The size distribution of dust toward HD 210121 as determined from extinction

Several observations suggest that the molecular cloud toward HD 210121 contains an enhanced relative abundance of small dust grains. In particular, the value of the ratio of total-to-selective extinction (R-v) is unusually low. In this paper, we estimate the size distribution of dust grains in this line of sight from the extinction curve observed in the near-infrared through the ultraviolet. We use the maximum entropy method (MEM) to find the smoothest possible size distribution consistent within a chi(2) confidence level fit to the extinction data. While MEM has been shown to be a powerful tool in modeling average extinction curves, we show in this paper that MEM can also be a useful technique in modeling individual lines of sight that differ significantly from Galactic averages. We review existing data for HD 210121 and calculate the dust-to-gas ratio in order to constrain the amount of material used in the grain model. We present size distributions for both two-component models of silicate and graphite and three-component models that include amorphous carbon. We compare HD 210121 with the average diffuse interstellar medium and with three other high-latitude lines of sight. The grain-size distribution toward HD 210121 contains a relative excess of grains with radius a < 0.1 mu m as well as a relative deficiency of grains with radius a > 0.1 mu m as compared with the average diffuse interstellar medium and other clouds at high latitude.