arXiv subjects

Zhao Gang

Publications and source records attributed to Zhao Gang.

8 recordsLinked to original sources

arXiv2026

Testing Chemical Tagging with LAMOST: Intrinsic Abundance Dispersion of Subgiant Stars in the Galactic Disk

The scatter in elemental abundances among stars of similar age and metallicity reflects chemical inhomogeneity in their birth environments, making abundance scatter a key observable for chemical-tagging studies of Galactic formation and evolution. Using a large sample of subgiant stars with precise ages and elemental abundances derived from LAMOST low-resolution spectra, we investigate the intrinsic chemical abundance scatter of the low-$\alpha$ thin disk near the solar neighborhood ($7 < R < 10$ kpc). We model the abundance ratio [X/Fe], for each of the 19 elements of concern, as a function of age, [Fe/H], and [Mg/Fe], and deduce the intrinsic dispersions with a forward modelling technique. Our results confirm previous findings that the intrinsic scatters are small, typically $\lesssim0.05$~dex, for light elements (C, Al), $\alpha$-elements (O, Mg, Si, Ca, Ti), and iron-peak elements (Mn, Ni). A dedicated analysis of M67 yields similarly small scatter values for these elements, implying limited discriminatory power from light-element abundances alone. In contrast, neutron-capture elements exhibit substantially larger scatters, typically $\gtrsim$0.1 dex, which are significantly larger than those of M67 member stars ($\sim$0.07~dex). In particular, our analysis suggests that the abundance variations of individual neutron-capture elements cannot be explained by a single tracer such as [Ba/Fe]. These findings clarify the utility of neutron-capture elements for chemical tagging and highlight the potential of low-resolution spectroscopy in such studies.

astro-ph.GA↗

arXiv2024

Detection of the actinide Th in an r-process-enhanced star with accretion origin

The thorium and six second-peak r-process element (56<Z<72) abundances are determined for the alpha-poor star LAMOST J1124+4535 based on a high-resolution spectrum obtained with the High Dispersion Spectrograph (HDS) on the Subaru telescope. The age of J1124+4535 is 11.3$\pm$4.4 Gyr using thorium and other r-process element abundances. J1124+4535 is confirmed to be a Galactic halo metal-poor ([Fe/H] = -1.27$\pm$0.1) star with extreme r-process element over-abundance ([Eu/Fe] = 1.13$\pm$0.08) and alpha element deficiency ([Mg/Fe] = -0.31$\pm$0.09) by the LAMOST-Subaru project. Along with the sub-solar alpha to iron ratios (e.g. [Mg/Fe], [Si/Fe], [Ca/Fe]), the relatively low abundances of Na, Cr, Ni and Zn in J1124+4535 show significant departure from the general trends of the Galactic halo but are in good agreement with those of dwarf galaxies. The chemical abundances and kinematics of J1124+4535 suggest it was formed in the late stage of star formation in a dwarf galaxy which has been disrupted by the Milky Way (MW). The star formation of its progenitor dwarf galaxy lasted more than 2 Gyr and has been affected by a rare r-process event before the occurrence of accretion event.

astro-ph.GA↗

arXiv2023

Timing the formation of the Galactic thin disc with asteroseismic stellar ages

The formation of the extended thin disc is the most spectacular event of our Galaxy in the past $\sim8$\,Gyr. To unveil this process, obtaining precise and accurate stellar ages for a large sample of stars is essential although challenging. In this work, we present the asteroseismic age determination of 5306 red giant branch stars using \kepler{} and LAMOST data, with a thorough examination of how the age determination is affected by the choice of different temperature scales and stellar models. Thanks to the high precision of the asteroseismic and spectroscopic parameters of our sample stars, we are able to achieve age determination with an average accuracy of 12 per cent. However, the age determination is sensitively dependent on the adopted temperature scale, as 50\,K difference in effective temperature may cause larger than 10 per cent systematic uncertainty in the age estimates. Using the ages derived with the most plausible set of the temperature scale, we study the age distribution of the chemical thin disc stars, and present an estimate of the formation epoch of the first Galactic thin disc stars. We find that the first (oldest) thin disc stars have an age of $9.5^{+0.5(\rm rand.)+0.5(\rm sys.)}_{-0.4(\rm rand.)-0.3(\rm sys.)}$\,Gyr, where the systematic uncertainties reflect ages estimated using different stellar evolutionary models. At this epoch, the Galactic thick disc was still forming stars, indicating there is a time window when both the thin and thick discs of our Galaxy were forming stars together. Moreover, we find that the first thin disc stars exhibit a broad distribution of Galactocentric radii, suggesting that the inner and outer thin discs began to form simultaneously.

astro-ph.GA↗

arXiv2021

Swivel: Hardening WebAssembly against Spectre

We describe Swivel, a new compiler framework for hardening WebAssembly (Wasm) against Spectre attacks. Outside the browser, Wasm has become a popular lightweight, in-process sandbox and is, for example, used in production to isolate different clients on edge clouds and function-as-a-service platforms. Unfortunately, Spectre attacks can bypass Wasm's isolation guarantees. Swivel hardens Wasm against this class of attacks by ensuring that potentially malicious code can neither use Spectre attacks to break out of the Wasm sandbox nor coerce victim code-another Wasm client or the embedding process-to leak secret data. We describe two Swivel designs, a software-only approach that can be used on existing CPUs, and a hardware-assisted approach that uses extension available in Intel 11th generation CPUs. For both, we evaluate a randomized approach that mitigates Spectre and a deterministic approach that eliminates Spectre altogether. Our randomized implementations impose under 10.3% overhead on the Wasm-compatible subset of SPEC 2006, while our deterministic implementations impose overheads between 3.3% and 240.2%. Though high on some benchmarks, Swivel's overhead is still between 9x and 36.3x smaller than existing defenses that rely on pipeline fences.

cs.CR↗

arXiv2018

Statistical Modeling of an astro-comb for high precision radial velocity observation

The advent of the laser frequency comb as the wavelength calibration unit allows us to measure the radial velocity at $cm\ s^{-1}$ precision level with high stability in long-term, which enable the possibility of the detection of Earth-twins around solar-like stars. Recent study shows that the laser frequency comb can also be used to measure and study the precision of the instrumental system including the variations of line profile and the systematic uncertainty and instrumental drift. In this paper, we present the stringent analysis of a laser frequency comb(LFC) system with 25GHz repetition frequency on a R$\sim$50,000 spectrograph with the wavelength spanning from 5085Å\ to 7380Å. We report a novel fitting model optimized for the comb line profile, the constrained double Gaussian. The constraint condition is set as $\left|μ_{1,2} - μ\right| <\sqrt{2ln2}σ$. We introduce Bayesian information criterion to test various models. Compared to the traditional Gaussian model, the CDG(Constrained Double Gaussians) model provides much better goodness of fit. We apply the CDG model to the observed comb data to demonstrate the improvement of RV precision with CDG model. We find that the improvement of CDG model is about 40\%$\sim$60\% for wavelength calibration precision. We also consider the application to use the LFC and CDG model as a tool to characterize the line shape variation across the detector. The motivation of this work is to measure and understand the details of the comb lines including their asymmetry and behaviors under various conditions, which plays a significant role in the simultaneous calibration process and cross-correlation function method to determine the Doppler shift at high precision level.

astro-ph.IM↗

arXiv2016

Code Definition Analysis for Call Graph Generation

Enterprise level software is implemented using multi-layer architecture. These layers are often implemented using de-coupled solutions with millions of lines of code. Programmers often have to track and debug a function call from user interface layer to the data access layer while troubleshooting an issue. They have to inspect the code based on search results or use design documents to construct the call graph. This process is time consuming and laborious. The development environment tools are insufficient or confined to analyzing only the code in the loaded solution. This paper proposes a method to construct a call graph of the call across several layers of the code residing in different code bases to help programmers better understand the design and architecture of the software. The signatures of class, methods, and properties were evaluated and then matched against the code files. A graph of matching functions was created. The recursive search stopped when there were no matches or the data layer code was detected. The method resulted in 78.26% accuracy when compared with manual search.

cs.SE↗

arXiv2016

Dynamic Probabilistic Network Based Human Action Recognition

This paper examines use of dynamic probabilistic networks (DPN) for human action recognition. The actions of lifting objects and walking in the room, sitting in the room and neutral standing pose were used for testing the classification. The research used the dynamic interrelation between various different regions of interest (ROI) on the human body (face, body, arms, legs) and the time series based events related to the these ROIs. This dynamic links are then used to recognize the human behavioral aspects in the scene. First a model is developed to identify the human activities in an indoor scene and this model is dependent on the key features and interlinks between the various dynamic events using DPNs. The sub ROI are classified with DPN to associate the combined interlink with a specific human activity. The recognition accuracy performance between indoor (controlled lighting conditions) is compared with the outdoor lighting conditions. The accuracy in outdoor scenes was lower than the controlled environment.

cs.CV↗

arXiv2006

High resolution spectroscopy of halo stars within the spectral region 3550-5000AA

An atlas of high resolution (R=60000) in the poor studied wavelength range 3550-5000 AA for 4 metal-deficient stars in the interval of metallicity -3.0<[Fe/H]< -0.6, effective temperature 4750<Teff<5900K, surface gravity 1.6<log g<5.0 is produced. Details of the method of producing a spectral atlas, line identifications, stellar atmospheric parameters determination are described. Based on these spectral data, we determined model atmosphere parameters and calculated abundances of 25 chemical elements.

astro-ph↗