928 research outputs found
The evolution of planetary nebulae VII. Modelling planetary nebulae of distant stellar systems
By means of hydrodynamical models we do the first investigations of how the
properties of planetary nebulae are affected by their metal content and what
can be learned from spatially unresolved spectrograms of planetary nebulae in
distant stellar systems. We computed a new series of 1D radiation-hydrodynamics
planetary nebulae model sequences with central stars of 0.595 M_sun surrounded
by initial envelope structures that differ only by their metal content. At
selected phases along the evolutionary path, the hydrodynamic terms were
switched off, allowing the models to relax for fixed radial structure and
radiation field into their equilibrium state with respect to energy and
ionisation. The analyses of the line spectra emitted from both the dynamical
and static models enabled us to systematically study the influence of
hydrodynamics as a function of metallicity and evolution. We also recomputed
selected sequences already used in previous publications, but now with
different metal abundances. These sequences were used to study the expansion
properties of planetary nebulae close to the bright cut-off of the planetary
nebula luminosity function. Our simulations show that the metal content
strongly influences the expansion of planetary nebulae: the lower the metal
content, the weaker the pressure of the stellar wind bubble, but the faster the
expansion of the outer shell because of the higher electron temperature. This
is in variance with the predictions of the interacting-stellar-winds model (or
its variants) according to which only the central-star wind is thought to be
responsible for driving the expansion of a planetary nebula. Metal-poor objects
around slowly evolving central stars become very dilute and are prone to depart
from thermal equilibrium because then adiabatic expansion contributes to gas
cooling. ...abridged abstract.Comment: 35 pages, 43 figures, accepted for publication by A&
Three-component modeling of C-rich AGB star winds. II. The effects of drift in long-period variables
We present three-component wind models for carbon rich pulsating AGB stars.
In particular we study the effects of drift in models of long-period variables,
meaning that the dust is allowed to move relative to the gas (drift models). In
addition we investigate the importance of the degree of variability of the wind
structures. The wind model contains separate conservation laws for each of the
three components of gas, dust and the radiation field. We use two different
representations for the gas opacity, resulting in models with different gas
densities in the wind. The effects which we investigate here are important for
the understanding of the wind mechanism and mass loss of AGB stars. This study
is hereby a necessary step towards more reliable interpretations of
observations. We find that the effects of drift generally are significant. They
cannot be predicted from models calculated without drift. Moreover, the
non-drift models showing the lowest mass loss rates, outflow velocities, and
the smallest variability in the degree of condensation do not form drift model
winds. The wind formation in drift models is, except for a few cases, generally
less efficient and the mass loss consequently lower than in the corresponding
non-drift models. The effects of drift are generally larger in the more
realistic models using that representation of the gas opacity which results in
lower densities. The outflow properties of these models are also -- for all
cases we have studied -- sensitive to the period of the stellar pulsations. A
check of the mass loss rates against a (recent) fit formula shows
systematically lower values, in particular in the more realistic models with a
low density. The fit is in its current form inapplicable to the new models
presented here.Comment: 19 pages, 8 figures, accepted by A&
Weak magnetic fields in central stars of planetary nebulae?
It is not yet clear whether magnetic fields play an essential role in shaping
planetary nebulae (PNe), or whether stellar rotation alone and/or a close
binary companion can account for the variety of the observed nebular
morphologies. In a quest for empirical evidence verifying or disproving the
role of magnetic fields in shaping PNe, we follow up on previous attempts to
measure the magnetic field in a representative sample of PN central stars. We
obtained low-resolution polarimetric spectra with FORS 2 at VLT for a sample of
twelve bright central stars of PNe with different morphology, including two
round nebulae, seven elliptical nebulae, and three bipolar nebulae. Two targets
are Wolf-Rayet type central stars. For the majority of the observed central
stars, we do not find any significant evidence for the existence of surface
magnetic fields. However, our measurements may indicate the presence of weak
mean longitudinal magnetic fields of the order of 100 Gauss in the central star
of the young elliptical planetary nebula IC 418, as well as in the Wolf-Rayet
type central star of the bipolar nebula Hen2-113 and the weak emission line
central star of the elliptical nebula Hen2-131. A clear detection of a 250 G
mean longitudinal field is achieved for the A-type companion of the central
star of NGC 1514. Some of the central stars show a moderate night-to-night
spectrum variability, which may be the signature of a variable stellar wind
and/or rotational modulation due to magnetic features. We conclude that strong
magnetic fields of the order of kG are not widespread among PNe central stars.
Nevertheless, simple estimates based on a theoretical model of magnetized wind
bubbles suggest that even weak magnetic fields below the current detection
limit of the order of 100 G may well be sufficient to contribute to the shaping
of PNe throughout their evolution.Comment: 16 pages, 11 figures, 3 tables, accepted for publication in A&A;
References updated, minor correction
p3d: a general data-reduction tool for fiber-fed integral-field spectrographs
The reduction of integral-field spectrograph (IFS) data is demanding work.
Many repetitive operations are required in order to convert raw data into,
typically a large number of, spectra. This effort can be markedly simplified
through the use of a tool or pipeline, which is designed to complete many of
the repetitive operations without human interaction. Here we present our
semi-automatic data-reduction tool p3d that is designed to be used with
fiber-fed IFSs. Important components of p3d include a novel algorithm for
automatic finding and tracing of spectra on the detector, and two methods of
optimal spectrum extraction in addition to standard aperture extraction. p3d
also provides tools to combine several images, perform wavelength calibration
and flat field data. p3d is at the moment configured for four IFSs. In order to
evaluate its performance we have tested the different components of the tool.
For these tests we used both simulated and observational data. We demonstrate
that for three of the IFSs a correction for so-called cross-talk due to
overlapping spectra on the detector is required. Without such a correction
spectra will be inaccurate, in particular if there is a significant intensity
gradient across the object. Our tests showed that p3d is able to produce
accurate results. p3d is a highly general and freely available tool. It is
easily extended to include improved algorithms, new visualization tools and
support for additional instruments. The program code can be downloaded from the
p3d-project web site http://p3d.sourceforge.netComment: 18 pages, 15 figures, 3 tables, accepted for publication in A&
Tilted two-fluid Bianchi type I models
In this paper we investigate expanding Bianchi type I models with two tilted
fluids with the same linear equation of state, characterized by the equation of
state parameter w. Individually the fluids have non-zero energy fluxes w.r.t.
the symmetry surfaces, but these cancel each other because of the Codazzi
constraint. We prove that when w=0 the model isotropizes to the future. Using
numerical simulations and a linear analysis we also find the asymptotic states
of models with w>0. We find that future isotropization occurs if and only if . The results are compared to similar models investigated previously
where the two fluids have different equation of state parameters.Comment: 14 pages, 3 figure
Perfect fluids and generic spacelike singularities
We present the conformally 1+3 Hubble-normalized field equations together
with the general total source equations, and then specialize to a source that
consists of perfect fluids with general barotropic equations of state.
Motivating, formulating, and assuming certain conjectures, we derive results
about how the properties of fluids (equations of state, momenta, angular
momenta) and generic spacelike singularities affect each other.Comment: Considerable changes have been made in presentation and arguments,
resulting in sharper conclusion
Measuring The Mass Loss Evolution at The Tip of The Asymptotic Giant Branch
In the final stages of stellar evolution low- to intermediate-mass stars lose
their envelope in increasingly massive stellar winds. Such winds affect the
interstellar medium and the galactic chemical evolution as well as the
circumstellar envelope where planetary nebulae form subsequently.
Characteristics of this mass loss depend on both stellar properties and
properties of gas and dust in the wind formation region. In this paper we
present an approach towards studies of mass loss using both observations and
models, focusing on the stage where the stellar envelope is nearly empty of
mass. In a recent study we measure the mass-loss evolution, and other
properties, of four planetary nebulae in the Galactic Disk. Specifically we use
the method of integral field spectroscopy on faint halos, which are found
outside the much brighter central parts of a planetary nebula. We begin with a
brief comparison between our and other observational methods to determine
mass-loss rates in order to illustrate how they differ and complement each
other. An advantage of our method is that it measures the gas component
directly requiring no assumptions of properties of dust in the wind. Thereafter
we present our observational approach in more detail in terms of its validity
and its assumptions. In the second part of this paper we discuss capabilities
and assumptions of current models of stellar winds. We propose and discuss
improvements to such models that will allow meaningful comparisons with our
observations. Currently the physically most complete models include too little
mass in the model domain to permit a formation of winds with as high mass-loss
rates as our observations show.Comment: 7 pages, workshop in honour of Agnes Acker, Legacies of the
Macquarie/AAO/Strasbourg Halpha Planetary Nebula project, ed. Q.Parker and
D.Frew, PASA, in press; clarified some parts and added some additional
reference
On the Epistemology of the Precautionary Principle: Reply to Steglich-Petersen
In a recent paper in this journal, we proposed two novel puzzles associated with the precautionary principle. Both are puzzles that materialise, we argue, once we investigate the principle through an epistemological lens, and each constitutes a philosophical hurdle for any proponent of a plausible version of the precautionary principle. Steglich-Petersen claims, also in this journal, that he has resolved our puzzles. In this short note, we explain why we remain skeptica
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