REASSESSMENT OF MILLIMETER-WAVE ABSORPTION-COEFFICIENTS IN INTERSTELLAR SILICATE GRAINS

ALMOST all of the elements heavier than helium in the interstellar med ium reside in small solid dust particles(1). Dust dominates the energy balance through its continuous absorption and emission of radiation. Its opacity provides the only direct means to assess the total mass in many interstellar regions: for example, in molecular clouds(2), in ci rcumstellar disks undergoing planet formation(3-5) and even in some ga laxies(6). But there are almost no laboratory measurements of small-pa rticle opacities at low temperatures for any of the principal constitu ents of interstellar dust at the far-infrared and millimetre wavelengt hs at which most observations are made. Here we report a laboratory st udy of absorption by forsterite (Mg2SiO4), the pure magnesian form of the mineral olivine, which is a main constituent of interstellar dust We have measured millimetre-wave absorption spectra of synthesized amo rphous and crystalline forsterite powders between 3.5 and 15 cm(-1) at temperatures below 30 K, the region most germane to estimates of inte rstellar dust mass. Our measurements indicate that the absorption per unit mass for this silicate is several times larger than that normally assumed in astronomical research(7-10). Estimates of the opacity of i nterstellar dust may have to be increased accordingly, causing the mas s of interstellar particles derived from observations of the opacity t o be lowered by a corresponding amount.