The Differential Doppler effect occurs when light is emitted from a rotating source.

In circumstellar

Circumstellar dust

Circumstellar Dust is astronomical dust around a star. It can be in the form of a spherical shell or a disk, e.g. an accretion disk. Circumstellar dust can be responsible for significant extinction and is usually the source of an infrared excess for stars that have it.The motion of circumstellar...

 environments it describes the difference in photons arriving at orbiting dust particles. Photons that originate from the limb that is rotating away from the particle are red-shifted

Redshift

In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...

, while photons emitted from the limb rotating toward the particle are blue-shifted.