139 research outputs found
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
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&
Post-AGB Stars in Globular Clusters and Galactic Halos
We discuss three aspects of post-AGB (PAGB) stars in old populations. (1) HST
photometry of the nucleus of the planetary nebula (PN) K 648 in the globular
cluster (GC) M15 implies a mass of 0.60 Msun, in contrast to the mean masses of
white dwarfs in GCs of ~0.5 Msun. This suggests that K 648 is descended from a
merged binary, and we infer that single Pop II stars do not produce visible
PNe. (2) Yellow PAGB stars are the visually brightest stars in old populations
(Mv ~ -3.3) and are easily recognizable because of their large Balmer jumps;
thus they show great promise as a Pop II standard candle. Two yellow PAGB stars
in the GC NGC 5986 have the same V magnitudes to within +/-0.05 mag, supporting
an expected narrow luminosity function. (3) Using CCD photometry and a u filter
lying below the Balmer jump, we have detected yellow PAGB stars in the halo of
M31 and in its dwarf elliptical companion NGC 205. With the Milky Way zero
point, we reproduce the Cepheid distance to M31, and find that NGC 205 is ~100
kpc further away than M31. The star counts imply a yellow PAGB lifetime of
about 25,000 yr, and their luminosities imply masses near 0.53 Msun.Comment: 6 pages, 2 figures. To appear in proceedings of Torun, Poland,
workshop on "Post-AGB Objects (Proto-Planetary Nebulae) as a Phase of Stellar
Evolution," ed. S.K. Gorn
The evolution of planetary nebulae IV. On the physics of the luminosity function
The nebular evolution is followed from the vicinity of the asymptotic-giant
branch across the Hertzsprung-Russell diagram until the white-dwarf domain is
reached, using various central-star models coupled to different initial
envelope configurations. Along each sequence the relevant line emissions of the
nebulae are computed and analysed. Maximum line luminosities in Hbeta and
[OIII] 5007A are achieved at stellar effective temperatures of about 65000K and
95000-100000K, respectively, provided the nebula remains optically thick for
ionising photons. In the optically thin case, the maximum line emission occurs
at or shortly after the thick/thin transition. Our models suggest that most
planetary nebulae with hotter (>~ 45000K) central stars are optically thin in
the Lyman continuum, and that their [OIII] 5007A emission fails to explain the
bright end of the observed planetary nebulae luminosity function. However,
sequences with central stars of >~ 0.6 Msun and rather dense initial envelopes
remain virtually optically thick and are able to populate the bright end of the
luminosity function. Individual luminosity functions depend strongly on the
central-star mass and on the variation of the nebular optical depth with time.
Hydrodynamical simulations of planetary nebulae are essential for any
understanding of the basic physics behind their observed luminosity function.
In particular, our models do not support the claim of Marigo et.al (2004)
according to which the maximum 5007A luminosity occurs during the recombination
phase well beyond 100 000K when the stellar luminosity declines and the nebular
models become, at least partially, optically thick. Consequently, there is no
need to invoke relatively massive central stars of, say > 0.7 Msun, to account
for the bright end of the luminosity function.Comment: 19 pages, 20 figures, A&A, in pres
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
Element Abundance Determination in Hot Evolved Stars
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class
PG1159 is probably caused by a (very) late helium-shell flash or a AGB final
thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden
intershell region. Thus, the photospheric element abundances of these stars
allow us to draw conclusions about details of nuclear burning and mixing
processes in the precursor AGB stars. We compare predicted element abundances
to those determined by quantitative spectral analyses performed with advanced
non-LTE model atmospheres. A good qualitative and quantitative agreement is
found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for
others (P, S, Fe) point at shortcomings in stellar evolution models for AGB
stars. Almost all of the chemical trace elements in these hot stars can only be
identified in the UV spectral range. The Far Ultraviolet Spectroscopic Explorer
and the Hubble Space Telescope played a crucial role for this research.Comment: To appear in: Recent Advances in Spectroscopy: Theoretical,
Astrophysical, and Experimental Perspectives, Proceedings, Jan 28 - 31, 2009,
Kodaikanal, India (Springer
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