Laboratory production of magnesium sulfide grains and their characteristic infrared spectra due to shape

Nanosized MgS grains, which have been considered the origin of the 30 μm emission feature of carbon-rich evolved objects, were produced from the gas phase using an advanced gas evaporation method. The far-infrared spectrum of cubic MgS grains showed a characteristic absorption peak at 311 cm^-1 (32.1 μm) with three shoulders at 460,400 and 262 cm^-1 (21.7, 25.0 and 38.2 μm). On the other hand, when the grains were roundish or network-like, the absorption peak at 250 cm^-1 became predominant. The cubic MgS grains were produced by direct nucleation from the gas phase. In the case of production via a gas-solid reaction, the MgS grains were network-like. Therefore, the formation environments of MgS grains around carbon-rich evolved objects may be predicted from the intensity of 310 and 250 cm^-1 bands. We suggest that the origins of the absorption band at 310 and 250 cm ^-1 are (100), (110) and/or (111) surfaces of MgS grains, respectively.