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

Amorphous silicate dust grains have been produced in the laboratory by mean s of laser ablation of solid targets in different ambient atmospheres. In t his work we show that, if the condensation occurs in the presence of hydrog en, 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 feature s, 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 co ndense in the atmospheres of oxygen-rich stars where hydrogen is also abund ant. This means that SiH functional groups may be present also in the circu mstellar and interstellar silicate dust grains. An absorption feature at 4. 6 mu m has already been observed in the absorbing dust of several protostel lar embedded sources. The observation of a similar feature in comets can gi ve important information on the origin and evolution of cometary material. (C) 1999 Elsevier Science Ltd. All rights reserved.