10,396 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. VIII. True expansion rates and visibility times
The visibility time of planetary nebulae (PNe) in stellar systems is an
essential quantity for estimating the size of a PN population in the context of
general population studies. For instance, it enters directly into the PN death
rate determination. The basic ingredient for determining visibility times is
the typical nebular expansion velocity, as a suited average over all PN sizes
of a PN population within a certain volume or stellar system. The true
expansion speed of the outer nebular edge of a PN is, however, not accessible
by spectroscopy -- a difficulty that we surmount by radiation-hydrodynamics
modelling. We find a mean true expansion velocity of 42 km/s, i.e. nearly twice
as high as the commonly adopted value to date. Accordingly, the time for a PN
to expand to a radius of, say 0.9 pc, is only 21000 +/- 5000 years. This
visibility time of a PN holds for all central star masses since a nebula does
not become extinct as the central star fades. There is, however, a dependence
on metallicity in the sense that the visibility time becomes shorter for lower
nebular metal content. With the higher expansion rate of PNe derived here we
determined their local death-rate density as (1.4 +/- 0.5) x E-12 PN pc^{-3}
yr^{-1}, using the local PN density advocated by Frew (2008).Comment: 20 pages, 10 Figures; accepted for publication in Astronomy &
Astrophysics / Note added in proo
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&
Static-test results from exhaust nozzles with novel features for supersonic-aircraft applications
Individual Microscopic Results Of Bottleneck Experiments
This contribution provides microscopic experimental study of pedestrian
motion in front of the bottleneck, explains the high variance of individual
travel time by the statistical analysis of trajectories. The analysis shows
that this heterogeneity increases with increasing occupancy. Some participants
were able to reach lower travel time due more efficient path selection and more
aggressive behavior within the crowd. Based on this observations, linear model
predicting travel time with respect to the aggressiveness of pedestrian is
proposed.Comment: Submitted to Traffic and Granullar Flow 2015, Springe
The Credibility Revolution in Empirical Economics: How Better Research Design is Taking the Con out of Econometrics
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