Abstract
Rotation is a critical physical process operating in the formation of Am stars. There is a strong correlation between low-velocity rotation and chemical peculiarity. However, the existence ...of many non-chemical-peculiar slow rotators challenges the understanding of Am stars. The purpose of our work is to search for low-velocity rotating non-chemical-peculiar A-type stars and Am stars and to make a comparative analysis. In this paper, we pick out a sample from the LAMOST–Kepler project, including 21 Am stars, 125 non-chemical-peculiar slow rotators, and 53 non-chemical-peculiar fast rotators. We calculate the rotational frequencies through a periodic change of light curves caused by inhomogeneous stellar surfaces and then obtain the rotational velocities. For slow rotators, the age of Am stars is statistically younger than that of non-chemical-peculiar stars in the same temperature bin. In the comparison of the period, the average amplitude, and stellar mass of Am and non-chemical-peculiar stars, we discover that there is no difference in the photometric variability between Am and non-chemical-peculiar stars, which implies similar inhomogeneities on the surfaces. The average amplitude of non-chemical-peculiar stars has a downward trend with the increase of effective temperature and stellar mass, which is consistent with the theoretical prediction caused by weak dynamo-generated magnetic fields in A-type stars. In addition, we confirm four non-chemical-peculiar stars that have flares by checking field-of-view images, pixel images, and pixel-level light curves.
We report on an experimental measurement of Coulomb drag in a double quantum well structure consisting of bilayer-bilayer graphene, separated by few layer hexagonal boron nitride. At low temperatures ...and intermediate densities, a novel negative drag response with an inverse sign is observed, distinct from the momentum and energy drag mechanisms previously reported in double monolayer graphene. By varying the device aspect ratio, the negative drag component is suppressed and a response consistent with pure momentum drag is recovered. In the momentum drag dominated regime, excellent quantitative agreement with the density and temperature dependence predicted for double bilayer graphene is found.
The fundamental stellar atmospheric parameters (Teff and log g) and 13 chemical abundances are derived for medium-resolution spectroscopy from Large Sky Area Multi-Object Fiber Spectroscopic ...Telescope (LAMOST) Medium Resolution Survey (MRS) data sets with a deep-learning method. The neural networks we designed, named SPCANet, precisely map LAMOST MRS spectra to stellar parameters and chemical abundances. The stellar labels derived by SPCANet have precisions of 119 K for Teff and 0.17 dex for log g. The abundance precision of 11 elements including C/H, N/H, O/H, Mg/H, Al/H, Si/H, S/H, Ca/H, Ti/H, Cr/H, Fe/H, and Ni/H are 0.06 ∼ 0.12 dex, while that of Cu/H is 0.19 dex. These precisions can be reached even for spectra with signal-to-noise ratios as low as 10. The results of SPCANet are consistent with those from other surveys such as APOGEE, GALAH, and RAVE, and are also validated with the previous literature values including clusters and field stars. The catalog of the estimated parameters is available at doi:10.12149/101012.
Amorphization by mechanical deformation Li, B.Y.; Li, A.C.; Zhao, S. ...
Materials science & engineering. R, Reports : a review journal,
June 2022, 2022-06-00, 20220601, Letnik:
149
Journal Article
Recenzirano
Amorphization of crystalline structures is a ubiquitous phenomenon in metals, ceramics, and intermetallic compounds. Although the amorphous phase generally has a higher Gibbs free energy than its ...crystalline counterpart, there are many methods by which amorphization can be generated. The requirement to create an amorphous phase from a solid crystalline one is to increase its free energy above a critical level which enables this transition. In this review, our focus is on amorphization induced by mechanical deformation which can be imparted by a variety of means, prominent among which are tribological processes, severe plastic deformation, nanoindentation, shock compression, diamond anvil cell and ball milling/mechanical alloying. The deformation introduces defects into the structure, raising its free energy to the level that it exceeds the one of the amorphous phase, thus propitiating conditions for amorphization. Experimental observations of amorphization in metallic alloys, intermetallic compounds, ionically and covalently bonded materials are presented and discussed. There is also an observation of amorphization in a biological material: it is generated by impact deformation of hydroxyapatite in the mantis shrimp club. We also focus on the fundamental mechanisms of plastic deformation of amorphous materials; this is a closely linked process by which deformation continues, beyond amorphization, in the new phase. Observations and analyses of amorphization are complemented by computational simulations that predict the process of mechanically-induced amorphization and address the mechanisms of this transformation.
In condensed-matter systems, higher temperatures typically disfavour ordered phases, leading to an upper critical temperature for magnetism, superconductivity and other phenomena. An exception is the ...Pomeranchuk effect in
He, in which the liquid ground state freezes upon increasing the temperature
, owing to the large entropy of the paramagnetic solid phase. Here we show that a similar mechanism describes the finite-temperature dynamics of spin and valley isospins in magic-angle twisted bilayer graphene
. Notably, a resistivity peak appears at high temperatures near a superlattice filling factor of -1, despite no signs of a commensurate correlated phase appearing in the low-temperature limit. Tilted-field magnetotransport and thermodynamic measurements of the in-plane magnetic moment show that the resistivity peak is connected to a finite-field magnetic phase transition
at which the system develops finite isospin polarization. These data are suggestive of a Pomeranchuk-type mechanism, in which the entropy of disordered isospin moments in the ferromagnetic phase stabilizes the phase relative to an isospin-unpolarized Fermi liquid phase at higher temperatures. We find the entropy, in units of Boltzmann's constant, to be of the order of unity per unit cell area, with a measurable fraction that is suppressed by an in-plane magnetic field consistent with a contribution from disordered spins. In contrast to
He, however, no discontinuities are observed in the thermodynamic quantities across this transition. Our findings imply a small isospin stiffness
, with implications for the nature of finite-temperature electron transport
, as well as for the mechanisms underlying isospin ordering and superconductivity
in twisted bilayer graphene and related systems.
Abstract
We derive a catalog of early-type emission-line stars including 30,023 spectra of 25,867 stars from LAMOST Data Release 7, in which 4189 have Simbad records. The spectra are classified into ...three morphological types (10 subtypes) based on H
α
emission-line profiles. Some spectra contaminated by nebula emission lines such as from H
ii
regions are flagged in the catalog. We also provide a specific catalog of 20 stars with stellar winds or accretion flows by calculating the terminal and peak velocities based on P-Cygni or inverse P-Cygni profiles. More important, with two color–color diagrams, (H-K
s
, J-H) and (K
s
-W1, H-K
s
), of a collection of known Herbig Ae/Be stars (HAeBes) and classical Ae/Be stars (CAeBes), we propose an updated criterion to separate HAeBes from CAeBes. By the criterion, we select 118 HAeBe candidates and 2636 CAeBe candidates from the sample. We confirm 71 of the 118 HAeBes based on the data from LAMOST (optical spectra) and WISE (photometry and images), 62 of which are newly identified. The 71 identified HAeBes are compiled into a specific catalog, and their SEDs and MIR images are also presented. The distances of 76% confirmed HAeBes are further than 1 kpc, which enlarges the number of known HAeBes in further distance. Most of the 71 HAeBes are located in the Galactic disk while 6 of them have ∣
Z
gal
∣ > 400 pc. Four HAeBes show the forbidden emission lines of Fe
ii
and O
i
, in which J051425.20+411310.7 is a newly discovered Be. In addition, four HAeBes having Spitzer IRS spectra all show PAH features.
IR emission spectra are calculated for dust heated by starlight, for mixtures of amorphous silicate and graphitic grains, including varying amounts of PAH particles. The models are constrained to ...reproduce the average Milky Way extinction curve. The calculations include the effects of single-photon heating. Updated IR absorption properties for the PAHs are presented that are consistent with observed emission spectra, including those newly obtained by Spitzer. We find a size distribution for the PAHs giving emission band ratios consistent with the observed spectra of the Milky Way and other galaxies. Emission spectra are presented for a wide range of starlight intensities. We calculate how the efficiency of emission into different IR bands depends on PAH size; the strong 7.7 km emission feature is produced mainly by PAH particles containing <10 super(3) C atoms. We also calculate how the emission spectrum depends on U, the starlight intensity relative to the local interstellar radiation field. The submillimeter and far-infrared emission is compared to the observed emission from the local interstellar medium. Using a simple distribution function, we calculate the emission spectrum for dust heated by a distribution of starlight intensities, such as occurs within galaxies. The models are parameterized by the PAH mass fraction q sub(PAH), the lower cutoff U sub(min) of the starlight intensity distribution, and the fraction g of the dust heated by starlight with U> U sub(min). We present graphical procedures using Spitzer IRAC and MIPS photometry to estimate the parameters q sub(PAH), U sub(min), and g, the fraction f sub(PDR) of the dust luminosity coming from photodissociation regions with U> 100, and the total dust mass M sub(dust).
A hybrid discretization scheme for solution of volume integral equation (VIE) by method of moments (MoM) for electromagnetic scattering from dielectric objects is proposed in this article. The ...Schaubert-Wilton-Glisson and edge (SWG-Edge) hybrid basis functions are used in this discretization scheme. According to the divergence-free condition of electric displacement vector, a kind of edge basis functions defined in elements including boundary faces which separate a dielectric object from the background is derived. As a result, we get a SWG-Edge hybrid basis set. Details for the calculation of the corresponding matrix elements for the edge basis and testing functions are presented. Numerical results show the validity and accuracy of the hybrid discretization scheme. Finally, the proposed method is used for efficient solution of VIE for inhomogeneous dielectric objects with multiboundary. It is shown that for multiboundary problems, the number of unknowns of the hybrid basis is only about 71% of the traditional SWG basis. This means that the memory for solution of VIE by the traditional SWG basis functions can be reduced by half. Therefore, the SWG-edge hybrid basis is much more efficient than the traditional SWG basis for solution of VIE with multiboundary problems.
Abstract In Data Release 9 of LAMOST, we present measurements of v sin i for a total of 121,698 stars measured using the Medium Resolution Spectrograph (MRS) and 80,108 stars using the Low Resolution ...Spectrograph (LRS). These values were obtained through a χ 2 minimization process, comparing LAMOST spectra with corresponding grids of synthetically broadened spectra. Due to the resolution and the spectral range of LAMOST, v sin i measurements are limited to stars with an effective temperature ( T eff ) ranging from 5000 to 8500 K for MRS and 7000 to 9000 K for LRS. The detectable v sin i for MRS is set between 27 and 350 km s −1 , and for LRS between 110 and 350 km s −1 . This limitation is because the convolved reference spectra become less informative beyond 350 km s −1 . The intrinsic precision of v sin i , determined from multiepoch observations, is approximately ∼4.0 km s −1 for MRS and ∼10.0 km s −1 for LRS at a signal-to-noise ratio greater than 50. Our v sin i values show consistency with those from APOGEE17, displaying a scatter of 8.79 km s −1 . They are also in agreement with measurements from the Gaia DR3 and Sun et al. catalogs. An observed trend in LAMOST MRS data is the decrease in v sin i with a drop in T eff , particularly transiting around 7000 K for dwarfs and 6500 K for giants, primarily observed in stars with near-solar abundances.
Abstract
We examine a sample of 340 cataclysmic variables (CVs) from the latest data release of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey, along with 18 objects ...that are newly classified as CVs. In this paper, we focus on investigating the photometric behaviors of these CVs using data from time-domain surveys. The orbital periods of three new and five previously known objects are determined from the long-term light curves displaying eclipses or ellipsoidal variations, and/or time-resolved spectra from LAMOST. For another 16 CVs with measured periods, it is more reliable to obtain consistent periods using light curves from different surveys, as the periods derived from single-site data are still questionable. Follow-up observations are needed to confirm whether the periods have physical meanings or are orbital-related. In our sample, we find that most of the objects have longer periods above the 2 ∼ 3 hr gap. Besides period estimates, we also carry out a separate detailed analysis of some valuable CVs, in terms of spectral characteristics and subtype determination. Finally, we discuss the observational properties of this sample, including the distributions of orbital periods, absolute magnitudes, and the statistical properties of each subclass of CVs. In addition, we pick out six non-CV systems, including five illumination-effect binaries, as well as one hot subdwarf, that we came across when searching for CVs, and we investigate their properties based on the spectra and photometric data.
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