364 research outputs found
Rings and arcs around evolved stars. II. The Carbon Star AFGL 3068 and the Planetary Nebulae NGC 6543, NGC 7009 and NGC 7027
We present a detailed comparative study of the arcs and fragmented ring-like
features in the haloes of the planetary nebulae (PNe) NGC 6543, NGC 7009, and
NGC 7027 and the spiral pattern around the carbon star AFGL 3068 using
high-quality multi-epoch HST images. This comparison allows us to investigate
the connection and possible evolution between the regular patterns surrounding
AGB stars and the irregular concentric patterns around PNe. The radial proper
motion of these features, ~15 km/s, are found to be consistent with the AGB
wind and their linear sizes and inter-lapse times (500-1900 yr) also agree with
those found around AGB stars, suggesting a common origin. We find evidence
using radiative-hydrodynamic simulations that regular patterns produced at the
end of the AGB phase become highly distorted by their interactions with the
expanding PN and the anisotropic illumination and ionization patterns caused by
shadow instabilities. These processes will disrupt the regular (mostly spiral)
patterns around AGB stars, plausibly becoming the arcs and fragmented rings
observed in the haloes of PNe.Comment: 13 pages, 9 figures, accepted for publication in MNRA
Disk Formation by AGB Winds in Dipole Magnetic Fields
We present a simple, robust mechanism by which an isolated star can produce
an equatorial disk. The mechanism requires that the star have a simple dipole
magnetic field on the surface and an isotropic wind acceleration mechanism. The
wind couples to the field, stretching it until the field lines become mostly
radial and oppositely directed above and below the magnetic equator, as occurs
in the solar wind. The interaction between the wind plasma and magnetic field
near the star produces a steady outflow in which magnetic forces direct plasma
toward the equator, constructing a disk. In the context of a slow (10 km/s)
outflow (10^{-5} M_sun/yr) from an AGB star, MHD simulations demonstrate that a
dense equatorial disk will be produced for dipole field strengths of only a few
Gauss on the surface of the star. A disk formed by this model can be
dynamically important for the shaping of Planetary Nebulae.Comment: 14 pages, 8 figures, 1 table, accepted by Ap
Abell 41: shaping of a planetary nebula by a binary central star?
We present the first detailed spatio-kinematical analysis and modelling of
the planetary nebula Abell 41, which is known to contain the well-studied
close-binary system MT Ser. This object represents an important test case in
the study of the evolution of planetary nebulae with binary central stars as
current evolutionary theories predict that the binary plane should be aligned
perpendicular to the symmetry axis of the nebula.
Deep narrowband imaging in the light of [NII], [OIII] and [SII], obtained
using ACAM on the William Herschel Telescope, has been used to investigate the
ionisation structure of Abell 41. Longslit observations of the H-alpha and
[NII] emission were obtained using the Manchester Echelle Spectrometer on the
2.1-m San Pedro M\'artir Telescope. These spectra, combined with the narrowband
imagery, were used to develop a spatio-kinematical model of [NII] emission from
Abell 41. The best fitting model reveals Abell 41 to have a waisted, bipolar
structure with an expansion velocity of ~40km\s at the waist. The symmetry axis
of the model nebula is within 5\degr of perpendicular to the orbital plane of
the central binary system. This provides strong evidence that the close-binary
system, MT Ser, has directly affected the shaping of its nebula, Abell 41.
Although the theoretical link between bipolar planetary nebulae and binary
central stars is long established, this nebula is only the second to have this
link, between nebular symmetry axis and binary plane, proved observationally.Comment: 7 pages, 6 figures, Accepted for publication in MNRA
The kinematics of the large western knot in the halo of the young planetary nebula NGC 6543
A detailed analysis is presented of the dominant ionised knot in the halo of
the planetary nebula NGC 6543. Observations were made at high spectral and
spatial resolution of the [OIII] 5007 line using the Manchester echelle
spectrometer combined with the 2.1-m San Pedro Martir Telescope. A 20-element
multislit was stepped across the field to give almost complete spatial coverage
of the knot and surrounding halo. The spectra reveal, for the first time, gas
flows around the kinematically inert knot. The gas flows are found to have
velocities comparable to the sound speed as gas is photo-evaporated off an
ionised surface. No evidence is found of fast wind interaction with the knot
and we find it likely that the fast wind is still contained in a
pressure-driven bubble in the core of the nebula. This rules out the
possibility of the knot having its origin in instabilities at the interface of
the fast and AGB winds. We suggest that the knot is embedded in the slowly
expanding Red Giant wind and that its surfaces are being continually
photoionised by the central star.Comment: 13 pages, 11 figures (all eps format). To be published in MNRAS,
accepted on 6/7/0
A Compact X-ray Source and Possible X-ray Jets within the Planetary Nebula Menzel 3
We report the discovery, by the Chandra X-ray Observatory, of X-ray emission
from the bipolar planetary nebula Menzel 3. In Chandra CCD imaging, Mz 3
displays hot (3-6x10^6 K) gas within its twin, coaxial bubbles of optical
nebulosity, as well as a compact X-ray source at the position of its central
star(s). The brightest diffuse X-ray emission lies along the polar axis of the
optical nebula, suggesting a jet-like configuration. The observed combination
of an X-ray-emitting point source and possible X-ray jet(s) is consistent with
models in which accretion disks and, potentially, magnetic fields shape bipolar
planetary nebulae via the generation of fast, collimated outflows.Comment: 12 pages, 3 figures; to appear in Astrophysical Journal (Letters
Large Magellanic Cloud Planetary Nebula Morphology: Probing Stellar Populations and Evolution
Planetary Nebulae (PNe) in the Large Magellanic Cloud (LMC) offer the unique
opportunity to study both the Population and evolution of low- and
intermediate-mass stars, by means of the morphological type of the nebula.
Using observations from our LMC PN morphological survey, and including images
available in the HST Data Archive, and published chemical abundances, we find
that asymmetry in PNe is strongly correlated with a younger stellar Population,
as indicated by the abundance of elements that are unaltered by stellar
evolution (Ne, Ar, S). While similar results have been obtained for Galactic
PNe, this is the first demonstration of the relationship for extra-galactic
PNe. We also examine the relation between morphology and abundance of the
products of stellar evolution. We found that asymmetric PNe have higher
nitrogen and lower carbon abundances than symmetric PNe. Our two main results
are broadly consistent with the predictions of stellar evolution if the
progenitors of asymmetric PNe have on average larger masses than the
progenitors of symmetric PNe. The results bear on the question of formation
mechanisms for asymmetric PNe, specifically, that the genesis of PNe structure
should relate strongly to the Population type, and by inference the mass, of
the progenitor star, and less strongly on whether the central star is a member
of a close binary system.Comment: The Astrophysical Journal Letters, in press 4 figure
From Bipolar to Elliptical: Simulating the Morphological Evolution of Planetary Nebulae
The majority of Proto-planetary nebulae (PPN) are observed to have bipolar
morphologies. The majority of mature PN are observed to have elliptical shapes.
In this paper we address the evolution of PPN/PN morphologies attempting to
understand if a transition from strongly bipolar to elliptical shape can be
driven by changes in the parameters of the mass loss process. To this end we
present 2.5D hydrodynamical simulations of mass loss at the end stages of
stellar evolution for intermediate mass stars. We track changes in wind
velocity, mass loss rate and mass loss geometry. In particular we focus on the
transition from mass loss dominated by a short duration jet flow (driven during
the PPN phase) to mass loss driven by a spherical fast wind (produced by the
central star of the PN). We address how such changes in outflow characteristics
can change the nebula from a bipolar to an elliptical morphology. Our results
show that including a period of jet formation in the temporal sequence of PPN
to PN produces realistic nebular synthetic emission geometries. More
importantly such a sequence provides insight, in principle, into the apparent
difference in morphology statistics characterizing PPN and PN systems. In
particular we find that while jet driven PPN can be expected to be dominated by
bipolar morphologies, systems that begin with a jet but are followed by a
spherical fast wind will evolve into elliptical nebulae. Furthermore, we find
that spherical nebulae are highly unlikely to ever derive from either bipolar
PPN or elliptical PN.Comment: Accepted for publication in the MNRAS, 15 pages, 7 figure
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