311 research outputs found
Modeling the diffuse X-ray emission of Planetary Nebulae with different chemical composition
Based on time-dependent radiation-hydrodynamics simulations of the evolution
of Planetary Nebulae (PNe), we have carried out a systematic parameter study to
address the non-trivial question of how the diffuse X-ray emission of PNe with
closed central cavities is expected to depend on the evolutionary state of the
nebula, the mass of the central star, and the metallicity of stellar wind and
circumstellar matter. We have also investigated how the model predictions
depend on the treatment of thermal conduction at the interface between the
central `hot bubble' and the `cool' inner nebula, and compare the results with
recent X-ray observations. Our study includes models whose properties resemble
the extreme case of PNe with Wolf-Rayet type central stars. Indeed, such models
are found to produce the highest X-ray luminosities.Comment: 5 pages, 2 figures, to appear in proceedings of the IAU Symposium
283: "Planetary Nebulae: An Eye to the Future", Eds.: A. Manchado, L.
Stanghellini and D. Schoenberne
Confronting expansion distances of planetary nebulae with Gaia DR2 measurements
Individual distances to planetary nebulae are of the utmost relevance for our
understanding of post-asymptotic giant-branch evolution because they allow a
precise determination of stellar and nebular properties. Also, objects with
individual distances serve as calibrators for the so-called statistical
distances based on secondary nebular properties. With independently known
distances, it is possible to check empirically our understanding of the
formation and evolution of planetary nebulae as suggested by existing
hydrodynamical simulations. We compared the expansion parallaxes that have
recently been determined for a number of planetary nebulae with the
trigonometric parallaxes provided by the Gaia Data Release 2. Except for two
out of 11 nebulae, we found good agreement between the expansion and the Gaia
trigonometric parallaxes without any systematic trend with distance. Therefore,
the Gaia measurements also prove that the correction factors necessary to
convert proper motions of shocks into Doppler velocities cannot be ignored.
Rather, the size of these correction factors and their evolution with time as
predicted by 1-D hydrodynamical models of planetary nebulae is basically
validated. These correction factors are generally greater than unity and are
different for the outer shell and the inner bright rim of a planetary nebula.
The Gaia measurements also confirm earlier findings that spectroscopic methods
often lead to an overestimation of the distance. They also show that even
modelling of the entire system of star and nebula by means of sophisticated
photoionization modeling may not always provide reliable results.
The Gaia measurements confirm the basic correctness of the present
radiation-hydrodynamics models, which predict that both the shell and the rim
of a planetary nebula are two independently expanding entities.Comment: Accepted by Astronomy & Astrophysics; 8 pages, 3 figures, 1 tabl
An Evaluation of the Excitation Class Parameter for the Central Stars of Planetary Nebulae
The three main methods currently in use for estimating the excitation class
of planetary nebulae (PNe) central stars are compared and evaluated using 586
newly discovered and previously known PNe in the Large Magellanic Cloud (LMC).
In order to achieve this we ran a series of evaluation tests using line ratios
derived from de-reddened, flux calibrated spectra. Pronounced differences
between the methods are exposed. Diagrams were created by comparing excitation
classes with H-beta line fluxes. The best methods are then compared to
published temperatures using the Zanstra method and assessed for their ability
to reflect central star effective temperatures and evolution. As a result we
call for a clarification of the term `excitation class' according to the
different input parameters used. The first method, which we refer to as Exneb
relies purely on the ratios of certain key emission lines. The second method,
which we refer to as Ex* includes modeling to create a continuous variable and,
for optically thick PNe in the Magellanic Clouds, is designed to relate more
closely to intrinsic stellar parameters. The third method, we refer to as Ex
[OIII]/H-beta since the [OIII]/H-beta ratio is used in isolation to other
temperature diagnostics. Each of these methods is shown to have serious
drawbacks when used as an indicator for central star temperature. Finally, we
suggest a new method (Exrho) for estimating excitation class incorporating both
the [OIII]/H-beta and the HeII4686 /H-beta ratios. Although any attempt to
provide accurate central star temperatures using the excitation class derived
from nebula lines will always be limited, we show that this new method provides
a substantial improvement over previous methods with better agreement to
temperatures derived through the Zanstra method.Comment: 14 pages, 18 figure. This peer reviewed paper has been accepted for
publication in PAS
The Ionization State of the Halo Planetary Nebula NGC 2438
NGC 2438 is a classical multiple shell or halo planetary nebula (PN). Its
central star and the main nebula are well studied. Also it was target of
various hydrodynamic simulations (Corradi et al. 2000). This initiated a
discussion whether the haloes are mainly containing recombined gas
(Schoenberner et al. 2002), or if they are still ionized (Armsdorfer et al.
2003). An analysis of narrow-band images and long slit spectra at multiple slit
positions was done to obtain a deeper look on morphological details and the
properties of the outer shell and halo. For this work there was data available
from ESO (direct imaging and long slit spectroscopy) and from SAAO
(spectroscopic observations using a small slit - scanning over the whole
nebula). Using temperature measurements from emission lines resulted in an
electron temperature which clearly indicates a fully ionized stage.
Additionally measurements of the electron density suggest a variation of the
filling factor.Comment: 3 pages, 1 figure, to appear in proceedings of the IAU Symposium 283:
"Planetary Nebulae: An Eye to the Future", Eds.: A. Manchado, L. Stanghellini
and D. Schoenberne
Hot bubbles of planetary nebulae with hydrogen-deficient winds I. Heat conduction in a chemically stratified plasma
Heat conduction has been found a plausible solution to explain discrepancies
between expected and measured temperatures in hot bubbles of planetary nebulae
(PNe). While the heat conduction process depends on the chemical composition,
to date it has been exclusively studied for pure hydrogen plasmas in PNe. A
smaller population of PNe show hydrogen-deficient and helium- and
carbon-enriched surfaces surrounded by bubbles of the same composition;
considerable differences are expected in physical properties of these objects
in comparison to the pure hydrogen case. The aim of this study is to explore
how a chemistry-dependent formulation of the heat conduction affects physical
properties and how it affects the X-ray emission from PN bubbles of
hydrogen-deficient stars. We extend the description of heat conduction in our
radiation hydrodynamics code to work with any chemical composition. We then
compare the bubble-formation process with a representative PN model using both
the new and the old descriptions. We also compare differences in the resulting
X-ray temperature and luminosity observables of the two descriptions. The
improved equations show that the heat conduction in our representative model of
a hydrogen-deficient PN is nearly as efficient with the chemistry-dependent
description; a lower value on the diffusion coefficient is compensated by a
slightly steeper temperature gradient. The bubble becomes somewhat hotter with
the improved equations, but differences are otherwise minute. The observable
properties of the bubble in terms of the X-ray temperature and luminosity are
seemingly unaffected.Comment: 11 pages, 11 figures, A&A in pres
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&
Recommended from our members
The dynamical evolution of planetary nebulae
Based on modern 1D-radiation-hydrodynamics simulations of formation and evolution of planetary nebulae, I discuss in detail the basic dynamical processes responsible for the "grand design" of most planetary nebulae, i.e. their double-shell morphology and their typical expansion properties. Special emphasis is given for a proper definition of a nebula's true expansion rate and its relation to spectroscopically measurable Doppler velocities of the expanding material. It is found that the typical nebular expansion is about twice as fast as hitherto assumed, viz. â45 kms-1
The Planetary Nebula Luminosity Function: Pieces of the Puzzle
Extragalactic surveys in the emission line of [O III] 5007 have provided us
with the absolute line strengths of large, homogeneous sets of planetary
nebulae. These data have been used to address a host of problems, from the
measurement of the extragalactic distance scale, to the study of stellar
populations. I review our current understanding of the [O III] planetary nebula
luminosity function (PNLF), and discuss some of the physical processes that
effect its structure. I also describe the features of the H-alpha PNLF, a
function that, upon first glance, looks similar to the [O III] PNLF, but which
includes a very different set of objects. Finally, I discuss recent
measurements of alpha, the number of PNe found in a stellar population,
normalized to that population's bolometric luminosity. I show that, contrary to
expectations, the values of alpha found in actively star-forming spirals is
essentially the same as those measured in late-type elliptical and lenticular
systems. I discuss how this result sheds light on the physics of the planetary
nebula phenomenon.Comment: 7 pages, including 7 figures; presentation at the workshop on the
Legacies of the Macquarie/AAO/Strasbourg H-alpha Planetary Nebula project,
accepted for publication in PAS
Hot bubbles of planetary nebulae with hydrogen-deficient winds - II. Analytical approximations with application to BD+303639
The first high-resolution X-ray spectroscopy of the planetary nebula
BD+303639 allowed to study X-ray emitting "hot bubbles" (HBs) of
planetary nebulae in unprecedented detail. We investigate (i) how diagnostic
line ratios are affected by the HB thermal structure and chemical profile, (ii)
if the HB chemical composition of BD+303639 is consistent with the
H-poor (H for hydrogen) composition of the stellar photosphere, and (iii) if
H-rich nebular matter has been added to this HB by evaporation. We apply an
analytical, 1D model for wind-blown HBs with temperature and density profiles
based on self-similar solutions including thermal conduction. We construct
heat-conduction HBs with chemical stratification. The X-ray emission is
computed using the CHIANTI code. Our HB models are used to re-analyse the
high-resolution X-ray spectrum of BD+303639. Our models reproduce the
observed line ratios much better than plasmas with single electron
temperatures. All the temperature- and abundance-sensitive line ratios are
consistent with BD+303639 X-ray observations for (i) an intervening
column density of neutral H, , (ii) a characteristic HB
X-ray temperature of MK together with (iii) a very
high neon mass fraction of about 0.05, virtually as high as that of oxygen. For
lower values of , we cannot exclude that the HB of BD+303639
contains a small amount of evaporated (or mixed) H-rich nebular matter. Given
the possible range of , the fraction of evaporated H-rich matter
cannot exceed 3% of the HB mass. The diffuse X-ray emission from
BD+303639 can well be explained by models of wind-blown HBs with
thermal conduction and a chemical composition equal to that of the H-poor and
carbon-, oxygen-, and neon-rich stellar surface.Comment: 24 pages, 19 figures (col and b/w), 4 tables, accepted for
publication in A&A, Fig. 18 adapted to accepted versio
- âŠ