arXiv subjects

Yanjun Guo

Publications and source records attributed to Yanjun Guo.

44 records · Page 3Linked to original sources

arXiv2021

The Binarity of Early-type Stars from LAMOST Medium-resolution Spectroscopic Survey

Massive binaries play significant roles in many fields. Identification of massive stars, particularly massive binaries, is of great importance. In this paper, by adopting the technique of measuring the equivalent widths of several spectral lines, we identified 9,382 early-type stars from LAMOST medium-resolution survey and divided the sample into four groups, T1 ($\sim$O-B4), T2 ($\sim$B5), T3 ($\sim$B7), and T4 ($\sim$B8-A). The relative radial velocities $RV_{\rm rel}$ were calculated using the Maximum Likelihood Estimation. The stars with significant changes of $RV_{\rm rel}$ and at least larger than 15.57km s$^{-1}$ were identified as spectroscopic binaries. We found that the observed spectroscopic binary fractions for the four groups are $24.6\%\pm0.5\%$, $20.8\%\pm0.6\%$, $13.7\%\pm0.3\%$, and $7.4\%\pm0.3\%$, respectively. Assuming that orbital period ($P$) and mass ratio ($q$) have intrinsic distributions as $f(P) \propto P^π$ (1\textless$P$\textless1000 days) and $f(q) \propto q^κ$ (0.1\textless$q$\textless1), respectively, we conducted a series of Monte-Carlo simulations to correct observational biases for estimating the intrinsic multiplicity properties. The results show that the intrinsic binary fractions for the four groups are 68$\%\pm8\%$, 52$\%\pm3\%$, 44$\%\pm6\%$, and 44$\%\pm6\%$, respectively. The best estimated values for $π$ are -1$\pm0.1$, -1.1$\pm0.05$, -1.1$\pm0.1$, and -0.6$\pm0.05$, respectively. The $κ$ cannot be constrained for groups T1 and T2 and is -2.4$\pm0.3$ for group T3 and -1.6$\pm0.3$ for group T4. We confirmed the relationship of a decreasing trend in binary fractions towards late-type stars. No correlation between the spectral type and the orbital period distribution has been found yet, possibly due to the limitation of observational cadence.

astro-ph.SR↗

arXiv2020

Numerical modeling of in-plane thermal conductivity measurement methods based on a suspended membrane setup

A numerical modeling study based on 3D finite element method (FEM) simulation and 1D analytical solutions has been carried out to evaluate the capabilities of two ac methods for measuring in-plane thermal conductivity of thin film deposited on the back of a suspended SiNx membrane setup. Two parallel metal strips are present on the top of the dielectric membrane. One strip (S1) serves as both heater and thermometer, while another one (S2) acts as thermometer only. For a modified phase shift (MPS) method, it is crucial to extract the in-plane thermal diffusivity from the phase shift of the temperature oscillation on S2. It was found that the frequency window for carrying out the data fitting became narrower as the in-plane thermal diffusivity of the composite membrane (${α_{\parallel ,C}}$) increased, primarily due to the failure of the semi-infinite width assumption in the low frequency region. To ensure the validity of the method, the upper limit of ${α_{\parallel ,C}}$ should not exceed ~1.8$ \times $10-5 m2 s-1 for the specific membrane dimension under consideration (1$\times $1 mm2). On the other hand, inspired by a modified Angstrom method proposed by Zhu recently, we suggest a new data reduction methodology which takes advantage of the phase shift on both S1 and S2 as well as the amplitude on S1. Based on the simulation results, it is expected that the non-ideality associated with the three observables may be at least partially cancelled out.Therefore, the frequency window selection for carrying out the data fitting is not sensitive to the magnitude of ${α_{\parallel ,C}}$. For typical specimen films whose in-plane thermal conductivity ranges from 0.84 W m-1 K-1 to 50 W m-1 K-1, the method proposed here yields a theoretical measurement uncertainty of less than 5%.

physics.app-ph↗

arXiv2018

Design of Novel 3D SERS Probes with Drastically Improved Detection Limit by Maximizing SPP - Based Multiple Coupling Effects

Quantifying formidable multiple coupling effects involved in Surface-enhanced Raman scattering (SERS) is a prerequisite for accurate design of SERS probes with superior detection limit and uniformity which are the targets for trace substance detection. Here, combining theory and experiments on novel 3D periodic Au/SiO2 hybrid nanogrids, we successfully develop a generalized methodology of accurately designing high performance SERS probes. Structural parameters and symmetry, Au roughness, and polarization are quantitatively correlated to intrinsic electromagnetic field (EMF) enhancements from surface plasmon polariton (SPP), localized surface plasmon resonance (LSPR), optical standing wave and their couplings theoretically, which is experimentally verified. The hexagonal SERS probes optimized by the methodology successfully detect 5*10^-11 M Hg ions in water, and 2.5*10^-11 M R6G with 40 times improvement of detection limit, an enhancement factor of 3.4*10^8 and uniformity of 5.56%, which results from the extra Au roughness - independent 144% contribution of LSPR effects excited by SPP interference waves as secondary sources, beyond the conventional recognization. This study opens up a pioneering way not only for providing the generalized design principles of SERS probe structures with high performance but for accurately designing their structures with particular purposes such as greatly improved detection limit and uniformity which are very significant for trace substance detection.

physics.optics↗

arXiv2017

Determination of the thermopower of microscale samples with an AC method

A modified AC method based on micro-fabricated heater and resistive thermometers has been applied to measure the thermopower of microscale samples. A sinusoidal current with frequency ω is passed to the heater to generate an oscillatory temperature difference across the sample at a frequency 2ω, which simultaneously induces an AC thermoelectric voltage, also at the frequency 2ω. A key step of the method is to extract amplitude and phase of the oscillatory temperature difference by probing the AC temperature variation at each individual thermometer. The sign of the thermopower is determined by examining the phase difference between the oscillatory temperature difference and the AC thermoelectric voltage. The technique has been compared with the popular DC method by testing both n-type and p-type thin film samples. Both methods yielded consistent results, which verified the reliability of the newly proposed AC method.

physics.ins-det↗

arXiv2016

Strange Matter: a state before black hole

Normal baryonic matter inside an evolved massive star can be intensely compressed by gravity after a supernova. General relativity predicts formation of a black hole if the core material is compressed into a singularity, but the real state of such compressed baryonic matter (CBM) before an event horizon of black hole appears is not yet well understood because of the non-perturbative nature of the fundamental strong interaction. Certainly, the rump left behind after a supernova explosion could manifest as a pulsar if its mass is less than the unknown maximum mass, $M_{\rm max}$. It is conjectured that pulsar-like compact stars are made of strange matter (i.e., with 3-flavour symmetry), where quarks are still localized as in the case of nuclear matter. In principle, different manifestations of pulsar-like objects could be explained in the regime of this conjecture. Besides compact stars, strange matter could also be manifested in the form of cosmic rays and even dark matter.

astro-ph.HE↗

arXiv2015

Massive Pulsars and Ultraluminous X-ray Sources

The detection of 1.37$\, $s pulsations from NuSTAR J095551+6940.8 implies the existence of an accreting pulsar, which challenges the conventional understanding of ultraluminous X-ray source. This kind of sources are proposed to be massive pulsars in this paper. Considering the general relativistic effect, stronger gravity of massive pulsars could lead to a larger maximum luminosity, scaled as the Eddington luminosity. The pseudo-Newtonian potential is employed to simulate the gravitational field in general relativity, and the Eddington luminosity is calculated for self-bound stars (quark star and quark-cluster star) and for the Tolman IV solution. It is found that, for a massive pulsar with radius close to the Schwarszchild radius, the Eddington luminosity could be as high as $2\times10^{39} \, {\rm erg\, s}^{-1}$. It is able to account for the X-ray luminosity of NuSTAR J095551+6940.8 with reasonable beaming factor. It is also suggested that massive pulsar-like compact stars could form via this super-Eddington phase of ultraluminous X-ray source.

astro-ph.HE↗

arXiv2014

An Ultra Low Mass and Small Radius Compact Object in 4U 1746-37?

Photospheric radius expansion (PRE) bursts have already been used to constrain the masses and radii of neutron stars. RXTE observed three PRE bursts in 4U 1746-37, all with low touchdown fluxes. We discuss here the possibility of low mass neutron star in 4U 1746-37 because the Eddington luminosity depends on stellar mass. With typical values of hydrogen mass fraction and color correction factor, a Monte-Carlo simulation was applied to constrain the mass and radius of neutron star in 4U 1746-37. 4U 1746-37 has a high inclination angle. Two geometric effects, the reflection of the far side accretion disc and the obscuration of the near side accretion disc have also been included in the mass and radius constraints of 4U 1746-37. If the reflection of the far side accretion disc is accounted, a low mass compact object (mass of $0.41\pm0.14~M_{\odot}$ and radius of $8.73\pm1.54~\rm km$ at 68% confidence) exists in 4U 1746-37. If another effect operated, 4U 1746-37 may contain an ultra low mass and small radius object ($M=0.21\pm0.06~M_{\odot},~R=6.26\pm0.99~\rm km$ at 68% confidence). Combined all possibilities, the mass of 4U 1746-37 is $0.41^{+0.70}_{-0.30}~M_\odot$ at 99.7% confidence. For such low mass NS, it could be reproduced by a self-bound compact star, i.e., quark star or quark-cluster star.

astro-ph.HE↗

arXiv2013

Compressed Baryonic Matter of Astrophysics

Baryonic matter in the core of a massive and evolved star is compressed significantly to form a supra-nuclear object, and compressed baryonic matter (CBM) is then produced after supernova. The state of cold matter at a few nuclear density is pedagogically reviewed, with significant attention paid to a possible quark-cluster state conjectured from an astrophysical point of view.

astro-ph.HE↗