Physics – Atomic Physics
Scientific paper
Sep 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004iibc.book....1m&link_type=abstract
In Edition Academia, Zaduzbina Andrejevic, vol. 114, p. 1-95.
Physics
Atomic Physics
Spectrosocpy, Spectral Line Broadening, Stark Broadening, Atomic Physics, Cp Stars, Hgmn Stars, Chi Lupi Star
Scientific paper
Stark broadening of spectral lines is dominant pressure broadening mechanism in hot, early-type, stars and white dwarf atmospheres. This type of broadening might also be important even in interstellar molecular and ionized hydrogen clouds, as e.g. for radio spectral lines in W51 emission nebulae, and in cooler stars as solar type ones for transitions involving higher principal quantum numbers. Present abundance analyses for early-type stars show that 10% - 20% of A and B stars have abundance anomalies, including anomalies in isotopic composition. The abundance anomalies in these stars, called CP stars, could be caused by diffusion occurring in the presence of selective radiative acceleration. The chemical species that absorb more of the outgoing photons are dragged by the photons to the stellar surface. Advanced calculation of the Stark broadening parameters using strong-coupling quantum-mechanical method are so complicated that only limited number of data for spectral lines originating from low laying transitions can be calculated in an adequate way. On the other hand, semiclassical method needs a set of large number of atomic data, energy levels and oscillator strengths. This method is not applicable in adequate way to the Stark broadening calculation of Zr II, Zr III and Cd III because there is no sufficient number of reliable atomic data. Here we use the modified semiempirical approach (MSE) which include explicitly only levels with δ n = 0 and l'if=lif +/- 1, where n is the principal quantum number, l is the orbital quantum number and i and f denote initial and final level, respectively. Levels with δ n ≠ 0 are lumped together and approximately estimated, so that for Stark broadening parameter calculation we need less atomic data then in the semiclassical method. The accuracy of the MSE calculations for spectral line widths is around +/- 50%. We present the computed electron-impact broadening parameters, Stark widths and shifts, for 30 multiplets of Zr III and for 6 spectral lines of Zr II and Zr III. Also we calculated Stark broadening parameters for 84 spectral lines (12 singlets and 72 triplets) of Cd III obtained by using the modified semiempircal method - MSE. The data are given for an electron density of dex(23) m^{-3} and temperatures from 5000 to 50000 K for Zr II and Zr III and temperatures from 5000 to 60000 K for Cd III. We have analyzed the influence of the electron-impact broadening mechanism on the equivalent width and consequently on the determination of zirconium abundance. We investigated the effect of the electron-impact broadening in the so called "zirconium conflict" in the HgMn star chi Lupi. In order to test the importance of the electron-impact broadening effect in determinations of zirconium abundance, we synthesized the line profiles of 2 Zr II and 6 Zr III spectral lines using SYNTH code where the LTE conditions are assumed. We have modified the SYNTH code, which uses log w (rad/s) per electron for T=10000 K as an input parameter, replacing it with the two parameters A0 and A1. We chose these lines because they have been commonly used for abundance determination, as they have a small wavelength displacement and are well resolved. We have calculated the equivalent widths with and without the electron-impact broadening effect for different abundances of zirconium.
The electron-broadening effect is more important in the case of higher abundances of zirconium. The equivalent width increases with abundances for both lines, but the equivalent widths for Zr III lines are more sensitive than Zr II lines. This may cause an error in abundance determination in cases where the electron-impact broadening effect is not taken into account. In any case, synthesizing of these two lines in order to measure the zirconium abundance without taking into account the electron-impact widths will result in Zr III lines suggesting an abundance of zirconium higher than that obtained with the Zr II lines. However, this effect is less than one order of magnitude of the abundance.
Although the "zirconium conflict" in the HgMn star chi Lupi cannot be explained only by this effect, one should be aware that the electron-impact broadening effect may cause errors in abundance determination. Our analysis of the influence of Stark broadening on Cd III 144.754 nm spectral line demonstrates the importance of this broadening mechanism for hot star atmospheres analysis. However, for very hot stars, of the spectral class O, the influence of Stark broadening is reduced due to lower pressure in lower atmospheric layers. We hope that the presented set of Zr II, Zr III and Cd III electron-impact broadening parameters will be of interest in future investigations of astrophysical and laboratory plasma, modelling of stellar atmospheres and stellar spectra synthesis and analysis.
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