科研之友
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摘要
Context. The freezeout of gas phase species onto cold dust mains can drastically alter the chemistry and the heating-cooling balance of protostellar material. In contrast to well-known species such as carbon monoxide (CO), the treezeout of various carriers with abundances <10(-5) has not yet been well studied.
Aims. Our aim here is to study the depletion of chlorine in the protosteliar core, OMC-2 FIR 4.
Methods. We observed transitions HCl of H2Cl+ and towards 0MG-2 FIR 4 using the Herschel Space Observatory and Caliech Submillimeter Observatory facilities. Our analysis makes use of state of the art chlorine gas grain chemical models and newly calculated HCl-H-2 hyperfine collisional excitation rate coefficients.
Results. A narrow emission component in the HCl lines traces the extended envelope, and a broad one traces a more compact central region. The gas phase HCl abundance in FIR 4 is 9 x 10(-11), a factor of only 10(-3) that of volatile elemental chlorine. The H2Cl+ lines are detected in absorption and trace a tenuous foreground cloud, where we find no depletion of volatile chlorine.
Conclusions. Gas-phase FICI is the tip of the chlorine iceberg in protostellar cores. Using a gas-grain chemical model, we show that the hydrogenation of atomic chlorine on grain surfaces in the dark cloud stage sequesters at least 90% of the volatile chlorine into HC1 ice, where it remains in the protostellar stage. About 10% of chlorine is in gaseous atomic form. Gas phase HC1 is a minor, but dingnostically key reservoir, with an abundance of less than or similar to 10(-10) in most of the protostellar core. We find the [Cl-35/[Cl-37] ratio in OMC-2 FIR 4 to be 3.2 +/- 0.1, consistent with the solar system value.
- 出版日期2015-2
