5 Conclusions

Non-LTE effects of silicon in the Sun were found to be small. The solar silicon abundance becomes $\log \epsilon_{\rm NLTE} = 7.550 \pm 0.056$ with a mean non-LTE correction of $\Delta \log \epsilon = -0.010$. This matches almost exactly the previous values (e.g. Becker et al. 1980; Holweger 1979).
The effect of horizontal temperature inhomogeneities associated with convection on the photospheric abundance of Si has also been considered. According to preliminary results by Steffen (2000b), based on 2D hydrodynamics simulations, a mean granulation abundance correction of +0.021 dex is probably a safe upper limit, leading to a silicon abundance of 7.571.
As a by-product of the solar analysis, an assessment of the accuracy of the f-values was possible. The internal accuracy of the f-values of Garz (1973) is superior to those compiled by Wiese et al. (1969) and Fuhr & Wiese (1998).
For Vega, a Si abundance of $\log \epsilon_{\rm NLTE} = 6.951 \pm 0.100$ and a non-LTE correction of only $\Delta \log \epsilon = -0.054$ was derived. This confirms the value $\log \epsilon_{\rm Si} = 6.94$ quoted by Lemke (1990). With respect to the Sun, an underabundance of -0.599 dex results, confirming the general metal deficiency of Vega.

Acknowledgements

The author wishes to thank H. Holweger for suggesting and supporting this work. Further thanks are due to I. Kamp and M. Hempel for their useful comments and help with non-LTE calculations and abundance analysis, M. Steffen for providing unpublished data for granulation abundance corrections and to the referee Y. Takeda for helpful comments.