogen by factors of 2-5; kinetic/thermal energy density-much higher kinetic energy per mass than thermal energy; and mass-typically 10(-4) to 10(-5) M., and yet FLIERs exhibit little difference in density or temperature from their surroundings. These properties and their small kinematic ages lead one to expect that FLIERs are high-density, opaque (i.e., partly neutral), and highly collimated material expelled recently from the nucleus. Their high ram pressure (10 times greater than the thermal pressure of the confining gas) should lead to the formation of bow shocks. Contrary to intuitive expectations, however, most FLIERs show the ionization structure expected of bow shocks. No simple model accounts naturally for the observed properties of FLIERs.
Highly collimated bipolar outflows and jets have been recently observed to be associated with some planetary nebulae. Assuming that the mechanism of jet formation requires the presence of an accretion disk, we examine the possibilities to form such disks in the case of planetary nebulae with binary nuclei. We show that at the emergence from the common envelope phase, some systems can form accretion disks. Planetary nebulae therefore form the newest addition to the classes of objects (which so far include active galactic nuclei, young stellar objects, and some X-ray binaries) which produce jets.
I propose that the destruction of brown dwarfs and massive planets
inside the envelopes of asymptotic giant branch (AGB) stars can lead to
the formation of jets and ansae in elliptical planetary nebulae. When at
˜1 Rsun from the AGB progenitors' cores, these very low
mass secondaries overflow their Roche lobes. Thick disks with jets on
their two sides are a plausible outcome of this process. The process is
likely to occur at late stages of the AGB, and the jets push their way
out of the envelopes in the course of a number of years. The Roche lobe
overflow continues for several hundred years and destroys the
secondaries. This scenario predicts that the ansae material will be
contaminated by two sources. One source is the AGB cores, from which
material can be mixed into the jets; the other consists of the destroyed
secondaries.
The arcsecond structure of OH maser clouds surrounding 20 Mira variables
and late-type supergiants has been measured using the VLA in a spectral
line mode at 1612 MHz. Angular radii of the maser clouds range from
<0arcsec.5 to 4arcsec and have been combined with phase lag
measurements of the shell size to estimate distances for some of the
stars. The outer radii range from 100 AU to 104AU, and the
authors estimate a ratio of outer to inner radii ⪉4 for the maser
regions. The previously discovered correlation between the mass loss
rate and the outer radius of the maser region is confirmed and used to
determine the mass loss rates for eight stars. The results of this work
are compared with current theoretical models of circumstellar shells and
the implications for mass loss mechanisms and the evolutionary fate of
these stars are considered. It is suggested that the rare cases of
nonspherical shells are indicating the presence of close binary
companions.
Spatially resolved, long-slit, echelle observations have been obtained in the light of the Halpha and [N II] lambda6548 emission lines for the symmetric strings of ionized knots emerging from the body of the elliptical planetary nebula Fleming 1. It is found that the magnitudes of the radial velocity differences of both strings systematically increase away from the nebular center. The north string is redshifted, whereas the south string is blue-shifted. The radial velocity difference from tip to tip of the north and south strings is 148 km s-1. The data are reasonably consistent with a simple ballistic model that assumes episodic ejections of collimated, bipolar flows emerging from a rotating source. These observations confirm some of the theoretical predictions of Soker (1992).
An atlas of CCD pictures of fifty-one planetary nebulae (PNs) taken in
the light of low-, moderate-, and high-ionization emission lines is
presented. The shapes of many of the PNs can be organized into an
empirical morphological sequence of round, elliptical, bipolar, and
butterfly classes. Many PNs, especially the ones of high surface
brightness such as NGC 2392, 3242, 6543, 6826, and 7662, show evidence
for thin inclusions of anomalously low ionization in a high-ionization
substrate. The morphological sequence of PNs is shown to be consistent
with the precepts of the shaping of PNs by interacting winds. This
consistency places the concepts of ongoing hydrodynamic shaping of PNs
on firm grounds, and justifies further observations that can lead to a
better understanding of the physical processes. PNs are proposed as an
ideal astrophysical laboratory for studying the interactions of winds on
a structured environment.
The transition from red giants to planetary nebulae is studied by comparing the spectral characteristics of red giant envelopes and planetary nebulae. Observational and theoretical evidence both suggest that remnants of red giant envelopes may still be present in planetary nebula systems and should have significant effects on their formation. The dynamical effects of the interaction of stellar winds from central stars of planetary nebulae with the remnant red giant envelopes are evaluated and the mechanism found to be capable of producing the observed masses and momenta of planetary nebulae. The observed mass-radii relation of planetary nebulae may also be best explained by the interacting winds model. The possibility that red giant mass loss, and therefore the production of planetary nebulae, is different between Population I and II systems is also discussed.
We present two-dimensional numerical simulations, which use techniques of radiation gasdynamics to simulate the structures of planetary nebulae (PNs). Our model incorporates realistic volume emissivities in order to fully account for the conversion of mechanical and thermal energy into radiation. The model also produces detailed predictions of observables such as projected structure (e.g., H-alpha and forbidden N II images) and kinematic patterns. Virtually the full range of PN morphologies are easily reproduced, as are the basic kinematics, ionization structures, and temperatures.
High- and medium-resolution images of SiO J = 2-1(V = 0) from the circumstellar envelopes (CSEs) of three oxygen-rich stars, Chi Cyg, RX Boo, and IK Tau, were obtained. The SIO images were found to be roughly circular, implying that the CSEs are spherically symmetric on angular-size scales of about 3-9 arcsec. The observed angular half-maximum intensity source radius is nearly independent of the LSR velocity for all three CSEs. Chi Cyg and RX Boo are argued to be less than 450 pc distant, and have mass-loss rates larger than about 10 exp -6 solar mass/yr. In Chi Cyg and RX Boo, the line profiles at the peak of the brightness distribution are rounded, typical of optically-thick emission from a spherical envelope expanding with a constant velocity. In the IK Tau line profiles, an additional narrower central component is present, probably a result of emission from an inner circumstellar shell with a significantly smaller expansion velocity than the extended envelope.