arXiv subjects

J. R. Shi

Publications and source records attributed to J. R. Shi.

27 records · Page 2Linked to original sources

arXiv2008

Statistical equilibrium of silicon in the solar atmosphere

The statistical equilibrium of neutral and ionised silicon in the solar photosphere is investigated. Line formation is discussed and the solar silicon abundance determined. High-resolution solar spectra were used to determine solar $\log gfε_{\rm Si}$ values by comparison with Si line synthesis based on LTE and NLTE level populations. The results will be used in a forthcoming paper for differential abundance analyses of metal-poor stars. A detailed analysis of silicon line spectra leads to setting up realistic model atoms, which are exposed to interactions in plane-parallel solar atmospheric models. The resulting departure coefficients are entered into a line-by-line analysis of the visible and near-infrared solar silicon spectrum. The statistical equilibrium of \ion{Si}{i} turns out to depend marginally on bound-free interaction processes, both radiative and collisional. Bound-bound interaction processes do not play a significant role either, except for hydrogen collisions, which have to be chosen adequately for fitting the cores of the near-infrared lines. Except for some near-infrared lines, the NLTE influence on the abundances is weak. Taking the deviations from LTE in silicon into account, it is possible to calculate the ionisation equilibrium from neutral and ionised lines. The solar abundance based on the experimental $f$-values of Garz corrected for the Becker et al.'s measurement is $7.52 \pm 0.05$. Combined with an extended line sample with selected NIST $f$-values, the solar abundance is $7.52 \pm 0.06$, with a nearly perfect ionisation equilibrium of $Δ\logε_\odot(\ion{Si}{ii}/\ion{Si}{i}) = -0.01$.

astro-ph↗

arXiv2007

Non-LTE line formation for heavy elements in four very metal-poor stars

Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba, and Eu are determined for four very metal-poor stars (-2.66 < [Fe/H] < -2.15) based on non-LTE line formation and analysis of high-resolution (R ~60000 and 90000) high signal-to-noise (S/N > 200) observed spectra. A model atom for H I is presented. An effective temperature was obtained from the Balmer Halpha and Hbeta line wing fits, the surface gravity from the Hipparcos parallax if available and the non-LTE ionization balance between Ca I and Ca II. Based on the hyperfine structure affecting the Ba II resonance line, the fractional abundance of the odd isotopes of Ba was derived for HD 84937 and HD 122563 from a requirement that Ba abundances from the resonance line and subordinate lines of Ba II must be equal. For each star, non-LTE leads to a consistency of Teff from two Balmer lines and to a higher temperature compared to the LTE case, by up to 60 K. Non-LTE effects are important in spectroscopic determination of surface gravity from Ca I/Ca II. For each star with a known trigonometric gravity, non-LTE abundances from the lines of two ionization stages agree within the error bars, while a difference in the LTE abundances consists of 0.23 dex to 0.40 dex for different stars. Departures from LTE are found to be significant for the investigated atoms, and they strongly depend on stellar parameters. For HD 84937, the Eu/Ba ratio is consistent with the relative solar system r-process abundances, and the fraction of the odd isotopes of Ba, f_odd, equals 0.43+-0.14. The latter can serve as a constraint on r-process models. The lower Eu/Ba ratio and f_odd = 0.22+-0.15 found for HD 122563 suggest that the s-process or the unknown process has contributed significantly to the Ba abundance in this star.

astro-ph↗

arXiv2006

The lithium-rotation correlation for WTTS in Taurus-Auriga

Surface lithium abundance and rotation velocity can serve as powerful and mutually complementary diagnostics of interior structure of stars. So far, the processes responsible for the lithium depletion during pre-main sequence evolution are still poorly understood. We investigate whether a correlation exists between equivalent widths of Li (EW(Li)) and rotation period (P$_{rot}$) for Weak-line T Tauri stars (WTTSs). We find that rapidly rotating stars have lower EW(Li) and the fast burning of Li begins at the phase when star's P$_{rot}$ evolves towards 3 days among 0.9M$_\odot$ to 1.4M$_\odot$ WTTSs in Taurus-Auriga. Our results support the conclusion by Piau & Turch-Chiéze about a model for lithium depletion with age of the star and by Bouvier et al. in relation to rotation evolution. The turn over of the curve for the correlation between EW(Li) and P$_{rot}$ is at the phase of Zero-Age Main Sequence (ZAMS). The EW(Li) decreases with decreasing P$_{rot}$ before the star reaches the ZAMS, while it decreases with increasing P$_{rot}$ (decreasing rotation velocity) for young low-mass main sequence stars. This result could be explained as an age effect of Li depletion and the rapid rotation does not inhibit Li destruction among low mass PMS stars.

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

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↗

arXiv2006

Doping dependence of the coupling of electrons to bosonic modes in the single-layer high-temperature Bi2Sr2CuO6 superconductor

A recent highlight in the study of high-Tc superconductors is the observation of band renormalization / self-energy effects on the quasiparticles. This is seen in the form of kinks in the quasiparticle dispersions as measured by photoemission and interpreted as signatures of collective bosonic modes coupling to the electrons. Here we compare for the first time the self-energies in an optimally doped and strongly overdoped, non-superconducting single-layer Bi-cuprate (Bi2Sr2CuO6). Besides the appearance of a strong overall weakening, we also find that weight of the self-energy in the overdoped system shifts to higher energies. We present evidence that this is related to a change in the coupling to c-axis phonons due to the rapid change of the c-axis screening in this doping range.

cond-mat.supr-con↗

arXiv2004

Sodium abundances in nearby disk stars

We present sodium abundances for a sample of nearby stars. All results have been derived from NLTE statistical equilibrium calculations. The influence of collisional interactions with electrons and hydrogen atoms is evaluated by comparison of the solar spectrum with very precise fits to the Na I line cores. The NLTE effects are more pronounced in metal-poor stars since the statistical equilibrium is dominated by collisions of which at least the electronic component is substantially reduced. The resulting influence on the determination of sodium abundances is in a direction opposite to that found previously for Mg and Al. The NLTE corrections are about -0.1 in thick-disk stars with [Fe/H] about -0.6. Our [Na/Fe] abundance ratios are about solar for thick- and thin-disk stars. The increase in [Na/Fe] as a function of [Fe/H] for metal-rich stars found by Edvardsson et al. (1993) is confirmed. Our results suggest that sodium yields increase with the metallicity, and quite large amounts of sodium may be produced by AGB stars. We find that [Na/Fe]ratios, together with either [Mg/Fe] ratio, kinematic data or stellar evolutionary ages, make possible the individual discrimination between thin- and thick-disk membership.

astro-ph↗

arXiv2001

Sources of carbon and the evolution of the abundance of CNO elements

Using the standard infall model of Galactic chemical evolution, we explore the origin of carbon and calculate the abundance evolution of CNO elements for 8 different models of stellar nucleosynthesis yields. The results show that, in the early stage of the Galaxy, massive stars are the main producer of carbon, and that as our Galaxy evolves to the late stage, the longer lived intermediate- and low-mass stars play an increasingly important role, while at the same time, metal-rich Wolf-Rayet stars eject a significant amount of carbon into the ISM by radiative-driven stellar winds. However, from the present published nucleosynthesis yields we cannot distinguish whether the main source of carbon in the late Galactic stage is just the massive stars ($M>8M_{\odot}$) alone, or just the intermediate-, low-mass stars and $M\leq 40M_{\odot}$ massive stars that do not go through the Wolf-Rayet stage. The $^{12}$C$(α,γ)^{16}$O reaction rate is very important in the stellar nucleosynthesis calculations: a lower rate will give a higher yield of carbon. The contribution to nitrogen is dominated by intermediate- and low-mass stars, and the secondary source of massive stars cannot explain the observed [N/Fe] in metal-poor stars. Most of oxygen is produced by massive stars. The fact that a higher O abundance in metal-poor stars is derived from the \ion{O}{i} 7771$-$7775 Åtriplet than from the forbidden [\ion{O}{i}] line at 6300 Åposes a problem.

astro-ph↗

arXiv2000

Cerenkov Line-like Radiation, The Extended and Improved Formulae System

You & Cheng (1980) argued that, for relativistic electrons moving through a dense gas, the Cerenkov effect will produce peculiar atomic and/or molecular emission lines--Cerenkov lines. They presented a series of formulae to describe the new line-mechanism. Elegant experimental confirmation has been obtained by Xu. et al. in the laboratory which definitely verified the existence of Cerenkov lines. Owing to the potential importance in high energy astrophysics, in this paper we give a more detailed physical discussion of the emission mechanisms and improve the previous formulae system into a form which is more convenient for astrophysical application. Specifically, the extended formulae in this paper can be applied to other species of atoms and/or ions rather than hydrogen as in the previous paper, and also to X-ray astronomy. They can also be used for the calculation of a nonuniform plane-parallel slab of the emissive gas.

astro-ph↗