300 research outputs found
An Infrared Imaging Study of the Bipolar Proto-Planetary Nebula IRAS 16594-4656
High-resolution mid-infrared images have been obtained in N-band and Q-band
for the proto-planetary nebula IRAS 16594-4656. A bright equatorial torus and a
pair of bipolar lobes can clearly be seen in the infrared images. The torus
appears thinner at the center than at the edges, suggesting that it is viewed
nearly edge-on. The infrared lobes correspond to the brightest lobes of the
reflection nebula seen in the Hubble Space Telescope (HST) optical image, but
with no sign of the point-symmetric structure seen in the visible image. The
lobe structure shows a close correspondence with a molecular hydrogen map
obtained with HST, suggesting that the dust emission in the lobes traces the
distribution of the shocked gas. The shape of the bipolar lobes shows clearly
that the fast outflow is still confined by the remnant circumstellar envelope
of the progenitor asymptotic giant branch (AGB) star. However, the
non-detection of the dust outside of the lobes suggests that the temperature of
the dust in the AGB envelope is too low for it to be detected at 20 microns.Comment: Accepted by the Astrophysical Journa
Where Are the Binaries? Results of a Long-Term Search for Radial Velocity Binaries in Proto-Planetary Nebulae
We present the results of an expanded, long-term radial velocity search (25
yrs) for evidence of binarity in a sample of seven bright proto-planetary
nebulae (PPNe). The goal is to investigate the widely-held view that the
bipolar or point-symmetric shapes of planetary nebulae (PNe) and PPNe are due
to binary interactions. Observations from three observatories were combined
from 2007-2015 to search for variations on the order of a few years and then
combined with earlier observations from 1991-1995 to search for variations on
the order of decades. All seven show velocity variations due to periodic
pulsation in the range of 35-135 days. However, in only one PPN, IRAS
22272+5435, did we find even marginal evidence found for multi-year variations
that might be due to a binary companion. This object shows
marginally-significant evidence of a two-year period of low semi-amplitude
which could be due to a low-mass companion, and it also displays some evidence
of a much longer period of >30 years. The absence of evidence in the other six
objects for long-period radial velocity variations due to a binary companion
sets significant constraints on the properties of any undetected binary
companions: they must be of low mass, 30 years.
Thus the present observations do not provide direct support for the binary
hypothesis to explain the shapes of PNe and PPNe and severely constrains the
properties of any such undetected companions.Comment: 28 pages, 5 figure
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
HST Snapshot Survey of Post-AGB Objects
The results from a Hubble Space Telescope (HST) snapshot survey of post-AGB
objects are shown. The aim of the survey is to complement existing HST images
of PPN and to connect various types of nebulosities with physical and chemical
properties of their central stars. Nebulosities are detected in 15 of 33
sources. Images and photometric and geometric measurements are presented. For
sources with nebulosities we see a morphological bifurcation into two groups,
DUPLEX and SOLE, as previous studies have found. We find further support to the
previous results suggesting that this dichotomy is caused by a difference in
optical thickness of the dust shell. The remaining 18 sources are classified as
stellar post-AGB objects, because our observations indicate a lack of
nebulosity. We show that some stellar sources may in fact be DUPLEX or SOLE
based on their infrared colors. The cause of the differences among the groups
are investigated. We discuss some evidence suggesting that high progenitor-mass
AGB stars tend to become DUPLEX post-AGB objects. Intermediate progenitor-mass
AGB stars tend to be SOLE post-AGB objects. Most of the stellar sources
probably have low mass progenitors and do not seem to develop nebulosities
during the post-AGB phase and therefore do not become planetary nebulae.Comment: 21 pages, 11 figure
A Herschel study of Planetary Nebulae
We present Herschel PACS and SPIRE images of the dust shells around the
planetary nebulae NGC 650, NGC 6853, and NGC 6720, as well as images showing
the dust temperature in their shells. The latter shows a rich structure, which
indicates that internal extinction in the UV is important despite the highly
evolved status of the nebulae.Comment: 2 pages, 1 figure, 2012, proceedings IAU Symposium 283 Planetary
Nebulae: An Eye to the Futur
Proper Motions of H2O Masers in the Water Fountain Source IRAS 19190+1102
We report on the results of two epochs of Very Long Baseline Array (VLBA)
observations of the 22 GHz water masers toward IRAS 19190+1102. The water maser
emission from this object shows two main arc-shaped formations perpendicular to
their NE-SW separation axis. The arcs are separated by ~280 mas in position,
and are expanding outwards at an angular rate of 2.35 mas/yr. We detect maser
emission at velocities between -53.3 km/s to +78.0 km/s and there is a distinct
velocity pattern where the NE masers are blueshifted and the SW masers are
redshifted. The outflow has a three-dimensional outflow velocity of 99.8 km/s
and a dynamical age of about 59 yr. A group of blueshifted masers not located
along the arcs shows a change in velocity of more than 35 km/s between epochs,
and may be indicative of the formation of a new lobe. These observations show
that IRAS 19190+1102 is a member of the class of "water fountain"'
pre-planetary nebulae displaying bipolar structureComment: Accepted for publication in ApJ, corrected typo
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