306 research outputs found
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
The Evolution of NGC 7027 at Radio Frequencies: A New Determination of the Distance and Core Mass
We present the results of a 25-year program to monitor the radio flux
evolution of the planetary nebula NGC7027. We find significant evolution of the
spectral flux densities. The flux density at 1465 MHz, where the nebula is
optically thick, is increasing at a rate of 0.251+-0.015 % per year, caused by
the expansion of the ionized nebula. At frequencies where the emission is
optically thin, the spectral flux density is changing at a rate of
-0.145+-0.005 % per year, caused by a decrease in the number of ionizing
photons coming from the central star. A distance of 980+-100 pc is derived. By
fitting interpolated models of post-AGB evolution to the observed changes, we
find that over the 25-yr monitoring period, the stellar temperature has
increased by 3900+-900 K and the stellar bolometric luminosity has decreased by
1.75+-0.38 %. We derive a distance-independent stellar mass of 0.655+-0.01
solar masses adopting the Bloecker stellar evolution models, or about 0.04
solar masses higher when using models of Vassiliadis & Wood which may provide a
better fit. A Cloudy photoionization model is used to fit all epochs at all
frequencies simultaneously. The differences between the radio flux density
predictions and the observed values show some time-independent residuals of
typically 1 %. A possible explanation is inaccuracies in the radio flux scale
of Baars et al. We propose an adjustment to the flux density scale of the
primary radio flux calibrator 3C286, based on the Cloudy model of NGC7027. We
also calculate precise flux densities for NGC7027 for all standard continuum
bands used at the VLA, as well as for some new 30GHz experiments.Comment: submitted to the Astrophysical Journa
Design and status of the detector block for the ISO-SWS
The Short Wave Spectrometer (SWS) is one of the two spectrometers for the Infrared Space Observatory (ISO). It consists of a pair of grating spectrometers and a Fabry-Perot interferometer. Together, the grating spectrometers cover the wavelength range 2.4 to 45 microns, at a resolution between 1000 and 2000. The Fabry-Perot interferometer, in series with one of the grating spectrometers, provides a resolution of about 20,000 at the wavelengths between 15 and 35 microns. The SWS is being built by the Space Research Organization of the Netherlands and the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. The spectrometer has 52 discrete detectors, most of which are bulk detectors. In the design of the spectrometer, the main emphasis is on the sensitivity of the individual channels, rather than on the number of detectors. This was one of the main reasons to select non-destructive read-out circuits, with a separate heated-JFET pre-amplifier for each individual detector. The signals are amplified and filtered in parallel. The engineering tests on the SWS detector block have not yet been completed. The design of the detector block is described and the present problem areas are indicated
Infrared High-Resolution Spectroscopy of Post-AGB Circumstellar Disks. I. HR 4049 - The Winnowing Flow Observed?
High-resolution infrared spectroscopy in the 2.3-4.6 micron region is
reported for the peculiar A supergiant, single-lined spectroscopic binary HR
4049. Lines from the CO fundamental and first overtone, OH fundamental, and
several H2O vibration-rotation transitions have been observed in the
near-infrared spectrum. The spectrum of HR 4049 appears principally in emission
through the 3 and 4.6 micron region and in absorption in the 2 micron region.
The 4.6 micron spectrum shows a rich 'forest' of emission lines. All the
spectral lines observed in the 2.3-4.6 micron spectrum are shown to be
circumbinary in origin. The presence of OH and H2O lines confirm the
oxygen-rich nature of the circumbinary gas which is in contrast to the
previously detected carbon-rich material. The emission and absorption line
profiles show that the circumbinary gas is located in a thin, rotating layer
near the dust disk. The properties of the dust and gas circumbinary disk and
the spectroscopic orbit yield masses for the individual stars, M_AI~0.58 Msolar
and M_MV~0.34 Msolar. Gas in the disk also has an outward flow with a velocity
of 1 km/s. The severe depletion of refractory elements but near-solar
abundances of volatile elements observed in HR 4049 results from abundance
winnowing. The separation of the volatiles from the grains in the disk and the
subsequent accretion by the star are discussed. Contrary to prior reports, the
HR 4049 carbon and oxygen isotopic abundances are typical AGB values:
12C/13C=6^{+9}_{-4} and 16O/17O>200.Comment: 42 pages, 14 figures, Accepted by Ap
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