S[CLC]i/O Abundance Ratios in Extragalactic H[/CLC] [CSC]ii[/CSC] Regions from [ITAL]Hubble Space Telescope[/ITAL] UV Spectroscopy

Open Access
Publisher Website
Google Scholar
Abstract
We have measured the Si III] intercombination doublet at 1883 Å and 1892 Å 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.

This publication has 24 references indexed in Scilit: