Astron. Astrophys. 363, 1177-1185 (2000)

1. Introduction

The ionized silicon spectrum is of considerable interest from the point of view of both stellar and laboratory plasma studies. In atmospheres of A, B and O type stars, and white dwarfs, a large number of ion lines has been observed (Peytremann 1972). In atmospheres of such stars, Stark broadening is the dominant pressure broadening mechanism and the knowledge of the Stark parameters in several ionization stages is very important for a number of astrophysical problems. Even in atmospheres of relative cool stars as the Sun, where line broadening caused by collisions with neutral perturbers is dominant, Stark broadening may compete with other broadening mechanisms in the line wings (Vince et al. 1985). Stark broadening of spectral lines has also regained interest for Astrophysics (Seaton 1987) with the development of researches on the physics of stellar interiors: in subphotospheric envelopes, the modellisation of energy transport requires the knowledge of radiative opacities and thus the relevant atomic process must be known with accuracy. At these high temperatures and densities the Stark broadening of multicharged ionic lines plays a non-negligible role in the calculations of the opacities.

Contrary to the case of singly ionized silicon, there are few Stark experimental data available for SiIII . Therefore, it is important to provide new measurements, specially for the multiplets more sensitive to this effect. As the final result of this work, Stark width and shift of twelve visible SiIII lines are given; for two of them previous experimental data did not exist.

All the results have been obtained from measurements performed in a linear discharge lamp, where a mixture of silane and helium has been purposely prepared so that self-absorption effects were minimum. This plasma source makes it possible to acquire a wide range of electron density and temperature values in a single discharge. This is particularly interesting for making calibrations of the Stark widths and shifts and will make possible other plasma diagnostics in the future.

Interferometric and spectroscopic measurements have been used to determine the electron density evolution curve, which ranges from 0.2 to m^-3. The temperature, which ranges from 17 500 to 21 000 K, has been determined from SiIII /SiII intensities ratio, the Boltzmann-plot of SiII and HeI lines and from absolute emission intensity measurements of HeI lines. Other broadening mechanisms such as Doppler broadening or the instrumental function have been also taken into account to obtain the Stark widths. A two-temperature (2-T) partial local thermodynamic equilibrium (pLTE) model has described the plasma. Different plots of the Stark coefficients with plasma parameters have resulted in values referred to an electron density of m^-3. These values have been compared with previously published data.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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