The 4.6 micron feature of -SiH groups in silicate dust grains and infrared cometary spectra

Amorphous silicate dust grains have been produced in the laboratory by means of laser ablation of solid targets in different ambient atmospheres. In this work we show that, if the condensation occurs in the presence of hydrogen, the spectra of silicate grains, together with the characteristic 10 and 20 mu m features, exhibit an absorption band around 4.6 mu m. Such features, absent in the spectra of the same silicate grains produced in an oxygen atmosphere, may be attributed to a fundamental stretching vibration of -SiH functional groups bound into the grains or on their surface. Based on the cosmic abundance of the elements, silicates are expected to condense in the atmospheres of oxygen-rich stars where hydrogen is also abundant. This means that SiH functional groups may be present also in the circumstellar and interstellar silicate dust grains. An absorption feature at 4.6 mu m has already been observed in the absorbing dust of several protostellar embedded sources. The observation of a similar feature in comets can give important information on the origin and evolution of cometary material. (C) 1999 Elsevier Science Ltd. All rights reserved.