14 research outputs found
Formation and Destiny of White Dwarf and Be Star Binaries
The binary systems consisting of a Be star and a white dwarf (BeWDs) are very
interesting.They can originate from the binaries composed of a Be star and a
subdwarf O or B star (BesdOBs), and they can merge into red giants via luminous
red nova or can evolve into double WD potentially detected by mission.
Using the method of population synthesis, we investigate the formation and the
destiny of BeWDs,and discuss the effects of the metallicity () and the
common envelope evolution parameters. We find that BesdOBs are significant
progenitors of BeWDs. About 30\% ()-50\% () of BeWDs come
from BesdOBs. About 60\% () -70\% () of BeWDs turn into red
giants via a merger between a WD and a non-degenerated star. About 30\%
() -40\% () of BeWDs evolve into double WDs which are
potential gravitational waves of mission at a frequency band between
about and Hz. The common envelope evolution
parameter introduces an uncertainty with a factor of about 1.3 on BeWD
populations in our simulations.Comment: 17 pages, 12 figures, 2 table, accepted for publication in RA
Hydrogen-free Wolf-Rayet stars: Helium stars with envelope-inflation structure and rotation
Observations have shown that the effective temperature of hydrogen-free
Wolf-Rayet (WR) stars is considerably lower than that of the standard model,
which means that the radius of the observed H-free WR stars is several times
larger than that estimated by the standard model. The envelope inflation
structure (EIS) caused by the radiation luminosity being close to the Eddington
luminosity in the iron opacity peak region of H-free WR stars may be the key to
resolve the radius problem of H-free WR stars. We try to explain the H-free WR
stars observed in the Milk Way (MW) and the Large Magellanic Cloud (LMC) by the
He stars. Using the Modules for Experiments in Stellar Astrophysics code, we
compute the evolution of He stars with and without MLT++ prescriptions and
discuss their effects on the EIS. We have calculated the evolution of He stars
using a new mass-loss rate formula and three different relative rotational
velocity and compared our results with observations on Hertzsprung-Russell
diagrams. The low luminosity (log) H-free WR stars in
the MW and the LMC can be explained by the helium giant phase in low-mass He
stars, the high and in WC stars can only evolve through
low-mass He stars with a rapid rotation. High-mass He stars with the EIS can
explain H-free WR stars with a luminosity exceeding and
an effective temperature above K in the MW. They can also explain
H-free WR stars on the right-hand side of the He zero-age main sequence in the
LMC. High-mass stars with the EIS evolve into WO stars at the final evolution
stage, and the shorter lifetime fraction is consistent with the small number of
observed WO stars.Comment: 9 pages, 7 figures 1 tables, Accepted to A&
Li-rich and super Li-rich giants produced by element diffusion
Context. About 0.2-2% of giant stars are Li-rich, whose lithium abundance
(A(Li)) is higher than 1.5 dex. Among them, near 6% are super Li-rich with
A(Li) exceeding 3.2 dex. Meanwhile, the formation mechanism of these Li-rich
and super Li-rich giants is still under debate. Aims. Considering the compact
He core of red giants, attention is paid to the effect of element diffusion on
A(Li). In particular, when the He core flash occurs, the element diffusion
makes the thermohaline mixing zone extend inward and connect to the inner
convection region of stars. Then, a large amount of 7Be produced by the He
flash can be transferred to stellar surface, finally turning into 7Li. Thus,
the goal of this work is to propose the mechanism of A(Li) enrichment and
achieve the consistency between the theoretical and observation data. Methods.
Using the Modules for Experiments in Stellar Astrophysics (MESA), we simulate
the evolution of low-mass stars, with considering the effects of element
diffusion on the Li abundances. The timescale ratio of Li-rich giants to normal
giants is estimated by population synthesis method. Then we get the theoretical
value of A(Li) and make a comparison with observations. Results. Considering
the influence of element diffusion in the model results in the increase of
lithium abundance up to about 1.8dex, which can reveal Li-rich giants.
Simultaneously, introducing high constant diffusive mixing coefficients (Dmix)
with the values from 10e11 to 10e15in the model allows A(Li) to increase from
2.4 to 4.5dex, which can explain the most of Li-rich and super Li-rich giant
stars. The population synthesis method reveals that the amount of Li-rich
giants among giants is about 0.2-2%, which is consistent with observation
estimated levels
Performance of Impregnated Paper Decorated Blockboard Manufactured Using HDF as Equilibrium Layer
In order to further improve the performance of impregnated paper decorated blockboard (ecological board), high-density fiberboard (HDF) was selected as the equilibrium layer to replace the commonly used poplar veneer. Results showed that the performance of HDF ecological board can be comparable to that of poplar veneer ecological board. It had good appearance quality, and its surface scratch resistance, surface wear resistance, water resistance and mechanical properties met the requirements of National Standard GB/T 34722-2017. The surface cracking resistance of the ecological board prepared with HDF as the equilibrium layer reached the highest level (grade 5), far better than that of the poplar veneer ecological board. This was because HDF was a relatively homogeneous material, and its dry shrinkage in both the transverse direction and along the grain direction was much lower than that of the poplar veneer. This characteristic of HDF made it possible to improve the dimensional stability and bending resistance of blockboard substrates under dry and hot conditions. The formaldehyde emission of the HDF ecological board was higher than that of the poplar veneer ecological board, but it met the formaldehyde emission requirements of indoor materials according to GB 18580-2001
Direct Structure Determination from Spherulites using 3D Electron Diffraction
The spherulitic morphology is considered to be the most common morphology of crystalline materials and is particularly apparent in melt-crystallized products. Yet, historically, the polycrystalline nature of spherulites has hindered successful crystal structure determination. Here, we report for the first time the direct structure determination of a small molecule organic compound in spherulite form using 3D electron diffraction (3D ED). We employed vemurafenib (VMN), a clinical drug used for the treatment of BRAF-mutant melanoma, as a model compound. VMN has four known polymorphs (α-, β-, γ-, and δ-VMN), three of which were discovered by melt crystallization. We first solved the crystal structures of α-, β-, and γ-VMN from both open and compact spherulite samples using 3D ED, and the resulting structures were highly consistent with those solved by single-crystal X-ray diffraction. We then determined the previously unknown crystal structure of δ-VMN—the least stable polymorph which cannot be cultivated as a single crystal—directly from the spherulite sample resulting from spontaneous nucleation. We unexpectedly discovered a new polymorph during our studies, denoted as Form ε. Single crystals of ε-VMN are extremely thin and are not suitable for study by X-ray diffraction. Again, we determined the structure of ε-VMN from both open and compact spherulite forms. This successful structure elucidation of all five VMN polymorphs demonstrates the possibility of removing the time-consuming step of single crystal growth and directly determining structures from spherulite samples. Thereby, this discovery will improve the efficiency and broaden the scope of polymorphism research, especially within the field of melt-crystallization
Nicotinamide: Seven New Polymorphic Structures Revealed by Melt Crystallization and Crystal Structure Prediction
Here, we reported nicotinamide (NIC), a long-known vitamin, was revealed in fact to be a highly polymorphic compound with nine solved single-crystal structures by performing melt crystallization. A CSP calculation successfully identified all six Z’ = 1 and 2 experimental structures. Melt crystallization has turned out to be an efficient tool for exploring polymorphic landscape, especially in regions inaccessbile by solution crystallization.</p
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Nonlinear co-generation of graphene plasmons for optoelectronic logic operations.
Surface plasmons in graphene provide a compelling strategy for advanced photonic technologies thanks to their tight confinement, fast response and tunability. Recent advances in the field of all-optical generation of graphene's plasmons in planar waveguides offer a promising method for high-speed signal processing in nanoscale integrated optoelectronic devices. Here, we use two counter propagating frequency combs with temporally synchronized pulses to demonstrate deterministic all-optical generation and electrical control of multiple plasmon polaritons, excited via difference frequency generation (DFG). Electrical tuning of a hybrid graphene-fibre device offers a precise control over the DFG phase-matching, leading to tunable responses of the graphene's plasmons at different frequencies across a broadband (0 ~ 50 THz) and provides a powerful tool for high-speed logic operations. Our results offer insights for plasmonics on hybrid photonic devices based on layered materials and pave the way to high-speed integrated optoelectronic computing circuits