1 Introduction

Emission lines of Na-like ions are useful in diagnostics of astrophysical and laboratory plasmas. The relative intensities of the 3p-3s and 3d-3s lines in those ions, for instance, are appropriate for the determination of temperature, because the energy separation of those levels is relatively large. (See Keenan 1993 for the general aspect of the line ratio diagnostics and Flower & Nussbaumer 1975, Keenan et al. 1986, and Keenan 1988 for the Na-like ions.) To derive reliable values of temperature from such line ratios, we need atomic data, particularly those on the electron-impact excitation rates of ions. The present paper presents those rate coefficients for five Na-like ions (Al III, Si IV, S VI, Ar VIII, Ca X) obtained by an accurate calculation (i.e., an eleven-state R-matrix method).

In 1987, Dufton and Kingston reported the rate coefficients for Al III, Si IV and S VI. These are based on a calculation of the excitation cross section with a 5-state R-matrix method. For Al III, Mitroy & Norcross (1989) made a close-coupling calculation with five and nine states taken into account. They calculated also the rate coefficients. Using the data obtained by Dufton & Kingston (1987) and more recent R-matrix type calculations for Ti XII, Fe XVI and Ni XVIII, Keenan et al. (1996) estimated rate coefficients for a number of Na-like ions, with an interpolation and scaling procedure. Keenan et al. (1996) explicitly reported the rate coefficients for the Na-like ions with Z = 15, 17-21, 23-25 and 27.

Recently, Nakazaki et al. (1996, referred to as Paper I) made an elaborate calculation (i.e., with a seven and eleven state R-matrix method) for Ar VIII. They showed quantitatively the importance of resonant collisions and some details of the relevant differential cross section.

The present paper extends their calculation to four other Na-like ions (Al III, Si IV, S VI and Ca X). From the collision cross sections obtained, the rate coefficients are calculated for all the transitions among 3s, 3p, 3d, 4s, 4p, 4d, 4f states for the four Na-like ions and Ar VIII. The resulting values are expected to be more accurate and more comprehensive than the previous ones cited above and will serve as a standard for future applications.