On Magnesium Sulfide as the Carrier of the 30 μm Emission Feature in Evolved Stars
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A large number of carbon-rich evolved objects (asymptotic giant branch stars, protoplanetary nebulae, and planetary nebulae) in both the Milky Way galaxy and the Magellanic Clouds exhibit an enigmatic broad emission feature at ~ 30 μm. This feature, extending from ~ 24 μm to ~ 45 μm, is very strong and accounts for up to ~ 30% of the total infrared luminosity of the object. In literature it is tentatively attributed to magnesium sulfide (MgS) dust. Using the prototypical protoplanetary nebula around HD 56126 for illustrative purpose, however, in this work we show that in order for MgS to be responsible for the 30 μm feature, one would require an amount of MgS mass substantially exceeding what would be available in this source. We therefore argue that MgS is unlikely the carrier of the 30 μm feature seen in this source and in other sources as well.