Poster

The Mysterious 30 Micrometer Emission Feature of Evolved Stars: MgS or Graphite?

¹Ke Zhang, ²Biwei Jiang, ³Aigen Li (¹Caltech, ²Beiing Normal University, ³University of Missouri)

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 micrometer. This feature, extending from ~24 to ~45 micrometer, 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 micrometer 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 micrometer feature seen in this source and in other sources as well. We further argue that graphite may be a promising candidate, as the free electrons in graphite seem to produce a strong resonance band at 30 micrometer.

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