Si abundance ratio as a function of stellar parameters. The arrows refer to upper limits; otherwise, the symbols are the same as in Fig. 2. The average error bar is shown in the lower right corner of each panel. The crosses are the results of Preston et al. (2006), while the open triangles are those of Lai et al. (2008). Besides, in the upper panel the dashed line, short dashed line, and dot-dashed line represent the least-square fits of the results of our observed data, Preston et al. (2006), and Lai et al. (2008), respectively.