423 research outputs found
Binary life after the AGB - towards a unified picture
A unified evolutionary scheme that includes post-AGB systems, barium stars,
symbiotics, and related systems, explaining their similarites as well as
differences. Can we construct it? We compare these various classes of objects
in order to construct a consistent picture. Special attention is given to the
comparison of the barium pollution and symbiotic phenomena. Finally, we outline
a `transient torus' evolutionary scenario that makes use of the various
observational and theoretical hints and aims at explaining the observed
characteristics of the relevant systems.Comment: 8 pages, 3 figures, uses baltlat5.sty. Invited review, to appear in a
special issue of the Baltic Astronomy - the proceedings of "Evolution and
chemistry of symbiotic stars, binary post-AGB and related objects" (Wierzba,
Poland, 28-30 August 2006). Also available at
http://www.astro.ulb.ac.be/Html/ps.html#PR
Spectroscopic binaries among Hipparcos M giants II. Binary frequency
This paper is the second one in a series devoted to the study of properties
of binaries involving M giants. The binary frequency of field M giants is
derived and compared with the binary fraction of K giants. Diagrams of the
CORAVEL spectroscopic parameter Sb (measuring the average line-width) vs.
radial-velocity standard deviation for our samples are used to define
appropriate binarity criteria. These then serve to extract the binarity
fraction among the M giants. Comparison is made to earlier data on K giants
binarity frequency. The Sb parameter is discussed in relation to global stellar
parameters and the Sb vs. stellar radius relation is used to identify fast
rotators. We find that the spectroscopic binary detection rate among field M
giants, in a sample with a low number of velocity measurements (~2), unbiased
toward earlier known binaries, is 6.3%. This is less than half of the analogous
rate for field K giants, likely resulting from a real difference. This
difference originates in the greater difficulty of finding binaries among M
giants because of their smaller orbital velocity amplitudes and larger
intrinsic jitter and in the different distributions of K and M giants in the
eccentricity-period diagram. A larger detection rate was obtained in a smaller
M giant sample with more radial velocity measurements per object: 11.1%
confirmed plus 2.7% possible binaries. The CORAVEL spectroscopic parameter Sb
was found to correlate better with the stellar radius than with either
luminosity or effective temperature separately. Two outliers of the Sb vs.
stellar radius relation, HD 190658 and HD 219654, have been recognized as fast
rotators. The rotation is companion-induced, as both objects turn out to be
spectroscopic binaries.Comment: 12 pages, 7 figures, accepted for publication in A&A, language
editing changes onl
Stabilizing the Retromer Complex in a Human Stem Cell Model of Alzheimer's Disease Reduces TAU Phosphorylation Independently of Amyloid Precursor Protein.
Developing effective therapeutics for complex diseases such as late-onset, sporadic Alzheimer's disease (SAD) is difficult due to genetic and environmental heterogeneity in the human population and the limitations of existing animal models. Here, we used hiPSC-derived neurons to test a compound that stabilizes the retromer, a highly conserved multiprotein assembly that plays a pivotal role in trafficking molecules through the endosomal network. Using this human-specific system, we have confirmed previous data generated in murine models and show that retromer stabilization has a potentially beneficial effect on amyloid beta generation from human stem cell-derived neurons. We further demonstrate that manipulation of retromer complex levels within neurons affects pathogenic TAU phosphorylation in an amyloid-independent manner. Taken together, our work demonstrates that retromer stabilization is a promising candidate for therapeutic development in AD and highlights the advantages of testing novel compounds in a human-specific, neuronal system
Radiation pressure and pulsation effects on the Roche lobe
Several observational pieces of evidence indicate that specific evolutionary
channels which involve Roche lobe overflow are not correctly accounted for by
the classical Roche model. We generalize the concept of Roche lobe in the
presence of extra forces (caused by radiation pressure or pulsations). By
computing the distortion of the equipotential surfaces, we are able to evaluate
the impact of these perturbing forces on the stability of Roche-lobe overflow
(RLOF). Radiative forces are parametrized through the constant reduction factor
that they impose on the gravitational force from the radiating star (neglecting
any shielding in case of large optical thickness). Forces imparted by
pulsations are derived from the velocity profile of the wind that they trigger.
We provide analytical expressions to compute the generalized Roche radius.
Depending on the extra force, the Roche-lobe radius may either stay unchanged,
become smaller, or even become meaningless (in the presence of a radiatively-
or pulsation-driven wind). There is little impact on the RLOF stability.Comment: 11 pages, 13 Postscript figure
Are planetary nebulae derived from multiple evolutionary scenarios?
Our understanding of planetary nebulae has been significantly enhanced as a
result of several recent large surveys (Parker et al., these proceedings).
These new discoveries suggest that the `PN phenomenon' is in fact more
heterogeneous than previously envisaged. Even after the careful elimination of
mimics from Galactic PN catalogues, there remains a surprising diversity in the
population of PNe and especially their central stars. Indeed, several
evolutionary scenarios are implicated in the formation of objects presently
catalogued as PNe. We provide a summary of these evolutionary pathways and give
examples of each. Eventually, a full census of local PNe can be used to
confront both stellar evolution theory and population synthesis models.Comment: 4 pages, 1 figure. To be published in Planetary Nebulae: an Eye to
the Future, Proceedings of IAU Symposium 283, held in Puerto de la Cruz,
Tenerife, Spain, July 25-29 201
Detection methods of binary stars with low- and intermediate-mass components
This paper reviews methods which can be used to detect binaries involving
low- and intermediate-mass stars, with special emphasis on evolved systems.
Besides the traditional methods involving radial-velocity or photometric
monitoring, the paper discusses as well less known methods involving astrometry
or maser (non-)detection. An extensive list of internet resources (mostly
catalogues/databases of orbits and individual measurements) for the study of
binary stars is provided at the end of the paper.Comment: 53 pages, 30 figures. v2: Eq. 7 modified. A file with full-resolution
figures is available at http://www.astro.ulb.ac.be/Html/ps.html#Binaries To
appear in P. Pellegrini (ed.), XII Special Courses at the National
Observatory of Rio de Janeiro, AIP Conference Proceedings. After it is
published, it will be found at http://link.aip.org/link/?ap
Statistics of multiphoton events in spontaneous parametric down-conversion
We present an experimental characterization of the statistics of multiple
photon pairs produced by spontaneous parametric down-conversion realized in a
nonlinear medium pumped by high-energy ultrashort pulses from a regenerative
amplifier. The photon number resolved measurement has been implemented with the
help of a fiber loop detector. We introduce an effective theoretical
description of the observed statistics based on parameters that can be assigned
direct physical nterpretation. These parameters, determined for our source from
the collected experimental data, characterize the usefulness of down-conversion
sources in multiphoton interference schemes that underlie protocols for quantum
information processing and communication
The Shapes of AGB Envelopes as Probes of Binary Companions
We describe how the large scale geometry of the circumstellar envelopes of
asymptotic giant branch stars can be used to probe the presence of unseen
stellar companions. A nearby companion modifies the mass loss by
gravitationally focusing the wind towards the orbital plane, and thereby
determines the shape of the envelope at large distances from the star. Using
available simulations, we develop a prescription for the observed shapes of
envelopes in terms of the binary parameters, envelope orientation, and type of
observation. The prescription provides a tool for the analysis of envelope
images at optical, infrared, and millimetre wavelengths, which can be used to
constrain the presence of companions in well observed cases. We illustrate this
approach by examining the possible role of binary companions in triggering the
onset of axi-symmetry in planetary nebula formation. If interaction with the
primary leads to axi-symmetry, the spherical halos widely seen around newly
formed nebulae set limits on the companion mass. Only low mass objects may
orbit close to the primary without observable shaping effects: they remain
invisible until the interaction causes a sudden change in the mass loss
geometry.Comment: 11 pages, 7 figures, to appear in MNRA
Spectroscopic binaries among Hipparcos M giants III. The eccentricity-period diagram and mass-transfer signatures
This paper is the third one in a series devoted to studying the properties of
binaries involving M giants. We use a new set of orbits to construct the first
(e-logP) diagram of an extensive sample of M giant binaries, to obtain their
mass-function distribution, and to derive evolutionary constraints for this
class of binaries and related systems. The orbital properties of binaries
involving M giants were analysed and compared with those of related families of
binaries (K giants, post-AGB stars, barium stars, Tc-poor S stars). The orbital
elements of post-AGB stars and M giants are not different, which may very
indicate that, for the considered sample of post-AGB binaries, the post-AGB
star left the AGB at quite an early stage (M4 or so). Neither are the orbital
elements of post-mass-transfer binaries like barium stars very different from
those of M giants, suggesting that the mass transfer did not alter the orbital
elements much, contrary to current belief. Finally, we show that binary systems
with e < 0.4 log P - 1 (with periods expressed in days) are predominantly
post-mass-transfer systems, because (i) the vast majority of barium and S
systems match this condition, and (ii) these systems have companion masses
peaking around 0.6 solar mass, as expected for white dwarfs. The latter
property has been shown to hold as well for open-cluster binaries involving K
giants, for which a lower bound on the companion mass may easily be set.Comment: 14 pages, 12 figures, accepted for publication in A&A, language
editing changes onl
Recommended from our members
Polyglutamine-expanded androgen receptor interferes with TFEB to elicit autophagy defects in SBMA.
Macroautophagy (hereafter autophagy) is a key pathway in neurodegeneration. Despite protective actions, autophagy may contribute to neuron demise when dysregulated. Here we consider X-linked spinal and bulbar muscular atrophy (SBMA), a repeat disorder caused by polyglutamine-expanded androgen receptor (polyQ-AR). We found that polyQ-AR reduced long-term protein turnover and impaired autophagic flux in motor neuron-like cells. Ultrastructural analysis of SBMA mice revealed a block in autophagy pathway progression. We examined the transcriptional regulation of autophagy and observed a functionally significant physical interaction between transcription factor EB (TFEB) and AR. Normal AR promoted, but polyQ-AR interfered with, TFEB transactivation. To evaluate physiological relevance, we reprogrammed patient fibroblasts to induced pluripotent stem cells and then to neuronal precursor cells (NPCs). We compared multiple SBMA NPC lines and documented the metabolic and autophagic flux defects that could be rescued by TFEB. Our results indicate that polyQ-AR diminishes TFEB function to impair autophagy and promote SBMA pathogenesis
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