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Excitation of metastable helium atoms by positron impact
No full textWe present measurements of the excitation of the 21S metastable state of helium by positron impact at energies between the threshold for excitation (20.6 eV) and 40 eV. The present results are obtained by directly measuring the yield of excited atoms that result from crossing a low-energy, high-resolution, magnetically confined and pulsed positron beam from a buffer gas trap with an effusive beam of He atoms. The measured excitation function is compared with previous measurements which used a scattering cell and with a recent state-of-the-art theoretical calculation
Attosecond pulse generation using high-order harmonic generation in argon gas based on the enhancement effect of multilayer plasmonics
No full textThis study explores the potential for high-harmonic generation (HHG) from argon atomic gas and single attosecond pulse generation by leveraging amplified and hyper-focused short laser pulses through a plasmonic nanostructure. The plasmonic nanostructure features triangular nanobowties with multilayer compositions of dielectrics and metals, supported by an insulating substrate. Within the nanobowtie gap, localized surface plasmons significantly enhance the laser field intensity over a substantial volume of the gap. Fine-tuning the geometric parameters of this structure achieves up to 45-fold amplification (< 17 dB) within the central wavelength of 800 nm of a standard titanium–sapphire laser. This enhancement enables the argon atoms introduced via a gas jet to exhibit a pronounced nonlinear response, leading to high-intensity HHG under incident pulses of relatively low intensity (1012 W/cm2). Based on the harmonic spectrum observed, the generation of isolated attosecond pulses with a temporal width of 33.37 attoseconds is achievable, notably without necessitating chirp mitigation techniques
Erratum to: Genotypic and environmental variability of tocopherols and phytosterols in linseed (Linum usitatissimum L.) oil
No full textThe dynamical and thermodynamic effects of turbulence on the cosmic baryonic fluid
No full textBoth simulations and observations indicate that the so-called missing baryons reside in the intergalactic medium known as the warm-hot intergalactic medium (WHIM). In this study we employed the IllustrisTNG50-1 simulation to demonstrate that knowledge of the turbulence in the cosmic baryonic fluid is crucial for correctly understanding both the spatial distribution and the physical origins of the missing baryons in the Universe. First, we find that dynamical effects cause the gas to be detained in low-density and intermediate-density regions, resulting in high baryon fractions, and prevent the convergence of the gas in high-density regions, leading to low baryon fractions. Second, turbulent energy is converted into thermal energy, and the injection and dissipation of turbulent energy have essentially reached a balance from z = 1 to 0. This indicates that the cosmic fluid is in a steady state within this redshift range. Due to turbulent heating, as the redshift decreases, an increasing amount of warm gas is heated and converted into the WHIM, and some even into hot gas. We find that, compared with turbulence in the cosmic fluid, shocks are unimportant in intermediate-density regions and even negligible in high-density regions, both dynamically and thermodynamically. This finding accounts for the origin of the WHIM in terms of both dynamics and thermodynamics, calls into question the traditional view of shock-heating, and highlights the importance of turbulence in shaping the large-scale structure of the Universe, particularly in the evolution of galaxies and galaxy clusters. In addition to TNG50-1, we validated our key findings with TNG50-2, TNG100-1, WIGEON, and EAGLE simulations, demonstrating that the spatial resolution, box size, and sub-grid-physics variations do not affect our main conclusions
The role of distant pulsars in the detectability of continuous gravitational waves
No full textContext. One of the imminent science goals of pulsar timing arrays (PTAs) is the detection of a continuous gravitational wave (CGW) emitted by an individual supermassive black hole binary (SMBHB). SMBHBs that cause CGWs with GW frequencies fGW > 10 nHz (high-frequency end of the nano-Hertz GW spectrum) have undergone significant orbital evolution and hence a change in fGW over time. In PTA datasets with a sufficiently long observational time span, this means that the contributions of the Earth and the pulsar terms to the CGW signal signature can eventually become resolvable. Since the pulsar term is accumulated incoherently and thus often treated as an additional source of noise, this separation can prove to be beneficial for the detection of the CGW signal in the PTA dataset.
Aims. We investigate to which extent resolvable Earth and pulsar terms affect currently used techniques for CGW searches with PTA datasets, which treat the pulsar term as an additional source noise. We focus on the dependence of the pulsar term frequencies on the pulsar distance. We aim to answer the question of whether adding more distant pulsars to a PTA dataset can mitigate biases and improve the detection of CGWs.
Methods. We used simulated PTA datasets based on the EPTA DR2 and IPTA DR2 pulsars in order to study the performance of the Earth-term-only Bayesian parameter estimation of the circular SMBHB model parameters and the frequentist narrow-band optimal statistic in the light of resolved pulsar terms due to larger pulsar distances.
Results. We show that under ideal conditions, more distant pulsars can facilitate the CGW search with PTA datasets. The Bayesian parameter estimation is yielding better parameter constraints, and the frequentist search becomes more stable. Based on realistic dataset simulations, however, we found that other configuration parameters of a PTA, such as the anisotropic distribution of pulsars and the effective number of pulsars in a PTA, can play a crucial role in the importance of this effect
Multi-objective optimization of the process combining ultrasonic vibration and age-forming of dissimilar friction-stir-welded Aluminium T-stiffened plates
No full textThis paper innovatively proposes a combined process combining ultrasonic vibration (abbreviated UV) with creep age forming (abbreviated CAF), named UVCAF, for high-quality forming of dissimilar 7055-T6/2197-T8 FSWed T-stiffened plates. And based on this, a multi-objective optimization quality comprehensive evaluation model of the AMOGA-EWM algorithm is proposed, obtaining the optimal process parameter solution combination as [164.8 °C, 9.2 h, 9.6 μm]. The reliability and feasibility of the above evaluation model in improving the forming rate and mechanical properties for the T-stiffened plates are verified. Meanwhile, through comparative analysis of the experimental results of UVCAF under the optimal process parameters, it is found that the forming rate, tensile strength, and elongation of T-stiffened plates under UVCAF are 5.4%, 12.3 MPa, and 0.82% higher than those under CAF. In addition, the microscopic analysis results indicate that the fracture mode of the specimens under CAF and UVCAF is a ductile fracture, and the weld nugget zone of the specimens under CAF and UVCAF is the combined reinforcement of the T1 and η phase. However, the introduction of UV through the strengthening effect of fine grains makes the η phase more abundant in UVCAF specimens, thereby improving the forming accuracy and performance of the specimens
Intermediate-mass black hole binary evolution in nuclear star clusters: The effect of the stellar-mass black hole population
No full textAims. In this study, we investigate the dynamics of intermediate-mass black hole (IMBH) binaries within nuclear star clusters (NSCs) that contain a population of stellar-mass black holes (BHs). We examine how these stellar and BH populations influence the dynamics of the IMBH binary and, in turn, how the evolving IMBH binary affects the surrounding stellar and BH populations.
Methods. We conducted high-resolution N-body simulations of NSCs constructed based on observational parameters from two local dwarf galaxies: NGC205 and NGC404. For the first time, we achieved a star particle mass resolution of 1 M⊙ and a BH mass resolution of 10 M⊙. This level of resolution is crucial for accurately modeling the collisional dynamics of these dense systems.
Results. Including stellar-mass BHs within the stellar population significantly influences the IMBH binary dynamics, nearly doubling the sinking rate and halving the merger time. During the initial phase of the inspiral, the IMBH binary disrupts both the stellar and BH cusps. However, the BH cusp quickly regains its steep slope due to its shorter relaxation time and continues to dominate the evolution of the IMBH binary, despite being much less massive than the stellar component. We uncover an interesting mechanism in which BHs first efficiently extract energy from the IMBH binary and then transfer this energy to the surrounding stars, allowing the BHs to spiral back toward the center of the system and restart the process. Our results imply that although stellar-mass BHs are a minor component of a stellar population they can significantly facilitate IMBH growth within NSCs via mergers. We also notice that these dense systems can potentially boost intermediate-mass ratio inspirals (IMRIs) predominantly on radial orbits
Experimental investigation of O2 diffusion and entrapment in interstellar amorphous solid water
No full textInterstellar ices are mainly composed of amorphous solid water (ASW) containing small amounts of hypervolatiles, such as O , whose diffusion-limited reactions play a key role in space chemistry. Although O is an important precursor molecule present during the early stages of ice formation, its surface diffusion in ASW remains poorly constrained. 2 2
In this study, we experimentally investigate the surface diffusion and the entrapment efficiency of O in porous ASW under astrophysically relevant conditions. 2
Experiments were conducted in an ultrahigh vacuum chamber and monitored using infrared (IR) spectroscopy and quadrupole mass spectrometry. Diffusion coefficients were extracted through a novel approach applicable to IR-inactive molecules, by fitting the mass spectrometer signal during the isothermal phase with a Fickian model. These coefficients were then used to derive the diffusion energy barrier of O in ASW. Entrapment efficiencies were measured by analyzing the subsequent temperature-programmed desorption phase. 2
We measured the surface diffusion coefficients at different temperatures (35 K, 40 K, 45 K) and water ice coverages (40 ML, 60 ML, 80 ML), yielding values on the order of 10 -10 cm s . From these values, we derived a diffusion energy barrier of E_ -16 -15 2 -1 diff = 10 ± 3 meV (116 ± 35 K), corresponding to a ̧hi ratio of about 0.1. Entrapment measurements revealed that a residual amount of ∼20% of O remains trapped in the ASW matrix at the highest temperatures investigated. 2
This work demonstrates that the surface diffusion of IR-inactive molecules can be experimentally quantified using mass spectrometry. Our findings show that O exhibits a low diffusion barrier, indicating high mobility in interstellar water ices. Moreover, we suggest these water ices likely retain a residual fraction of hypervolatiles entrapped within their structure.
The dependence of asteroid rotation on composition. A spectral class database for MP3C
No full textThe rotational properties of asteroids provide critical information about not only their internal structure, but also their collisional and thermal histories. Previous work has revealed a bimodal distribution of asteroid spin rates, dividing populations into fast and slow rotators; however, this separation remains poorly understood, for example, with regard to its dependence on composition.
We investigated whether the valley that separates fast and slow rotators in rotational period-diameter space depends on asteroid composition. We approximated the composition using the asteroids' spectral class.
First, we extended the Minor Planet Physical Properties Catalogue (MP3C) to include the available spectral classes of asteroids. For each asteroid, we then selected the best diameter, rotational period, and spectral class. Building upon a semi-supervised machine-learning method, we quantified the valley between fast and slow rotators for S- and C-complex asteroids, which are linked to different types of meteorites: ordinary and carbonaceous chondrites, respectively. The method iteratively fits a linear boundary between the two populations in a rotational period-diameter space to maximise separation between them.
We find a clear compositional dependence of the valley: for C-complex asteroids, the transition occurs at longer periods than for S-complex, with P_ ̊m km ^ 0.739 (C-complex) and P_ = 11.6,D_ ̊m km ^ 0.718 (S-complex), where the period and diameter are given in hours and kilometres, respectively. This corresponds to μ Q ≃ 2 and 13 ̊m GPa, respectively, where μ is rigidity, which measures how strongly a body resists shear deformation under applied stress, and Q is the quality factor, which measures how efficiently a body dissipates mechanical energy when cyclically deformed.
The dependence of the valley on spectral classes likely reflects compositional and structural differences: C-complex asteroids, being more porous and weaker, dissipate angular momentum more efficiently than stronger, more coherent S-complex asteroids. This represents quantitative evidence of class-dependent rotational valleys within asteroid populations
One-dimensional and time-dependent modelling of complex organic molecules in protostars
No full textComplex organic molecules (COMs), the building blocks of life, have been extensively detected under various physical conditions, from quiescent clouds to star-forming regions. They therefore serve as excellent tracers of the local physical and chemical properties of these environments. Proper models that are capable of grasping the formation and destruction of COMs are crucial to understanding observations. However, given that distinct COMs can be detected from different locations and at varying times, we improved UCLCHEM -- a gas-grain chemical code -- to a 1D, time-dependent model tailored to protostars. In this update, we examine two stages of a protostar, the prestellar and heating stages, incorporating a simple radiative mechanism for both the internal and external radiation fields of the cloud. This approach relies on the key assumption that the dust and gas temperatures are completely coupled. Ultimately, we implemented an updated version of our model to interpret observations obtained through both single-dish and interferometry under varying conditions, including a SgrB2(N1) hot core, massive Galactic clumps, and a hot core in Orion. We show that our model can reproduce these observations well. We highlight that some COMs are positioned at a higher temperature in the envelope, and others at a lower temperature, which could potentially leading to misinterpretations when using a single-point model. In the case of SgrB2(N1), the best model indicates that the cosmic-ray ionisation rate significantly exceeds the value typically used for the standard interstellar medium. Our model is as an efficient computational tool that will be particularly useful for gaining better insights into COM observations