Complex concentrated solutions of multiple principal elements are being widely investigated as high- or medium-entropy alloys (HEAs or MEAs)
, often assuming that these materials have the high ...configurational entropy of an ideal solution. However, enthalpic interactions among constituent elements are also expected at normal temperatures, resulting in various degrees of local chemical order
. Of the local chemical orders that can develop, chemical short-range order (CSRO) is arguably the most difficult to decipher and firm evidence of CSRO in these materials has been missing thus far
. Here we discover that, using an appropriate zone axis, micro/nanobeam diffraction, together with atomic-resolution imaging and chemical mapping via transmission electron microscopy, can explicitly reveal CSRO in a face-centred-cubic VCoNi concentrated solution. Our complementary suite of tools provides concrete information about the degree/extent of CSRO, atomic packing configuration and preferential occupancy of neighbouring lattice planes/sites by chemical species. Modelling of the CSRO order parameters and pair correlations over the nearest atomic shells indicates that the CSRO originates from the nearest-neighbour preference towards unlike (V-Co and V-Ni) pairs and avoidance of V-V pairs. Our findings offer a way of identifying CSRO in concentrated solution alloys. We also use atomic strain mapping to demonstrate the dislocation interactions enhanced by the CSROs, clarifying the effects of these CSROs on plasticity mechanisms and mechanical properties upon deformation.
Eclipsing binary millisecond pulsars (MSPs; the so-called black widows and redbacks) can provide important information about accretion history, pulsar irradiation of their companion stars, and the ...evolutionary link between accreting X-ray pulsars and isolated MSPs. However, the formation of such systems is not well understood, nor the difference in progenitor evolution between the two populations of black widows and redbacks. Whereas both populations have orbital periods between 0.1 and 1.0 days, their companion masses differ by an order of magnitude. In this paper, we investigate the formation of these systems via the evolution of converging low-mass X-ray binaries by employing the MESA stellar evolution code. Our results confirm that one can explain the formation of most of these eclipsing binary MSPs using this scenario. More notably, we find that the determining factor for producing either black widows or redbacks is the efficiency of the irradiation process, such that the redbacks absorb a larger fraction of the emitted spin-down energy of the radio pulsar (resulting in more efficient mass loss via evaporation) compared to that of the black widow systems. We argue that geometric effects (beaming) are responsible for the strong bimodality of these two populations. Finally, we conclude that redback systems do not evolve into black widow systems with time.
ABSTRACT In the limit of extremely rapid mass transfer, the response of a donor star in an interacting binary becomes asymptotically one of adiabatic expansion. We survey here adiabatic mass loss ...from Population I stars (Z = 0.02) of mass 0.10 M -100 M from the zero-age main sequence to the base of the giant branch, or to central hydrogen exhaustion for lower main sequence stars. The logarithmic derivatives of radius with respect to mass along adiabatic mass-loss sequences translate into critical mass ratios for runaway (dynamical timescale) mass transfer, evaluated here under the assumption of conservative mass transfer. For intermediate- and high-mass stars, dynamical mass transfer is preceded by an extended phase of thermal timescale mass transfer as the star is stripped of most of its envelope mass. The critical mass ratio qad (throughout this paper, we follow the convention of defining the binary mass ratio as q Mdonor/Maccretor) above which this delayed dynamical instability occurs increases with advancing evolutionary age of the donor star, by ever-increasing factors for more massive donors. Most intermediate- or high-mass binaries with nondegenerate accretors probably evolve into contact before manifesting this instability. As they approach the base of the giant branch, however, and begin developing a convective envelope, qad plummets dramatically among intermediate-mass stars, to values of order unity, and a prompt dynamical instability occurs. Among low-mass stars, the prompt instability prevails throughout main sequence evolution, with qad declining with decreasing mass, and asymptotically approaching qad = 2/3, appropriate to a classical isentropic n = 3/2 polytrope. Our calculated qad values agree well with the behavior of time-dependent models by Chen & Han of intermediate-mass stars initiating mass transfer in the Hertzsprung gap. Application of our results to cataclysmic variables, as systems that must be stable against rapid mass transfer, nicely circumscribes the range in qad as a function of the orbital period in which they are found. These results are intended to advance the verisimilitude of population synthesis models of close binary evolution.
Al-Mg alloys are a series of low cost and low density Al alloys, which show remarkable strengthening during deformation. In this work, study of commercial purity Al, Al-0.5Mg and Al-4.1Mg alloys was ...carried out to investigate the effect of Mg on materials strengthening. Slight solid solution strengthening by Mg addition is found in the as-cast alloys. While further significant strengthening effect is achieved in the alloys produced by high pressure torsion. An extraordinarily high strength of ~800 MPa is achieved in the Al-4.1Mg alloy, as a result of deformation induced ultrafine grains, high density stacking faults and Mg segregation.
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Binary population synthesis shows that mass transfer from a giant star to a main-sequence (MS) companion may account for some observed long-orbital-period blue stragglers. However, little attention ...is paid to this blue straggler formation scenario as dynamical instability often happens when the mass donor is a giant star. In this paper, we have studied the critical mass ratio, qc, for dynamically stable mass transfer from a giant star to a MS companion using detailed evolution calculations. The results show that a more evolved star is generally less stable for Roche lobe overflow. Meanwhile, qc almost linearly increases with the amount of the mass and angular momentum lost during mass transfer, but has little dependance on stellar wind. To conveniently use the result, we give a fit of qc as a function of the stellar radius at the onset of Roche lobe overflow and of the mass-transfer efficiency during the Roche lobe overflow. To examine the formation of blue stragglers from mass transfer between giants and MS stars, we have performed Monte Carlo simulations with various qc. The simulations show that some binaries with the mass donor on the first giant branch may contribute to blue stragglers with qc obtained in this paper but will not from previous qc. Meanwhile, from our qc, blue stragglers from the mass transfer between an asymptotic giant branch star and a MS companion may be more numerous and have a wider range of orbital periods than those from the other qc.
The asymptotic response of donor stars in interacting binary systems to very rapid mass loss is characterized by adiabatic expansion throughout their interiors. In this limit, energy generation and ...heat flow through the stellar interior can be neglected. We model this response by constructing model sequences, beginning with a donor star filling its Roche lobe at an arbitrary point in its evolution, holding its specific entropy and composition profiles fixed as mass is removed from the surface. The stellar interior remains in hydrostatic equilibrium. Luminosity profiles in these adiabatic models of mass-losing stars can be reconstructed from the specific entropy profiles and their gradients. These approximations are validated by comparison with time-dependent binary mass transfer calculations. We describe how adiabatic mass-loss sequences can be used to quantify threshold conditions for dynamical timescale mass transfer, and to establish the range of post-common envelope binaries that are allowed energetically. In dynamical timescale mass transfer, the adiabatic response of the donor star drives it to expand beyond its Roche lobe, leading to runaway mass transfer and the formation of a common envelope with its companion star. For donor stars with surface convection zones of any significant depth, this runaway condition is encountered early in mass transfer, if at all; but for main-sequence stars with radiative envelopes, it may be encountered after a prolonged phase of thermal timescale mass transfer, a so-called delayed dynamical instability. We identify the critical binary mass ratio for the onset of dynamical timescale mass transfer as that ratio for which the adiabatic response of the donor star radius to mass loss matches that of its Roche lobe at some point during mass transfer; if the ratio of donor to accretor masses exceeds this critical value, dynamical timescale mass transfer ensues. In common envelope evolution, the dissipation of orbital energy of the binary provides the energy to eject the common envelope; the energy budget for this process consists essentially of the initial orbital energy of the binary and the initial self-energies of the binary components. We emphasize that, because the stellar core and envelope contribute mutually to each other's gravitational potential energy, proper evaluation of the total energy of a star requires integration over the entire stellar interior, and not just over the ejected envelope alone as commonly assumed. We show that the change in total energy of the donor star, as a function of its remaining mass along an adiabatic mass-loss sequence, can be calculated either by integration over initial and final models, or by a path integral along the mass-loss sequence. That change in total energy of the donor star, combined with the requirement that both remnant donor and its companion star fit within their respective Roche lobes, then circumscribes energetically possible survivors of common envelope evolution.
Context.
Mass transfer stability is a key issue in studies of binary evolution. Critical mass ratios for dynamically stable mass transfer have been analyzed on the basis of an adiabatic mass loss ...model, finding that the donor stars on the giant branches tend to be more stable than that based on the composite polytropic stellar model. Double white dwarfs (DWDs) are of great importance in many fields and their properties would be significantly affected under the new mass transfer stability criterion.
Aims.
We seek to investigate the influence of mass transfer stability on the formation and properties of DWD populations and discuss the implications in supernova Type Ia (SN Ia) and gravitational wave (GW) sources.
Methods.
We performed a series of binary population synthesis, adopting the critical mass ratios from the adiabatic mass loss model (i.e., Ge’s model) and that of the composite polytropic model, respectively. In each simulation, 5 × 10
6
binaries were included and evolved from zero-age main sequence to the end of their evolution and the DWDs were gradually obtained.
Results.
For Ge’s model, most of the DWDs are produced from the stable non-conservative Roche lobe (RL) overflow, along with a common-envelope (CE) ejection channel (RL+CE channel), regardless of the CE ejection efficiency,
α
CE
. Conversely, the results of the polytropic model strongly depend on the adopted value of
α
CE
. We find DWDs produced from the RL+CE channel have comparable WD masses and the mass ratio distribution peaks at around 1. Based on the magnitude-limited sample of DWDs, the space densities for the detectable DWDs and those with extremely low-mass WD (ELM WD) companions in Ge’s model is: 1347 kpc
−3
and 473 kpc
−3
, respectively, which is close to what has been shown in observations. On the other hand, the polytropic model overpredicts space density of DWDs by a factor of about 2−3. We also find that the results of DWD merger rate distribution per Galaxy in Ge’s model reproduce the observations better than that of the polytropic model, and the merger rate of DWDs with ELM WD companions in the Galaxy is about 1.8 × 10
−3
yr
−1
in Ge’s model. This result is comparable to the observation estimation of 2 × 10
−3
yr
−1
. The findings from Ge’s model predict a Galactic SN Ia rate of ∼6 × 10
−3
yr
−1
from DWDs, supporting observations of (5.4 ± 1.2)×10
−3
yr
−1
. For the fiducial model of
α
CE
= 1, the number of detectable GW sources in the polytropic model is larger than that in Ge’s model by about 35%.
Conclusions.
We confirm that mass transfer stability plays an important role in the formation and properties of DWD populations as well as in the progenitors of SNe Ia and detectable GW sources. The results of Ge’s model support the observational DWD merger rate distribution per Galaxy and the space density of DWDs in the Galaxy.
The 4.2 ka BP Event is an abrupt climate change that might have contributed to the collapse of ancient civilizations and marks the transition between the mid‐ and late‐Holocene. Despite considerable ...research on this event, our understanding remains primarily based on terrestrial paleoclimate reconstructions, leaving a significant gap in understanding the role of the ocean in this event. Here, we present paired sea surface temperature (SST) and seawater δ18O reconstructions based on four fossil corals from the South China Sea. Our results demonstrate that the climate during the event was cooler, and there were meridional dry‐wet patterns in East Asia, indicating a weakened summer monsoon. Furthermore, our examination of additional coral records from the Pacific and Indian Oceans suggests that low‐latitude forcing (i.e., SST anomalies in the tropical Pacific) plays a crucial role in driving hydrology shifts in East Asia over the 4.2 ka BP interval.
Plain Language Summary
Around 4,200 years ago, a major climate event occurred, resulting in a global megadrought that might have caused the collapse of ancient civilizations. However, most evidence for this event comes from land‐based paleoclimate studies, leaving a gap in our understanding of how it impacted the ocean. In this study, we analyzed fossil coral samples from the South China Sea and found that the area was cooler and wetter during the event. While the cooling appears to have been consistent across East Asia, the hydrological changes exhibited a meridional pattern, with Central China also experiencing wetter conditions while North and South China were dry. By examining coral records from the Pacific and Indian Oceans, we concluded that the Central Pacific El Niño was likely occurring during this event, weakening the East Asian Summer Monsoon and causing the associated climate and hydrology changes in East Asia.
Key Points
Paired fossil coral Sr/Ca and δ18O records indicate a cold and wet climate in the northern South China Sea during the 4.2 ka BP event
Coral and terrestrial reconstructions reveal consistent cooling but meridionally‐varied hydrological changes in East Asia around 4.2 ka BP
Low‐latitude sea surface temperature anomalies in tropical Pacific significantly contribute to the shift in hydroclimate in East Asia
Gravitational Waves (GWs) provide a unique way to explore our Universe. The ongoing ground-based detectors, e.g., LIGO, Virgo, and KAGRA, and the upcoming next-generation detectors, e.g., Cosmic ...Explorer and Einstein Telescope, as well as the future space-borne GW antennas, e.g., LISA, TianQin, and TaiJi, cover a wide range of GW frequencies from ∼10−4Hz to ∼103Hz and almost all types of compact objects in close orbits serve as the potential target sources for these GW detectors. The synergistic multi-band GW and EM observations would allow us to study fundamental physics from stars to cosmology. The formation of stellar GW sources has been extensively explored in recent years, and progress on physical processes in binary interaction has been made as well. Furthermore, some studies have shown that the progress in binary evolution may significantly affect the properties of the stellar GW sources. In this article, we review the formation channels of compact objects in close orbits and discuss their implications for GW observations.
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