Si and Fe Depletion in Star-forming Regions Probed by Infrared Spectroscopy

Abstract

Listen

Translate article

Si and Fe are major constituents of silicate and their gas phase abundance are 5% and 0.5% solar, respectively, in cool clouds [3]. We made observations of 11 Galactic star-forming regions by Infrared Spectrograph (IRS) on board the Spitzer Space Telescope to examine the depletion pattern of Si and Fe. We detected [Si II]35μm emission for all the observed positions and [Fe II]26μm and [Fe III]23μm in most regions. For the regions with Nii122μm or [O I]146μm data available by previous observations, we derived the gas-phase Si and Fe abundance relative to N or O from line ratios such as [Si II]35μm/Nii 122μm. The derived Si abundance is 20% solar or larger in Hiiregions and 10–20% solar in photodissociation regions (PDRs). From the results of all the observed regions, the gas phase Si abundance in Hiiregions is found to be several tens of percent in a wide range of exciting energy. This suggests that a significant fraction of Si-bearing dust grains are destructed in Hiiregions. Since large Si abundance in PDRs is reported in this work and in [1, 2], Si-bearing dust is more easily destroyed than silicate. In some regions gas phase Fe abundance is compatible with that in cool interstellar clouds, whereas some other regions have Fe abundance larger by about one order of magnitude than that in cool clouds. The ratio of Fe atoms released from dust grains to those remaining in dust is smaller than that of Si atoms. This suggests that a fraction of Si and Fe atoms reside in different components of interstellar dust.