15,664 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
Evaluating local correlation tracking using CO5BOLD simulations of solar granulation
Flows on the solar surface are linked to solar activity, and LCT is one of
the standard techniques for capturing the dynamics of these processes by
cross-correlating solar images. However, the link between contrast variations
in successive images to the underlying plasma motions has to be quantitatively
confirmed. Radiation hydrodynamics simulations of solar granulation
(e.g.,CO5BOLD) provide access to both the wavelength-integrated, emergent
continuum intensity and the 3D velocity field at various heights in the solar
atmosphere. Thus, applying LCT to continuum images yields horizontal proper
motions, which are then compared to the velocity field of the simulated
(non-magnetic) granulation. In this study, we evaluate the performance of an
LCT algorithm previously developed for bulk-processing Hinode G-band images,
establish it as a quantitative tool for measuring horizontal proper motions,
and clearly work out the limitations of LCT or similar techniques designed to
track optical flows. Horizontal flow maps and frequency distributions of the
flow speed were computed for a variety of LCT input parameters including the
spatial resolution, the width of the sampling window, the time cadence of
successive images, and the averaging time used to determine persistent flow
properties. Smoothed velocity fields from the hydrodynamics simulation at three
atmospheric layers (log tau=-1,0,and +1) served as a point of reference for the
LCT results. LCT recovers many of the granulation properties, e.g.,the shape of
the flow speed distributions, the relationship between mean flow speed and
averaging time, and also--with significant smoothing of the simulated velocity
field--morphological features of the flow and divergence maps. However, the
horizontal proper motions are grossly underestimated by as much as a factor of
three. The LCT flows match best the flows deeper in the atmosphere at log
tau=+1.Comment: 11 pages, 16 figures, accepted for publication in Astronomy and
Astrophysic
Optimal decision making for sperm chemotaxis in the presence of noise
For navigation, microscopic agents such as biological cells rely on noisy
sensory input. In cells performing chemotaxis, such noise arises from the
stochastic binding of signaling molecules at low concentrations. Using
chemotaxis of sperm cells as application example, we address the classic
problem of chemotaxis towards a single target. We reveal a fundamental
relationship between the speed of chemotactic steering and the strength of
directional fluctuations that result from the amplification of noise in the
chemical input signal. This relation implies a trade-off between slow, but
reliable, and fast, but less reliable, steering.
By formulating the problem of optimal navigation in the presence of noise as
a Markov decision process, we show that dynamic switching between reliable and
fast steering substantially increases the probability to find a target, such as
the egg. Intriguingly, this decision making would provide no benefit in the
absence of noise. Instead, decision making is most beneficial, if chemical
signals are above detection threshold, yet signal-to-noise ratios of gradient
measurements are low. This situation generically arises at intermediate
distances from a target, where signaling molecules emitted by the target are
diluted, thus defining a `noise zone' that cells have to cross.
Our work addresses the intermediate case between well-studied perfect
chemotaxis at high signal-to-noise ratios close to a target, and random search
strategies in the absence of navigation cues, e.g. far away from a target. Our
specific results provide a rational for the surprising observation of decision
making in recent experiments on sea urchin sperm chemotaxis. The general theory
demonstrates how decision making enables chemotactic agents to cope with high
levels of noise in gradient measurements by dynamically adjusting the
persistence length of a biased persistent random walk.Comment: 9 pages, 5 figure
ER-mitochondria contacts: Actin dynamics at the ER control mitochondrial fission via calcium release.
The formin-like protein INF2 is an important player in the polymerization of actin filaments. In this issue, Chakrabarti et al. (2018. J. Cell Biol. https://doi.org/10.1083/jcb.201709111) demonstrate that INF2 mediates actin polymerization at the endoplasmic reticulum (ER), resulting in increased ER-mitochondria contacts, calcium uptake by mitochondria, and mitochondrial division
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
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