5,836 research outputs found
C/O white dwarfs of very low mass: 0.33-0.5 Mo
The standard lower limit for the mass of white dwarfs (WDs) with a C/O core
is roughly 0.5 Mo. In the present work we investigated the possibility to form
C/O WDs with mass as low as 0.33 Mo. Both the pre-WD and the cooling evolution
of such nonstandard models will be described.Comment: Submitted to the "Proceedings of the 16th European White Dwarf
Workshop" (to be published JPCS). 7 pages including 13 figure
The Araucaria Project. OGLE-LMC-CEP-1718: An exotic eclipsing binary system composed of two classical overtone Cepheids in a 413-day orbit
We have obtained extensive high-quality spectroscopic observations of the
OGLE-LMC-CEP-1718 eclipsing binary system in the Large Magellanic Cloud which
Soszynski et al. (2008) had identified as a candidate system for containing two
classical Cepheids in orbit. Our spectroscopic data clearly demonstrate binary
motion of the Cepheids in a 413-day eccentric orbit, rendering this eclipsing
binary system the first ever known to consist of two classical Cepheid
variables. After disentangling the four different radial velocity variations in
the system we present the orbital solution and the individual pulsational
radial velocity curves of the Cepheids. We show that both Cepheids are
extremely likely to be first overtone pulsators and determine their respective
dynamical masses, which turn out to be equal to within 1.5 %. Since the
secondary eclipse is not observed in the orbital light curve we cannot derive
the individual radii of the Cepheids, but the sum of their radii derived from
the photometry is consistent with overtone pulsation for both variables.
The existence of two equal-mass Cepheids in a binary system having different
pulsation periods (1.96 and 2.48 days, respectively) may pose an interesting
challenge to stellar evolution and pulsation theories, and a more detailed
study of this system using additional datasets should yield deeper insight
about the physics of stellar evolution of Cepheid variables. Future analysis of
the system using additional near-infrared photometry might also lead to a
better understanding of the systematic uncertainties in current Baade-Wesselink
techniques of distance determinations to Cepheid variables.Comment: accepted to be published in Ap
Design of Allosteric Stimulators of the Hsp90 ATPase as New Anticancer Leads
Allosteric compounds that stimulate Hsp90 adenosine triphosphatase (ATPase) activity were rationally designed, showing anticancer potencies in the low micromolar to nanomolar range. In parallel, the mode of action of these compounds was clarified and a quantitative model that links the dynamic ligand-protein cross-talk to observed cellular and in vitro activities was developed. The results support the potential of using dynamics-based approaches to develop original mechanism-based cancer therapeutics
Thermodynamical features of multifragmentation in peripheral Au + Au Collisions at 35 A.MeV
The distribution of fragments produced in events involving the
multifragmentation of excited sources is studied for peripheral Au + Au
reactions at 35 A.MeV.
The Quasi-Projectile has been reconstructed from its de-excitation products.
An isotropic emission in its rest frame has been observed, indicating that an
equilibrated system has been formed. The excitation energy of the
Quasi-Projectile has been determined via calorimetry.
A new event by event effective thermometer is proposed based on the energy
balance. A peak in the energy fluctuations is observed related to the heat
capacity, suggesting that the system undergoes a liquid-gas type phase
transition at an excitation energy about 5 A.MeV and a temperature 4 - 6 MeV,
dependent on the freeze-out hypothesis. By analyzing different regions of the
Campi-plot, the events associated with the liquid and gas phases as well as the
critical region are thermodynamically characterized.
The critical exponents, tau, beta,gamma, extracted from the high moments of
the charge distribution are consistent with a liquid-gas type phase transition.Comment: 44 pages, 16 Postscript figures, Fig14_nucl-ex.eps in colors, to be
published in Nucl.Phys.A (1999
Physical parameters and the projection factor of the classical Cepheid in the binary system OGLE-LMC-CEP-0227
A novel method of analysis of double-lined eclipsing binaries containing a
radially pulsating star is presented. The combined pulsating-eclipsing light
curve is built up from a purely eclipsing light curve grid created using an
existing modeling tool. For every pulsation phase the instantaneous radius and
surface brightness are taken into account, being calculated from the
disentangled radial velocity curve of the pulsating star and from its
out-of-eclipse pulsational light curve and the light ratio of the components,
respectively. The best model is found using the Markov Chain Monte Carlo
method.
The method is applied to the eclipsing binary Cepheid OGLE-LMC-CEP-0227
(P_puls = 3.80 d, P_orb = 309 d). We analyze a set of new spectroscopic and
photometric observations for this binary, simultaneously fitting OGLE V-band,
I-band and Spitzer 3.6 {\mu}m photometry. We derive a set of fundamental
parameters of the system significantly improving the precision comparing to the
previous results obtained by our group. The Cepheid mass and radius are M_1 =
4.165 +/- 0.032 M_solar and R_1 = 34.92 +/- 0.34 R_solar, respectively.
For the first time a direct, geometrical and distance-independent
determination of the Cepheid projection factor is presented. The value p = 1.21
+/- 0.03(stat.) +/- 0.04(syst.) is consistent with theoretical expectations for
a short period Cepheid and interferometric measurements for {\delta} Cep. We
also find a very high value of the optical limb darkening coefficients for the
Cepheid component, in strong disagreement with theoretical predictions for
static atmospheres at a given surface temperature and gravity.Comment: 16 pages, 17 figures, accepted for publication in MNRA
Chloride Ions in the Pore of Glycine and GABA Channels Shape the Time Course and Voltage Dependence of Agonist Currents
In the vertebrate CNS, fast synaptic inhibition is mediated by GABA and glycine receptors. We recently reported that the time course of these synaptic currents is slower when intracellular chloride is high. Here we extend these findings to measure the effects of both extracellular and intracellular chloride on the deactivation of glycine and GABA currents at both negative and positive holding potentials. Currents were elicited by fast agonist application to outside-out patches from HEK-293 cells expressing rat glycine or GABA receptors. The slowing effect of high extracellular chloride on current decay was detectable only in low intracellular chloride (4 mM). Our main finding is that glycine and GABA receptors "sense" chloride concentrations because of interactions between the M2 pore-lining domain and the permeating ions. This hypothesis is supported by the observation that the sensitivity of channel gating to intracellular chloride is abolished if the channel is engineered to become cation selective or if positive charges in the external pore vestibule are eliminated by mutagenesis. The appropriate interaction between permeating ions and channel pore is also necessary to maintain the channel voltage sensitivity of gating, which prolongs current decay at depolarized potentials. Voltage dependence is abolished by the same mutations that suppress the effect of intracellular chloride and also by replacing chloride with another permeant ion, thiocyanate. These observations suggest that permeant chloride affects gating by a foot-in-the-door effect, binding to a channel site with asymmetrical access from the intracellular and extracellular sides of the membrane
Towards an in vitro model mimicking the foreign body response: tailoring the surface properties of biomaterials to modulate extracellular matrix
Despite various studies to minimize host reaction following a biomaterial implantation, an appealing strategy in regenerative medicine is to actively use such an immune response to trigger and control tissue regeneration. We have developed an in vitro model to modulate the host response by tuning biomaterials' surface properties through surface modifications techniques as a new strategy for tissue regeneration applications. Results showed tunable surface topography, roughness, wettability, and chemistry by varying treatment type and exposure, allowing for the first time to correlate the effect of these surface properties on cell attachment, morphology, strength and proliferation, as well as proinflammatory (IL-1β, IL-6) and antiflammatory cytokines (TGF-β1, IL-10) secreted in medium, and protein expression of collagen and elastin. Surface microstructuring, derived from chloroform partial etching, increased surface roughness and oxygen content. This resulted in enhanced cell adhesion, strength and proliferation as well as a balance of soluble factors for optimum collagen and elastin synthesis for tissue regeneration. By linking surface parameters to cell activity, we could determine the fate of the regenerated tissue to create successful soft tissue-engineered replacement
Negative heat capacity in the critical region of nuclear fragmentation: an experimental evidence of the liquid-gas phase transition
An experimental indication of negative heat capacity in excited nuclear
systems is inferred from the event by event study of energy fluctuations in
quasi-projectile sources formed in collisions at 35 A.MeV. The
excited source configuration is reconstructed through a calorimetric analysis
of its de-excitation products. Fragment partitions show signs of a critical
behavior at about 5 A.MeV excitation energy. In the same energy range the heat
capacity shows a negative branch providing a direct evidence of a first order
liquid gas phase transition.Comment: 4 Postscript figures, submitted to Phys. Rev. Lett. on 14-apr-199
Nonlocal density functionals and the linear response of the homogeneous electron gas
The known and usable truly nonlocal functionals for exchange-correlation
energy of the inhomogeneous electron gas are the ADA (average density
approximation) and the WDA (weighted density approximation). ADA, by design,
yields the correct linear response function of the uniform electron gas. WDA is
constructed so that it is exact in the limit of one-electron systems. We derive
an expression for the linear response of the uniform gas in the WDA, and
calculate it for several flavors of WDA. We then compare the results with the
Monte-Carlo data on the exchange-correlation local field correction, and
identify the weak points of conventional WDA in the homogeneous limit. We
suggest how the WDA can be modified to improve the response function. The
resulting approximation is a good one in both opposite limits, and should be
useful for practical nonlocal density functional calculations.Comment: 4 pages, two eps figures embedde
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