198 research outputs found
Radio Observations of New Galactic Bulge Planetary Nebulae
We observed 64 newly identified galactic bulge planetary nebulae in the radio
continuum at 3 and 6 cm with the Australia Telescope Compact Array. We present
their radio images, positions, flux densities, and angular sizes. The survey
appears to have detected a larger ratio of more extended planetary nebulae with
low surface brightness than in previous surveys. We calculated their distances
according to Van de Steene & Zijlstra (1995). We find that most of the new
sample is located on the near side around the galactic center and closer in
than the previously known bulge PNe. Based on H-alpha images and spectroscopic
data, we calculated the total H-alpha flux. We compare this flux value with the
radio flux density and derive the extinction. We confirm that the distribution
of the extinction values around the galactic center rises toward the center, as
expected.Comment: accepted for publication in A&
Reduce, Reuse, Recycle: Planetary Nebulae as Green Galactic Citizens
We review gas-phase abundances in PNe and describe their dual utility as
archives of original progenitor metallicity via the alpha elements, as well as
sources of processed material from nucleosynthesis during the star's evolution,
i.e., C, N, and s-process elements. We describe the analysis of PN spectra to
derive abundances and discuss the discrepancies that arise from different
choices at each step. Abundance results for the Milky Way and Magellanic Clouds
from various groups of investigators are presented; the observational results
are compared with theoretical predictions of AGB stellar yields. Finally, we
suggest areas where more work is needed to improve our abilities to determine
abundances in PNe.Comment: 8 pages, 7 figures, 1 table. Invited review presented at the IAU
Symposium No. 283, Planetary Nebulae: an Eye to the Futur
Detection of the Central Star of the Planetary Nebula NGC 6302
NGC 6302 is one of the highest ionization planetary nebulae known and shows
emission from species with ionization potential >300eV. The temperature of the
central star must be >200,000K to photoionize the nebula, and has been
suggested to be up to ~ 400,000K. On account of the dense dust and molecular
disc, the central star has not convincingly been directly imaged until now. NGC
6302 was imaged in six narrow band filters by Wide Field Camera 3 on HST as
part of the Servicing Mission 4 Early Release Observations. The central star is
directly detected for the first time, and is situated at the nebula centre on
the foreground side of the tilted equatorial disc. The magnitudes of the
central star have been reliably measured in two filters(F469N and F673N).
Assuming a hot black body, the reddening has been measured from the
(4688-6766\AA) colour and a value of c=3.1, A_v=6.6 mag determined. A G-K main
sequence binary companion can be excluded. The position of the star on the HR
diagram suggests a fairly massive PN central star of about 0.64,M_sun close to
the white dwarf cooling track. A fit to the evolutionary tracks for
(T,L,t)=(200,000K, 2000L_sun, 2200yr), where t is the nebular age, is obtained;
however the luminosity and temperature remain uncertain. The model tracks
predict that the star is rapidly evolving, and fading at a rate of almost 1 %
per year. Future observations could test this prediction.Comment: 13 pages, 5 figures, submitted to ApJ Letters on 25.09.2009 accepted
on 19.10.200
Properties of Dust Grains in Planetary Nebulae -- I. The Ionized Region of NGC 6445
In this paper we study new infrared spectra of the evolved planetary nebula
NGC 6445 obtained with ISO. These data show that the thermal emission from the
grains is very cool and has a low flux compared to H beta. A model of the
ionized region is constructed, using the photo-ionization code CLOUDY 90.05.
Based on this model, we show from depletions in the gas phase elements that
little grain destruction can have occurred in the ionized region of NGC 6445.
We also argue that dust-gas separation in the nebula is not plausible. The most
likely conclusion is that grains are residing inside the ionized region of NGC
6445 and that the low temperature and flux of the grain emission are caused by
the low luminosity of the central star and the low optical depth of the grains.
This implies that the bulk of the silicon-bearing grains in this nebula were
able to survive exposure to hard UV photons for at least several thousands of
years, contradicting previously published results. A comparison between optical
and infrared diagnostic line ratios gives a marginal indication for the
presence of a t^2-effect in the nebula. However, the evidence is not convincing
and other explanations for the discrepancy are also plausible. The off-source
spectrum taken with ISO-LWS clearly shows the presence of a warm cirrus
component with a temperature of 24 K as well as a very cold component with a
temperature of 7 K. Since our observation encompasses only a small region of
the sky, it is not clear how extended the 7 K component is and whether it
contributed significantly to the FIRAS spectrum taken by COBE. Because our line
of sight is in the galactic plane, the very cold component could be a starless
core.Comment: 36 pages, 8 tables, 7 figures, accepted for publication in Ap
Thermal instability of an expanding dusty plasma with equilibrium cooling
We present an analysis of radiation induced instabilities in an expanding
plasma with considerable presence of dust particles and equilibrium cooling. We
have shown that the equilibrium expansion and cooling destabilize the radiation
condensation modes and the presence of dust particles enhances this effect. We
have examined our results in the context of ionized, dusty-plasma environments
such as those found in planetary nebulae (PNe). We show that due to the
non-static equilibrium and finite equilibrium cooling, small-scale localized
structures formed out of thermal instability, become transient, which agrees
with the observational results. The dust-charge fluctuation is found to heavily
suppress these instabilities, though in view of non-availability of convincing
experimental data, a definitive conclusion could not be made.Comment: 23 pages, 14 figure
Trigonometric Parallaxes of Central Stars of Planetary Nebulae
Trigonometric parallaxes of 16 nearby planetary nebulae are presented,
including reduced errors for seven objects with previous initial results and
results for six new objects. The median error in the parallax is 0.42 mas, and
twelve nebulae have parallax errors less than 20 percent. The parallax for
PHL932 is found here to be smaller than was measured by Hipparcos, and this
peculiar object is discussed. Comparisons are made with other distance
estimates. The distances determined from these parallaxes tend to be
intermediate between some short distance estimates and other long estimates;
they are somewhat smaller than estimated from spectra of the central stars.
Proper motions and tangential velocities are presented. No astrometric
perturbations from unresolved close companions are detected.Comment: 24 pages, includes 4 figures. Accepted for A
Physical Structure of Planetary Nebulae. I. The Owl Nebula
The Owl Nebula is a triple-shell planetary nebula with the outermost shell
being a faint bow-shaped halo. We have obtained deep narrow-band images and
high-dispersion echelle spectra in the H-alpha, [O III], and [N II] emission
lines to determine the physical structure of each shell in the nebula. These
spatio-kinematic data allow us to rule out hydrodynamic models that can
reproduce only the nebular morphology. Our analysis shows that the inner shell
of the main nebula is slightly elongated with a bipolar cavity along its major
axis, the outer nebula is a filled envelope co-expanding with the inner shell
at 40 km/s, and the halo has been braked by the interstellar medium as the Owl
Nebula moves through it. To explain the morphology and kinematics of the Owl
Nebula, we suggest the following scenario for its formation and evolution. The
early mass loss at the TP-AGB phase forms the halo, and the superwind at the
end of the AGB phase forms the main nebula. The subsequent fast stellar wind
compressed the superwind to form the inner shell and excavated an elongated
cavity at the center, but has ceased in the past. At the current old age, the
inner shell is backfilling the central cavity.Comment: 10 pages, 6 figures, 1 table, to appear in the Astronomical Journa
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