arXiv subjects

G. Y. Liang

Publications and source records attributed to G. Y. Liang.

9 recordsLinked to original sources

arXiv2022

X-ray morphology due to charge-exchange emissions used to study the global structure around Mars

Soft x-ray emissions induced by solar wind ions that collide with neutral material in the solar system have been detected around planets, and were proposed as a remote probe for the solar wind interaction with the Martian exosphere. A multi-fluid three-dimensional magneto-hydrodynamic model is adopted to derive the global distributions of solar wind particles. Spherically symmetric exospheric H, H$_2$, He, O, and CO$_2$ density profiles and a sophisticated hybrid model that includes charge-exchange and proton/neutral excitation processes are used to study the low triplet line ratio $G=\frac{i+f}{r}$ (0.77$\pm$0.58) of O VII and total x-ray luminosity around Mars. We further calculate the emission factor $α$-value with different neutrals over a wide ion abundance and velocity ranges. Our results are in good agreement with those of previous reports. The evolution of the charge stage of solar wind ions shows that sequential recombination due to charge-exchange can be negligible at the interaction region. This only appears below the altitude of 400~km. The anonymous low disk $G$ ratio can be easily explained by the collisional quenching effect at neutral densities higher than 10$^{11}$cm$^{-3}$. However, the quenching contribution is small in Mars' exosphere and only appears below 400~km. Charge-exchange with H$_2$ and N$_2$ is still the most likely reason for this low $G$-ratio. X-ray emissivity maps in collisions with different neutrals differ from each other. A clear bow shock in the collision with all the neutrals is in accordance with previous reports. The resulting total x-ray luminosity of 6.55~MW shows a better agreement with the XMM-Newton observation of 12.8$\pm$1.4~MW than that of previous predictions.

astro-ph.EP↗

arXiv2021

Measurement of n-resolved State-Selective Charge Exchange in Ne(8,9)+ Collision with He and H2

Charge exchange between highly charged ions and neutral atoms and molecules has been considered as one of the important mechanisms controlling soft X ray emissions in many astrophysical objects and environments. However, for modeling charge exchange soft X ray emission, the data of n and l resolved state selective capture cross sections are often obtained by empirical and semiclassical theory calculations. With a newly built cold target recoil ion momentum spectroscopy (COLTRIMS) apparatus, we perform a series of measurements of the charge exchange of Ne(8,9)+ ions with He and H2 for collision energy ranging from 1 to 24.75 keV/u. n resolved state selective capture cross-sections are reported. By comparing the measured state selective capture cross sections to those calculated by the multichannel Landau Zener method (MCLZ), it is found that MCLZ calculations are in good agreement with the measurement for the dominant n capture for He target. Furthermore, by using nl resolved cross sections calculated by MCLZ and applying l distributions commonly used in the astrophysical literature to experimentally derived n resolved cross sections, we calculate the soft X ray emissions in the charge exchange between 4 keV/u Ne8+ and He by considering the radiative cascade from the excited Ne7+ ions. Reasonable agreement is found in comparison to the measurement for even and separable models, and MCLZ calculations give results in a better agreement.

astro-ph.GA↗

arXiv2014

X-ray and EUV spectroscopy of various astrophysical and laboratory plasmas -- Collisional, photoionization and charge-exchange plasmas

Several laboratory facilities were used to benchmark theoretical spectral models those extensively used by astronomical communities. However there are still many differences between astrophysical environments and laboratory miniatures that can be archived. Here we setup a spectral analysis system for astrophysical and laboratory (SASAL) plasmas to make a bridge between them, and investigate the effects from non-thermal electrons, contribution from metastable level-population on level populations and charge stage distribution for coronal-like, photoionized, and geocoronal plasmas. Test applications to laboratory measurement (i.e. EBIT plasma) and astrophysical observation (i.e. Comet, Cygnus X-3) are presented. Time evolution of charge stage and level population are also explored for collisional and photoionized plasmas.

astro-ph.SR↗

arXiv2011

R-matrix electron-impact excitation data for astrophysically abundant sulphur ions

We present results for the electron-impact excitation of highly-charged sulphur ions (S8+ - S11+) obtained using the intermediate-coupling frame transformation R-matrix approach. A detailed comparison of the target structure has been made for the four ions to assess the uncertainty on collision strengths from the target structure. Effective collision strengths (Υs) are presented at temperatures ranging from 2\times10^2(z+1)^2 K to 2\times10^6(z+1)^2 K (where z is the residual charge of ions. Detailed comparisons for the Υare made with the results of previous calculations for these ions, which will pose insight on the uncertainty in their usage by astrophysical and fusion modelling codes.

astro-ph.SR↗

arXiv2008

Soft X-ray spectroscopy of highly charged silicon ions in dense plasmas

Rich soft X-ray emission lines of highly charged silicon ions (Si VI--Si XII) were observed by irradiating an ultra-intense laser pulse with width of 200 fs and energy of $\sim$90 mJ on the solid silicon target. The high resolution spectra of highly charged silicon ions with full-width at half maximum (FWHM) of $\sim$0.3--0.4Åis analyzed in wavelength range of 40--90 Å. The wavelengths of 53 prominent lines are determined with statistical uncertainties being up to 0.005 Å. Collisional-radiative models were constructed for Si VI -- Si XII ions, which satisfactorily reproduces the experimental spectra, and helps the line identification. Calculations at different electron densities reveal that the spectra of dense plasmas are more complicate than the spectra of thin plasmas. A comparison with the Kelly database reveals a good agreement for most peak intensities, and differences for a few emission lines.

astro-ph↗

arXiv2007

Soft X-ray emissions of highly charged Si VII--Si XII in cool star--Procyon

Different observation data for cool star--Procyon (Obs\_IDs of 63, 1461 and 1224) available from {\it Chandra Data Public Archive} were co-added and analyzed. Emissivities of emission lines of highly charged silicon ions (Si VII--Si XII) were calculated over temperatures by adopting the published data of Liang et al. (2007, {\it Atom. Data and Nucl. Data Tables}, {\bf 93}, 375). Using the emission measure derived by Raassen et al. (2002, A&A, {\bf 389}, 228), the theoretical line fluxes are predicted, and the theoretical spectra are constructed by assuming the Gaussian profile with instrumental broadening (0.06 Å). By detailed comparison between observation and predictions, several emissions lines are identified firstly such as emissions at 43.663 Å(Si XI), 45.550 Å(Si XII), 46.179 Å(Si VIII), 50.874 Å(Si X), 64.668 Å(Si IX), and 73.189 Å(Si VII) etc. Several emission lines are re-assigned in this work, such as the emission line at 52.594 Åto Si X (52.612 Å), at 69.641 Åto the blending of Si VII (69.632 Å) and Si VIII (69.664 Å) lines, as well as at 70.050 Åto Si VII (70.027 Å). The prediction reveals the large discrepancies between the $3s$--$2p$ line (63.715 Å) {\it versus} 3d--$2p$ line (61.012 Å) again for lower charge stage Si VIII. Solar flare observation is also added for the assessment of present calculation. Different assignments for some lines between Procyon and solar flare, have been found to be due to the hotter emitter in Sun than in Procyon coronae.

astro-ph↗

arXiv2006

Electron density diagnostic potential of Ar XIV soft X-ray emission lines

Theoretical electron density-sensitive line ratios $R_1 - R_6$ of Ar XIV soft X-ray emission lines are presented. We found that these line ratios are sensitive to electron density $n_e$, and the ratio $R_1$ is insensitive to electron temperature $T_e$. Recent work has shown that accurate atomic data, such as electron impact excitation rates, is very important for reliable determination of the electron density of laboratory and astrophysical plasmas. Present work indicates that the maximum discrepancy of line ratios introduced from different atomic data calculated with distorted wave and R-matrix approximations, is up to 18% in the range of $n_e=10^{9-13}$cm$^{-3}$. By comparison of these line ratios with experiment results carried out in electron beam ion trap (EBIT-II), electron density of the laboratory plasma is diagnosed, and a consistent result is obtained from $R_1$, $R_2$ and $R_3$. Our result is in agreement with that diagnosed by Chen et al using triplet of N VI. A relative higher diagnosed electron density from $R_2$ is due to its weak sensitivity to electron temperature. A better consistency at lower $T_e$ indicates that temperature of the laboratory plasma is lower than log$T_e$(K)=6.5. Comparison between the measured and theoretical ratios reveals that 32.014~Åline is weakly blended by lines from other Ar ions, while 30.344~Åline is strongly contaminated.

physics.plasm-ph↗

arXiv2006

Determinations of temperature and density for solar-like Stars using Si XI soft X-ray emission lines

We study temperature and density sensitivities of ratios of Si XI soft X-ray emission lines, in the wavelength range of 43--54Å. The typical temperature of the formation of the analyzed lines is around 1.6~MK, which makes this analysis complementary to the analysis of He-like triplets being sensitive to hotter plasma. We present theoretical calculations and compare them with ratios obtained from high-resolution X-ray spectra of five solar-like stars: Procyon, $α$ Cen A$&$B, $ε$ Eri, and Capella. We find that our results are in good agreement with results obtained by other authors through different diagnostics, namely the analysis of density- and temperature-sensitive He-like triplet lines. We further estimate the coronal pressure and filling factors from Si~XI lines in this study.

astro-ph↗

arXiv2006

Coronal density diagnostics with Si X: CHANDRA/LETGS observations of Procyon, $α$ Cen A$&$B, Capella and $ε$ Eri

Electron density diagnostics based on a line intensity ratio of Si X are applied to the X-ray spectra of Procyon, $α$ Cen A$&$B, Capella and $ε$ Eri measured with the Low Energy Transmission Grating Spectrometer (LETGS) combined with High-resolution Camera (HRC) on board the {\it Chandra X-ray Observatory}. The ratio $R_1$ of the intensities of the Si X lines at 50.524 Åand 50.691 Åis adopted. A certain of the temperature effect in $R_1$ appears near the low-density limit region, which is due to the contamination of Si X line at 50.703 Å. Using the emission measure distribution (EMD) model derived by Audard et al. (2001) for Capella and emissivities calculated with APEC model by Smith et al. (2001), we successfully estimate contributions of Fe XVI lines at 50.367 Åand 50.576 Å(73% and 62%, respectively). A comparison between observed ratios and theoretical predictions constrains the electron densities (in logarithmic) for Procyon to be 8.61$^{+0.24}_{-0.20}$ cm$^{-3}$, while for $α$ Cen A$&$B, Capella and $ε$ Eri to be 8.81$^{+0.27}_{-0.23}$ cm$^{-3}$, 8.60$^{+0.39}_{0.32}$ cm$^{-3}$, 9.30$_{-0.48}$ cm$^{-3}$ and 9.11$^{+1.40}_{-0.38}$ cm$^{-3}$, respectively. The comparison of our results with those constrained by the triplet of He-like carbon shows a good agreement. For normal stars, our results display a narrow uncertainty, while for active stars, a relatively larger uncertainty, due to the contamination from Fe XVI lines, is found. Another possible reason may be that the determination of the continuum level, since the emission lines of Si X become weak for the active stars. For $ε$ Eri, an electron density in the C V forming region was estimated firstly through Si X emissions.

astro-ph↗