Si/o abundance ratios in extragalactic h ii regions from hubble space telescope uv spectroscopy1 2 3 4 5 6 7 8 9

We have measured the Si iii] intercombination doublet at 1883 A and 1892 A in seven extragalactic H II regions using the Faint Object Spectrograph on the Hubble Space Telescope. These measurements were used to derive the Si/C and Si/O gas phase abundance ratios in the ionized gas. We find that Si/O shows no systematic variation with O/H over the range —4.8 < log O/H < —3.4. The weighted mean value for Si/O over this range is log Si/O = —1.59 ± 0.07. For comparison, the solar value is log Si/O = —1.37, while for Galactic B stars and supergiants —1.6 < log Si/O < —1.2. Uncertainty in the stellar reference value for Si/O prevents a straightforward interpretation of our Si/O ratios. However, if the solar Si/O ratio represents the intrinsic cosmic ratio, our results imply that, on average, only about 50% of the total abundance of silicon in these H II regions is incorporated into dust grains, a value significantly smaller than is typically measured in dense interstellar clouds. Our results suggest that some grain modification is occurring within the H II region environment.